ALBERT

Beschreibung: Aim Seamounts are conspicuous geological features with an important ecological role and can be considered vulnerable marine ecosystems (VMEs). Since many deep‐sea regions remain largely unexplored, investigating the occurrence of VME taxa on seamounts is challenging. Our study aimed to predict the distribution of four cold‐water coral (CWC) taxa, indicators for VMEs, in a region where occurrence data are scarce. Location Seamounts around the Cabo Verde archipelago (NW Africa). Methods We used species presence–absence data obtained from remotely operated vehicle (ROV) footage collected during two research expeditions. Terrain variables calculated using a multiscale approach from a 100‐m‐resolution bathymetry grid, as well as physical oceanographical data from the VIKING20X model, at a native resolution of 1/20°, were used as environmental predictors. Two modelling techniques (generalized additive model and random forest) were employed and single‐model predictions were combined into a final weighted‐average ensemble model. Model performance was validated using different metrics through cross‐validation. Results Terrain orientation, at broad scale, presented one of the highest relative variable contributions to the distribution models of all CWC taxa, suggesting that hydrodynamic–topographic interactions on the seamounts could benefit CWCs by maximizing food supply. However, changes at finer scales in terrain morphology and bottom salinity were important for driving differences in the distribution of specific CWCs. The ensemble model predicted the presence of VME taxa on all seamounts and consistently achieved the highest performance metrics, outperforming individual models. Nonetheless, model extrapolation and uncertainty, measured as the coefficient of variation, were high, particularly, in least surveyed areas across seamounts, highlighting the need to collect more data in future surveys. Main Conclusions Our study shows how data‐poor areas may be assessed for the likelihood of VMEs and provides important information to guide future research in Cabo Verde, which is fundamental to advise ongoing conservation planning.

Beschreibung: This study addresses the lithospheric structure of the West and Central African rift system (WCARS) and explores its origin and development in relation to the enigmatic Cameroon volcanic line (CVL). Based on a recent seismic tomography model, we subdivide the areas in tectonic domains. We perform integrated 3D geophysical and petrological forward modeling. By exploring the thickness and composition of different domains, we compare the model response to the observed topography and gravity anomalies, under consideration of the available seismic Moho depth points. Our model reveals three distinct domains within the study area: The WCARS is predominantly underlain by a Phanerozoic‐type lithospheric mantle, surrounded by the West African and the Congo Cratons, where the lithospheric mantle has a Proterozoic‐type signature. Between these domains, we identify a transition area where lithospheric thickness changes rapidly. Our preferred model shows significant variability of crustal thickness from 20 km in the rift area to 50 km beneath the cratons accompanied by thin lithosphere of 80 km in the rift area to thick lithosphere of up to 240 km beneath the cratons. The final model confirms that the WCARS' origin is passive, and suggests that the origin of the CVL, particularly its continental part, is the result of two tectonic events: (a) V‐shaped opening of the lithospheric mantle beneath the WCARS, resulting in (b) a strong variation of the lithosphere thickness at the transition between the rift zone and the northwestern part of the Congo craton. Plain Language Summary In this study, we describe the current structure of the subsurface (from the surface to a depth of 300 km) in Central and Western Africa. The aim is to understand the formation of the Central African Rift zone during the opening of the Atlantic Ocean, and how this relates to the linear chain of volcanoes that cross Cameroon, known as the Cameroon Volcanic Line. To achieve these objectives, we divide the study area into tectonic domains reflecting their seismological signature, and then, establish a three‐dimensional representation of the subsurface structure, based on fitting topography and gravity data. Our model confirms the geological subdivision of the study area into three blocks corresponding to two cratons and a rift zone, with transitional areas between them. Our model is compatible with a passive origin of the rifts in the region. We propose that the origin of the volcanic line of Cameroon is related to magma ascent during the separation of the African and South American plate in connection with the opening of the Atlantic and channeled by the lithospheric architecture. Key Points We present a new 3D model of the lithosphere for the West and Central African Rift System (WCARS) Our model confirms that the WCARS has a passive origin Our model suggest that the origin of the Cameroon volcanic line is linked to the architecture of the WCARS and adjoining cratons

Beschreibung: Despite impressive results achieved by many on‐land visual mapping algorithms in the recent decades, transferring these methods from land to the deep sea remains a challenge due to harsh environmental conditions. Images captured by autonomous underwater vehicles, equipped with high‐resolution cameras and artificial illumination systems, often suffer from heterogeneous illumination and quality degradation caused by attenuation and scattering, on top of refraction of light rays. These challenges often result in the failure of on‐land Simultaneous Localization and Mapping (SLAM) approaches when applied underwater or cause Structure‐from‐Motion (SfM) approaches to exhibit drifting or omit challenging images. Consequently, this leads to gaps, jumps, or weakly reconstructed areas. In this work, we present a navigation‐aided hierarchical reconstruction approach to facilitate the automated robotic three‐dimensional reconstruction of hectares of seafloor. Our hierarchical approach combines the advantages of SLAM and global SfM that are much more efficient than incremental SfM, while ensuring the completeness and consistency of the global map. This is achieved through identifying and revisiting problematic or weakly reconstructed areas, avoiding to omit images and making better use of limited dive time. The proposed system has been extensively tested and evaluated during several research cruises, demonstrating its robustness and practicality in real‐world conditions.

Beschreibung: The Arctic Ocean plays an important role in the regulation of the earth's climate system, for instance by storing large amounts of carbon dioxide within its interior. It also plays a critical role in the global thermohaline circulation, transporting water entering from the Atlantic Ocean to the interior and initializing the southward transport of deep waters. Currently, the Arctic Ocean is undergoing rapid changes due to climate warming. The resulting consequences on ventilation patterns, however, are scarce. In this study we present transient tracer (CFC-12 and SF6) measurements, in conjunction with dissolved oxygen concentrations, to asses ventilation and circulation changes in the Eurasian Arctic Ocean over three decades (1991–2021). We constrained transit time distributions of water masses in different areas and quantified temporal variability in ventilation. Specifically, mean ages of intermediate water layers in the Eurasian Arctic Ocean were evaluated, revealing a decrease in ventilation in each of the designated areas from 2005 to 2021. This intermediate layer (250–1,500 m) is dominated by Atlantic Water entering from the Nordic Seas. We also identify a variability in ventilation during the observation period in most regions, as the data from 1991 shows mean ages comparable to those from 2021. Only in the northern Amundsen Basin, where the Arctic Ocean Boundary Current is present at intermediate depths, the ventilation in 1991 is congruent to the one in 2005, increasing thereafter until 2021. This suggests a reduced ventilation and decrease in the strength of the Boundary Current during the last 16 years. Key Points Temporal variability of ventilation in the Eurasian Arctic Ocean during the past 30 years is estimated by observations of transient tracers We found a slow down of the ventilation between 2005 and 2021 in the intermediate waters Evidence of multidecadal variability of ventilation in the intermediate waters of the Eurasian Arctic Ocean is present Plain Language Summary The Eurasian Arctic Ocean, the region of the Arctic Ocean connected to the European and Asian continents, is an important pathway for recently ventilated water from the Nordic Seas. These waters are exported back to the North Atlantic following their travel through the Arctic Ocean. Ventilation describes the process of surface waters being transported into the interior ocean due to increasing density, which affects the underlying water masses. In this study we investigate how the ventilation patterns have evolved in the Eurasian Arctic Ocean over the past three decades, using transient tracer (CFC-12 and SF6) measurements. We observed a significant change in the intermediate layer (250–1,500 m) with older waters found in measurements in 1991 and 2021 compared to 2005 and 2015. Moreover, our data suggest a slowdown in ventilation throughout the three decades in the northern Amundsen Basin, implying a decrease in the circulation time-scale of the Arctic Ocean Boundary Current over the past 16 years. This has potentially important implications for the transport of, for example, heat, salt or oxygen from the Atlantic Ocean around the Arctic Ocean, and back.

Beschreibung: Freshwater input from Greenland ice sheet melt has been increasing in the past decades from warming temperatures. To identify the impacts from enhanced meltwater input into the subpolar North Atlantic from 1997 to 2021, we use output from two nearly identical simulations in the eddy-rich model VIKING20X (1/20°) only differing in the freshwater input from Greenland: one with realistic interannually varying runoff increasing in the early 2000s and the other with climatologically (1961–2000) continued runoff. The majority of the additional freshwater remains within the boundary current enhancing the density gradient toward the warm and salty interior waters yielding increased current velocities. The accelerated boundary current shows a tendency to enhanced, upstream shifted eddy shedding into the Labrador Sea interior. Further, the experiments allow to attribute higher stratification and shallower mixed layers southwest of Greenland and deeper mixed layers in the Irminger Sea, particularly in 2015–2018, to the runoff increase in the early 2000s. Key Points The West Greenland Current (WGC) freshens and cools with the observed recent increase in meltwater runoff from Greenland The density gradient across the boundary current intensifies, strengthening the WGC and increasing local eddy formation Enhanced meltwater runoff contributed to an eastward shift in deep convection towards the Irminger Sea (2015–2018) Plain Language Summary Global warming has accelerated the melting of the Greenland ice sheet over the past few decades resulting in enhanced freshwater input into the North Atlantic. The additional freshwater can potentially inhibit deep water formation and have future implications on ocean circulation. To determine the influence from Greenland melt, we compare two high-resolution model experiments all with the same forcing but differing input of Greenland freshwater fluxes from 1997 to 2021. We find that in the experiment with realistically increasing Greenland meltwater, the water becomes fresher and cooler along the continental shelf and boundary of the subpolar gyre. The density difference between the shelf and interior increases with more freshwater, resulting in faster West Greenland Current speeds and enhanced eddy formation. Deeper mixed layers are found in the eastern Irminger Sea, particularly in 2015–2018. From 2009 to 2013, there were shallower mixed layers in the Labrador Sea where less Greenland meltwater was mixed downwards and spread eastward, causing mixed layers to deepen in the Irminger Sea.

Beschreibung: Volcanic flank collapses, especially those in island settings, have generated some of the most voluminous mass transport deposits on Earth and can trigger devastating tsunamis. Reliable tsunami hazard assessments for flank collapse-driven tsunamis require an understanding of the complex emplacement processes involved. The seafloor sequence southeast of Montserrat (Lesser Antilles) is a key site for the study of volcanic flank collapse emplacement processes that span subaerial to submarine environments. Here, we present new 2D and 3D seismic data as well as MeBo drill core data from one of the most extensive mass transport deposits offshore Montserrat, which exemplifies multi-phase landslide deposition from volcanic islands. The deposits reveal emplacement in multiple stages including two blocky volcanic debris avalanches, secondary seafloor failure and a late-stage erosive density current that carved channel-like incisions into the hummocky surface of the deposit about 15 km from the source region. The highly erosive density current potentially originated from downslope-acceleration of fine-grained material that was suspended in the water column earlier during the slide. Late-stage erosive turbidity currents may be a more common process following volcanic sector collapse than has been previously recognized, exerting a potentially important control on the observed deposit morphology as well as on the runout and the overall shape of the deposit. Key Points Landslide emplacement offshore Montserrat included volcanic flank collapses, sediment incorporation, and a late-stage erosive flow Highly erosive flows are likely to be common processes during volcanic flank collapse deposition Pre-existing topography plays a major role in shaping flank collapse-associated mass transport deposits Plain Language Summary Disintegration of volcanic islands can cause very large landslides and destructive tsunamis. To assess the tsunami hazard of such events, it is crucial to understand the processes that are involved in their formation. We present new insights from seismic data and drill cores from a landslide deposit offshore Montserrat, a volcanic island in the Lesser Antilles Arc in the Caribbean. Our analysis reveals the emplacement of landslide material in several stages, including multiple volcanic flank collapses, incorporation of seafloor sediments and an erosive flow that carved channels into the top of the deposit right after its emplacement. We suggest that highly erosive flows are a common process during volcanic flank collapse deposition and that they play a significant role in the shaping of the deposit's appearance.

Beschreibung: Phytoplankton primary productivity (PP) varies significantly over environmental gradients, particularly in physically‐dynamic systems such as estuaries and coastal seas. During summer, runoff peaks in the Changjiang River driving large environmental gradients in both the Changjiang estuary and adjacent East China Sea (ECS), likely driving significant variability in PP. As satellite models of PP often underperform in coastal waters, we aimed to develop a novel approach for assessing net PP variability in such a dynamic environment. Parallel in situ measurements of Fast Repetition Rate (FRR) fluorometry and carbon (C) uptake rates were conducted for the first time in this region during two summer cruises in 2019 and 2021. A series of 13 C‐incubations ( n = 31) were performed, with measured PP ranging from ∼6 to 1,700 mgC m −3 d −1 . Net PP values were significantly correlated with salinity ( r = 0.45), phytoplankton chlorophyll a (Chl‐ a , r = 0.88), Photosystem II (PSII) functional absorption cross‐section ( σ PSII , r = −0.76) and maximum PSII quantum yield ( F v / F m , r = 0.59). Stepwise regression analysis showed that Chl‐ a and σ PSII were the strongest predictors of net PP. A generalized additive model (GAM) was also used to estimate net PP considering nonlinear effects of Chl‐ a and σ PSII . We demonstrate that GAM outperforms linear modeling approaches in estimating net PP in this study, as evidenced by a lower root mean square error (∼140 vs. 250 mgC m −3 d −1 ). Our novel approach provides a valuable tool to examine carbon cycling dynamics in this important region. Plain Language Summary The East China Sea has a complex current system that creates a highly dynamic physical environment for phytoplankton, particularly during the summer months. Net primary productivity (PP) is highly variable in this region, yet characterizing these spatial patterns in PP is difficult due to the lack of a high‐resolution data collecting method. Therefore, a strong need exists for a quick and easily implemented method for monitoring PP in this dynamic system. Based on parallel measurements of phytoplankton biomass and photophysiology, we present a novel approach that allows us to rapidly and easily assess regional PP at a high resolution. The high data volume potentially afforded by our net PP estimation method could not only contribute to a better understanding of PP variations in such a dynamic environment, but also help fill the large gaps in field data needed for validating satellite‐based PP models. Key Points Parallel in situ measurements of net primary productivity (PP) and Fast Repetition Rate fluorometry were conducted in the Changjiang estuary Productivity was highest at stations with high Chl and low σ PSII , typically located along the Chiangjiang river plume front A generalized additive model was developed to estimate net PP, providing an approach for assessing regional C‐cycling dynamics

Beschreibung: Submarine landslides pose a hazard to coastal communities and critical seafloor infrastructure, occurring on all of the world's continental margins, from coastal zones to hadal trenches. Offshore monitoring has been limited by the largely unpredictable occurrence of submarine landslides and the need to cover large regions. Recent subsea monitoring has provided new insights into the preconditioning and run-out of submarine landslides using active geophysical techniques. However, these tools measure a small spatial footprint and are power- and memory-intensive, thus limiting long-duration monitoring. Most landslide events remain unrecorded. In this chapter, we first show how passive acoustic and seismologic techniques can record acoustic emissions and ground motions created by terrestrial landslides. This terrestrial-focused research has catalyzed advances in characterizing submarine landslides using onshore and offshore networks of broadband seismometers, hydrophones, and geophones. We discuss new insights into submarine landslide preconditioning, timing, location, velocity, and down-slope evolution arising from these advances. Finally, we outline challenges, emphasizing the need to calibrate seismic and acoustic signals generated by submarine landslides. Passive seismic and acoustic sensing has a strong potential to enable more complete hazard catalogs to be built and open the door to emerging techniques (such as fiber-optic sensing) to fill key knowledge gaps.

Beschreibung: While basaltic volcanism is dominate during rifting and continental breakup, felsic magmatism may also comprise important components of some rift margins. During International Ocean Discovery Program (IODP) Expedition 396 on the continental margin of Norway, a graphite-garnet-cordierite bearing dacitic, pyroclastic unit was recovered within early Eocene sediments on Mimir High (Site U1570), a marginal high on the Vøring transform margin. Here, we present a comprehensive textural, mineralogical, and petrological study of the dacite in order to assess its melting origin and emplacement. The major mineral phases (garnet, cordierite, quartz, plagioclase, alkali feldspar) are hosted in a fresh rhyolitic, highly vesicular, glassy matrix, locally mingled with sediments. The xenocrystic major element chemistry of garnet and cordierite, the presence of zircon inclusions with inherited cores, and thermobarometric calculations all support a crustal metapelite origin. While most magma-rich margin models favor crustal anatexis in the lower crust, thermobarometric calculations performed here show that the dacite was produced at upper-crustal depths (〈 5 kbar) and high temperature (750–800 °C) with up to 3 wt% water content. In situ U-Pb analyses on zircon inclusions give a magmatic age of 54.6 ± 1.1 Ma, revealing the emplacement of the dacite post-dates the Paleocene-Eocene Thermal Maximum (PETM). Our results suggest that the opening of the North Atlantic was associated with a phase of low-pressure, high-temperature crustal melting at the onset of the main phase of magmatism.

Beschreibung: Marginal seas influenced by large rivers are characterized by complex hydrodynamic and organic matter cycling processes. However, the impacts of hydrodynamics on the composition and reactivity of particulate organic matter (POM) remain unclear. Here we conducted a comprehensive study on the bulk, molecular and biological properties of suspended POM in the Changjiang Estuary and adjacent area subjected to strong currents, eddies as well as typhoons during spring and autumn. D/L‐enantiomers of particulate amino acids (PAA) were analyzed to evaluate the bioreactivity of POM and quantify bacterial‐derived organic carbon. We found that POM bioavailability as indicated by carbon‐normalized yields of PAA (PAA‐C%) reflected the ecosystem productivity. Relatively high PAA‐C% values (20−35%) were observed in productive areas influenced by Changjiang River plume, cyclonic eddies and typhoons, likely related to the enhanced nutrient availability arising from hydrodynamic processes. In contrast, the oligotrophic Taiwan Warm Current‐influenced regions featured relatively low POM bioavailability (PAA‐C% 〈 10%) despite typhoons facilitating water mixing. The PAA‐C% values showed a significant positive correlation with extracellular enzyme activity, indicating that bioavailable POM can rapidly stimulate heterotrophic transformation. Hot spots of elevated bioavailable POM showed high contributions of bacterial organic carbon. A large portion (∼2/3) of bacterial organic carbon was present in the form of bacterial detritus, suggesting that patches of these biological hot spots represent important sites of carbon sequestration. Together, our findings indicate that fresh POM production is largely controlled by nutrient supply driven by hydrodynamic processes, with important implications for carbon sequestration in the dynamic ocean margins. Plain Language Summary Marginal seas are subject to complex hydrodynamic processes and play an important role in carbon sequestration. Disentangling the linkages between hydrodynamics and organic carbon reactivity and composition is crucial to understanding the regional carbon cycle. Here we collected suspended particulate organic matter (POM) in the Changjiang Estuary and adjacent coastal areas. Based on the biomarker D/L‐amino acids, we assessed the bioavailability of POM and quantified the organic carbon originating from bacteria. We found that high bioactivity of POM occurred in productive Changjiang River plume, cyclonic eddy, and typhoon influenced areas. These hydrodynamic processes appear to increase nutrient availability, therefore promoting phytoplankton growth. Bioavailable POM can rapidly stimulate heterotrophic activity and facilitate the transformation of algal‐derived organic carbon to bacterial detritus, thus contributing to carbon sequestration. Our findings suggest that the production of bioavailable POM is largely controlled by hydrodynamically driven nutrient supply. Key Points We use D/L‐amino acids to assess the bioreactivity and bacterial origins of particulate organic matter (POM) in the dynamic Changjiang Estuary and adjacent area High bioavailability of POM occurs in productive regions affected by Changjiang River plume, cyclonic eddies and typhoons Hot spots of bioavailable POM represent important sites for carbon sequestration

Beschreibung: In the boreal summer of 2021, the equatorial Atlantic experienced the strongest warm event, that is, Atlantic Niño, since the beginning of satellite observations in the 1970s. Such events have far‐reaching impacts on large‐scale wind patterns and rainfall over the surrounding continents. Yet, developing a paradigm of how Atlantic Niño interacts with the upper‐ocean currents and intraseasonal waves remains elusive. Here we show that the equatorial Kelvin wave associated with the onset of the 2021 Atlantic Niño modulated both the background flow and the eddy flux of the equatorial upper‐ocean circulation, causing an extremely weak and delayed tropical instability wave (TIW) season. TIW‐induced variations of sea surface temperature (SST), sea surface salinity, sea surface height, and eddy temperature advection were exceptionally weak during May to July, the climatological peak of TIW activity, but rebounded in August when higher than normal variability was observed. Moored velocity data at 23°W show that during the peak of the 2021 Atlantic Niño from June to August, the Equatorial Undercurrent was deeper and stronger than usual. An anomalously weak eddy momentum flux strongly suppressed barotropic energy conversion north of the equator from May to July, likely contributing to low TIW activity. Reduced baroclinic energy conversion also might have played a role, as the meridional gradient of SST was sharply reduced during the Atlantic Niño. Despite extremely weak TIW velocities, modest intraseasonal variability of chlorophyll‐a (Chl‐ a ) was observed during the Atlantic Niño, due to pronounced meridional Chl‐ a gradients that partly compensated for the weak TIWs. Plain Language Summary Every few years the eastern equatorial Atlantic Ocean is significantly warmer than usual during boreal summer. Such warm events are referred to as Atlantic Niño events, and share similarities with El Niño events in the Pacific. In 2021, the strongest Atlantic Niño in at least four decades was observed in the equatorial Atlantic. This study is the first that investigates the complex interaction between Atlantic Niño, tropical Atlantic upper‐ocean currents, and equatorial waves based on various observational data sets. We show that the developing 2021 Atlantic Niño weakened both the background flow and the variability of near‐surface currents in May, which in turn largely reduced the strength of intraseasonal (20–50 days) waves that are usually generated by instability of the upper‐ocean zonal currents. As a consequence, the cooling effect that these waves usually have north of the equator and the warming effect along the equator vanished from May to July 2021. Interestingly, variability of chlorophyll concentration was enhanced, suggesting that enhanced meridional chlorophyll gradients compensated for reduced wave activity. Key Points The developing 2021 Atlantic Niño led to weaker equatorial surface currents and reduced vertical shear of upper‐ocean horizontal velocity Strong reduction of the surface flow, eddy flux, and meridional temperature gradient in May caused extremely weak and delayed tropical instability wave (TIW) season Reduced meridional TIW advection contributed to sharpen the north equatorial Chl‐ a front resulting in modest intraseasonal Chl‐ a variability

Beschreibung: Three volcanic arcs have been the source of New Zealand's volcanic activity since the Neogene: Northland arc, Coromandel Volcanic Zone (CVZ) and Taupō Volcanic Zone (TVZ). The eruption chronology for the Quaternary, sourced by the TVZ, is well studied and established, whereas the volcanic evolution of the precursor arc systems, like the CVZ (central activity c. 18 to 2 Ma), is poorly known due to limited accessibility to, or identification of, onshore volcanic deposits and their sources. Here, we investigate the marine tephra record of the Neogene, mostly sourced by the CVZ, of cores from IODP Exp. 375 (Sites U1520 and U1526), ODP Leg 181 (Sites 1123, 1124 and 1125), IODP Leg 329 (Site U1371) and DSDP Leg 90 (Site 594) offshore of New Zealand. In total, we identify 306 primary tephra layers in the marine sediments. Multi-approach age models (e.g. biostratigraphy, zircon ages) are used in combination with geochemical fingerprinting (major and trace element compositions) and the stratigraphic context of each marine tephra layer to establish 168 tie-lines between marine tephra layers from different holes and sites. Following this approach, we identify 208 explosive volcanic events in the Neogene between c. 17.5 and 2.6 Ma. This is the first comprehensive study of New Zealand's Neogene explosive volcanism established from tephrochronostratigraphic studies, which reveals continuous volcanic activity between c. 12 and 2.6 Ma with an abrupt compositional change at c. 4.5 Ma, potentially associated with the transition from CVZ to TVZ. Key Points New Zealand's Neogene explosive volcanism based on the marine tephra record Geochemical fingerprinting of marine tephra layers across the study area to establish volcanic events Insights into geochemical variations with time, repose times and spatiotemporal distribution

Beschreibung: Mapping and sampling four sections of the slow-spreading Reykjanes Ridge provide insight into how tectonic and volcanic activity varies with distance from the Iceland plume. The studied areas are characterized by significant variations in water depth, lava chemistry, crustal thickness, thermal structure, and ridge morphology. For each study area, fault pattern and dimension, tectonic strain, seamount morphology, and density are inferred from 15 m-resolution bathymetry. These observations are combined with geochemical analysis from glass samples and sediment thickness estimations along Remotely Operated Vehicle-dive videos. They reveal that (a) tectonic and volcanic activity along the Reykjanes Ridge, do not systematically vary with distance from the plume center. (b) The tectonic geometry appears directly related to the deepening of the brittle/ductile transition and the maximum change in tectonic strain related to the rapid change in crustal thickness and the transition between axial-high and axial valley (∼59.5°N). (c) Across-axis variations in the fault density and sediment thickness provide similar widths for the neo-volcanic zone except in regions of increased seamount emplacement. (d) The variations in seamount density (especially strong for flat-topped seamounts) are not related to the distance from the plume but appear to be correlated with the interaction between the V-shape ridges (VSR) flanking the ridge and the ridge axis. These observations are more compatible with the buoyant upwelling melting instability hypothesis for VSR formation and suggest that buoyant melting instabilities create many small magma batches which by-pass the normal subaxial magmatic plumbing system, erupting over a wider-than-normal area. Key Points The distance from the plume center is not the only factor controlling tectonic and volcanic activity along the Reykjanes Ridge Fault dimensions are primarily controlled by the variation of crustal thermal structure with distance from the hotspot Flat-topped seamount abundances peak where a V-shaped ridge intersects the axis, consistent with a buoyant upwelling melting instability

Beschreibung: The simulation of deep-sea conditions in laboratories is technically challenging but necessary for experiments that aim at a deeper understanding of physiological mechanisms or host-symbiont interactions of deep-sea organisms. In a proof-of-concept study, we designed a recirculating system for long-term culture (〉2 yr) of deep-sea mussels Gigantidas childressi (previously Bathymodiolus childressi). Mussels were automatically (and safely) supplied with a maximum stable level of ~60 μmol L−1 methane in seawater using a novel methane–air mixing system. Experimental animals also received daily doses of live microalgae. Condition indices of cultured G. childressi remained high over the years, and low shell growth rates could be detected, too, which is indicative of positive energy budgets. Using stable isotope data, we demonstrate that G. childressi in our culture system gained energy, both, from the digestion of methane-oxidizing endosymbionts and from digesting particulate food (microalgae). Limitations of the system, as well as opportunities for future experimental approaches involving deep-sea mussels, are discussed.

Beschreibung: Air-sea interaction in late boreal winter is studied over the extratropical North Atlantic (NA) during 1960–2020 by examining the relationship between sea-surface temperature (SST) and total turbulent heat flux (THF). The two quantities are positively correlated on interannual timescales over the central-midlatitude and subpolar NA, suggesting the atmosphere on average drives SST and THF variability is independent of SST. On decadal timescales and over the central-midlatitude NA the correlation is negative, suggesting ocean processes on average drive SST and THF variability is sensitive to SST. The correlation is positive over the subpolar NA. There, interannual and decadal THF variability is governed by the North Atlantic Oscillation (NAO). During the major late 20th and early 21st century SST increase in the subpolar NA diminishing oceanic heat loss associated with a weakening NAO was observed. This study suggests that the atmosphere is more sensitive to SST over the central-midlatitude than subpolar NA. Key Points: - Regional variation in the nature of air-sea interaction over the extratropical North Atlantic (NA) north of 35°N - Timescale dependence in relationship between sea-surface temperature (SST) and turbulent heat flux over the central-midlatitude NA - The atmosphere is more sensitive to SST variability over the central-midlatitude than subpolar NA

Beschreibung: The disturbance of marine organism phenology due to climate change and the subsequent effects on recruitment success are still poorly understood, especially in migratory fish species, such as the Atlantic herring (Clupea harengus; Clupeidae). Here we used the commercial catch data from a local fisher over a 50-year period (1971–2020) to estimate western Baltic spring-spawning (WBSS) herring mean arrival time Q50 (i.e., the week when 50% of the total fish catches had been made) at their spawning ground within the Kiel Fjord, southwest Baltic Sea, and the duration of the spawning season for each year. The relationship between the seawater temperature in the Kiel Bight and other environmental parameters (such as water salinity, North Atlantic and Atlantic multidecadal oscillations) and Q50 was evaluated using a general linear model to test the hypothesis that fish arrived earlier after warm than cold winters. We also estimated the accumulated thermal time to Q50 during gonadal development to estimate the effects of seawater temperature on the variations of Q50. The results of this study revealed a dramatic decrease in herring catches within the Kiel Fjord since the mid-1990s, as documented for the whole southwestern Baltic Sea. Warmer winter seawater temperature was the only factor related to an earlier arrival (1 week for one January seawater temperature degree increase) of herring at their spawning ground. The relationship was found for the first time on week 52 of the year prior to spawning and was the strongest (50% of the variability explained) from the fourth week of January (8 weeks before the mean Q50 among the studied years). A thermal constant to Q50 (~316°C day) was found when temperatures were integrated from the 49th week of the year prior to spawning. These results indicate that seawater temperature enhanced the speed of gonadal maturation during the latest phases of gametogenesis, leading to an early fish arrival under warm conditions. The duration of the spawning season was elongated during warmer years, therefore potentially mitigating the effects of trophic mismatch when fish spawn early. The results of this study highlight the altering effects of climate change on the spawning activity of a migratory fish species in the Baltic Sea where fast global changes presage that in other coastal areas worldwide

Beschreibung: The Black Sea is a permanently anoxic, marine basin serving as model system for the deposition of organic-rich sediments in a highly stratified ocean. In such systems, archaeal lipids are widely used as paleoceanographic and biogeochemical proxies; however, the diverse planktonic and benthic sources as well as their potentially distinct diagenetic fate may complicate their application. To track the flux of archaeal lipids and to constrain their sources and turnover, we quantitatively examined the distributions and stable carbon isotopic compositions (delta 13C) of intact polar lipids (IPLs) and core lipids (CLs) from the upper oxic water column into the underlying sediments, reaching deposits from the last glacial. The distribution of IPLs responded more sensitively to the geochemical zonation than the CLs, with the latter being governed by the deposition from the chemocline. The isotopic composition of archaeal lipids indicates CLs and IPLs in the deep anoxic water column have negligible influence on the sedimentary pool. Archaeol substitutes tetraether lipids as the most abundant IPL in the deep anoxic water column and the lacustrine methanic zone. Its elevated IPL/CL ratios and negative delta 13C values indicate active methane metabolism. Sedimentary CL- and IPL-crenarchaeol were exclusively derived from the water column, as indicated by non-variable delta 13C values that are identical to those in the chemocline and by the low BIT (branched isoprenoid tetraether index). By contrast, in situ production accounts on average for 22% of the sedimentary IPL-GDGT-0 (glycerol dibiphytanyl glycerol tetraether) based on isotopic mass balance using the fermentation product lactate as an endmember for the dissolved substrate pool. Despite the structural similarity, glycosidic crenarchaeol appears to be more recalcitrant in comparison to its non-cycloalkylated counterpart GDGT-0, as indicated by its consistently higher IPL/CL ratio in sediments. The higher TEX86, CCaT, and GDGT-2/-3 values in glacial sediments could plausibly result from selective turnover of archaeal lipids and/or an archaeal ecology shift during the transition from the glacial lacustrine to the Holocene marine setting. Our in-depth molecular-isotopic examination of archaeal core and intact polar lipids provided new constraints on the sources and fate of archaeal lipids and their applicability in paleoceanographic and biogeochemical studies.

Beschreibung: The crises of climate change and biodiversity loss are interlinked and must be addressed jointly. A proposed solution for reducing reliance on fossil fuels, and thus mitigating climate change, is the transition from conventional combustion-engine to electric vehicles. This transition currently requires additional mineral resources, such as nickel and cobalt used in car batteries, presently obtained from land-based mines. Most options to meet this demand are associated with some biodiversity loss. One proposal is to mine the deep seabed, a vast, relatively pristine and mostly unexplored region of our planet. Few comparisons of environmental impacts of solely expanding land-based mining versus extending mining to the deep seabed for the additional resources exist and for biodiversity only qualitative. Here, we present a framework that facilitates a holistic comparison of relative ecosystem impacts by mining, using empirical data from relevant environmental metrics. This framework (Environmental Impact Wheel) includes a suite of physicochemical and biological components, rather than a few selected metrics, surrogates, or proxies. It is modified from the “recovery wheel” presented in the International Standards for the Practice of Ecological Restoration to address impacts rather than recovery. The wheel includes six attributes (physical condition, community composition, structural diversity, ecosystem function, external exchanges and absence of threats). Each has 3–5 sub attributes, in turn measured with several indicators. The framework includes five steps: (1) identifying geographic scope; (2) identifying relevant spatiotemporal scales; (3) selecting relevant indicators for each sub-attribute; (4) aggregating changes in indicators to scores; and (5) generating Environmental Impact Wheels for targeted comparisons. To move forward comparisons of land-based with deep seabed mining, thresholds of the indicators that reflect the range in severity of environmental impacts are needed. Indicators should be based on clearly articulated environmental goals, with objectives and targets that are specific, measurable, achievable, relevant, and time bound.

Beschreibung: The Banda Sea is of crucial importance for the circulation of the world's oceans, as it is part of the connection between the Pacific to the Indian Ocean. One peculiarity of the upper ocean hydrography in the Banda Sea is the occurrence of barrier layers. The regionality and temporal variability of barrier layer thickness (BLT) in the Banda Sea are examined in this study utilizing in-situ observations and ocean reanalysis output. It is found that a barrier layer occurs in over 90 % of the observational data profiles, and in over 72 % of those profiles, the BLT is shallower than 10 m. Furthermore, we find a seasonal cycle in BLT with a maximum thickness of about 60 m occurring during austral autumn and winter and coinciding with the presence of low saline waters fed by the regional river discharge and rainfall from the Java Sea and Makassar Strait. In addition, we identify the existence of a quasi-permanent anticyclonic circulation cell in the Banda Sea that may support the trapping of surface freshwater by retention. The anticyclonic circulation is most likely wind-driven because it coincides with the regional Ekman pumping pattern. Modulation of the anticyclone is via seasonal variability in the wind stress curl which in turn may explain the efficiency of freshwater retention and thus the BLT. The annual mean BLT distribution in the Banda Sea shows a preferential region of thickened barrier layers around 6o-8oS and 124o-126oE and resampling the pattern of the monthly mean climatology. Key Points: - First study estimating barrier layer thickness (BLT) in the Banda Sea using comprehensive observations - A quasi-permanent barrier layer exists in the Banda Sea with seasonal variation in occurrence and thickness - The intrusion of low saline waters and anticyclonic circulation are identified as the main mechanisms for creating and modulating the local BLT

Beschreibung: Key Points: - We reconstruct the temporal evolution of seawater isotope ratios of boron, strontium, lithium, and osmium over the last 65 million years - The evolution of seawater boron isotope ratio shows similarity to the evolution of strontium, lithium and osmium isotope ratios - Randomly drawn, smooth time series are provided for use in uncertainty propagation in calculation of palaeo pH The boron isotope ratio of seawater (δ11Bsw) is a parameter which must be known to reconstruct palaeo pH and CO2 from boron isotope measurements of marine carbonates. Beyond a few million years ago, δ11Bsw is likely to have been different to modern. Palaeo δ11Bsw can be estimated by simultaneously constraining the vertical gradients in foraminiferal δ11B (Δδ11B) and pH (ΔpH). A number of subtly different techniques have been used to estimate ΔpH in the past, all broadly based on assumptions about vertical gradients in oxygen, and/or carbon, or other carbonate system constraints. In this work we pull together existing data from previous studies, alongside a constraint on the rate of change of δ11Bsw from modeling. We combine this information in an overarching statistical framework called a Gaussian Process. The Gaussian Process technique allows us to bring together data and constraints on the rate of change in δ11Bsw to generate random plausible evolutions of δ11Bsw. We reconstruct δ11Bsw, and by extension palaeo pH, across the last 65Myr using this novel methodology. Reconstructed δ11Bsw is compared to other seawater isotope ratios, namely ,87/86 Sr, 187/188 Os , and δ7Li, which we also reconstruct with Gaussian Processes. Our method provides a template for incorporation of future δ11Bsw constraints, and a mechanism for propagation of uncertainty in δ11Bsw into future studies.

Beschreibung: The spectral composition of light is an important factor for the metabolism of photosynthetic organisms. Several blue light-regulated metabolic processes have already been identified in the industrially relevant microalga Monoraphidium braunii. However, little is known about the spectral impact on this species' growth, fatty acid (FA), and pigment composition. In this study, M. braunii was cultivated under different light spectra (white light: 400–700 nm, blue light: 400–550 nm, green light: 450–600 nm, and red light: 580–700 nm) at 25°C for 96 h. The growth was monitored daily. Additionally, the FA composition, and pigment concentration was analyzed after 96 h. The highest biomass production was observed upon white light and red light irradiation. However, green light also led to comparably high biomass production, fueling the scientific debate about the contribution of weakly absorbed light wavelengths to microalgal biomass production. All light spectra (white, blue, and green) that comprised blue-green light (450–550 nm) led to a higher degree of FA unsaturation and a greater concentration of all identified pigments than red light. These results further contribute to the growing understanding that blue-green light is an essential trigger for maximized pigment concentration and FA unsaturation in green microalgae.

Beschreibung: This article presents risk factors that are associated with the handling of unexploded ordnance (UXO) during explosive ordnance disposal (EOD) operations in German waters. The construction of offshore wind parks and the German immediate action program are expected to increase the number of EOD operations. Existing literature and guidelines do not offer a structured and reproducible framework for assessing EOD risk. To fill this gap, a network of EOD risk factors was developed by means of a literature review and validation via expert consultation. The study was scoped to “personnel and equipment at the EOD location” as the risk receptor and “undesired detonation” as the undesired event under investigation. Factors are subdivided into UXO factors that depend on the object that should be handled and factors that describe the object's surrounding environment. While the former can be researched by an EOD expert, the latter must be measured on site or acquired from a model. Each of these factors contributes to risk, some directly and others indirectly via other factors. The complexity of the resulting network, with its 33 factors, demonstrates the need for a reliable and reproducible model to quantify EOD risk. Its purpose is not to replace EOD experts but to aid them in their decision‐making process. Such a tool can provide valuable support for the high‐cost and high‐risk EOD operations.

Beschreibung: Infections by filamentous phages, which are usually nonlethal to the bacterial cells, influence bacterial fitness in various ways. While phage-encoded accessory genes, for example virulence genes, can be highly beneficial, the production of viral particles is energetically costly and often reduces bacterial growth. Consequently, if costs outweigh benefits, bacteria evolve resistance, which can shorten phage epidemics. Abiotic conditions are known to influence the net-fitness effect for infected bacteria. Their impact on the dynamics and trajectories of host resistance evolution, however, remains yet unknown. To address this, we experimentally evolved the bacterium Vibrio alginolyticus in the presence of a filamentous phage at three different salinity levels, that is (1) ambient, (2) 50% reduction and (3) fluctuations between reduced and ambient. In all three salinities, bacteria rapidly acquired resistance through super infection exclusion (SIE), whereby phage-infected cells acquired immunity at the cost of reduced growth. Over time, SIE was gradually replaced by evolutionary fitter surface receptor mutants (SRM). This replacement was significantly faster at ambient and fluctuating conditions compared with the low saline environment. Our experimentally parameterized mathematical model explains that suboptimal environmental conditions, in which bacterial growth is slower, slow down phage resistance evolution ultimately prolonging phage epidemics. Our results may explain the high prevalence of filamentous phages in natural environments where bacteria are frequently exposed to suboptimal conditions and constantly shifting selections regimes. Thus, our future ocean may favour the emergence of phage-born pathogenic bacteria and impose a greater risk for disease outbreaks, impacting not only marine animals but also humans.

Beschreibung: The deep-sea water column below 200 m is a vast three-dimensional habitat with an enormous but largely unexplored biodiversity (Robison, 2009). Cephalopod mollusks are abundant in the deep sea and are important prey for many kinds of predators. Still, most deep-sea cephalopods have never been observed alive in their natural habitat and their reproductive biology remains poorly documented. In March of 2015, at a depth of 2566 m, we observed a female squid of an undescribed species but likely belonging to the Gonatidae, carrying few but exceptionally large eggs in her arms. This raises questions as to how these and other related animals reproduce in the deep sea, an environment that is generally characterized by darkness, low temperature, reduced oxygen, limited food availability, and low population densities. The authors were conducting dives with deep-sea robots (remotely operated vehicles or ROVs) equipped with cameras in the deep basins of the Gulf of California, to investigate how deep-sea fauna are distributed in relation to the extensive low-oxygen zones in the region (Gilly et al., 2013). The squid (Individual 1, Table 1) we observed with an ROV at 2566 m in the Gulf of California in 2015 was carrying 30–40 large eggs (average maximum diameter 11.2 mm, n = 5; measured eggs were 11.4, 10.4, 11.5, 11.7 and 11 mm maximum diameter) embedded in a small external egg sheet that did not extend beyond the arm tips. The size of the eggs was similar to those of two squid (Individuals 2 and 3, Table 1) that were observed close to the seafloor during earlier expeditions in the same region.

Beschreibung: Low-level jets (LLJs), vertical profiles with a wind speed maxima in the lowest hundred meters of the troposphere, have multiple impacts in the Earth system, but a global present-day climatology based on contemporary data does not exist. We use the spatially and temporally complete data set from ERA5 reanalysis to compile a global climatology of LLJs for studying the formation mechanisms, characteristics, and trends during the period of 1992–2021. In the global mean, LLJs are detected 21% of the time with more cases over land (32%) than over the ocean (15%). We classified the LLJs into three categories: non-polar land (LLLJ), polar land (PLLJ), and coastal (CLLJ) LLJs. For LLLJ, the averaged frequency of occurrence is 20% and 75% of them are associated with a near-surface temperature inversion as a prerequisite for an inertial oscillation. PLLJs are also associated with a temperature inversion and occur even more frequently with 59% of the time. These are also the lowest and the strongest LLJs among the three categories. CLLJs are particularly frequent in some marine hotspots, situated along the west coast of continents, with neutral to unstable stratification close to the surfaces and a stably stratified layer aloft. We found distinct regional trends in both the frequency and intensity of LLJs over the past decades, which can have implications for the emission and transport of aerosols, and the transport of atmospheric moisture. Future studies could address changes in LLJs and the associated implications in more detail, based on the here released ERA5-based LLJ data. Key Points: - First global comprehensive low-level jet (LLJ) climatology using ERA5 - Polar LLJs are the strongest and most frequent among the detected types - Distinct past trends in regional LLJ frequency and intensity

Beschreibung: 1. The expansion of scientific image data holds great promise to quantify individuals, size distributions and traits. Computer vision tools are especially powerful to automate data mining of images and thus have been applied widely across studies in aquatic and terrestrial ecology. Yet marine benthic communities, especially infauna, remain understudied despite their dominance of marine biomass, biodiversity and playing critical roles in ecosystem functioning. 2. Here, we disaggregated infauna from sediment cores taken throughout the spring transition (April-June) from a near-natural mesocosm setup under experimental warming (Ambient, +1.5 degrees C, +3.0 degrees C). Numerically abundant mudsnails were imaged in batches under stereomicroscopy, from which we automatically counted and sized individuals using a superpixel-based segmentation algorithm. Our segmentation approach was based on clustering superpixels, which naturally partition images by low-level properties (e.g., colour, shape and edges) and allow instance-based segmentation to extract all individuals from each image. 3. We demonstrate high accuracy and precision for counting and sizing individuals, through a procedure that is robust to the number of individuals per image (5-65) and to size ranges spanning an order of magnitude (〈750 mu m to 7.4 mm). The segmentation routine provided at least a fivefold increase in efficiency compared with manual measurements. Scaling this approach to a larger dataset tallied 〉40k individuals and revealed overall growth in response to springtime warming. 4. We illustrate that image processing and segmentation workflows can be built upon existing open-access R packages, underlining the potential for wider adoption of computer vision tools among ecologists. The image-based approach also generated reproducible data products that, alongside our scripts, we have made freely available. This work reinforces the need for next-generation monitoring of benthic communities, especially infauna, which can display differential responses to average warming.

Beschreibung: Biological invasions pose a rapidly expanding threat to the persistence, functioning and service provisioning of ecosystems globally, and to socio-economic interests. The stages of successful invasions are driven by the same mechanism that underlies adaptive changes across species in general-via natural selection on intraspecific variation in traits that influence survival and reproductive performance (i.e., fitness). Surprisingly, however, the rapid progress in the field of invasion science has resulted in a predominance of species-level approaches (such as deny lists), often irrespective of natural selection theory, local adaptation and other population-level processes that govern successful invasions. To address these issues, we analyse non-native species dynamics at the population level by employing a database of European freshwater macroinvertebrate time series, to investigate spreading speed, abundance dynamics and impact assessments among populations. Our findings reveal substantial variability in spreading speed and abundance trends within and between macroinvertebrate species across biogeographic regions, indicating that levels of invasiveness and impact differ markedly. Discrepancies and inconsistencies among species-level risk screenings and real population-level data were also identified, highlighting the inherent challenges in accurately assessing population-level effects through species-level assessments. In recognition of the importance of population-level assessments, we urge a shift in invasive species management frameworks, which should account for the dynamics of different populations and their environmental context. Adopting an adaptive, region-specific and population-focused approach is imperative, considering the diverse ecological contexts and varying degrees of susceptibility. Such an approach could improve and refine risk assessments while promoting mechanistic understandings of risks and impacts, thereby enabling the development of more effective conservation and management strategies. Biological invasions increasingly threaten global ecosystems and socio-economic interests, advancing through mechanisms like natural selection that enhance survival and reproductive traits. Our study focuses on population-level analyses of non-native European freshwater macroinvertebrates to better understand their spread and impact. We found significant variability in invasion dynamics across populations and regions, suggesting that current species-level risk assessments may overlook crucial population-specific factors.image

Beschreibung: Persistently high marine temperatures are escalating and threating marine biodiversity. The Baltic Sea, warming faster than other seas, is a good model to study the impact of increasing sea surface temperatures. Zostera marina, a key player in the Baltic ecosystem, faces susceptibility to disturbances, especially under chronic high temperatures. Despite the increasing number of studies on the impact of global warming on seagrasses, little attention has been paid to the role of the holobiont. Using an outdoor benthocosm to replicate near-natural conditions, this study explores the repercussions of persistent warming on the microbiome of Z. marina and its implications for holobiont function. Results show that both seasonal warming and chronic warming, impact Z. marina roots and sediment microbiome. Compared with roots, sediments demonstrate higher diversity and stability throughout the study, but temperature effects manifest earlier in both compartments, possibly linked to premature Z. marina die-offs under chronic warming. Shifts in microbial composition, such as an increase in organic matter-degrading and sulfur-related bacteria, accompany chronic warming. A higher ratio of sulfate-reducing bacteria compared to sulfide oxidizers was found in the warming treatment which may result in the collapse of the seagrasses, due to toxic levels of sulfide. Differentiating predicted pathways for warmest temperatures were related to sulfur and nitrogen cycles, suggest an increase of the microbial metabolism, and possible seagrass protection strategies through the production of isoprene. These structural and compositional variations in the associated microbiome offer early insights into the ecological status of seagrasses. Certain taxa/genes/pathways may serve as markers for specific stresses. Monitoring programs should integrate this aspect to identify early indicators of seagrass health. Understanding microbiome changes under stress is crucial for the use of potential probiotic taxa to mitigate climate change effects. Broader-scale examination of seagrass–microorganism interactions is needed to leverage knowledge on host–microbe interactions in seagrasses.

Beschreibung: The recent severe European summer heat waves of 2015 and 2018 co-occurred with cold subpolar North Atlantic (NA) sea surface temperatures (SSTs). However, a significant connection between this oceanic state and European heat waves was not yet established. We performed two AMIP-like model experiments: (a) employing daily 2018 SSTs as observed and (b) applying a novel approach to remove the negative NA SST anomaly, while keeping SST daily and small-scale variability. Comparing these experiments, we find that cold subpolar NA SSTs significantly increase heat wave duration and magnitude downstream over the European continent. Surface temperature and circulation anomalies are connected by the upper-tropospheric summer wave pattern of meridional winds over the North Atlantic European sector, which is enhanced with cold NA SSTs. Our results highlight the relevance of the subpolar NA region for European summer conditions, a region that is marked by large biases in current coupled climate model simulations. Key Points: - Model study designed to investigate the ocean impact on European heat waves by prescribing observed and realistic ocean surface conditions - Cold subpolar North Atlantic sea surface temperatures significantly enhance heat wave intensity and duration over the European continent - North Atlantic ocean and European surface temperature and circulation anomalies are bridged by the upper-tropospheric summer mean wave

Beschreibung: In geoscience and other fields, researchers use models as a simplified representation of reality. The models include processes that often rely on uncertain parameters that reduce model performance in reflecting real-world processes. The problem is commonly addressed by adapting parameter values to reach a good match between model simulations and corresponding observations. Different optimization tools have been successfully applied to address this task of model calibration. However, seeking one best value for every single model parameter might not always be optimal. For example, if model equations integrate over multiple real-world processes which cannot be fully resolved, it might be preferable to consider associated model parameters as random parameters. In this paper, a random parameter is drawn from a wide probability distribution for every singe model simulation. We developed an optimization approach that allows us to declare certain parameters random while optimizing those that are assumed to take fixed values. We designed a corresponding variant of the well known Covariance Matrix Adaption Evolution Strategy (CMA-ES). The new algorithm was applied to a global biogeochemical circulation model to quantify the impact of zooplankton mortality on the underlying biogeochemistry. Compared to the deterministic CMA-ES, our new method converges to a solution that better suits the credible range of the corresponding random parameter with less computational effort.

Beschreibung: Flow of dense shelf water provide an efficient mechanism for pumping CO 2 to the deep ocean along the continental shelf slope, particularly around the Antarctic bottom water (AABW) formation areas where much of the global bottom water is formed. However, the contribution of the formation of AABW to sequestering anthropogenic carbon ( C ant ) and its consequences remain unclear. Here, we show prominent transport of C ant (25.0 ± 4.7 Tg C yr −1 ) into the deep ocean (〉2,000 m) in four AABW formation regions around Antarctica based on an integrated observational data set (1974–2018). This maintains a lower C ant in the upper waters than that of other open oceans to sustain a stronger CO 2 uptake capacity (16.9 ± 3.8 Tg C yr −1 ). Nevertheless, the accumulation of C ant can further trigger acidification of AABW at a rate of −0.0006 ± 0.0001 pH unit yr −1 . Our findings elucidate the prominent role of AABW in controlling the Southern Ocean carbon uptake and storage to mitigate climate change, whereas its side effects (e.g., acidification) could also spread to other ocean basins via the global ocean conveyor belt. Plain Language Summary The Southern Ocean is thought to uptake and store a large amount of anthropogenic CO 2 ( C ant ), but little attention has been paid to the Antarctic coastal regions in the south of 60°S, mainly due to the lack of observations. Based on an integrated data set, we discovered the deep penetration of C ant and a visible pattern of relatively high concentration of C ant along the AABW formation pathway, and the concentration of C ant along the shelf‐slope is higher than that of other marginal seas at low‐mid latitudes, implying a highly effective C ant transport in AABW formation areas. We also found strong upper‐layer CO 2 uptake and a significant acidification rate in the deep waters of the Southern Ocean due to the AABW‐driven CO 2 transport, which is 3 times faster than those in other deep oceans. It is therefore crucial to understand how the Antarctic shelf regions affect the global carbon cycle through the uptake and transport of anthropogenic CO 2 , which also drives acidification in the other ocean basins. Key Points We show evidence for the accumulation of C ant along the Antarctic shelf‐slope into the deep ocean The process of AABW formation drives C ant downward transport at 25.0 ± 4.7 Tg C yr −1 , sustaining the CO 2 uptake in the surface ocean This further triggers acidification of AABW at a rate of −0.0006 ± 0.0001 pH unit yr −1 , which is faster than in other deep oceans

Beschreibung: To reach their net-zero targets, countries will have to compensate hard-to-abate CO2 emissions through carbon dioxide removal (CDR). Yet, current assessments rarely include socio-cultural or institutional aspects or fail to contextualize CDR options for implementation. Here we present a context-specific feasibility assessment of CDR options for the example of Germany. We assess 14 CDR options, including three chemical carbon capture options, six options for bioenergy combined with carbon capture and storage (BECCS), and five options that aim to increase ecosystem carbon uptake. The assessment addresses technological, economic, environmental, institutional, social-cultural and systemic considerations using a traffic-light system to evaluate implementation opportunities and hurdles. We find that in Germany CDR options like cover crops or seagrass restoration currently face comparably low implementation hurdles in terms of technological, economic, or environmental feasibility and low institutional or social opposition but show comparably small CO2 removal potentials. In contrast, some BECCS options that show high CDR potentials face significant techno-economic, societal and institutional hurdles when it comes to the geological storage of CO2. While a combination of CDR options is likely required to meet the net-zero target in Germany, the current climate protection law includes a limited set of options. Our analysis aims to provide comprehensive information on CDR hurdles and possibilities for Germany for use in further research on CDR options, climate, and energy scenario development, as well as an effective decision support basis for various actors. Key Points: - More context-specific assessments of carbon dioxide removal (CDR) options are needed to guide national net-zero decision making - Ecosystem-based CDR options with comparably low implementation hurdles in Germany show relatively small CO2 removal potentials - High CDR potential options in Germany face high institutional, technological and societal hurdles linked in many ways to geological storage

Beschreibung: The Northwest Tropical Atlantic (NWTA) is a region of complex surface ocean circulation. The most prominent feature is the North Brazil Current (NBC) and its retroflection at 8°N, which leads to the formation of numerous mesoscale eddies known as NBC rings. The NWTA also receives the outflow of the Amazon River, generating freshwater plumes that can extend up to 100,000 km2. We show that these two processes influence the spatial variability of the region's surface latent heat flux (LHF). On the one hand, the presence of surface freshwater modifies the vertical stratification of the ocean, the mixed layer heat budget, and thus the air-sea heat exchanges. On the other hand, NBC rings create a highly heterogeneous mesoscale sea surface temperature (SST) field that directly influences the near-surface atmospheric circulation. These effects are illustrated by observations from the ElUcidating the RolE of Cloud-Circulation Coupling in ClimAte - Ocean Atmosphere (EUREC4A-OA) and Atlantic Tradewind Ocean-Atmosphere Interaction Campaign (ATOMIC) experiments, satellite and reanalysis data. We decompose the LHF budget into several terms controlled by different atmospheric and oceanic processes to identify the mechanisms leading to LHF changes. We find LHF variations of up to 160 W m2, of which 100 W m2 are associated with wind speed changes and 40 W m2 with SST variations. Surface currents or heat release associated with stratification changes remain as second-order contributions with LHF variations of less than 10 W m2 each. This study highlights the importance of considering these three components to properly characterize LHF variability at different spatial scales, although it is limited by the scarcity of collocated observations. Key Points: - Latent heat flux (LHF) presents strong spatial variations in the northwest tropical Atlantic (NWTA), which has a complex ocean circulation - Surface winds and sea surface temperature are the major drivers of LHF changes. The Amazon plume remains as a second-order contributor - It is necessary to distinguish between spatial scales (mesoscale and below vs. large-scale) when assessing the ocean's influence on LHF

Beschreibung: Pattern-triggered immunity (PTI) is an integral part of the innate immune system of many eukaryotic hosts, assisting in the defence against pathogen invasions. In plants and animals, PTI exerts a selective pressure on the microbiota that can alter community composition. However, the effect of PTI on the microbiota for non-model hosts, including seaweeds, remains unknown. Using quantitative polymerase chain reaction complemented with 16S rRNA gene and transcript amplicon sequencing, this study profiled the impact that PTI of the red seaweed Gracilaria gracilis has on its microbiota. PTI elicitation with agar oligosaccharides resulted in a significant reduction in the number of bacteria (by 〉75% within 72 h after treatment). However, the PTI elicitation did not cause any significant difference in the community diversity or structure. These findings demonstrated that PTI can be non-selective, and this might help to maintain a stable microbiota by uniformly reducing bacterial loads.

Beschreibung: Significance Particulate organic carbon (POC) formed by photosynthesis in the sunlit surface ocean fuels the ecosystems in the dark ocean below. We show that mesoscale fronts and eddies, which are ubiquitous physical features in subtropical oceans, generate three-dimensional intrusions connecting the surface to deep ocean. Intrusions are enriched in total POC due to enhancement of small, nonsinking photosynthetic plankton and free-living bacteria that resemble surface microbial communities. Flow-driven export of POC, estimated using an approximation of eddy physics, is the same order of magnitude as export by sinking POC, which was previously thought to dominate export. These observations reveal coupling of surface and deep ocean productivity and biodiversity and give insight into mechanisms by which the ocean transports carbon to depth. Abstract Subtropical oceans contribute significantly to global primary production, but the fate of the picophytoplankton that dominate in these low-nutrient regions is poorly understood. Working in the subtropical Mediterranean, we demonstrate that subduction of water at ocean fronts generates 3D intrusions with uncharacteristically high carbon, chlorophyll, and oxygen that extend below the sunlit photic zone into the dark ocean. These contain fresh picophytoplankton assemblages that resemble the photic-zone regions where the water originated. Intrusions propagate depth-dependent seasonal variations in microbial assemblages into the ocean interior. Strikingly, the intrusions included dominant biomass contributions from nonphotosynthetic bacteria and enrichment of enigmatic heterotrophic bacterial lineages. Thus, the intrusions not only deliver material that differs in composition and nutritional character from sinking detrital particles, but also drive shifts in bacterial community composition, organic matter processing, and interactions between surface and deep communities. Modeling efforts paired with global observations demonstrate that subduction can flux similar magnitudes of particulate organic carbon as sinking export, but is not accounted for in current export estimates and carbon cycle models. Intrusions formed by subduction are a particularly important mechanism for enhancing connectivity between surface and upper mesopelagic ecosystems in stratified subtropical ocean environments that are expanding due to the warming climate.

Beschreibung: Tropical sea surface temperature (SST) biases can cause atmospheric biases on global scales, hence SST needs to be represented well in climate models. A major source of uncertainties is the representation of turbulent mixing in the oceanic boundary layer, or mixed layer (ML). In the present study we focus on near-inertial wave (NIW) induced mixing. The performance of two mixing schemes, Turbulent Kinetic Energy and K-profile parameterization (KPP), is assessed at two sites (11.5°N, 23°W and 15°N, 38°W) in the tropical Atlantic. At 11.5°N, turbulence observations (eddy diffusivities, shear and stratification) are available for comparison. We find that the schemes differ in their representation of NIWs, but both under-represent the observed enhanced diffusivities below the observed ML. However, we find that the models do mix below the ML at 15°N when a storm passes nearby. The near-inertial oscillations remain below the ML for the following 10 days. Near-inertial kinetic energy (NIKE) biases in the models are not directly correlated with the wind speed, the MLD biases, or the stratification at the ML base. Instead, NIKE biases are sensitive to the vertical mixing scheme parameterization. NIKE biases are lowest when the KPP scheme is used. Key Points: - Observations of inertial oscillations are used to evaluate the performance of two vertical mixing schemes in two high-resolution models - Both the K-profile parameterization and the Turbulent Kinetic Energy closure underestimate the NIW-induced mixing - Near-inertial kinetic energy biases are sensitive to the vertical mixing parameterization

Beschreibung: Global coupled climate models are in continuous need for evaluation against independent observations to reveal systematic model deficits and uncertainties. Changes in terrestrial water storage (TWS) as measured by satellite gravimetry missions GRACE and GRACE-FO provide valuable information on wetting and drying trends over the continents. Challenges arising from a comparison of observed and modelled water storage trends are related to gravity observations including non-water related variations such as, for example, glacial isostatic adjustment (GIA). Therefore, correcting secular changes in the Earth's gravity field caused by ongoing GIA is important for the monitoring of long-term changes in terrestrial water from GRACE in particular in former ice-covered regions. By utilizing a new ensemble of 56 individual realizations of GIA signals based on perturbations of mantle viscosities and ice history, we find that many of those alternative GIA corrections change the direction of GRACE-derived water storage trends, for example, from gaining mass into drying conditions, in particular in Eastern Canada. The change in the sign of the TWS trends subsequently impacts the conclusions drawn from using GRACE as observational basis for the evaluation of climate models as it influences the dis-/agreement between observed and modelled wetting/drying trends. A modified GIA correction, a combined GRACE/GRACE-FO data record extending over two decades, and a new generation of climate model experiments leads to substantially larger continental areas where wetting/drying trends currently observed by satellite missions coincide with long-term predictions obtained from climate model experiments.

Beschreibung: The Cabo Verde Archipelago is related to a mantle plume located close to the rotational pole of the African Plate. It consists of islands and seamounts arranged in a horseshoe‐shaped pattern open to the west, thus forming two volcanic chains, each with a weak east‐west age progression. High‐resolution swath bathymetry of 12 Cabo Verde seamounts is used here to assign each seamount to its pre‐shield, shield or post‐shield evolutionary stage, respectively. The eastern seamounts exhibit degraded and partially eroded morphologies, and are mainly in their post‐shield stage. A new 40 Ar‐ 39 Ar date for Senghor Seamount at 14.872 ± 0.027 Ma supports old ages for the eastern seamounts. The western seamounts generally exhibit younger volcanic‐edifice‐construction morphologies, showing fresh effusive and explosive volcanics, including rarely observed deep‐water explosive volcanism in the Charles Darwin Volcanic Field. Furthermore, the two previously unknown seamounts Sodade and Tavares in the westernmost termini of both volcanic chains exhibit pristine volcanic morphologies, in agreement with present‐day volcanism and seismic activity recorded from the western seamounts. The islands and seamounts rest on three submarine platforms to the east, northwest and southwest, respectively. Taken together, the seamount and island data suggest a shift in igneous activity from the eastern to the other platforms at about 8–6 Ma. However, the complex evolution pattern for both volcanic chains includes the simultaneous occurrence of pre‐shield or shield edifices at any time, followed by erosional and rejuvenation stages. The new seamount data still demonstrate ongoing westward submarine‐growth in both volcanic chains. Plain Language Summary The Cabo Verde volcanic islands and seamounts are located in the central Atlantic Ocean, ∼570 km off the west coast of Africa. They form a horseshoe‐shaped archipelago with two volcanic chains, which were formed by the African plate moving very slowly over a mantle hotspot (the Cabo Verde Plume). Both the northern and southern volcanic chains show weak east‐to‐west age progressions from ∼26 million years to the present day. This study uses underwater topographic data and observations/rock sampling via remotely operated vehicles from 12 submarine volcanic seamounts, including two previously unknown seamounts, collected during four research cruises in the Cabo Verde Archipelago. Geomorphology is used to classify each seamount as being in its pre‐shield, shield or post‐shield evolutionary stage, respectively. Cabo Verde islands and seamounts rest on three submarine morphological platforms, reflecting westward jumps of the main igneous activity, and also confirming the westward migration of the Cabo Verde hotspot beneath both volcanic chains. Key Points We present bathymetrical maps of 12, in part previously uncharted Cabo Verde seamounts Geomorphology reflects various evolutionary seamount stages and relative ages. Four older seamounts indicate late Quaternary sea level lowstands Islands and seamounts rest on three morphological platforms, indicating westward jumps of the main igneous activity

Beschreibung: Mineral dust is one of the most abundant atmospheric aerosol species and has various far-reaching effects on the climate system and adverse impacts on air quality. Satellite observations can provide spatio-temporal information on dust emission and transport pathways. However, satellite observations of dust plumes are frequently obscured by clouds. We use a method based on established, machine-learning-based image in-painting techniques to restore the spatial extent of dust plumes for the first time. We train an artificial neural net (ANN) on modern reanalysis data paired with satellite-derived cloud masks. The trained ANN is applied to cloud-masked, gray-scaled images, which were derived from false color images indicating elevated dust plumes in bright magenta. The images were obtained from the Spinning Enhanced Visible and Infrared Imager instrument onboard the Meteosat Second Generation satellite. We find up to 15% of summertime observations in West Africa and 10% of summertime observations in Nubia by satellite images miss dust plumes due to cloud cover. We use the new dust-plume data to demonstrate a novel approach for validating spatial patterns of the operational forecasts provided by the World Meteorological Organization Dust Regional Center in Barcelona. The comparison elucidates often similar dust plume patterns in the forecasts and the satellite-based reconstruction, but once trained, the reconstruction is computationally inexpensive. Our proposed reconstruction provides a new opportunity for validating dust aerosol transport in numerical weather models and Earth system models. It can be adapted to other aerosol species and trace gases. Key Points: - We present the first fast reconstruction of cloud-obscured Saharan dust plumes through novel machine learning applied to satellite images - The reconstruction algorithm utilizes partial convolutions to restore cloud-induced gaps in gray-scaled Meteosat Second Generation-Spinning Enhanced Visible and Infrared Imager Dust RGB images - World Meteorological Organization dust forecasts for North Africa mostly agree with the satellite-based reconstruction of the dust plume extent

Beschreibung: Orbital cyclicity is a fundamental pacemaker of Earth’s climate system. The Newark–Hartford Basin (NHB) lake sediment record of eastern North America contains compelling geologic expressions of this cyclicity, reflecting variations of climatic conditions in tropical Pangea during the Late Triassic and earliest Jurassic (~233 to 199 Ma). Climate modeling enables a deeper mechanistic understanding of Earth system modulation during this unique greenhouse and supercontinent period. We link major features of the NHB record to the combined climatic effects of orbital forcing, paleogeographic changes, and atmospheric p CO 2 variations. An ensemble of transient, orbitally driven climate simulations is assessed for nine time slices, three atmospheric p CO 2 values, and two paleogeographic reconstructions. Climatic transitions from tropical humid to more seasonal and ultimately semiarid are associated with tectonic drift of the NHB from ~ 5 ° N to 20 ° N . The modeled orbital modulation of the precipitation–evaporation balance is most pronounced during the 220 to 200 Ma interval, whereas it is limited by weak seasonality and increasing aridity before and after this interval. Lower p CO 2 at around 205 Ma contributes to drier climates and could have led to the observed damping of sediment cyclicity. Eccentricity-modulated precession dominates the orbitally driven climate response in the NHB region. High obliquity further amplifies summer precipitation through the seasonal shifts in the tropical rainfall belt. Regions with other proxy records are also assessed, providing guidance toward an integrated picture of global astronomical climate forcing in the Late Triassic and ultimately of other periods in Earth history.

Beschreibung: Observation‐based quantification of ocean carbon dioxide (CO 2 ) uptake relies on synthesis data sets such as the Surface Ocean CO 2 ATlas (SOCAT). However, the data collection effort has dramatically declined and the number of annual data sets in SOCATv2023 decreased by ∼35% from 2017 to 2021. This decline has led to a 65% increase (from 0.15 to 0.25 Pg C yr −1 ) in the standard deviation of seven SOCAT‐based air‐sea CO 2 flux estimates. Reducing the availability of the annual data to that in the year 2000 creates substantial bias (50%) in the long‐term flux trend. The annual mean CO 2 flux is insensitive to the seasonal skew of the SOCAT data and to the addition of the lower accuracy data set available in SOCAT. Our study highlights the need for sustained data collection and synthesis, to inform the Global Carbon Budget assessment, the UN‐led climate negotiations, and measurement, reporting, and verification of ocean‐based CO 2 removal projects. Plain Language Summary The Surface Ocean CO 2 ATlas (SOCAT) data set plays a crucial role in estimating the ocean carbon sink component of the Global Carbon Budget. However, the number of data sets available in SOCAT each year has drastically decreased since 2017. This study shows that the uncertainty in the data‐based ocean CO 2 flux estimate has increased by 65% due to this decline in data availability. The estimated fluxes, especially the long‐term flux trend, are remarkably affected by the data availability in SOCAT, reducing the reliability of ocean CO 2 uptake estimates in years and regions with sparse observations. Key Points Lower surface ocean f CO 2 data availability leads to higher uncertainty in data‐based estimates of ocean CO 2 uptake The long‐term trend in the ocean CO 2 flux increases by 1.5 times for subsequent years if the data availability is reduced to that in 2000 The annual mean CO 2 flux is not sensitive to the seasonal skew in the data and to the addition of low accuracy data

Beschreibung: Ocean warming and species exploitation have already caused large‐scale reorganization of biological communities across the world. Accurate projections of future biodiversity change require a comprehensive understanding of how entire communities respond to global change. We combined a time‐dynamic integrated food web modeling approach (Ecosim) with previous data from community‐level mesocosm experiments to determine the independent and combined effects of ocean warming, ocean acidification and fisheries exploitation on a well‐managed temperate coastal ecosystem. The mesocosm parameters enabled important physiological and behavioral responses to climate stressors to be projected for trophic levels ranging from primary producers to top predators, including sharks. Through model simulations, we show that under sustainable rates of fisheries exploitation, near‐future warming or ocean acidification in isolation could benefit species biomass at higher trophic levels (e.g., mammals, birds, and demersal finfish) in their current climate ranges, with the exception of small pelagic fishes. However, under warming and acidification combined, biomass increases at higher trophic levels will be lower or absent, while in the longer term reduced productivity of prey species is unlikely to support the increased biomass at the top of the food web. We also show that increases in exploitation will suppress any positive effects of human‐driven climate change, causing individual species biomass to decrease at higher trophic levels. Nevertheless, total future potential biomass of some fisheries species in temperate areas might remain high, particularly under acidification, because unharvested opportunistic species will likely benefit from decreased competition and show an increase in biomass. Ecological indicators of species composition such as the Shannon diversity index decline under all climate change scenarios, suggesting a trade‐off between biomass gain and functional diversity. By coupling parameters from multilevel mesocosm food web experiments with dynamic food web models, we were able to simulate the generative mechanisms that drive complex responses of temperate marine ecosystems to global change. This approach, which blends theory with experimental data, provides new prospects for forecasting climate‐driven biodiversity change and its effects on ecosystem processes.

Beschreibung: We report the first catalog of low‐frequency earthquakes in the Hikurangi subduction zone, located beneath the Kaimanawa Range of the North Island at 50 km depth, downdip of regularly recurring (every 4–5 years) deep M7 slow slip events. To systematically detect low‐frequency earthquakes within the regional continuous seismic data, we utilized a matched‐filter approach with template waveforms derived from previous observations of tectonic tremor. We built our catalog of 36 low‐frequency earthquake sources, that produced almost 21,000 events over more than a decade, with two matched‐filter search iterations. In each iteration, the detections were gathered into families and their coherent waveforms processed and stacked to extract high‐quality waveforms, allowing us to pick seismic phase arrivals to locate the low‐frequency earthquakes. We highlight three characteristic features to validate that our detected events are indeed low‐frequency earthquakes: the eponymous deficit of high frequencies in their seismic waveforms, the episodic swarms of activity that define their activity through time, and their location at the plate boundary with a double‐couple source mechanism and geometry consistent with the subduction interface. Considering the observed low‐frequency earthquakes' relationship to neighboring slow slip, we observe the event swarms to occur much more frequently than the M7 slow slip events located just updip. Similar to other deep low‐frequency earthquakes in other subduction zones, we suggest that this characteristic clustering in time is driven by more frequent, smaller slow slip events that are not clearly observable at the surface. Plain Language Summary Slow slip is episodic fault slip that lasts days, weeks or months, rather than the rapid ruptures of regular earthquakes. Geodetic observations of the surface displacement produced by slow slip suggest that their timing and location influence the seismic cycle of nearby faults and may even trigger large earthquakes. Although slow slip does not produce seismic radiation itself, slow slip is often accompanied by tiny repetitive seismic signals. These tiny seismic events, called low‐frequency earthquakes, can act as a powerful indicator of when and where slow slip is happening. In this study, we develop a new approach to detect low‐frequency earthquakes within continuous seismic waveforms, revealing the first observations of low‐frequency earthquakes in the Hikurangi subduction zone beneath the North Island of New Zealand. Our catalog of low‐frequency earthquakes suggests a complex pattern of slow fault slip at depth, with more frequent activity than geodetic data alone would suggest. The observed low‐frequency earthquake activity in the Hikurangi subduction zone thus represents a unique opportunity to study the slip history at depth beneath the North Island of New Zealand. Key Points 36 low‐frequency earthquake sources are extracted from continuous waveforms through template matching, deblurring, and unsupervised learning Low‐frequency earthquake sources locate close to the plate boundary with source mechanisms consistent with the subduction interface Detected low‐frequency earthquakes are likely triggered by small, frequent, and deep slow slip not geodetically observable at the surface

Beschreibung: In order not to significantly overshoot maximum levels of warming like the 1.5 and 2°C target we must stay within a fixed emissions budget. How to fairly distribute the entitlements to emit within such a budget is perhaps the most intensely discussed question in all of climate justice. In our review we discuss the most prominent proposals in moral and political philosophy on how to solve this question and put a special emphasis on scholarly contributions from the last decade. We canvass the arguments for and against emissions egalitarianism, emissions sufficientarianism, and emissions grandfathering as well as the debates surrounding them. These are how to deal with non‐compliance, how to split emissions between producers and consumers, how to best account for terrestrial carbon sinks, and whether emissions from having children should be subtracted from parents' emissions budgets. From the viewpoint of justice, it matters not only that we act against climate change but also how we do so. This review aims to elucidate one of the major ways in which our reaction to climate change could be just or unjust.

Beschreibung: The potential for future earthquakes on faults is often inferred from inversions of geodetically derived surface velocities for locking on faults using kinematic models such as block models. This can be challenging in complex deforming zones with many closely spaced faults or where deformation is not readily described with block motions. Furthermore, surface strain rates are more directly related to coupling on faults than surface velocities. We present a methodology for estimating slip deficit rate directly from strain rate and apply it to New Zealand for the purpose of incorporating geodetic data in the 2022 revision of the New Zealand National Seismic Hazard Model. The strain rate inversions imply slightly higher slip deficit rates than the preferred geologic slip rates on sections of the major strike‐slip systems including the Alpine Fault, the Marlborough Fault System and the northern part of the North Island Fault System. Slip deficit rates are significantly lower than even the lowest geologic estimates on some strike‐slip faults in the southern North Island Fault System near Wellington. Over the entire plate boundary, geodetic slip deficit rates are systematically higher than geologic slip rates for faults slipping less than one mm/yr but lower on average for faults with slip rates between about 5 and 25 mm/yr. We show that 70%–80% of the total strain rate field can be attributed to elastic strain due to fault coupling. The remaining 20%–30% shows systematic spatial patterns of strain rate style that is often consistent with local geologic style of faulting. Plain Language Summary The potential for future earthquakes on faults is often inferred from velocities of the ground surface derived from satellite geodesy, but this approach can be challenging in complex deforming zones with many closely spaced faults. We present a new methodology for estimating the rate at which energy is accumulating on faults using measurements of surface strain rates. The method is applied to New Zealand for the purpose of incorporating geodetic data in the 2022 revision of the New Zealand National Seismic Hazard Model. We show that 70%–80% of the total deformation field can be attributed to energy accumulation on known active faults while the source of the remaining 20%–30% remains unknown. Along some of the major faults in New Zealand we find some important differences in rates of energy accumulation from what is expected from geologic data. Estimated rates are significantly lower than even the lowest geologic estimates on some faults in the fault system near highly‐populated Wellington. Key Points We develop a method to invert geodetically derived strain rates for slip deficit rates on faults We find small but systematic differences between slip deficit rates and geologic slip rates About 70%–80% of the surface strain can be attributed to elastic strain due to coupling on faults

Beschreibung: Oceanic detachment faulting, a major mode of seafloor accretion at slow and ultraslow spreading ridges, is thought to occur during magma‐poor phases and be abandoned when magmatism increases. In this framework, detachment faulting is the result of temporal variations in magma flux, which is inconsistent with recent geophysical observations at the Longqi segment on the Southwest Indian Ridge (49°42′E). In this paper, we focus on this sequentially active detachment faulting system that includes an old, inactive detachment fault and a younger, active detachment fault. We investigate the mechanisms controlling the temporal evolution of this tectonomagmatic system by using 2D mid‐ocean ridge spreading models that simulate faulting and magma intrusion into a visco‐elasto‐plastic continuum. Our models show that temporal variations in magma flux alone are insufficient to match the inferred temporal evolution of the sequentially active detachment system. Rather we find that sequentially active detachment faulting spontaneously occurs at the Longqi segment as a function of lithospheric thickness. This finding is in agreement with an analytical model, which shows that a thicker axial lithosphere results in a smaller fault heave and that a flatter angle in lithosphere thickening away from the accretion axis stabilizes the active fault. A thicker axial lithosphere and its flatter off‐axis angle combined have the potential to modulate sequentially active detachment faulting at the Longqi segment. Our results thus suggest that temporal changes of magmatism are not necessary for the development and abandonment of detachment faults at ultraslow spreading ridges.

Beschreibung: Mesoscale eddies are common in the subtropical Northwest Pacific, however, relatively little is known about their spatial variability and temporal evolution, and how these impact upper ocean biogeochemistry. Here we investigate these using observations of a cyclonic eddy carried out along four sequential transects. Consistent with previous observations of cyclonic eddies, the eddy core had doming isopycnals, bringing elevated nutrient waters nearer to the surface. However, we also found that the upper layer of the eddy above the nutricline had significantly lower phosphate concentrations within its core relative to its edge. We attributed this to elevated N 2 fixation within the eddy core, which was likely driven by enhanced subsurface iron supply, ultimately resulting in increased phosphate consumption. Eddy‐enhanced N 2 fixation was additionally supported by the elevation of nitrate + nitrite to phosphate ratios below the euphotic zone. Moreover, we observed that while the upward displacement of isopycnals within the eddy core led to an increase in phytoplankton biomass in the lower euphotic zone, there was no significant increase in total phytoplankton biomass across the entire euphotic zone. Cyclonic eddies in the subtropical North Pacific are projected to be becoming more frequent, implying that such dynamics could become increasingly important for regulating nutrient biogeochemistry and ultimately productivity of the region. Key Points Lower phosphate concentrations were observed above the nutricline within the eddy core in comparison to the edge Enhanced N2 fixation within the eddy core is proposed to have driven increased phosphate consumption No substantial total phytoplankton biomass increase was found within the eddy core

Beschreibung: Warmer temperatures and higher sea level than today characterized the Last Interglacial interval [Pleistocene, 128 to 116 thousand years ago (ka)]. This period is a remarkable deep-time analog for temperature and sea-level conditions as projected for 2100 AD, yet there has been no evidence of fossil assemblages in the equatorial Atlantic. Here, we report foraminifer, metazoan (mollusks, bony fish, bryozoans, decapods, and sharks among others), and plant communities of coastal tropical marine and mangrove affinities, dating precisely from a ca. 130 to 115 ka time interval near the Equator, at Kourou, in French Guiana. These communities include ca. 230 recent species, some being endangered today and/or first recorded as fossils. The hyperdiverse Kourou mollusk assemblage suggests stronger affinities between Guianese and Caribbean coastal waters by the Last Interglacial than today, questioning the structuring role of the Amazon Plume on tropical Western Atlantic communities at the time. Grassland-dominated pollen, phytoliths, and charcoals from younger deposits in the same sections attest to a marine retreat and dryer conditions during the onset of the last glacial (ca. 110 to 50 ka), with a savanna-dominated landscape and episodes of fire. Charcoals from the last millennia suggest human presence in a mosaic of modern-like continental habitats. Our results provide key information about the ecology and biogeography of pristine Pleistocene tropical coastal ecosystems, especially relevant regarding the—widely anthropogenic—ongoing global warming.

Beschreibung: Scientific Significance Statement Millions of predator–prey interactions between deep-diving toothed whales and cephalopods occur daily in the dark deep sea. While predatory whales developed traits to detect and hunt their prey, cephalopods had to expand their anti-predatory strategies specialized for visual predators, to counteract acoustic predators. Since toothed whale-cephalopod interactions have never been directly observed in the deep sea, it remains unknown what selective pressures and traits evolved from this arms race. Combining current knowledge, we formalize four hypotheses and associated research approaches that will guide future investigation on oceanic predator–prey systems. We identify whale echolocation as an unprecedented armament to hunt distant prey and propose that deep-sea squids avoid acoustic predators by (1) reducing their acoustic cross-section through body shape and posture, (2) deep-sea migration, and (3) not schooling. Toothed whale predation emerges as a potential driver of the cephalopod live-fast-die-young strategy—which may now leave cephalopods at competitive advantage under global change.

Beschreibung: Aims Within the intensively‐studied, well‐documented latitudinal diversity gradient, the deep‐sea biodiversity of the present‐day Norwegian Sea stands out with its notably low diversity, constituting a steep latitudinal diversity gradient in the North Atlantic. The reason behind this has long been a topic of debate and speculation. Most prominently, it is explained by the deep‐sea glacial disturbance hypothesis, which states that harsh environmental glacial conditions negatively impacted Norwegian Sea diversities, which have not yet fully recovered. Our aim is to empirically test this hypothesis. Specific research questions are: (1) Has deep‐sea biodiversity been lower during glacials than during interglacials? ( 2) Was there any faunal shift at the Mid‐Brunhes Event (MBE) when the mode of glacial–interglacial climatic change was altered? Location Norwegian Sea, deep sea (1819–2800 m), coring sites MD992277, PS1243, and M23352. Time period 620.7–1.4 ka (Middle Pleistocene–Late Holocene). Taxa studied Ostracoda (Crustacea). Methods We empirically test the deep‐sea glacial disturbance hypothesis by investigating whether diversity in glacial periods is consistently lower than diversity in interglacial periods. Additionally, we apply comparative analyses to determine a potential faunal shift at the MBE, a Pleistocene event describing a fundamental shift in global climate. Results The deep Norwegian Sea diversity was not lower during glacial periods compared to interglacial periods. Holocene diversity was exceedingly lower than that of the last glacial period. Faunal composition changed substantially between pre‐ and post‐MBE. Main conclusions These results reject the glacial disturbance hypothesis, since the low glacial diversity is the important precondition here. The present‐day‐style deep Norwegian Sea ecosystem was established by the MBE, more specifically by MBE‐induced changes in global climate, which has led to more dynamic post‐MBE conditions. In a broader context, this implies that the MBE has played an important role in the establishment of the modern polar deep‐sea ecosystem and biodiversity in general.

Beschreibung: The marine cyanobacterium Trichodesmium has the remarkable ability to interact with and utilize air‐borne dust as a nutrient source. However, dust may adversely affect Trichodesmium through buoyancy loss and exposure to toxic metals. Our study explored the effect of desert dust on buoyancy and mortality of natural Red Sea puff‐shaped Trichodesmium thiebautii . Sinking velocities and ability of individual colonies to stay afloat with increasing dust loads were studied in sedimentation chambers. Low dust loads of up to ∼400 ng per colony did not impact initial sinking velocity and colonies remained afloat in the chamber. Above this threshold, sinking velocity increased linearly with the colony dust load at a slope matching prediction based on Stoke's law. The potential toxicity of dust was assessed with regards to metal dissolution kinetics, differentiating between rapidly released metals, which may impact surface blooms, and gradually released metals that may impact dust‐centering colonies. Incubations with increasing dust concentrations revealed colony death, but the observed lethal dose far exceeded dust concentrations measured in coastal and open ocean systems. Removal of toxic particles as a mechanism to reduce toxicity was explored using SEM‐EDX imaging of colonies incubated with Cu‐minerals, yet observations did not support this pathway. Combining our current and former experiments, we suggest that in natural settings the nutritional benefits gained by Trichodesmium via dust collection outweigh the risks of buoyancy loss and toxicity. Our data and concepts feed into the growing recognition of the significance of dust for Trichodesmium 's ecology and subsequently to ocean productivity. Plain Language Summary Trichodesmium spp. are abundant cyanobacteria, forming extensive blooms in low latitude warm oceans, and contribute significantly to carbon (C) and nitrogen (N) fixation, recycling and export. Desert dust deposited on the ocean surface was shown to supply Trichodesmium with the scarce micronutrient iron. Spherical, millimeter‐sized colonies of Trichodesmium from different ocean basins were reported to actively accumulate dust in their cores. While dust accumulation likely helps Trichodesmium obtain nutrients, it may come at a cost. Metals released from dust may induce toxicity and the dust weight could send Trichodesmium to the ocean depth. Our experimental study with natural Red Sea colonies examined some trade‐offs of dust accumulation. Links between dust load and colony buoyancy were examined in sedimentation experiments. Toxicity thresholds for surface blooms and dust‐accumulating colonies were determined from mortality assays and dust dissolution measurements. We found that metal‐induced toxicity to Trichodesmium is unlikely at typical oceanic dust fluxes, and that dust‐containing colonies can remain buoyant. At high loads, dust weight determined the colony's sinking velocity. Our findings and concepts can be extended to additional aerosols and Trichodesmium ‐rich habitats, and may assist in assessing Trichodesmium 's distribution, ecophysiology, and contribution to C or N transport to the deep ocean. Key Points Dust collected by Trichodesmium colonies from seawater as a nutrient source may result in metal toxification and buoyancy loss At moderate dust loads, colonies kept their buoyancy, but above 400 ng, sinking velocities increased linearly with dust loads Desert dust induced Trichodesmium mortality through toxic metal release, yet the lethal dose far exceeded oceanic dust concentrations

Beschreibung: Every piece of plastic is made up of a unique combination of the host polymer, with some residual monomers or catalysts, as well as chemical additives added during processing of the plastic. This chapter aims to introduce plastic additives with a focus on their chemistry and function, transport and fate, detection in marine environments, and toxicities. The extensive list of additives can be simplified by dividing the types of additives into three groups: functional additives, colorants, and fillers/reinforcements. Plasticizers are added to plastics to improve their flexibility, durability, and elasticity over a broad range of temperatures while also reducing the glass transition temperature and the melt flow. Additives are well known to leach from plastics in the marine environment. Like their plastic counterparts, plastic additives are also susceptible to oxidative degradation and biodegradation. The toxicity of plastic additives is quite variable given the diversity of their chemical classes.

Beschreibung: Bacterial sulfate reduction (SR) is often determined by radiotracer techniques using 35S‐labeled sulfate. In environments featuring simultaneous sulfide oxidation, SR can be underestimated due to re‐oxidation of 35S‐sulfide. Recycling of 35S‐tracer is expected to be high in sediment with low concentrations of pore‐water sulfide and high abundance of giant filamentous sulfur‐oxidizing bacteria (GFSOB). Here, we applied a sulfide‐spiking method, originally developed for water samples, to sediments along a shelf‐slope transect (72, 128, 243, 752 m water depth) traversing the Peruvian oxygen minimum zone. Sediment spiked with unlabeled sulfide prior to 35S‐sulfate injection to prevent radiotracer recycling was compared to unspiked sediment. At stations characterized by low natural sulfide and abundant GFSOB (128 and 243 m), the method revealed 1–3 times higher SR rates in spiked sediment. Spiking had no effect on SR in sediment with high natural sulfide despite presence of GFSOB (72 m). Bioturbated sediment devoid of GFSOB (752 m) showed elevated SR in spiked samples, likely from artificial introduction of sulfidic conditions. Sulfide oxidation rates at the 128 and 243 m station, derived from the difference in SR between spiked and unspiked sediment, approximated rates of dissimilatory nitrate reduction to ammonium by GFSOB. Gross SR contributed considerably to benthic dissolved inorganic carbon fluxes at the three shallowest station, confirming that SR is an important process for benthic carbon respirations within the oxygen minimum zone. We recommend to further explore the spiking method to capture SR in sediment featuring low sulfide concentrations and high sulfur cycling by GFSOB.

Beschreibung: In the northeastern tropical Atlantic, a region of high potential vorticity (PV) determines the size of the exchange window for the interior thermocline flow of the subtropical cell via its variations in strength and extent. Variability of this PV barrier has the potential to impact the ventilation of the tropical Atlantic on decadal timescales. Here, the impact of the North Atlantic Oscillation (NAO) on the PV barrier related to isopycnals within the thermocline of the subtropical-tropical Atlantic Ocean is assessed from Argo observations for the time period of 2006-2022. Relative to the negative NAO phase (2009-2010), during the positive NAO phase (2014-2019), the North Atlantic subtropical high and the northeast trades are intensified. Satellite-derived wind stress curl shows increased upwelling/downwelling on the equatorward/poleward side of the trade wind zone, respectively. In the subtropical-tropical Atlantic, a symmetric pattern of isopycnal heave is observed: rising isopycnals within 20 degrees N and 20 degrees S and sinking poleward of that. With rising isopycnals, the PV barrier in the northeastern tropical Atlantic becomes stronger. Analyses of geostrophic velocities and the Sverdrup streamfunction show that during the positive NAO phase there are increased equatorward velocities at thermocline level along the western boundary and reduced velocities through the interior as a result of intensified northeast trades and therefore a strengthened PV barrier. Intensified trades lead to enhanced subduction of thermocline waters and, independent of that, to a strengthened Equatorial Undercurrent transport as observed at the mooring site at 0 degrees, 23 degrees W, likely via the pulling effect of the subtropical cells. In the North Atlantic Ocean, subducted water from the subtropics has two possible pathways within the thermocline toward the equatorial region: the interior pathway and the pathway along the western boundary. The size of the exchange window between subtropics and tropics depends on the extent of a barrier zone in the eastern part of the basin that is associated with wind-driven upwelling of density surfaces. The North Atlantic Oscillation (NAO) is the dominant atmospheric climate mode in the North Atlantic and in this study, we show how the NAO impacts the barrier for the equatorward thermocline flow in the tropical Atlantic Ocean. During positive NAO phases (e.g., 2014-2019), density surfaces become shallower and strengthen the barrier, while during negative NAO phases (e.g., 2009-2010) the barrier weakens. Geostrophic velocity analysis reveals that during positive NAO phases more thermocline water is transported equatorward via the western boundary and less via the interior pathway. Additionally, observations from a mooring site at 0 degrees, 23 degrees W show stronger Equatorial Undercurrent transport as a result of intensified trade winds during positive NAO phases. Trade winds in the northeastern tropical Atlantic strengthen during positive phases of the North Atlantic Oscillation (NAO+) Potential vorticity barrier for the interior equatorward thermocline flow of the North Atlantic Subtropical Cell strengthens during NAO+ Annual subduction of thermocline water and Equatorial Undercurrent transport increase simultaneously from 2008 to 2018

Beschreibung: Viscosity in the momentum equation is needed for numerical stability, as well as to arrest the direct cascade of enstrophy at grid scales. However, a viscous momentum closure tends to over-dissipate eddy kinetic energy. To return excessively dissipated energy to the system, the viscous closure is equipped with what is called dynamic kinetic energy backscatter. The amplitude of backscatter is based on the amount of unresolved kinetic energy (UKE). This energy is tracked through space and time via a prognostic equation. Our study proposes to add advection of UKE by the resolved flow to that equation to explicitly consider the effects of nonlocality on the subgrid energy budget. UKE can consequently be advected by the resolved flow before it is reinjected via backscatter. Furthermore, we suggest incorporating a stochastic element into the UKE equation to account for missing small-scale variability, which is not present in the purely deterministic approach. The implementations are tested on two intermediate complexity setups of the global ocean model FESOM2: an idealized channel setup and a double-gyre setup. The impacts of these additional terms are analyzed, highlighting increased eddy activity and improved flow characteristics when advection and carefully tuned, stochastic sources are incorporated into the UKE budget. Additionally, we provide diagnostics to gain further insights into the effects of scale separation between the viscous dissipation operator and the backscatter operator responsible for the energy injection. Oceanic swirls or "eddies" have a typical size of 10-100 km, which is close to the smallest scales that global ocean models commonly resolve. For physical and numerical reasons, these models require the addition of artificial terms that influence the flow near its smallest scales. Common approaches have the drawback of introducing systematic loss of kinetic energy contained in the eddies, which leads to errors that also affect the oceanic circulation on global scales. In our research, we compensate for this error by returning some of the missing energy back into the simulation, using a so-called kinetic energy backscatter scheme. In this work, we continue the development of an already existing and successful backscatter scheme, adding certain improvements to the way energy is budgeted and returned to the flow: we ensure that the local energy budget is attached to each fluid parcel as it is transported by the large-scale flow, and we also add a random forcing term that mimics unknown sources of such energy to bring its statistical properties closer to reality. We demonstrate that these modifications effectively improve the characteristics of the simulated flow. Extension of the subgrid energy equation of the kinetic energy backscatter parameterization by adding advection and a stochastic term Both additional terms improve several flow characteristics in two idealized test cases, a channel and a double-gyre Scale analysis reveals the necessity of sufficient scale separation between viscous energy dissipation and energy injection via backscatter. Key Points: - Extension of the subgrid energy equation of the kinetic energy backscatter parameterization by adding advection and a stochastic term - Both additional terms improve several flow characteristics in two idealized test cases, a channel and a double-gyre - Scale analysis reveals the necessity of sufficient scale separation between viscous energy dissipation and energy injection via backscatter

Beschreibung: The sinking of particulate matter from the upper ocean dominates the export and sequestration of organic carbon by the biological pump, a critical component of the Earth's carbon cycle. Controls on carbon export are thought to be driven by ecological processes that produce and repackage sinking biogenic particles. Here, we present observations during the demise of the Northeast Atlantic Ocean spring bloom illustrating the importance of storm-induced turbulence on the dynamics of sinking particles. A sequence of four large storms caused upper layer mean turbulence levels to vary by more than three orders of magnitude. Large particle (>0.1 to 10 mm) abundance and size changed accordingly: increasing via shear coagulation when turbulence was moderate and decreasing rapidly when turbulence was intense due to shear disaggregation. Particle export was also tied to storm forcing as large particles were mixed to depth during mixed layer deepening. After the mixed layer shoaled, these particles, now isolated from intense surface mixing, grew larger and subsequently sank. This sequence of events matched the timing of sinking particle flux observations. Particle export was influenced by increases in aggregate abundance and porosity, which appeared to be enhanced by the repeated creation and destruction of aggregates. Last, particle transit efficiency through the mesopelagic zone was reduced by presumably biotic processes that created small particles (〈0.5 mm) from larger ones. Our results demonstrate that ocean turbulence significantly impacts the nature and dynamics of sinking particles, strongly influencing particle export and the efficiency of the biological pump.

Beschreibung: We present a continuous ∼6.2 Ma long record of explosive activity from the Northwest Pacific volcanic arcs based on a composite tephra sequence derived from Ocean Drilling Program Sites 882A and 884B, and core MD01‐2416 on the Detroit Seamount. Geochemical fingerprinting of tephra glass using major and trace element analyses and correlations of tephra layers between the three cores allowed the identification of 119 unique tephras, suggesting eruptions of magnitude (M) of 5.8–7.8. Age estimates for all the identified eruptions were obtained with the help of published and further refined age models for the studied cores, direct 40 Ar/ 39 Ar dating of four ash layers, and Bayesian age modeling. The glass compositions vary from low‐ to high‐K 2 O basaltic andesite to rhyolite and exhibit typical subduction‐related affinity. The majority of the tephras originated from Kamchatka, only a few tephras—from the neighboring Kuril and Aleutian arcs. The glass compositions revealed no temporal trends but made it possible to identify their source volcanic zones in Kamchatka and, in some cases, to determine their source eruptive centers. Our data indicates episodes of explosive activity recorded in the Detroit tephra sequence at ∼6,200, 5,600–5,000, 4,300–3,700 ka, and almost continuous activity since ∼3,000 ka. Within the latter episode, the most active intervals can be identified at 1,700–1,600, 1,150–1,050, and 600–50 ka. Geochemically fingerprinted and dated Detroit tephra sequence form a framework for dating and correlating diverse paleoenvironmental archives across the Northwest Pacific and for studies of geochemical evolution of the adjacent volcanic arcs. Plain Language Summary Explosive volcanic eruptions produce defragmented material named tephra, which can be spread over large distances and form layers in sediments on ocean floor and continents. Long continuous tephra sequences preserved in marine sediments provide one of the best chronicles of the explosive eruptions, and allow detailed evaluation of their timing relative to climatic changes. We studied one of such natural records of explosive volcanism preserved in the sediments covering the Detroit Seamount in the Northwest Pacific. We identified 119 tephra layers, which have been buried in the sediments during the last 6.2 Ma and represent volcanic eruptions with ≥7 km 3 tephra volume. We analyzed geochemical composition and determined age of each tephra. Most tephras were found to originate from volcanoes in Kamchatka, a few from the Kuril and Aleutian volcanoes. We found that the explosive activity recorded in the Detroit tephra sequence was not uniform over time. It peaked at ∼6,200, 5,600–5,000, 4,300–3,700, has continued since ∼3,000 thousand years ago until present. All tephra layers from our study can be used as unique isochrons for dating and correlating paleoenvironmental archives across the Northwest Pacific and for the reconstruction of the detailed volcanic record in the Earth history. Key Points We report age and composition for 119 tephras from sediment cores representing ∼6.2 Ma record of explosive volcanism in the NW Pacific The tephras have subduction‐related origin and mostly originate from volcanic eruptions with magnitude (M) of 5.8–7.8 in Kamchatka The data indicates episodes of explosive activity at ∼6,200, 5,600–5,000, 4,300–3,700 ka, and almost continuous activity since ∼3,000 ka

Beschreibung: Due to the complexity of 2D magnetic anomaly maps north of 18°S and the sparsity of seismic data, the tectonic evolution of the northern Lau Basin has not yet been unraveled. We use a multi-method approach to reconstruct the formation of the basin at ∼16°S by compiling seismic, magnetic, gravimetric and geochemical data along a 185 km-long crustal transect. We identified a crustal zonation which preserves the level of subduction input at the time of the crust's formation. Paired with the seafloor magnetization, the crustal zonation enabled us to qualitatively approximate the dynamic spreading history of the region. Further assessment of the recent tectonic activity and the degree of tectonic overprinting visible in the crust both suggest a complex tectonic history including a dynamically moving spreading center and the reorganizing of the local magma supply. Comparing the compiled data sets has revealed substantial differences in the opening mechanisms of the two arms of the Overlapping Spreading Center (OSC) that is made up by the northernmost tip of the Fonualei Rift and Spreading Center in the east and the southernmost segment of the Mangatolu Triple Junction in the west. The observed transition from a predominantly tectonic opening mechanism at the eastern OSC arm to a magmatic opening mechanism at the western OSC arm coincides with an equally sharp transition from and strongly subduction influenced crust to a crust with virtually no subduction input. The degree of subduction input alters the geochemical composition, as well as the lithospheric stress response. Key Points Oceanic crust in the north-eastern Lau Basin formed at the now reorganized FRSC-MTJ system The position and the opening mechanisms of back-arc basin spreading center's change more dynamically at mid-ocean ridges Different opening mechanisms at the southern Mangatolu Triple Junction and northern Fonualei Rift Spreading Center despite their proximity

Beschreibung: Promoting effects of aluminum addition on chlorophyll biosynthesis and growth of two cultured iron‐limited marine diatoms Linbin Zhou CAS Key Laboratory of Tropical Marine Bio‐resources and Ecology, Guangdong Provincial Key Laboratory of Applied Marine Biology, South China Sea Institute of Oceanology Chinese Academy of Sciences Guangzhou China Marine Biogeochemistry Division GEOMAR Helmholtz Centre for Ocean Research Kiel Germany University of Chinese Academy of Sciences Beijing China https://orcid.org/0000-0001-7230-4116 Fengjie Liu Marine Biogeochemistry Division GEOMAR Helmholtz Centre for Ocean Research Kiel Germany Grantham Institute—Climate Change and the Environment, Department of Life Sciences Imperial College London London UK Eric P. Achterberg Marine Biogeochemistry Division GEOMAR Helmholtz Centre for Ocean Research Kiel Germany Anja Engel Marine Biogeochemistry Division GEOMAR Helmholtz Centre for Ocean Research Kiel Germany https://orcid.org/0000-0002-1042-1955 Peter G.C. Campbell Institut National de la Recherche Scientifique Centre Eau Terre Environnement Quebec Canada https://orcid.org/0000-0001-7160-4571 Claude Fortin Institut National de la Recherche Scientifique Centre Eau Terre Environnement Quebec Canada https://orcid.org/0000-0002-2479-1869 Liangmin Huang CAS Key Laboratory of Tropical Marine Bio‐resources and Ecology, Guangdong Provincial Key Laboratory of Applied Marine Biology, South China Sea Institute of Oceanology Chinese Academy of Sciences Guangzhou China University of Chinese Academy of Sciences Beijing China Yehui Tan CAS Key Laboratory of Tropical Marine Bio‐resources and Ecology, Guangdong Provincial Key Laboratory of Applied Marine Biology, South China Sea Institute of Oceanology Chinese Academy of Sciences Guangzhou China University of Chinese Academy of Sciences Beijing China Abstract Aluminum (Al) may play a role in the ocean's capacity for absorbing atmospheric CO 2 via influencing carbon fixation, export, and sequestration. Aluminum fertilization, especially in iron (Fe)‐limited high‐nutrient, low‐chlorophyll ocean regions, has been proposed as a potential CO 2 removal strategy to mitigate global warming. However, how Al addition would influence the solubility and bioavailability of Fe as well as the physiology of Fe‐limited phytoplankton has not yet been examined. Here, we show that Al addition (20 and 100 nM) had little influence on the Fe solubility in surface seawater and decreased the Fe bio‐uptake by 11–22% in Fe‐limited diatom Thalassiosira weissflogii in Fe‐buffered media. On the other hand, the Al addition significantly increased the rate of chlorophyll biosynthesis by 45–60% for Fe‐limited T. weissflogii and 81–102% for Fe‐limited Thalassiosira pseudonana , as well as their cell size, cellular chlorophyll content, photosynthetic quantum efficiency ( F v / F m ) and growth rate. Under Fe‐sufficient conditions, the Al addition still led to an increased growth rate, though the beneficial effects of Al addition on chlorophyll biosynthesis were no longer apparent. These results suggest that Al may facilitate chlorophyll biosynthesis and benefit the photosynthetic efficiency and growth of Fe‐limited diatoms. We speculate that Al addition may enhance intracellular Fe use efficiency for chlorophyll biosynthesis by facilitating the superoxide‐mediated intracellular reduction of Fe(III) to Fe(II). Our study provides new evidence and support for the iron–aluminum hypothesis.

Beschreibung: Submarine groundwater discharge (SGD) is a globally important process supplying nutrients and trace elements to the coastal environment, thus playing a pivotal role in sustaining marine primary productivity. Along with nutrients, groundwater also contains allochthonous microbes that are discharged from the terrestrial subsurface into the sea. Currently, little is known about the interactions between groundwater‐borne and coastal seawater microbial populations, and groundwater microbes' role upon introduction to coastal seawater populations. Here, we investigated seawater microbial abundance, activity and diversity in a site strongly influenced by SGD. In addition, through laboratory‐controlled bottle incubations, we mimicked different mixing scenarios between groundwater and seawater. Our results demonstrate that the addition of 0.1 μm filtered groundwater stimulated heterotrophic activity and increased microbial abundance compared to control coastal seawater, whereas 0.22 μm filtration treatments induced primary productivity and Synechococcus growth. 16S rRNA gene sequencing showed a strong shift from a SAR11‐rich community in the control samples to Rhodobacteraceae dominance in the 〈0.1 μm treatment, in agreement with Rhodobacteraceae enrichment in the SGD field site. These results suggest that microbes delivered by SGD may affect the abundance, activity and diversity of intrinsic microbes in coastal seawater, highlighting the cryptic interplay between groundwater and seawater microbes in coastal environments, which has important implications for carbon cycling. Plain Language Summary Submarine groundwater discharge (SGD) is an important process where groundwater flows into the ocean along the coast. When the groundwater mixes with seawater, the microbes from both sources interact with each other, which can impact the diversity, activity, and amount of microbes in the coastal environment. Currently, little is known about how groundwater‐borne microbes affect marine microbial populations. Our research shows that when groundwater microbes are removed before mixing groundwater with seawater, the abundance and activity of certain microbes that consume organic matter significantly increase. Additionally, we noticed a significant difference in the types of microbes present between the sites where SGD occurs versus background (uninfluenced) coastal water, especially in terms of the microbes that consume organic matter. Overall, this study suggests that there is a connection between groundwater and seawater microbes, which can influence the delicate balance between organisms that produce carbon and those that consume it. This has important implications for how carbon cycles globally. Key Points Groundwater discharge into the coastal zone delivers both nutrients and allochthonous microbes Groundwater microbes interact with seawater populations, by which affecting the delicate autotroph‐heterotroph balance Subterranean microbial processes are key drivers of food webs, potentially affecting biogenic carbon fluxes in the ocean

Beschreibung: With almost 700 Pg of carbon, marine dissolved organic carbon (DOC) stores more carbon than all living biomass on Earth combined. However, the controls behind the persistence and the spatial patterns of DOC concentrations on the basin scale remain largely unknown, precluding quantitative assessments of the fate of this large carbon pool in a changing climate. Net removal rates of DOC along the overturning circulation suggest lifetimes of millennia. These net removal rates are in stark contrast to the turnover times of days to weeks of heterotrophic microorganisms, which are the main consumers of organic carbon in the ocean. Here, we present a dynamic “MICrobial DOC” model (MICDOC) with an explicit representation of picoheterotrophs to test whether ecological mechanisms may lead to observed decadal to millennial net removal rates. MICDOC is in line with 〉40,000 DOC observations. Contrary to other global models, the reactivity of DOC fractions is not prescribed, but emerges from a dynamic feedback between microbes and DOC governed by carbon and macronutrient availability. A colimitation of macronutrients and organic carbon on microbial DOC uptake explains 〉70% of the global variation of DOC concentrations, and governs characteristic features of its distribution. Here, decadal to millennial net removal rates emerge from microbial processes acting on time scales of days to weeks, suggesting that the temporal variability of the marine DOC inventory may be larger than previously thought. With MICDOC, we provide a foundation for assessing global effects on DOC related to changes in heterotrophic microbial communities in a future ocean Plain Language Summary The ocean stores more carbon as dissolved organic compounds (DOC) than all animals and plants on land and the oceans combined. However, numerical models used for future climate scenarios lack an implementation of processes transforming DOC back to CO 2 by marine microorganisms. Here, we present a global dynamical ocean model that explicitly considers the processes of DOC degradation by marine microorganisms. In the present ocean, the availability of organic carbon but also nitrogen and phosphorus control the amount of carbon stored as DOC, as the lack of these nutrients inhibits its degradation by bacteria. The identification of these ecological controls allows a quantitative assessment of the fate of this large carbon reservoir in the future. The findings indicate that the marine DOC reservoir is potentially more dynamic than previously thought, since decadal to millennial scale net removal rates might be a result of microbial processes acting on shorter time scales Key Points A model to reconcile millennial‐scale bulk dissolved organic carbon degradation rates and short‐term microbial turnover times is presented Macronutrient colimitation can explain observed concentration patterns of dissolved organic carbon in the surface ocean Continuous microbial reworking suggests a higher temporal variability of the marine dissolved organic matter inventory than previously thought

Beschreibung: The zebra mussel Dreissena polymorpha is one of the most successful, notorious, and detrimental aquatic invasive non-native species worldwide, having invaded Europe and North America while causing substantial ecological and socio-economic impacts. Here, we investigated the spatiotemporal trends in this species' invasion success using 178 macroinvertebrate abundance time series, containing 1451 records of D. polymorpha collected across nine European countries between 1972–2019. Using these raw (absolute) abundance data, we examined trends and drivers of occurrences and relative abundances of D. polymorpha within invaded communities. Meta-regression models revealed non-significant trends both at the European level and for the majority of the invaded countries, except for France (significant decreasing trend) and Hungary (marginally positive trend). At the European level, the number of D. polymorpha occurrences over time followed a flat-top bell-shaped distribution, with a steep increase between 1973–1989 followed by a plateau phase prior to significantly declining post-1998. Using a series of climatic and hydromorphological site-specific characteristics of invaded and uninvaded sites from two periods (1998–2002; 2011–2015), we found that native richness, non-native abundance, distance to the next barrier, and elevation were associated with the occurrence of D. polymorpha. We also found that higher native richness and lower latitude were related to lower relative abundances. Using Cohen's D as a measure of D. polymorpha impact, we found that biodiversity within the invaded sites was initially higher than in uninvaded ones, but then declined, suggesting differences in biodiversity trends across invaded and uninvaded sites. While our results emphasise the high invasion success of D. polymorpha, increasing stressors within the context of global change – particularly ongoing climate change – are likely to enhance invasion rates and the impact of D. polymorpha in the near future, exacerbated by the lack of timely and effective management actions.

Beschreibung: The study of offshore freshened groundwater (OFG) is gaining importance due to population growth and environmental pressure on coastal water resources. Marine controlled source electromagnetic (CSEM) methods can effectively map the spatial extent of OFG systems using electrical resistivity as a proxy. Integrating these resistivity models with sub-surface properties, such as host-rock porosity, allows for estimates of pore-water salinity. However, evaluating the uncertainty in pore-water salinity using resistivity models obtained from deterministic inversion approaches presents challenges, as they provide only one best-fit model, with no associated estimate of uncertainty. To address this limitation, we employ trans-dimensional Markov-Chain Monte-Carlo inversion on marine time-domain CSEM data, acquired in the Canterbury Bight, New Zealand. We integrate resistivity posterior probability distributions with borehole and seismic reflection data to estimate pore-water salinity with corresponding uncertainty estimates. The results highlight a low-salinity groundwater body in the center of the survey area, hosted by consecutive silty- and fine-sand layers approximately 20–60 km from the coast. The posterior probability distribution of resistivity models indicates freshening of the OFG body toward the shoreline within a permeable, coarse-sand layer 40–150 m beneath the seafloor, suggesting an active connection between the OFG body and the terrestrial groundwater system. The approach demonstrates how Bayesian inversion constrains the uncertainties in resistivity models and subsequently in pore-water salinity estimates. Our findings highlight the potential of Bayesian inversion to enhance our understanding of OFG systems and provide uncertainty constraints for hydrogeological modeling, thereby contributing to sustainable water resource development. Key Points A Bayesian workflow is employed to evaluate uncertainty in pore-water salinity estimates Offshore groundwater in Canterbury Bight stores freshened pore-water in fine-grained sediments, likely extending from the onshore aquifer Correlation between pore-water salinities and seismic-derived stratigraphy provides boundary conditions for hydrogeological modeling Plain Language Summary Geophysical methods that measure the electromagnetic properties of the Earth are effective in investigating freshwater sources beneath the seafloor. By combining the geophysical and geological information, we can better assess the quality of this groundwater. In this study, we develop a workflow that uses statistical methods to integrate electromagnetic observations with borehole and acoustic measurements along the eastern coast of the South Island of New Zealand. We aim to improve our understanding of the groundwater quality beneath the seafloor. Our research confirms the presence of freshened groundwater within the sandy seafloor up to 60 km from the coastline. Importantly, our observations indicate that the groundwater quality increases toward the coast. These findings are significant as they enhance the hydrogeological modeling of the groundwater system and suggest its potential as a source of freshwater.

Beschreibung: Detecting phase arrivals and pinpointing the arrival times of seismic phases in seismograms is crucial for many seismological analysis workflows. For land station data, machine learning methods have already found widespread adoption. However, deep learning approaches are not yet commonly applied to ocean bottom data due to a lack of appropriate training data and models. Here, we compiled an extensive and labeled ocean bottom seismometer (OBS) data set from 15 deployments in different tectonic settings, comprising ∼90,000 P and ∼63,000 S manual picks from 13,190 events and 355 stations. We propose PickBlue, an adaptation of the two popular deep learning networks EQTransformer and PhaseNet. PickBlue joint processes three seismometer recordings in conjunction with a hydrophone component and is trained with the waveforms in the new database. The performance is enhanced by employing transfer learning, where initial weights are derived from models trained with land earthquake data. PickBlue significantly outperforms neural networks trained with land stations and models trained without hydrophone data. The model achieves a mean absolute deviation of 0.05 s for P-waves and 0.12 s for S-waves, and we apply the picker on the Hikurangi Ocean Bottom Tremor and Slow Slip OBS deployment offshore New Zealand. We integrate our data set and trained models into SeisBench to enable an easy and direct application in future deployments. Key Points We assembled a database of ocean Bottom Seismometer (OBS) waveforms and manual P and S picks, on which we train PickBlue, a deep learning picker Our picker significantly outperforms pickers trained with land-based data with confidence values reflecting the likelihood of outlier picks The picker and database are available in the SeisBench platform, allowing easy and direct application to OBS traces and hydrophone records

Beschreibung: The Bay of Bengal (BoB) spans 〉2.2 million km(2) in the northeastern Indian Ocean and is bordered by dense populations that depend upon its resources. Over recent decades, a shift from larger phytoplankton to picoplankton has been reported, yet the abundance, activity, and composition of primary producer communities are not well-characterized. We analysed the BoB regions during the summer monsoon. Prochlorococcus ranged up to 3.14 x 10(5) cells mL(-1 )in the surface mixed layer, averaging 1.74 +/- 0.46 x 10(5) in the upper 10 m and consistently higher than Synechococcus and eukaryotic phytoplankton. V1-V2 rRNA gene amplicon analyses showed the High Light II (HLII) ecotype formed 98 +/- 1% of Prochlorococcus amplicons in surface waters, comprising six oligotypes, with the dominant oligotype accounting for 65 +/- 4% of HLII. Diel sampling of a coherent water mass demonstrated evening onset of cell division and rapid Prochlorococcus growth between 1.5 and 3.1 div day-1, based on cell cycle analysis, as confirmed by abundance-based estimates of 2.1 div day(-1). Accumulation of Prochlorococcus produced by ultradian growth was restricted by high loss rates. Alongside prior Arabian Sea and tropical Atlantic rates, our results indicate Prochlorococcus growth rates should be reevaluated with greater attention to latitudinal zones and influences on contributions to global primary production.

Beschreibung: Seismic data from the North Sea commonly show vertical acoustic blanking (VAB) often interpreted as fluid conduits with implications for Quaternary development. The robustness of this interpretation has long been controversial as the infill of tunnel valleys can also cause vertical blanking. Using 2D and 3D seismic data and sediment echosounder data from the German North Sea, we investigate VAB to determine a geological or imaging origin of these anomalies. We detected multiple VAB occurrences throughout the North Sea. 3D data from the Ducks Beak (‘Entenschnabel’) reveal a correlation of VAB with bright spots in incised channels directly below the seafloor. Large source–receiver distances allow imaging the subsurface below the channel without signal penetrating through it (undershooting). This method removes the blanking. Energy absorption by shallow biogenic gas trapped within the channels explains the observed VAB. Hence, the blanking represents an imaging artifact, highlighting the need for careful seismic processing with sufficient offset before interpreting such anomalies as fluid pathways. The channels belong to a postglacial channel system related to the now submerged lowlands of Doggerland. This work demonstrates the usability of mapping VAB to detect shallow features for paleo‐landscape reconstruction and identification of shallow gas for hazard assessments, for example.

Beschreibung: The long-term dynamics of microbial communities across geographic, hydrographic, and biogeochemical gradients in the Arctic Ocean are largely unknown. To address this, we annually sampled polar, mixed, and Atlantic water masses of the Fram Strait (2015–2019; 5–100 m depth) to assess microbiome composition, substrate concentrations, and oceanographic parameters. Longitude and water depth were the major determinants (~30%) of microbial community variability. Bacterial alpha diversity was highest in lower-photic polar waters. Community composition shifted from west to east, with the prevalence of, for example, Dadabacteriales and Thiotrichales in Arctic- and Atlantic-influenced waters, respectively. Concentrations of dissolved organic carbon peaked in the western, compared to carbohydrates in the chlorophyll-maximum of eastern Fram Strait. Interannual differences due to the time of sampling, which varied between early (June 2016/2018) and late (September 2019) phytoplankton bloom stages, illustrated that phytoplankton composition and resulting availability of labile substrates influence bacterial dynamics. We identified 10 species clusters with stable environmental correlations, representing signature populations of distinct ecosystem states. In context with published metagenomic evidence, our microbial-biogeochemical inventory of a key Arctic region establishes a benchmark to assess ecosystem dynamics and the imprint of climate change.

Beschreibung: Standardised terminology in science is important for clarity of interpretation and communication. In invasion science - a dynamic and rapidly evolving discipline - the proliferation of technical terminology has lacked a standardised framework for its development. The result is a convoluted and inconsistent usage of terminology, with various discrepancies in descriptions of damage and interventions. A standardised framework is therefore needed for a clear, universally applicable, and consistent terminology to promote more effective communication across researchers, stakeholders, and policymakers. Inconsistencies in terminology stem from the exponential increase in scientific publications on the patterns and processes of biological invasions authored by experts from various disciplines and countries since the 1990s, as well as publications by legislators and policymakers focusing on practical applications, regulations, and management of resources. Aligning and standardising terminology across stakeholders remains a challenge in invasion science. Here, we review and evaluate the multiple terms used in invasion science (e.g. 'non-native', 'alien', 'invasive' or 'invader', 'exotic', 'non-indigenous', 'naturalised', 'pest') to propose a more simplified and standardised terminology. The streamlined framework we propose and translate into 28 other languages is based on the terms (i) 'non-native', denoting species transported beyond their natural biogeographic range, (ii) 'established non-native', i.e. those non-native species that have established self-sustaining populations in their new location(s) in the wild, and (iii) 'invasive non-native' - populations of established non-native species that have recently spread or are spreading rapidly in their invaded range actively or passively with or without human mediation. We also highlight the importance of conceptualising 'spread' for classifying invasiveness and 'impact' for management. Finally, we propose a protocol for classifying populations based on (i) dispersal mechanism, (ii) species origin, (iii) population status, and (iv) impact. Collectively and without introducing new terminology, the framework that we present aims to facilitate effective communication and collaboration in invasion science and management of non-native species.

Beschreibung: The Hikurangi Margin east of New Zealand's North Island hosts an extensive gas hydrate province with numerous gas hydrate accumulations related to the faulted structure of the accretionary wedge. One such hydrate feature occurs in a small perched upper‐slope basin known as Urutī Basin. We investigated this hydrate accumulation by combining a long‐offset seismic line (10‐km‐long receiver array) with a grid of high‐resolution seismic lines acquired with a 600‐m‐long hydrophone streamer. The long‐offset data enable quantitative velocity analysis, while the high‐resolution data constrain the three‐dimensional geometry of the hydrate accumulation. The sediments in Urutī Basin dip landward due to ongoing deformation of the accretionary wedge. These strata are clearly imaged in seismic data where they cross a distinct bottom simulating reflection (BSR) that dips counterintuitively in the opposite direction to the regional dip of the seafloor. BSR‐derived heat flow estimates reveal a distinct heat flow anomaly that coincides spatially with the upper extent of a landward‐verging thrust fault. We present a conceptual model of this gas hydrate system that highlights the roles of fault‐controlled fluid flow at depth merging into strata‐controlled fluid flow into the hydrate stability zone. The result is a layer‐constrained accumulation of concentrated gas hydrate in the dipping strata. Our study provides new insight into the interplay between deep faulting, fluid flow and gas hydrate formation within an active accretionary margin. Plain Language Summary Gas hydrates are ice‐like substances in which natural gas molecules are trapped in a cage of water molecules. They exist where the pressure is high, temperature is cold, and enough methane is present. These conditions exist in the marine environment at water depths greater than 300–500 m near sediment‐rich continental margins and in polar regions. It is important to study gas hydrates because they represent a significant part of the Earth's carbon budget and influence the flow of methane into the oceans and atmosphere. In this study, we use the seismic reflection method to generate images of gas‐hydrate‐bearing marine sediments east of New Zealand. Our data reveal an intriguing relationship between deep‐sourced fluid flow upward along a tectonic fault, and shallower flow through dipping sediments. This complex fluid flow pattern has led to disruption of the gas hydrate system and the formation of concentrated gas hydrate deposits within the dipping sediments. Our study highlights the relationships between relatively deep tectonic processes (faulting and fluid flow) and the shallow process of gas hydrate formation in an active subduction zone. Key Points A distinct gas‐hydrate to free‐gas transition is mapped using high‐ and low‐frequency seismic data Gas and hydrate accumulations in the Urutī Basin are controlled by the structural setting, ongoing deep‐sourced fluid flow, and near surface stratigraphy Regions of high modeled heat flow can be directly related to accumulations of gas and gas hydrates

Beschreibung: We conducted two‐dimensional numerical simulations to investigate the mechanisms underlying the strong spatiotemporal correlation observed between submarine landslides and gas hydrate dissociation due to glacial sea‐level drops. Our results suggest that potential plastic deformation or slip could occur at localized and small scales in the shallow‐water portion of the gas hydrate stability zone (GHSZ). This shallow‐water portion of the GHSZ typically lies within the area enclosed by three points: the BGHSZ–seafloor intersection, the seafloor at ∼600 m below sea level (mbsl), and the base of the GHSZ (BGHSZ) at ∼1,050 mbsl in low‐latitude regions. The deep BGHSZ (〉1,050 mbsl) could not slip; therefore, the entire BGHSZ was not a complete slip surface. Glacial hydrate dissociation alone is unlikely to cause large‐scale submarine landslides. Observed deep‐water (much greater than 600 mbsl) turbidites containing geochemical evidence of glacial hydrate dissociation potentially formed from erosion or detachment in the GHSZ pinch‐out zone. Plain Language Summary Many submarine landslides spatiotemporally correlate with gas hydrate dissociation. However, direct mechanical evidence supporting whether the overpressure and deformation due to glacial sea‐level drop‐induced hydrate dissociation are adequate for triggering submarine landslides is lacking. Here, we present two‐dimensional thermal‐hydraulic‐chemical and geomechanical models of a gas‐hydrate system in response to glacial sea‐level drops and conduct sensitivity analyses of the model behavior under a wide range of key conditions from a global perspective. Our simulations suggest that glacial hydrate dissociation might induce plastic deformation or slip at localized and small scales only possibly within the shallow‐water portion of the hydrate stability zone. The deep part (〉1,050 m below sea level) of the bottom boundary of the hydrate stability zone could not slip; therefore, the entire bottom boundary of the hydrate stability zone was not a complete slip surface. We demonstrate that glacial hydrate dissociation alone is unlikely to trigger large‐scale submarine landslides. Our work highlights the vicinity of the upper limit of the hydrate stability zone (where the base of the hydrate stability zone intersects the seafloor) as an important area for investigating overpressure and focused fluid flow, localized plastic deformation or slip, and downslope sediment transport related to glacial hydrate dissociation. Key Points Glacial hydrate dissociation might cause potential plastic deformation or slip at localized and small scales in shallow parts of the GHSZ The large deformation surface at the BGHSZ boundary of the potential plastic deformation zone was not a complete slip surface Glacial sea‐level drop‐induced gas hydrate dissociation alone is unlikely to have caused large‐scale submarine landslides

Beschreibung: Carbon disulfide (CS2) has recently gained attention as an important precursor for the atmospheric trace gas carbonyl sulfide (OCS), which delivers sulfur to the stratospheric sulfur layer and impacts the radiative budget of the Earth. CS2 is naturally produced in the ocean and emitted to the atmosphere. However, the magnitude of its marine emissions is only poorly constrained due to lacking understanding of its production and consumption processes. Here, we present incubation experiments with and without UV light treatment and provide evidence for a previously not considered UV-light-driven degradation process of CS2 in seawater, following first-order kinetics. In addition to its already known photochemical production process, CS2 production is found in the dark, depending on the amount of dissolved organic sulfur present in seawater. We provide novel production and consumption rates of CS2 in seawater that pave the way toward mechanistically quantifying marine emissions of this important trace gas. Key Points: - Carbon disulfide in seawater is degraded by UV light at time scales of days - Carbon disulfide is produced in seawater without UV light at rates comparable to photochemical production - Carbon disulfide dark production is limited by dissolved organic sulfur

Beschreibung: Retrieving the physical properties and water content of marine aerosols requires understanding the links between the particles' optical and microphysical properties. By using a morphologically realistic model with varying salt mass fractions fm, describing the transition from irregularly shaped, dry salt crystals to brine-coated geometries, optical properties relevant to polarimetric remote sensing are computed at wavelengths of 532 and 1,064 nm. The extinction cross section and its color ratio depend on particle size, but are insensitive to changes in fm; thus, measured extinction coefficients at two wavelengths contain information on both particle number and size. The lidar ratio's dependence on both size and wavelength has implications for inverting the lidar equation. The results suggest that active observations of the backscattering cross section's color ratio and the depolarization ratio, as well as, passive observations of the degree of linear polarization offer avenues to obtain the water content of marine aerosols. Key Points: - Modeled extinction coefficient of marine aerosol depends on particle radius and wavelength, but not on water content - The depolarization ratio and the color ratio of the backscattering cross section generally decrease with growing aerosol water content - The linear polarization peak near backscattering angles at NIR wavelength could be used in passive polarimetry to retrieve water content

Beschreibung: Current earthquake forecasting approaches are mainly based on probabilistic assumptions, as earthquakes seem to occur randomly. Such apparent randomness can however be caused by deterministic chaos, rendering deterministic short‐term forecasts possible. Due to the short historical and instrumental record of earthquakes, chaos detection has proven challenging, but more frequently occurring slow slip events (SSE) are promising candidates to probe for determinism. Here, we characterize the SSE signatures obtained from GNSS position time series in the Hikurangi Subduction Zone (New Zealand) to investigate whether the seemingly random SSE occurrence is governed by chaotic determinism. We find evidence for deterministic chaos for stations recording shallow SSEs, suggesting that short‐term deterministic forecasting of SSEs, similar to weather forecasts, might indeed be possible over timescales of a few weeks. We anticipate that our findings could open the door for next‐generation SSE forecasting, adding new tools to existing probabilistic approaches. Plain Language Summary Since earthquakes appear to occur randomly, the currently available probabilistic predictions are based on past earthquake records. These predictions estimate the likelihood of an earthquake of a given magnitude occurring within a defined time period. In contrast to such probabilistic approaches, deterministic systems are fully predictable, albeit often confined to short time scales due to their potential chaotic behavior. Probing for deterministic predictability in the earthquake cycle is intractable due to the limited historical instrumental record. However, frequently occurring slow slip events ‐ captured by transient GNSS displacements that can last several weeks ‐ provide a unique opportunity to explore deterministic predictability in these types of slow earthquakes. By studying GNSS time series from various stations on New Zealand’s North Island, we have discovered evidence suggesting that these irregularly occurring slow slip events might be governed by chaotic determinism. This implies the potential to forecast both timing and magnitude of slow slip events a few weeks in advance using deterministic methods, much like we predict weather patterns. Consequently, our theoretical findings could therefore pave the way for innovative approaches to short‐term slow slip forecasting. Key Points Nonlinear analysis of GNSS displacement time series unveils evidence for deterministic chaos in slow slip events in New Zealand Our theoretical findings imply that irregularly occurring slow slip events could potentially be forecasted a few weeks in advance

Beschreibung: Nutrient transfer into the sunlit surface ocean by cyclonic eddies is potentially crucial for sustaining primary productivity in the stratified subtropical gyres. However, the nature of productivity enhancements, including the flow of matter to higher trophic levels and its impact on carbon fluxes, remain poorly resolved. Here, we report a detailed assessment of the biogeochemical response to a cyclonic eddy in the subtropical Northwest Pacific via a combination of ship‐based and autonomous platforms. Primary production was enhanced twofold within the eddy core relative to reference sites outside, whereas phytoplankton biomass even decreased. Pico‐phytoplankton (〈 2 μ m) dominated (〉 80%) both phytoplankton biomass and primary production inside and outside the eddy. The stimulated primary production in the eddy core was accompanied by an approximately twofold increase in mesozooplankton abundance, an approximately threefold increase in particle formation in the deep chlorophyll maximum layer, as well as significantly enhanced surface oceanic CO 2 uptake and net community production. We suggest these observations carry important implications for understanding carbon export in the subtropical ocean and highlight the need to include such subtropical eddy features in ocean carbon budget analyses.

Beschreibung: Concentrations of bioavailable inorganic nitrogen (N) and phosphorus (P) are simultaneously depleted in the (sub)tropical North Atlantic Ocean, but it remains unclear if phytoplankton growth rates are N limited or N–P co‐limited. Here we present findings from three bottle‐scale experiments using a four‐by‐four matrix of low‐level N and P additions, conducted at one site in the subtropical North Atlantic Ocean. Phytoplankton responses were assessed both in terms of bulk chlorophyll a (Chl a ) concentrations and intracellular Chl a of dominant Prochlorococcus and Synechococcus groups. Two matrix experiments suggested that N was independently limiting in situ growth, with no co‐limiting role for P, while the third showed co‐limitation by both N and P in this region. This switch from N limitation to N–P co‐limitation was attributed to an episodic wet deposition event that supplied N, thereby stimulating phytoplankton growth and consuming available P. Such rapid transitions in nutrient limitation in response to environmental forcing might be common in oceanic systems with multiple depleted nutrients.

Beschreibung: Rationale Potassium (K) is a major component of several silicate minerals and seawater, and, therefore, constraining past changes in the potassium cycle is a promising way of tracing large‐scale geological processes on Earth. However, [K] measurement using inductively coupled plasma mass spectrometry (ICP‐MS) is challenging due to an ArH + interference, which may be of a similar magnitude to the K + ion beam in samples with 〈0.1% m/m [K]. Methods In this work, we investigated the effect of the ArH + interference on K/Ca data quality by comparing results from laser‐ablation (LA)‐ICP‐MS measured in medium and high mass resolution modes and validating our LA results via solution ICP‐optical emission spectroscopy (OES) and solution ICP‐MS measurements. To do so, we used a wide range of geological reference materials, with a particular focus on marine carbonates, which are potential archives of past changes in the K cycle but are typically characterised by [K] 〈 200 μg/g. In addition, we examine the degree to which trace‐element data quality is driven by downhole fractionation during LA‐ICP‐MS measurements. Results Our results show that medium mass resolution (MR) mode is sufficiently capable of minimising the effect of the ArH + interference on K + . However, the rate of downhole fractionation for Na and K varies between different samples as a result of their differing bulk composition, resulting in matrix‐specific inaccuracy. We show how this can be accounted for via downhole fractionation corrections, resulting in an accuracy of better than 1% and a long‐term reproducibility (intermediate precision) of 〈6% (relative standard deviation) in JCp‐1NP using LA‐ICP‐MS in MR mode. Conclusion Our [K] measurement protocol is demonstrably precise and accurate and applicable to a wide range of materials. The measurement of K/Ca in relatively low‐[K] marine carbonates is presented here as a key example of a new application opened up by these advances.

Beschreibung: The increasing demand for metals is pushing forward the progress of deep‐sea mining industry. The abyss between the Clarion and Clipperton Fracture Zones (CCFZ), a region holding a higher concentration of minerals than land deposits, is the most targeted area for the exploration of polymetallic nodules worldwide, which may likely disturb the seafloor across large areas and over many years. Effects from nodule extraction cause acute biodiversity loss of organisms inhabiting sediments and polymetallic nodules. Attention to deep‐sea ecosystems and their services has to be considered before mining starts but the lack of basic scientific knowledge on the methodologies for the ecological surveys of fauna in the context of deep‐sea mining impacts is still scarce. We review the methodology to sample, process and investigate metazoan infauna both inhabiting sediments and nodules dwelling on these polymetallic‐nodule areas. We suggest effective procedures for sampling designs, devices and methods involving gear types, sediment processing, morphological and genetic identification including metabarcoding and proteomic fingerprinting, the assessment of biomass, functional traits, fatty acids, and stable isotope studies within the CCFZ based on both first‐hand experiences and literature. We recommend multi‐ and boxcorers for the quantitative assessments of meio‐ and macrofauna, respectively. The assessment of biodiversity at species level should be focused and/or the combination of morphological with metabarcoding or proteomic fingerprinting techniques. We highlight that biomass, functional traits, and trophic markers may provide critical insights for biodiversity assessments and how statistical modeling facilitates predicting patterns spatially across point‐source data and is essential for conservation management.

Beschreibung: Mixed turbidite–contourite depositional systems result from interactions between down‐slope turbidity currents and along‐slope bottom currents, comprising excellent records of past oceanographic currents. Modern and ancient systems have been widely documented along the continental margins of the Atlantic Ocean. Yet, few examples have so far been identified on the North‐west African continental margin, limiting understanding of the sedimentary and palaeoceanographic evolution in this area. This work uses two‐dimensional seismic reflection profiles to report, for the first time, the presence of three giant sediment mounds beneath the headwall region of the Sahara Slide Complex. The sediment mounds are elongated and separated by two broad canyons, showing a north‐west/south‐east orientation that is roughly perpendicular to the continental margin. These mounds are 24 to 37 km long and 12 to 17 km wide, reaching a maximum height of ca 1000 m. Numerous slide scarps are observed within and along the flanks of the mounds, hinting at the occurrence of submarine landslides during their development. Based on their geometries, external shapes, internal seismic architecture and stratigraphic stacking patterns, it is proposed that these sediment mounds comprise down‐slope elongated mounded drifts formed in a mixed turbidite–contourite system during four evolutionary stages: onset, growth, maintenance and burial. The significance of this work is that it demonstrates the gradual transition from a turbidite system to a full mixed turbidite–contourite system to be associated, in the study area, with the establishment of strong ocean currents along north‐west Africa.

Beschreibung: “Flip‐flop” detachment mode represents an endmember type of lithosphere‐scale faulting observed at almost amagmatic sections of ultraslow‐spreading mid‐ocean ridges. Recent numerical experiments using an imposed steady temperature structure show that an axial temperature maximum is essential to trigger flip‐flop faults by focusing flexural strain in the footwall of the active fault. However, ridge segments without significant melt budget are more likely to be in a transient thermal state controlled, at least partly, by the faulting dynamics themselves. Therefore, we investigate which processes control the thermal structure of the lithosphere and how feedbacks with the deformation mechanisms can explain observed faulting patterns. We present results of 2‐D thermo‐mechanical numerical modeling including serpentinization reactions and dynamic grain size evolution. The model features a novel form of parametrized hydrothermal cooling along fault zones as well as the thermal and rheological effects of periodic sill intrusions. We find that the interplay of hydrothermal fault zone cooling and periodic sill intrusions in the footwall facilitates the flip‐flop detachment mode. Hydrothermal cooling of the fault zone pushes the temperature maximum into the footwall, while intrusions near the temperature maximum further weaken the rock and promote the formation of new faults with opposite polarity. Our model allows us to put constraints on the magnitude of two processes, and we obtain most reasonable melt budgets and hydrothermal heat fluxes if both are considered. Furthermore, we frequently observe two other faulting modes in our experiments complementing flip‐flop faulting to yield a potentially more robust alternative interpretation for existing observations. Plain Language Summary At mid‐ocean ridges, two plates diverge and new seafloor is created. The nature and appearance of this new seafloor strongly depend on spreading velocity and the availability of magmatic melts. At one of the melt‐poorest and slowest‐spreading ridges, a special form of large‐scale tectonic faults, so‐called flip‐flop detachments, can be observed. Tectonic faults can act as pathways for fluids circulating through the seafloor, which provides a significant cooling effect for the young plate. The interplay of magmatic activity, faulting and fluid circulation is evident at many different ridges with different magmatic activity and spreading rates. Flip‐flop faulting is restricted to only a few ridge sections worldwide, and we here investigate the prerequisites for this special spreading mode. To do so, we set up a computer model of an ultraslow‐spreading mid‐ocean ridge including the effects of sparse magmatism as well as the cooling effect associated with fluid circulation. We find that feedbacks between faulting dynamics, hydrothermal cooling and magmatic activity control the magnitude and spatial location of each individual process. Seafloor and subsurface observations are best explained by calculations with moderate melt input and hydrothermal circulation acting together. Key Points We implemented hydrothermal cooling and magmatic intrusion in a thermo‐mechanical model to explain detachment faulting at ultraslow ridges Stable flip‐flop detachment faulting is observed for setups considering both melt input and hydrothermal heat fluxes at realistic magnitudes Two other faulting modes frequently observed in our model offer potential alternative interpretations for existing seafloor observations

Beschreibung: Key Points: - North Atlantic biases are alleviated by an eddying nested ocean configuration embedded in a global climate model, FOCI-VIKING10 - It is indicated that reduction of the North Atlantic biases could improve the representation of NAO sub-decadal (8 years) variability - For detecting weak external imprints with limited computational resources, an ensemble with a coarse-resolution model is favorable Increasing the horizontal resolution of an ocean model is frequently seen as a way to reduce the model biases in the North Atlantic, but we are often limited by computational resources. Here, a two-way nested ocean model configuration (VIKING10) that consists of a high-resolution (1/10°) component and covers the northern North Atlantic, is embedded in a 1/2° ocean grid as part of the global chemistry-climate model, FOCI (called FOCI-VIKING10). This configuration yields a significantly improved path of the North Atlantic current (NAC), which here reduces the North Atlantic cold bias by ∼50%. Compared with the coarse-resolution, non-eddying model, the improved thermal state of upper ocean layers and surface heat fluxes in a historical simulation based on FOCI-VIKING10 are beneficial for simulating the subdecadal North Atlantic Oscillation (NAO) variability (i.e., a period of 8 years). A northward drift of the NAO-forced ocean thermal anomalies as seen in observations and the eddying FOCI-VIKING10, provide a lagged ocean feedback to the NAO via changes in the net surface heat flux, leading to the NAO periodicity of 8 years. This lagged feedback and the 8 years variability of the NAO cannot be captured by the non-eddying standard FOCI historical simulation. Furthermore, the argumentative responses of the North Atlantic to the 11-year solar cycle are re-examined in this study. The reported solar-induced NAO-like responses are confirmed in the 9-member ensemble mean based on FOCI but with low robustness among individual members. A lagged NAO-like response is only found in the nested eddying simulation but absent from the non-eddying reference simulation, suggesting North Atlantic biases importantly limit climate model capability to realistically solar imprints in North Atlantic climate.

Beschreibung: Three boreholes drilled during the International Ocean Discovery Program (IODP) Expedition 396 have yielded unexpected findings of altered granitic rocks covered by basalt flows, interbedded sediments and glacial mud near the continent‐ocean transition of the mid‐Norwegian margin. U‐Pb and K‐Ar geochronological analyses were conducted on both protolithic and authigenically formed K‐bearing minerals to determine the age of granite crystallisation and subsequent alteration episodes. The granite's crystallisation age based on 104 zircons is 56.3 ± 0.2 Ma, and subsequent exhumation along with alteration/weathering events took place between 54.7 ± 1 and 37.1 ± 1 Ma. This intrusion represents the youngest granite discovered in Norway and intruded at an extremely shallow crustal level before a rapid rift‐to‐drift transition. The shallow emplacement of granitic rock and its fast exhumation before and during the onset of volcanism holds significant implications for the syn‐ and post‐breakup tectonic evolution of volcanic margins.

Beschreibung: The past ∼200 million years of Earth's geomagnetic field behavior have been recorded within oceanic basalts, many of which are only accessible via scientific ocean drilling. Obtaining the best possible paleomagnetic measurements from such valuable samples requires an a priori understanding of their magnetic mineralogies when choosing the most appropriate protocol for stepwise demagnetization experiments (either alternating field or thermal). Here, we present a quick, and non‐destructive method that utilizes the amplitude‐dependence of magnetic susceptibility to screen submarine basalts prior to choosing a demagnetization protocol, whenever conducting a pilot study or other detailed rock‐magnetic characterization is not possible. We demonstrate this method using samples acquired during International Ocean Discovery Program Expedition 391. Our approach is rooted in the observation that amplitude‐dependent magnetic susceptibility is observed in basalt samples whose dominant magnetic carrier is multidomain titanomagnetite (∼TM 60–65 , (Ti 0.60–0.65 Fe 0.35–0.40 )Fe 2 O 4 ). Samples with low Ti contents within titanomagnetite or samples that have experienced a high degree of oxidative weathering do not display appreciable amplitude dependence. Due to their low Curie temperatures, basalts that possess amplitude‐dependence should ideally be demagnetized either using alternating fields or via finely‐spaced thermal demagnetization heating steps below 300°C. Our screening method can enhance the success rate of paleomagnetic studies of oceanic basalt samples. Plain Language Summary Oceanic basalts are ideal recorders of the Earth's magnetic field. To decipher magnetic histories recorded in rocks, paleomagnetists need to isolate the magnetization directions and intensities within rocks by one of two possible methods. One method typically involves progressively heating the samples to high temperatures. The other method involves exposing samples to alternating magnetic fields with increasing peak field intensities. Both of these methods are ultimately destructive to the original magnetization preserved within rocks. However, without knowledge of a given rock's magnetic mineralogy, randomly choosing thermal or alternating field demagnetization methods may result in high failure rates. We developed a pre‐screening method to help decide which cleaning method will likely be more successful for a given sample based on low‐field magnetic susceptibility measurements. These measurements do not affect the original magnetic information recorded in a rock, thereby permitting subsequent paleomagnetic studies on the same sample. Our technique can be performed as rapidly as 2 min per sample, is non‐destructive, and does not require complicated sample preparation. Key Points Paleomagnetic studies utilize either alternating field or thermal demagnetization, but it is difficult to choose the best protocol a priori Amplitude‐dependence of magnetic susceptibility measurements permits preliminary magnetic mineralogy characterization in submarine basalts Rapid amplitude‐dependence measurements may aid in deciding upon the best demagnetization protocol for submarine basalt samples

Beschreibung: Significance Oceans represent 70% of our planet’s surface, housing a large spectrum of microorganisms that interact with the above atmosphere. Ocean microorganisms were proposed in the late 80’s to be at the center of a climate feedback loop involving dimethyl sulfide (DMS) that would form aerosols and modify cloud properties (CLAW hypothesis). In the present paper, we report observational evidence from semicontrolled experiments in the South Pacific that nitrate ions, yet hitherto not considered, is a key species involved in aerosol nucleation in the pristine marine atmosphere and which precursors are coemitted with DMS. Our results further indicate that nitrate ion formation would be related to short-term microbial processes, sensitive to environmental stressors, therefore potentially “closing the loop”. Abstract Our understanding of ocean–cloud interactions and their effect on climate lacks insight into a key pathway: do biogenic marine emissions form new particles in the open ocean atmosphere? Using measurements collected in ship-borne air–sea interface tanks deployed in the Southwestern Pacific Ocean, we identified new particle formation (NPF) during nighttime that was related to plankton community composition. We show that nitrate ions are the only species for which abundance could support NPF rates in our semicontrolled experiments. Nitrate ions also prevailed in the natural pristine marine atmosphere and were elevated under higher sub-10 nm particle concentrations. We hypothesize that these nucleation events were fueled by complex, short-term biogeochemical cycling involving the microbial loop. These findings suggest a new perspective with a previously unidentified role of nitrate of marine biogeochemical origin in aerosol nucleation.

Beschreibung: Key Points: - Freshwater input has significantly contributed to the surface warming at the peak of the 1995 Benguela Niño - Anomalously high river discharge and precipitation increased stratification and reduced turbulent heat loss by creating barrier layers - Combination of high freshwater input and strong poleward surface current might play a role in temperature variability off Angola Benguela Nino events are characterized by strong warm sea surface temperature (SST) anomalies off the Angolan and Namibian coasts. In 1995, the strongest event in the satellite era took place, impacting fish availability in both Angolan and Namibian waters. In this study, we use direct observations, satellite data, and reanalysis products to investigate the impact that the up-until-now unnoticed mechanism of freshwater input from Congo River discharge (CRD) and precipitation had on the evolution of the 1995 Benguela Nino. In the onset phase of the event, anomalous rainfall in November/December 1994 at around 6 degrees S, combined with a high CRD, generated a low salinity plume. The plume was advected into the Angola-Namibia region in the following February/March 1995 by an anomalously strong poleward surface current generated by the relaxation of the southerly winds and shifts in the coastal wind stress curl. The presence of this low surface salinity anomaly of about -2 psu increased ocean stability by generating barrier layers, thereby reducing the turbulent heat loss, since turbulent mixing acted on a weak vertical temperature gradient. A mixed layer heat budget analysis demonstrates that southward advection of Angolan waters drove the warming at the onset, while reduced mixing played the main role at the event's peak. We conclude that a freshwater input contributed to the SST increase in this exceptionally strong event and suggest that this input can influence the SST variability in Angola-Namibia waters through a combination of high CRD, precipitation, and the presence of a strong poleward surface current. Benguela Nino events are characterized by excessive warming of the sea surface temperature off the Angolan and Namibian coasts. One of the strongest-ever recorded warm events dates back to 1995, impacting fish availability in both Angolan and Namibian waters. In our research, we investigate if freshwater from rain and from the Congo River could have impacted the evolution of this 1995 Benguela Nino. In the event's early stage, high precipitation and river discharge generated a low salinity pool at the Congo River mouth, which in February/March 1995 was taken to the south by an exceptionally strong surface current, generated by changes in wind strength and direction at the African coast. This low sea surface salinity in a shallow layer in the upper meters of the ocean increased the ocean's stability. As the stabilized waters diminished the usual mixing from the depths below which cools down the surface waters, it contributed to an increase in warming in the surface layer of the ocean. We conclude that the warming of the surface waters in the region was indeed influenced by the combination of high precipitation and high Congo River discharge with a strong surface current toward the south. Freshwater input has significantly contributed to the surface warming at the peak of the 1995 Benguela Nino Anomalously high river discharge and precipitation increased stratification and reduced turbulent heat loss by creating barrier layers Combination of high freshwater input and strong poleward surface current might play a role in temperature variability off Angola

Beschreibung: The Congo River supplies vast quantities of trace metals (TMs) to the South Atlantic Ocean, but TM budgets for the Congo plume derived using radium isotopes for GEOTRACES cruise GA08 suggest additional input other than the river outflow. Considering the tight correlations between most dissolved TMs and salinity in the plume and the high rainfall during the wet season over the Congo shelf, we hypothesized that wet atmospheric deposition is a TM source to the Congo plume. Observed TM concentrations in rainwaters across the Congo shelf were mostly comparable to values from previous work in the North Atlantic and Mediterranean Sea. Wet deposition contributed the equivalent of 43% dCd, 21% dCu, 20% dPb and 68% dZn of the Congo River fluxes. Our findings show an important role of wet deposition in supplying TMs to the South Atlantic overlapping with the region that receives substantial TM fluxes from the Congo River. Key Points The Congo River is an important source of trace metals (TMs) to the South Atlantic Ocean revealed by data from GEOTRACES cruise GA08 Wet deposition (rainfall) is identified as an additional TM source to the Congo plume by concurrently considering river and rain data Rainfall supplies anthropogenic dTMs (Cd, Cu, Pb and Zn) with fluxes equivalent to 20%–68% of those from the Congo River on the Congo shelf Plain Language Summary The Congo River has the second largest freshwater discharge volume globally and creates an extensive near-equatorial plume into the Atlantic Ocean. The Congo plume constitutes an important source of trace metals (TMs) to the ocean, which impacts biogeochemical cycles in the tropical and subtropical ocean. However, existing work suggests a discrepancy within the TM budgets in the Congo plume and points to unknown source other than the Congo River or shelf sediments. Most TM concentrations across the Congo plume remain tightly correlated with salinity, suggesting that any additional sources are likely also freshwater-derived or enter the ocean at the river mouth coincidently with direct riverine TM inputs. Here, TM concentrations in ocean, river and rainwater collected during the GEOTRACES GA08 cruise are combined to suggest that wet deposition augmented some Congo TM fluxes to the ocean. Fluxes of anthropogenic Cd, Cu, Pb and Zn to the Congo shelf from wet deposition are of the same order of magnitude as the Congo River. Concentrations of these elements in rainwater are similar to prior observations reported for the North Atlantic and Mediterranean Sea, suggesting that a large fraction of the global range of rainwater concentrations over the ocean has been captured in our observations.

Beschreibung: Iodine cycling in the ocean is closely linked to productivity, organic carbon export, and oxygenation. However, iodine sources and sinks at the seafloor are poorly constrained, which limits the applicability of iodine as a biogeochemical tracer. We present pore water and solid phase iodine data for sediment cores from the Peruvian continental margin, which cover a range of bottom water oxygen concentrations, organic carbon rain rates and sedimentation rates. By applying a numerical reaction‐transport model, we evaluate how these parameters determine benthic iodine fluxes and sedimentary iodine‐to‐organic carbon ratios (I:C org ) in the paleo‐record. Iodine is delivered to the sediment with organic material and released into the pore water as iodide (I − ) during early diagenesis. Under anoxic conditions in the bottom water, most of the iodine delivered is recycled, which can explain the presence of excess dissolved iodine in near‐shore anoxic seawater. According to our model, the benthic I − efflux in anoxic areas is mainly determined by the organic carbon rain rate. Under oxic conditions, pore water dissolved I − is oxidized and precipitated at the sediment surface. Much of the precipitated iodine re‐dissolves during early diagenesis and only a fraction is buried. Particulate iodine burial efficiency and I:C org burial ratios do increase with bottom water oxygen. However, multiple combinations of bottom water oxygen, organic carbon rain rate and sedimentation rate can lead to identical I:C org , which limits the utility of I:C org as a quantitative oxygenation proxy. Our findings may help to better constrain the ocean's iodine mass balance, both today and in the geological past. Key Points The impact of early diagenesis on benthic iodine fluxes and iodine burial was quantitatively evaluated using a reaction‐transport model Dissolved iodine anomalies in the water column are indicative of benthic efflux from anoxic sediments with high organic carbon turnover Not only bottom water oxygen but also organic carbon delivery and sedimentation rate determine sedimentary iodine‐to‐organic carbon ratios

Beschreibung: The understanding of silicate weathering and its role as a sink for atmospheric CO 2 is important to get a better insight into how the Earth shifts from warm to cool climates. The lithium isotope composition (δ 7 Li) of marine carbonates can be used as a proxy to track the past chemical weathering of silicates. A high‐resolution δ 7 Li record would be helpful to evaluate the role of silicate weathering during the late Cretaceous climate cooling. Here, we assess chalk as a potential archive for reconstructing Late Cretaceous seawater Li isotope composition by comparing Maastrichtian chalk from Northern Germany (Hemmoor, Kronsmoor) to a Quaternary coccolith ooze from the Manihiki Plateau (Pacific Ocean) as a lithological analog to modern conditions. We observe a negative offset of 3.9 ± 0.6‰ for the coccolith ooze relative to the modern seawater Li isotope composition (+31.1 ± 0.3‰; 2SE; n = 54), a value that falls in the range of published offsets for modern core‐top samples and for brachiopod calcite. Further, the negative offset between the Li isotope compositions of Manihiki coccolith ooze and modern planktonic foraminifera is 2.3 ± 0.6‰. Although chalk represents a diagenetically altered modification of pelagic nannofossil ooze, manifested by changes in the composition of trace elements, we observe a consistent offset of Li isotope data between Maastrichtian chalk and Maastrichtian planktonic foraminiferal data (−1.4 ± 0. 5‰) that lies within the uncertainty of modern values. We therefore suggest that chalk can be used as a reliable archive for δ 7 Li reconstructions. Key Points Chalk is a reliable archive for the Li isotope composition of seawater Coccolith ooze has a negative offset of 3.9 ± 0.6‰ from modern seawater for Li isotope ratios The estimated mean value for the late Maastrichtian seawater Li isotope composition is +27.5 ± 1.0‰

Beschreibung: The coastal ocean contributes to regulating atmospheric greenhouse gas concentrations by taking up carbon dioxide (CO2) and releasing nitrous oxide (N2O) and methane (CH4). In this second phase of the Regional Carbon Cycle Assessment and Processes (RECCAP2), we quantify global coastal ocean fluxes of CO2, N2O and CH4 using an ensemble of global gap-filled observation-based products and ocean biogeochemical models. The global coastal ocean is a net sink of CO2 in both observational products and models, but the magnitude of the median net global coastal uptake is similar to 60% larger in models (-0.72 vs. -0.44 PgC year-1, 1998-2018, coastal ocean extending to 300 km offshore or 1,000 m isobath with area of 77 million km2). We attribute most of this model-product difference to the seasonality in sea surface CO2 partial pressure at mid- and high-latitudes, where models simulate stronger winter CO2 uptake. The coastal ocean CO2 sink has increased in the past decades but the available time-resolving observation-based products and models show large discrepancies in the magnitude of this increase. The global coastal ocean is a major source of N2O (+0.70 PgCO2-e year-1 in observational product and +0.54 PgCO2-e year-1 in model median) and CH4 (+0.21 PgCO2-e year-1 in observational product), which offsets a substantial proportion of the coastal CO2 uptake in the net radiative balance (30%-60% in CO2-equivalents), highlighting the importance of considering the three greenhouse gases when examining the influence of the coastal ocean on climate. The coastal ocean regulates greenhouse gases. It acts as a sink of carbon dioxide (CO2) but also releases nitrous oxide (N2O) and methane (CH4) into the atmosphere. This synthesis contributes to the second phase of the Regional Carbon Cycle Assessment and Processes (RECCAP2) and provides a comprehensive view of the coastal air-sea fluxes of these three greenhouse gases at the global scale. We use a multi-faceted approach combining gap-filled observation-based products and ocean biogeochemical models. We show that the global coastal ocean is a net sink of CO2 in both observational products and models, but the coastal uptake of CO2 is similar to 60% larger in models than in observation-based products due to model-product differences in seasonality. The coastal CO2 sink is strengthening but the magnitude of this strengthening is poorly constrained. We also find that the coastal emissions of N2O and CH4 counteract a substantial part of the effect of coastal CO2 uptake in the atmospheric radiative balance (by 30%-60% in CO2-equivalents), highlighting the need to consider these three gases together to understand the influence of the coastal ocean on climate. We synthesize air-sea fluxes of CO2, nitrous oxide and methane in the global coastal ocean using observation-based products and ocean models The coastal ocean CO2 sink is 60% larger in ocean models than in observation-based products due to systematic differences in seasonality Coastal nitrous oxide and methane emissions offset 30%-60% of the CO2 coastal uptake in the net radiative balance

Beschreibung: Accessible seafloor minerals located near mid‐ocean ridges are noticed to mitigate the projected metal demands of the net‐zero energy transition, promoting growing interest in quantifying the global distributions of seafloor massive sulfides (SMS). Mineral potentials are commonly estimated using geophysical and geological data that lastly rely on additional confirmation studies using sparsely available, locally limited, seafloor imagery, grab samples, and coring data. This raises the challenge of linking in situ confirmation data to geophysical data acquired at disparate spatial scales to obtain quantitative mineral predictions. Although multivariate data sets for marine mineral research are incessantly acquired, robust, integrative data analysis requires cumbersome workflows and experienced interpreters. We introduce an automated two‐step machine learning approach that integrates the mound detection through image segmentation with geophysical data. SMS predictors are subsequently clustered into distinct classes to infer marine mineral potentials that help guide future exploration. The automated workflow employs a U‐Net convolutional neural network to identify mound structures in bathymetry data and distinguishes different mound classes through the classification of mound architectures and magnetic signatures. Finally, controlled source electromagnetic data are utilized together with in situ sampling data to reassess predictions of potential SMS volumes. Our study focuses on the Trans‐Atlantic Geotraverse area, which is among the most explored SMS areas worldwide and includes 15 known SMS sites. The automated workflow classifies 14 of the 15 known mounds as exploration targets of either high or medium priority. This reduces the exploration area to less than 7% of the original survey area from 49 to 3.1 km 2 . Key Points A two‐step machine learning workflow identifies mound structures in bathymetry data and classifies their origins based on auxiliary data Significant increase in potential seafloor massive sulfides (SMS) edifices detected within the trans‐Atlantic geo‐traverse hydrothermal field distributed within latitudinal bands SMS mineral potential is likely lower than previously assumed due to heterogeneously distributed mineralization within mounds

Beschreibung: Burial driven recycling is an important process in the natural gas hydrate (GH) systems worldwide, characterized by complex multiphysics interactions like gas migration through an evolving gas hydrate stability zone (GHSZ), competing gas-water-hydrate (i.e., fluid-fluid-solid) phase transitions, locally appearing and disappearing phases, and evolving sediment properties (e.g., permeability, reaction surface area, and capillary entry pressure). Such a recycling process is typically studied in homogeneous or layered sediments. However, there is mounting evidence that structural heterogeneity and anisotropy linked to normal and inclined fault systems or anomalous sediment layers have a strong impact on the GH dynamics. Here, we consider the impacts of such a structurally complex media on the recycling process. To capture the properties of the anomalous layers accurately, we introduce a fully mass conservative, high-order, discontinuous Galerkin (DG) finite element based numerical scheme. Moreover, to handle the rapidly switching thermodynamic phase states robustly, we cast the problem of phase transitions as a set of variational inequalities, and combine our DG discretization scheme with a semi-smooth Newton solver. Here, we present our new simulator, and demonstrate using synthetic geological scenarios, (a) how the presence of an anomalous high-permeability layer, like a fracture or brecciated sediment, can alter the recycling process through flow-localization, and more importantly, (b) how an incorrect or incomplete approximation of the properties of such a layer can lead to large errors in the overall prediction of the recycling process. Key Points Structural heterogeneity linked to inclined fault systems or anomalous sediment layers have a strong impact on the gas hydrate dynamics The presence of anomalous high-permeability layers within gas hydrate stability zone alters the recycling process through flow-localization The presented discontinuous Galerkin scheme is able to accurately capture the gas hydrate recycling processes through strongly anisotropic materials

Beschreibung: The Nazca Ridge (NR) was formed near the interaction of a hotspot mantle plume and an active spreading center. We use active-source wide-angle seismic data to obtain 2-D Vp and Vs tomographic models, and hence the Poisson's ratio (ν) structure beneath the NR. Results show a ∼2 km thick seismic layer 2A with ν values of 0.25–0.32 in the uppermost crust interpreted as pillow basalts with a low degree of fracturing and/or hydrothermal alteration. The 2A/B boundary layer presents ν values of 0.27–0.29 consistent with pillow basalts/sheeted dykes units. A ∼3 km layer 2B overlies a ∼10 km layer 3 with ν values of 0.24–0.3 at the 2/3 boundary layer. The lowermost layer 3 presents ν values of 0.28 ± 0.02 suggesting an increase in Mg content (≥10% wt). The NR crust (∼15 km thick) requires an increment of the asthenospheric mantle potential temperature in ∼100°C formed by passive adiabatic decompression melting.

Beschreibung: Upwelling of subsurface waters injects macronutrients (fixed N, P and Si) and micronutrient trace metals (TMs) into surface waters supporting elevated primary production in Eastern Boundary Upwelling Regions (EBUR). The eastern South Atlantic features a highly productive shelf sea transitioning to a low productivity N-Fe (co)limited open ocean. Whilst a gradient in most TM concentrations is expected in any off-shelf transect, the factors controlling the magnitude of cross-shelf TM fluxes are poorly constrained. Here, we present dissolved TM concentrations of Fe, Co, Mn, Cd, Ni and Cu within the Benguela Upwelling System (BUS) from the coastal section of the GEOTRACES GA08 cruise. Elevated dissolved Fe, Co, Mn, Cd, Ni, Cu and macronutrient concentrations were observed near shelf sediments. Benthic sources supplied 2.22 ± 0.99 μmol Fe m-2 d-1, 0.05 ± 0.03 μmol Co m-2 d-1, 0.28 ± 0.11 μmol Mn m-2 d-1 and were found to be the dominant source to shallow shelf waters compared to atmospheric depositions. Similarly, off-shelf transfer was a more important source of TMs to the eastern South Atlantic Ocean compared to atmospheric deposition. Assessment of surface (shelf, upper 200 m) and subsurface (shelf edge, 200 - 500 m) fluxes of Fe and Co indicated TM fluxes from subsurface were 2 - 5 times larger than those from surface into the eastern South Atlantic Ocean. Under future conditions of increasing ocean deoxygenation, these fluxes may increase further, potentially contributing to a shift towards more extensive regional limitation of primary production by fixed N availability. Key Points Shelf sediments release redox-sensitive trace metals (TMs) to overlying oxygen-depleted waters in the Benguela Upwelling System (BUS) Sediment-derived TMs are upwelled and laterally transported constituting a major source to shelf waters and to the eastern South Atlantic Subsurface fluxes of dissolved Fe and Co from the shelf edge play an important role in supplying Fe and Co to the eastern South Atlantic

Beschreibung: Sub-seabed fluid flow, gas hydrate accumulation and seafloor methane seepage are tightly interwoven processes with implications for marine biodiversity, ocean chemistry and seafloor stability. We combine long-offset seismic reflection data with high-resolution seismic data to investigate shallow structural deformation and its relationship to focused gas migration and hydrate accumulation in the southern Hikurangi subduction wedge. Anticlines, effective traps for focusing free gas, are characterized by both normal faults and vertical zones of hydraulic fracturing within the hydrate stability zone. The normal faults form as a result of sediment layer folding and gravitational collapse of ridges during uplift. We document both longitudinal (ridge-parallel) and transverse (ridge-perpendicular) extensional structures (normal faults and elongated hydraulic fracture zones) in the sub-seafloor of anticlinal ridges. Intriguingly, gas flow through ridges close to the deformation front of the wedge exploits longitudinal structures, while ridges further inboard are characterized by gas flow along transverse structures. This highlights pronounced changes in the shallow deformation of ridges in different parts of the wedge, associated with a switching of the least and intermediate principal stress directions. It is critical to understand these shallow stress fields because they control fluid flow patterns and methane seepage out of the seafloor. Key Points Gas migration through ridges occurs along both longitudinal (ridge-parallel) and transverse (ridge-perpendicular) zones of fracturing Shallow stress fields differ significantly between ridges, reflecting differences in ridge evolution and deformation Seismic reflection images of the base of gas hydrate stability and gas-water contacts are strongly affected by seismic frequency content

Beschreibung: The transmission of microbes from mother to offspring is an ancient, advantageous, and widespread feature of metazoan life history. Despite this, little is known about the quantitative strategies taken to maintain symbioses across generations. The quantity of maternal microbes that is provided to each offspring through vertical transmission could theoretically be stochastic (no trend), consistent (an optimal range is allocated), or provisioned (a trade-off with fecundity). Examples currently come from animals that release free-living eggs (oviparous) and suggest that offspring are provided a consistent quantity of symbionts. The quantity of maternal microbes that is vertically transmitted in other major reproductive strategies has yet to be assessed. We used the brooding (viviparous) sponge Halichondria panicea to test whether offspring receive quantitatively similar numbers of maternal microbes. We observed that H. panicea has a maternal pool of the obligate symbiont Candidatus Halichondribacter symbioticus and that this maternal pool is provisioned proportionally to reproductive output and allometrically by offspring size. This pattern was not observed for the total bacterial community. Experimental perturbation by antibiotics could not reduce the abundance of Ca. H. symbioticus in larvae, while the total bacterial community could be reduced without affecting the ability of larvae to undergo metamorphosis. A trade-off between offspring size and number is, by definition, maternal provisioning and parallel differences in Ca. H. symbioticus abundance would suggest that this obligate symbiont is also provisioned.