ALBERT

Description: The Denmark Strait Overflow (DSO) contributes roughly half to the total volume transport of the Nordic overflows. The overflow increases its volume by entraining ambient water as it descends into the subpolar North Atlantic, feeding into the deep branch of the Atlantic Meridional Overturning Circulation. In June 2012, a multiplatform experiment was carried out in the DSO plume on the continental slope off Greenland (180 km downstream of the sill in Denmark Strait), to observe the variability associated with the entrainment of ambient waters into the DSO plume. In this study, we report on two high-dissipation events captured by an autonomous underwater vehicle (AUV) by horizontal profiling in the interfacial layer between the DSO plume and the ambient water. Strong dissipation of turbulent kinetic energy of O( math formula) W kg−1 was associated with enhanced small-scale temperature variance at wavelengths between 0.05 and 500 m as deduced from a fast-response thermistor. Isotherm displacement slope spectra reveal a wave number-dependence characteristic of turbulence in the inertial-convective subrange ( math formula) at wavelengths between 0.14 and 100 m. The first event captured by the AUV was transient, and occurred near the edge of a bottom-intensified energetic eddy. Our observations imply that both horizontal advection of warm water and vertical mixing of it into the plume are eddy-driven and go hand in hand in entraining ambient water into the DSO plume. The second event was found to be a stationary feature on the upstream side of a topographic elevation located in the plume pathway. Flow-topography interaction is suggested to drive the intense mixing at this site.

Description: To maintain the chance of keeping the average global temperature increase below 2 degrees C and to limit long-term climate change, removing carbon dioxide from the atmosphere (carbon dioxide removal, CDR) is becoming increasingly necessary. We analyze optimal and cost-effective climate policies in the dynamic integrated assessment model (IAM) of climate and the economy (DICE2016R) and investigate (1) the utilization of (ocean) CDR under different climate objectives, (2) the sensitivity of policies with respect to carbon cycle feedbacks, and (3) how well carbon cycle feedbacks are captured in the carbon cycle models used in state-of-the-art IAMs. Overall, the carbon cycle model in DICE2016R shows clear improvements compared to its predecessor, DICE2013R, capturing much better long-term dynamics and also oceanic carbon outgassing due to excess oceanic storage of carbon from CDR. However, this comes at the cost of a (too) tight short-term remaining emission budget, limiting the model suitability to analyze low-emission scenarios accurately. With DICE2016R, the compliance with the 2 degrees C goal is no longer feasible without negative emissions via CDR. Overall, the optimal amount of CDR has to take into account (1) the emission substitution effect and (2) compensation for carbon cycle feedbacks.

Description: During the summer monsoon, the western tropical Indian Ocean is predicted to be a hot spot for dimethylsulfide emissions, the major marine sulfur source to the atmosphere, and an important aerosol precursor. Other aerosol relevant fluxes, such as isoprene and sea spray, should also be enhanced, due to the steady strong winds during the monsoon. Marine air masses dominate the area during the summer monsoon, excluding the influence of continentally derived pollutants. During the SO234-2/235 cruise in the western tropical Indian Ocean from July to August 2014, directly measured eddy covariance DMS fluxes confirm that the area is a large source of sulfur to the atmosphere (cruise average 9.1 μmol m−2 d−1). The directly measured fluxes, as well as computed isoprene and sea spray fluxes, were combined with FLEXPART backward and forward trajectories to track the emissions in space and time. The fluxes show a significant positive correlation with aerosol data from the Terra and Suomi-NPP satellites, indicating a local influence of marine emissions on atmospheric aerosol numbers.

Description: Anthropogenic activities have resulted in enhanced lead (Pb) emissions to the environment over the past century, mainly through the combustion of leaded gasoline. Here, we present the first combined dissolved (DPb), labile (LpPb) and particulate (PPb) Pb dataset from the Northeast Atlantic (Celtic Sea) since the phasing out of leaded gasoline in Europe. Concentrations of DPb in surface waters have decreased by 4-fold over the last four decades. We demonstrate that anthropogenic Pb is transported from the Mediterranean Sea over long distances (〉2500 km). Benthic DPb fluxes exceeded the atmospheric Pb flux in the region, indicating the importance of sediments as a contemporary Pb source. A strong positive correlation between DPb, PPb and LpPb indicates a dynamic equilibrium between the phases and the potential for particles to ‘buffer’ the DPb pool. This study provides insights into Pb biogeochemical cycling and demonstrates the potential of Pb in constraining ocean circulation patterns.

Description: The variability of the Atlantic Meridional Overturning Circulation (AMOC) may play a role in sea surface temperature predictions on seasonal to decadal time scales. Therefore, AMOC seasonal cycles are a potential baseline for interpreting predictions. Here we present estimates for the seasonal cycle of transports of volume, temperature, and freshwater associated with the upper limb of the AMOC in the eastern subpolar North Atlantic on the Extended Ellett Line hydrographic section between Scotland and Iceland. Due to weather, ship‐based observations are primarily in summer. Recent glider observations during other seasons present an opportunity to investigate the seasonal variability in the upper layer of the AMOC. First, we document a new method to quality control and merge ship, float, and glider hydrographic observations. This method accounts for the different spatial sampling rates of the three platforms. The merged observations are used to compute seasonal cycles of volume, temperature, and freshwater transports in the Rockall Trough. These estimates are similar to the seasonal cycles in two eddy‐resolving ocean models. Volume transport appears to be the primary factor modulating other Rockall Trough transports. Finally, we show that the weakest transports occur in summer, consistent with seasonal changes in the regional‐scale wind stress curl. Although the seasonal cycle is weak compared to other variability in this region, the amplitude of the seasonal cycle in the Rockall Trough, roughly 0.5–1 Sv about a mean of 3.4 Sv, may account for up to 7–14% of the heat flux between Scotland and Greenland.

Description: We reanalyze existing paleodata of global mean surface temperature ΔTg and radiative forcing ΔR of CO2 and land ice albedo for the last 800,000 years to show that a state‐dependency in paleoclimate sensitivity S, as previously suggested, is only found if ΔTg is based on reconstructions, and not when ΔTg is based on model simulations. Furthermore, during times of decreasing obliquity (periods of land‐ice sheet growth and sea level fall) the multi‐millennial component of reconstructed ΔTg diverges from CO2, while in simulations both variables vary more synchronously, suggesting that the differences during these times are due to relatively low rates of simulated land ice growth and associated cooling. To produce a reconstruction‐based extrapolation of S for the future we exclude intervals with strong ΔTg‐CO2 divergence and find that S is less state‐dependent, or even constant (state‐independent), yielding a mean equilibrium warming of 2–4 K for a doubling of CO2.

Description: Chlorophyll (Chl) is a distinctive component of autotrophic organisms, often used as an indicator of phytoplankton biomass in the ocean. However, assessment of phytoplankton biomass from Chl relies on the accurate estimation of the Chl:carbon(C) ratio. Here we present global patterns of Chl:C ratios in the surface ocean obtained from a phytoplankton growth model that accounts for the optimal acclimation of phytoplankton to ambient nutrient, light, and temperature conditions. The model agrees largely with observed/expected global patterns of Chl:C. Combining our Chl:C estimates with satellite Chl and particulate organic carbon (POC), we infer phytoplankton C concentration in the surface ocean and its contribution to the total POC pool. Our results suggest that the portion of POC corresponding to living phytoplankton is higher in subtropical latitudes and less productive regions (∼30–70%) and decreases to ∼10–30% toward high latitudes and productive regions. An important caveat of our model is the lack of iron limiting effects on phytoplankton physiology. Comparison of our predicted phytoplankton biomass with an independent estimate of total POC reveals a positive correlation between nitrate concentrations and nonphotosynthetic POC in the surface ocean. This correlation disappears when a constant Chl:C is applied. Our analysis is not constrained by assumptions of constant Chl:C or phytoplankton:POC ratio, providing a novel independent analysis of phytoplankton biomass in the surface ocean. These results highlight the importance of accounting for the variability in Chl:C and its application in distinguishing the autotrophic and heterotrophic components in the assemblage of the marine plankton ecosystem.

Description: Provenance studies of widely distributed tephras, integrated within a well-defined temporal framework, are important to deduce systematic changes in the source, scale, distribution and changes in regional explosive volcanism. Here, we establish a robust tephro-chronostratigraphy for a total of 157 marine tephra layers collected during IODP Expedition 352. We infer at least three major phases of highly explosive volcanism during Oligocene to Pleistocene time. Provenance analysis based on glass composition assigns 56 of the tephras to a Japan source, including correlations with 12 major and widespread tephra layers resulting from individual eruptions in Kyushu, Central Japan and North Japan between 115 ka and 3.5 Ma. The remaining 101 tephras are assigned to four source regions along the Izu-Bonin arc. One, of exclusively Oligocene age, is proximal to the Bonin Ridge islands; two reflect eruptions within the volcanic front and back-arc of the central Izu-Bonin arc, and a fourth region corresponds to the Northern Izu-Bonin arc source. First-order volume estimates imply eruptive magnitudes ranging from 6.3 to 7.6 for Japan-related eruptions and between 5.5 and 6.5 for IBM eruptions. Our results suggest tephras between 30 and 22 Ma that show a subtly different Izu-Bonin chemical signature compared to the recent arc. After a ∼11 m.y. gap in eruption, tephra supply from the Izu-Bonin arc predominates from 15 to 5 Ma, and finally a subequal mixture of tephra sources from the (palaeo)Honshu and Izu-Bonin arcs occurs within the last ∼5 Ma.

Description: Subsurface coherent eddies are well-known features of ocean circulation, but the sparsity of observations prevents an assessment of their importance for biogeochemistry. Here, we use a global eddying (0.1° ) ocean-biogeochemical model to carry out a census of subsurface coherent eddies originating from eastern boundary upwelling systems (EBUS), and quantify their biogeochemical effects as they propagate westward into the subtropical gyres. While most eddies exist for a few months, moving over distances of 100s of km, a small fraction (〈 5%) of long-lived eddies propagates over distances greater than 1000km, carrying the oxygen-poor and nutrient-rich signature of EBUS into the gyre interiors. In the Pacific, transport by subsurface coherent eddies accounts for roughly 10% of the offshore transport of oxygen and nutrients in pycnocline waters. This "leakage" of subsurface waters can be a significant fraction of the transport by nutrient-rich poleward undercurrents, and may contribute to the well-known reduction of productivity by eddies in EBUS. Furthermore, at the density layer of their cores, eddies decrease climatological oxygen locally by close to 10%, thereby expanding oxygen minimum zones. Finally, eddies represent low-oxygen extreme events in otherwise oxygenated waters, increasing the area of hypoxic waters by several percent and producing dramatic short-term changes that may play an important ecological role. Capturing these non-local effects in global climate models, which typically include non-eddying oceans, would require dedicated parameterizations.

Description: Climate models depict large diversity in the strength of the El Niño/Southern Oscillation (ENSO) (ENSO amplitude). Here we investigate ENSO-amplitude diversity in the Coupled Model Intercomparison Project Phase 5 (CMIP5) by means of the linear recharge oscillator model, which reduces ENSO dynamics to a two-dimensional problem in terms of eastern equatorial Pacific sea surface temperature anomalies (T) and equatorial Pacific upper ocean heat content anomalies (h). We find that a large contribution to ENSO-amplitude diversity originates from stochastic forcing. Further, significant interactions exist between the stochastic forcing and the growth rates of T and h with competing effects on ENSO amplitude. The joint consideration of stochastic forcing and growth rates explains more than 80% of the ENSO-amplitude variance within CMIP5. Our results can readily explain the lack of correlation between the Bjerknes Stability index, a measure of the growth rate of T, and ENSO amplitude in a multimodel ensemble.

Description: Monthly mean sea level anomalies in the tropical Pacific for the period 1961-2002 are reconstructed using a linear, multi-mode model driven by monthly mean wind stress anomalies from the NCEP/NCAR and ERA-40 reanalysis products. Overall, the sea level anomalies reconstructed by both wind stress products agree well with the available tide gauge data, although with poor performance at Kanton Island in the western-central equatorial Pacific and reduced amplitude at Christmas Island. The reduced performance is related to model error in locating the pivot point in sea level variability associated with the so-called “tilt” mode. We present evidence that the pivot point was further west during the period 1993-2014 than during the period 1961-2002 and attribute this to a persistent upward trend in the zonal wind stress variance along the equator west of 160° W throughout the period 1961-2014. Experiments driven by the zonal component of the wind stress alone reproduce much of the trend in sea level found in the experiments driven by both components of the wind stress. The experiments show an upward trend in sea level in the eastern tropical Pacific over the period 1961-2002, but with a much stronger upward trend when using the NCEP/NCAR product. We argue that the latter is related to an overly strong eastward trend in zonal wind stress in the eastern-central Pacific that is believed to be a spurious feature of the NCEP/NCAR product.

Description: Continental margin ecosystems in the western North Pacific Ocean are subject to strong climate forcing and anthropogenic impacts. To evaluate mechanisms controlling phytoplankton biomass and community structure variations in marginal sea‐open ocean boundary regions, brassicasterol, dinosterol and C37 alkenones were measured in suspended particles in summer and autumn from 2012 to 2013 in the northeastern East China Sea and the western Tsushima Strait (NEECS‐WTS). In summer, the concentrations of brassicasterol (40 ‐ 1535 ng L‐1) and dinosterol (4.2 ‐ 94 ng L‐1) were higher in the southwest of Cheju Island, while C37 alkenones (0 ‐ 30 ng L‐1) were higher in the south of Cheju Island. In autumn, brassicasterol (12 ‐ 106 ng L‐1), dinosterol (2.4 ‐ 21 ng L‐1) and C37 alkenones (0.7 – 7.0 ng L‐1) were higher in the southwest of Cheju Island and the WTS, and higher C37 alkenones also occurred in the Okinawa Trough. Correlation analysis of biomarkers and environmental conditions (temperature, salinity and inorganic nutrient concentrations) clearly demonstrated that phytoplankton biomass and community structure variations can be well elucidated by water masses as indexed by temperature and salinity. High nutrients from the Changjiang River were the main cause of high biomass in summer, while nutrients from subsurface water were likely the key factor regulating phytoplankton biomass in open ocean water stations in autumn. This study indicates that mechanisms controlling phytoplankton biomass in marginal sea‐open ocean boundary regions should be classified by various water masses with different nutrient concentrations, instead of by geography.

Description: A submesoscale coherent vortex (SCV) with a low oxygen core is characterized from underwater glider and mooring observations from the eastern tropical North Atlantic, north of the Cape Verde Islands. The eddy crossed the mooring with its center and a 1 month time series of the SCV's hydrographic and upper 100 m currents structure was obtained. About 45 days after, and ∼100 km west, the SCV frontal zone was surveyed in high temporal and spatial resolution using an underwater glider. Satellite altimetry showed the SCV was formed about 7 months before at the Mauritanian coast. The SCV was located at 80-100 m depth, its diameter was ∼100 km and its maximum swirl velocity ∼0.4 m s-1. A Burger number of 0.2 and a vortex Rossby number 0.15 indicate a flat lens in geostrophic balance. Mooring and glider data show in general comparable dynamical and thermohaline structures, the glider in high spatial resolution, the mooring in high temporal resolution. Surface maps of chlorophyll concentration suggest high productivity inside and around the SCV. The low potential vorticity (PV) core of the SCV is surrounded by filamentary structures, sloping down at different angles from the mixed layer base and with typical width of 10-20 km and a vertical extent of 50-100 m.

Description: We present an improved neotectonic numerical model of the complex NW Africa-SW Eurasia plate boundary segment that runs from west to east along the Gloria Fault up to the northern Algerian margin. We model the surface velocity field and the ongoing lithospheric deformation using the most recent version of the thin-shell code SHELLS and updated lithospheric model and fault map of the region. To check the presence versus the absence of an independently driven Alboran domain, we develop two alternative plate models: one does not include an Alboran plate; another includes it and determines the basal shear tractions necessary to drive it with known velocities. We also compare two alternative sets of Africa-Eurasia velocity boundary conditions, corresponding to geodetic and geological-scale averages of plate motion. Finally, we perform an extensive parametric study of fault friction coefficient, trench resistance, and velocities imposed in Alboran nodes. The final run comprises 5240 experiments, each scored to geodetic velocities (estimated for 250 stations and here provided), stress direction data, and seismic strain rates. The model with the least discrepancy to the data includes the Alboran plate driven by a basal WSW directed shear traction, slightly oblique to the westward direction of Alboran motion. We provide estimates of long-term strain rates and slip rates for the modeled faults, which can be useful for further hazard studies. Our results support that a mechanism additional to the Africa-Eurasia convergence is required to drive the Alboran domain, which can be related to subduction processes occurring within the mantle.

Description: Earth is 180 Myr into the current supercontinent cycle, and the next supercontinent is predicted to form in 250 Myr. The continuous changes in continental configuration can move the ocean between resonant states, and the semidiurnal tides are currently large compared to the past 252 Myr due to tidal resonance in the Atlantic. This leads to the hypothesis that there is a “supertidal” cycle linked to the supercontinent cycle. Here this is tested using new tectonic predictions for the next 250 Myr as bathymetry in a numerical tidal model. The simulations support the following hypothesis: a new tidal resonance will appear 150 Myr from now, followed by a decreasing tide as the supercontinent forms 100 Myr later. This affects the dissipation of tidal energy in the oceans, with consequences for the evolution of the Earth‐Moon system, ocean circulation and climate, and implications for the ocean's capacity of hosting and evolving life.

Description: The Antarctic Circumpolar Current (ACC) has highly energetic mesoscale phenomena, but their impacts on phytoplankton biomass, productivity, and biogeochemical cycling are not understood well. We analyze satellite observations and an eddy‐rich ocean model to show that they drive chlorophyll anomalies of opposite sign in winter versus summer. In winter, deeper mixed layers in positive sea surface height (SSH) anomalies reduce light availability, leading to anomalously low chlorophyll concentrations. In summer with abundant light, however, positive SSH anomalies show elevated chlorophyll concentration due to higher iron level, and an iron budget analysis reveals that anomalously strong vertical mixing enhances iron supply to the mixed layer. Features with negative SSH anomalies exhibit the opposite tendencies: higher chlorophyll concentration in winter and lower in summer. Our results suggest that mesoscale modulation of iron supply, light availability and vertical mixing plays an important role in causing systematic variations in primary productivity over the seasonal cycle.

Description: Halogen- and sulfur-containing compounds are supersaturated in the surface ocean, which results in their emission to the atmosphere. These compounds can be transported to the stratosphere, where they impact ozone, the background aerosol layer, and climate. In this study we calculate the seasonal and interannual variability of transport from the West Indian Ocean (WIO) surface to the stratosphere for 2000-2016 with the Lagrangian transport model FLEXPART using ERA-Interim meteorological fields. We investigate the transport relevant for very short lived substances (VSLS) with tropospheric lifetimes corresponding to dimethylsulfide (1 day), methyl iodide (CH3I, 3.5 days), bromoform (CHBr3, 17 days), and dibromomethane (CH2Br2, 150 days). The stratospheric source gas injection of VSLS tracers from the WIO shows a distinct annual cycle associated with the Asian monsoon. Over the 16-year time series, a slight increase in source gas injection from the WIO to the stratosphere is found for all VSLS tracers and during all seasons. The interannual variability shows a relationship with sea surface temperatures in the WIO as well as the El Niño-Southern Oscillation. During boreal spring of El Niño, enhanced stratospheric injection of VSLS from the tropical WIO is caused by positive sea surface temperature anomalies and enhanced vertical uplift above the WIO. During boreal fall of La Niña, strong injection is related to enhanced atmospheric upward motion over the East Indian Ocean and a prolonged Indian summer monsoon season. Related physical mechanisms and uncertainties are discussed in this study

Description: The notion of a shallow northern sourced intermediate water mass is a well evidenced feature of the Atlantic circulation scheme of the Last Glacial Maximum (LGM). However, recent observations from stable carbon isotopes (δ13C) at the Corner Rise in the deep northwest Atlantic suggested a significant contribution of a Northern Component Water mass to the abyssal northwest Atlantic basin that has not been described before. Here we test the hypothesis of this northern sourced water mass underlying the southern sourced glacial Antarctic Bottom Water by measuring the authigenic neodymium (Nd) isotopic composition from the same sediments from 5,010-m water depth. Neodymium isotopes act as a semiconservative water mass tracer capable of distinguishing between Northern and Southern Component Waters at the northwest Atlantic. Our new Nd isotopic record resolves various water mass changes from the LGM to the early Holocene in agreement with existing Nd-based reconstructions from across the west Atlantic Ocean. Especially pronounced are the Younger Dryas and Bølling-Allerød with unprecedented changes in the Nd isotopic composition. For the LGM we found Nd isotopic evidence for a northern sourced water mass contributing to abyssal depths, thus being in agreement with previous δ13C data from Corner Rise. Overall, however, the deep northwest Atlantic was still dominated by southern sourced water, since we found signatures that are intermediate between northern and southern end member compositions. Furthermore, this new record indicates that C and Nd isotopes were partly decoupled, pointing to nonconservative behavior of one or more likely of both water mass proxies during the LGM.

Description: Pre-stack depth migration data across the Hikurangi margin, East Coast of the North Island, New Zealand, are used to derive subducting slab geometry, upper crustal structure and seismic velocities resolved to ∼14 km depth. We investigate the potential relationship between the crustal architecture, fluid migration and short-term geodetically determined slow-slip events. The subduction interface is a shallow dipping thrust at 〈 7 km depth near the trench and steps down to 14 km depth along an ∼18 km long ramp, beneath Porangahau Ridge. This apparent bend in the décollement is associated with splay fault branching and coincides with a zone of maximum slip (90 mm) inferred on the subduction interface during slow slip events in June and July 2011. A low-velocity zone beneath the plate interface, up-dip of the plate interface ramp, is interpreted as fluid-rich overpressured sediments capped with a low permeability condensed layer of chalk and interbedded mudstones. Fluid rich sediments have been imbricated by splay faults in a region that coincides with the step down in the décollement from the top of subducting sediments to the oceanic crust and contribute to spatial variation in frictional properties of the plate interface that may promote slow slip behavior in the region. Further, transient fluid migration along splay faults at Porangahau Ridge may signify stress changes during slow slip.

Description: Salt marshes provide numerous valuable ecological services. In particular, nitrogen (N) removal in salt marsh sediments alleviates N loading to the coastal ocean. N removal reduces the threat of eutrophication caused by increased N inputs from anthropogenic sources. It is unclear, however, whether chronic nutrient over-enrichment alters the capacity of salt marshes to remove anthropogenic N. To assess the effect of nutrient enrichment on N cycling in salt marsh sediments, we examined important N cycle pathways in experimental fertilization plots in a New England salt marsh. We determined rates of nitrification, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) using sediment slurry incubations with 15 N labeled ammonium or nitrate tracers under oxic headspace (20% oxygen / 80% helium). Nitrification and denitrification rates were more than ten-fold higher in fertilized plots compared to control plots. By contrast, DNRA, which retains N in the system, was high in control plots but not detected in fertilized plots. The relative contribution of DNRA to total nitrate reduction largely depends on the carbon/nitrate ratio in the sediment. These results suggest that long-term fertilization shifts N cycling in salt marsh sediments from predominantly retention to removal. Long-term fertilization alters the relative importance of nitrate reduction pathways in salt marsh sediments: NO 3 - reduction in salt marsh sediments (PDF Download Available). Available from: https://www.researchgate.net/publication/305480944_Long-term_fertilization_alters_the_relative_importance_of_nitrate_reduction_pathways_in_salt_marsh_sediments_NO_3_-_reduction_in_salt_marsh_sediments [accessed Jun 6, 2017].

Description: Bioassay incubation experiments conducted with nutrients and local atmospheric aerosol amendments indicate that phosphorus (P) availability limited phytoplankton growth in the low-nutrient low-chlorophyll (LNLC) ocean off Barbados. Atmospheric deposition provides a relatively large influx of new nutrients and trace metals to the surface ocean in this region in comparison to other nutrient sources. However, the impact on native phytoplankton is muted due to the high ratio of nitrogen (N) to P (NO3:SRP 〉 40) and the low P solubility of these aerosols. Atmospheric deposition induces P limitation in this LNLC region by adding more N and iron (Fe) relative to P. This favors the growth of Prochlorococcus, a genus characterized by low P requirements and highly efficient P acquisition mechanisms. A global three-dimensional marine ecosystem model that includes species-specific phytoplankton elemental quotas/stoichiometry and the atmospheric deposition of N, P, and Fe supports this conclusion. Future increases in aerosol N loading may therefore influence phytoplankton community structure in other LNLC areas, thereby affecting the biological pump and associated carbon sequestration.

Description: Subduction of a narrow slab of oceanic lithosphere beneath a tightly curved orogenic arc requires the presence of at least one lithospheric scale tear fault. While the Calabrian subduction beneath southern Italy is considered to be the type example of this geodynamic setting, the geometry, kinematics and surface expression of the associated lateral, slab tear fault offshore eastern Sicily remain controversial. Results from a new marine geophysical survey conducted in the Ionian Sea, using high‐resolution bathymetry and seismic profiling reveal active faulting at the seafloor within a 140 km long, two‐branched fault system near Alfeo Seamount. The previously unidentified 60 km long NW trending North Alfeo Fault system shows primarily strike‐slip kinematics as indicated by the morphology and steep‐dipping transpressional and transtensional faults. Available earthquake focal mechanisms indicate dextral strike‐slip motion along this fault segment. The 80 km long SSE trending South Alfeo fault system is expressed by one or two steeply dipping normal faults, bounding the western side of a 500+ m thick, 5 km wide, elongate, syntectonic Plio‐Quaternary sedimentary basin. Both branches of the fault system are mechanically capable of generating magnitude 6–7 earthquakes like those that struck eastern Sicily in 1169, 1542, and 1693.

Description: The MARCAN project, launched last January, is working to fill a gap in our knowledge of how freshwater flowing underground shapes and alters the continental margins.

Description: Recent evidence from mooring data in the equatorial Atlantic reveals that semi-annual and longer time scale ocean current variability is close to being resonant with equatorial basin modes. Here we show that intraseasonal variability, with time scales of 10's of days, provides the energy to maintain these resonant basin modes against dissipation. The mechanism is analogous to that by which storm systems in the atmosphere act to maintain the atmospheric jet stream. We demonstrate the mechanism using an idealised model set-up that exhibits equatorial deep jets. The results are supported by direct analysis of available mooring data from the equatorial Atlantic Ocean covering a depth range of several thousand meters. The analysis of the mooring data suggests that the same mechanism also helps maintain the seasonal variability.

Description: Fluid pressure plays an important role in the stability of tectonic faults. However, the in situ mechanical response of faults to fluid pressure variations is still poorly known. To address this question, we performed a fluid injection experiment in a fault zone in shales while monitoring fault movements at the injection source and seismic velocity variations from a near‐distance (〈10 m) monitoring network. We measured and located the P and S wave velocity perturbations in and around the fault using repetitive active sources. We observed that seismic velocity perturbations dramatically increase above 1.5 MPa of injection pressure. This is consistent with an increase of fluid flow associated with an aseismic dilatant shearing of the fault as shown by numerical modeling. We find that seismic velocity changes are sensitive to both fault opening by fluid invasion and effective stress variations and can be an efficient measurement for monitoring fluid‐driven aseismic deformations of faults.

Description: Stable carbon and oxygen isotopes (δ13C and δ18O) of foraminiferal tests are amongst the most important tools in paleoceanography, but the extent to which recrystallization can alter the isotopic composition of the tests is not well known. Here we compare three middle Miocene (16–13 Ma) benthic foraminiferal stable isotope records from eastern equatorial Pacific sites with different diagenetic histories to investigate the effect of recrystallization. To test an extreme case, we analyzed stable isotope compositions of benthic foraminifera from Integrated Ocean Drilling Program Site U1336, for which the geochemistry of bulk carbonates and associated pore waters indicates continued diagenetic alteration in sediments 〉 14.7 Ma. Despite this diagenetic overprinting, the amplitudes and absolute values of the analyzed U1336 stable isotopes agree well with high-resolution records from better preserved Sites U1337 and U1338 nearby. Our results suggest that although benthic foraminiferal tests of all three sites show some degree of textural changes due to recrystallization, they have retained their original stable isotope signatures. The good agreement of the benthic foraminiferal stable isotope records demonstrates that recrystallization occurred extremely rapidly (〈 100 kyr) after deposition. This is confirmed by the preservation of orbital cyclicities in U1336 stable isotope data and δ18O values being different to inorganic calcite that would precipitate from U1336 pore waters during late recrystallization. The close similarity of the benthic foraminiferal stable isotope records between the sites allows the well-resolved paleomagnetic results of Site U1336 to be transferred to Sites U1337 and U1338 improving the global geological timescale.

Description: We examine the interannual variability of the seasonal mean atmospheric circulation in the Southern Hemisphere during austral winter. The three major modes are identified by rotated EOF (REOF) analysis. As expected, REOF1 is associated with the Southern Annular Mode which is dominated by internal atmospheric dynamics. REOF2 displays a wave train, linked to the western North Pacific monsoon and the Pacific-Japan pattern in East Asia in the same season; REOF3 resembles the Pacific-South American pattern. Externally-forced variability strongly projects on both REOF2 and REOF3 so that, in the ensemble mean, an atmospheric model with prescribed observed sea surface temperature (SST) captures considerable parts of the time evolution of REOF2 (50%) and REOF3 (25%), suggesting a potential predictability for the two modes.

Description: We present a deep electrical resistivity image from the passive continental margin in Namibia. The approximately 700 km long magnetotelluric profile follows the Walvis Ridge offshore, continues onshore across the Kaoko Mobile Belt and reaches onto the Congo Craton. Two-dimensional inversion reveals moderately resistive material offshore, atypically low for oceanic lithosphere, reaching depths of 15–20 km. Such moderate resistivities are consistent with seismic P wave velocity models, which suggest up to 35 km thick crust. The Neoproterozoic rocks of the Kaoko Mobile Belt are resistive, but NNW-striking major shear-zones are imaged as subvertical, conductive structures in the upper and middle crust. Since the geophysical imprint of the shear zones is intact, opening of the South Atlantic in the Cretaceous did not alter the middle crust. The transition into the cratonic region coincides with a deepening of the high-resistive material to depths of more than 60 km.

Description: Volcanic ash deposition to the ocean forms a natural source of iron (Fe) to surface water microbial communities. Inputs of lithogenic material may also facilitate Fe removal through scavenging. Combining dissolved Fe (dFe) and thorium-234 observations alongside modelling, we investigate scavenging of Fe in the North Atlantic following the Eyjafjallajökull volcanic eruption. Under typical conditions biogenic particles dominate scavenging, whereas ash particles dominate during the eruption. The size of particles is important as smaller scavenging particles can become saturated with surface-associated ions. Model simulations indicate that ash deposition associated with Eyjafjallajökull likely led to net Fe removal. Our model suggests a three-fold greater stimulation of biological activity if ash deposition had occurred later in the growing season when the region was Fe-limited. The implications of ash particle-scavenging, eruption timing and particle saturation need to be considered when assessing the impact of ash deposition on the ocean Fe cycle and productivity.

Description: Prior to the 2000s, the North Atlantic was the basin showing the greatest warming. However, since the mid-2000s during the so-called global warming hiatus, large amounts of heat were transferred in this basin from upper to deeper levels while the dominance in terms of atmospheric heat capture moved into the Indo-Pacific. Here we show that a large transformation of modal waters in the eastern North Atlantic (ENA) played a crucial role in such contrasting behavior. First, strong winter mixing in 2005 transformed ENA modal waters into a much saltier, warmer, and denser variety, transferring upper ocean heat and salt gained slowly over time to deeper layers. The new denser waters also altered the zonal dynamic height gradient reversing the southward regional flow and enhancing the access of saltier southern waters to higher latitudes. Then, the excess salinity in northern regions favored additional heat injection through deep convection events in later years.

Description: The Arctic sea ice cover is thinning and retreating, causing changes in surface roughness that in turn modify the momentum flux from the atmosphere through the ice into the ocean. New model simulations comprising variable sea ice drag coefficients for both the air and water interface demonstrate that the heterogeneity in sea ice surface roughness significantly impacts the spatial distribution and trends of ocean surface stress during the last decades. Simulations with constant sea ice drag coefficients as used in most climate models show an increase in annual mean ocean surface stress (0.003 N/m2 per decade, 4.6%) due to the reduction of ice thickness leading to a weakening of the ice and accelerated ice drift. In contrast, with variable drag coefficients our simulations show annual mean ocean surface stress is declining at a rate of -0.002 N/m2 per decade (3.1%) over the period 1980-2013 because of a significant reduction in surface roughness associated with an increasingly thinner and younger sea ice cover. The effectiveness of sea ice in transferring momentum does not only depend on its resistive strength against the wind forcing but is also set by its top and bottom surface roughness varying with ice types and ice conditions. This reveals the need to account for sea ice surface roughness variations in climate simulations in order to correctly represent the implications of sea ice loss under global warming

Description: Monowai is an active submarine volcanic center in the Kermadec Arc, Southwest Pacific Ocean. During May 2011, it erupted over a period of 5 days, with explosive activity directly linked to the generation of seismoacoustic T phases. We show, using cross-correlation and time-difference-of-arrival techniques, that the eruption is detected as far as Ascension Island, equatorial South Atlantic Ocean, where a bottom moored hydrophone array is operated as part of the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization. Hydroacoustic phases from the volcanic center must therefore have propagated through the Sound Fixing and Ranging channel in the South Pacific and South Atlantic Oceans, a source-receiver distance of ~15,800 km. We believe this to be the furthest documented range of a naturally occurring underwater signal above 1 Hz. Our findings, which are consistent with observations at regional broadband stations and long-range, acoustic parabolic equation modeling, have implications for submarine volcano monitoring.

Description: Marine sediments deposited off the Zambezi River that drains a considerable part of the southeast African continent provide continuous records of the continental climatic and environmental conditions. Here we present time series of neodymium (Nd) isotope signatures of the detrital sediment fraction during the past ~45,000 years, to reconstruct climate-driven changes in the provenance of clays deposited along the Mozambique Margin. Coherent with the surface current regime, the Nd isotope distribution in surface sediments reveals mixing of the alongshore flowing Zambezi suspension load with sediments supplied by smaller rivers located further north. To reconstruct past changes in sediment provenances, Nd isotope signatures of clays that are not significantly fractionated during weathering processes have been obtained from core 64PE304-80, which was recovered just north of the Zambezi mouth at 1329 m water depth. Distinctly unradiogenic clay signatures (ENd values 〈214.2) are found during the Last Glacial Maximum, Heinrich Stadial 1, and Younger Dryas. In contrast, the Nd isotope record shows higher, more radiogenic isotope signatures during Marine Isotope Stage 3 and between ~15 and ~5 ka BP, the latter coinciding with the timing of the northern hemisphere African Humid Period. The clay-sized sediment fraction with the least radiogenic Nd isotope signatures was deposited during the Holocene, when the adjacent Mozambique Shelf became completely flooded. In general, the contribution of the distinctly unradiogenic Zambezi suspension load has followed the intensity of precession-forced monsoonal precipitation and enhanced during periods of increased southern hemisphere insolation and high-latitude northern hemispheric climate variability.

Description: The rising temperature of the world’s oceans is affecting coral reef ecosystems by increasing the frequency and severity of bleaching and mortality events. The susceptibility of corals to temperature stress varies on local and regional scales. Insights into potential controlling parameters are hampered by a lack of long term in situ data in most coral reef environments and sea surface temperature (SST) products often do not resolve reef-scale variations. Here we use 42 years (1970–2012) of coral Sr/Ca data to reconstruct seasonal- to decadal-scale SST variations in two adjacent but distinct reef environments at Little Cayman, Cayman Islands. Our results indicate that two massive Diploria strigosa corals growing in the lagoon and in the fore reef responded differently to past warming events. Coral Sr/Ca data from the shallow lagoon successfully record high summer temperatures confirmed by in situ observations (〉338C). Surprisingly, coral Sr/Ca from the deeper fore reef is strongly affected by thermal stress events, although seasonal temperature extremes and mean SSTs at this site are reduced compared to the lagoon. The shallow lagoon coral showed decadal variations in Sr/Ca, supposedly related to the modulation of lagoonal temperature through varying tidal water exchange, influenced by the 18.6 year lunar nodal cycle. Our results show that reef-scale SST variability can be much larger than suggested by satellite SST measurements. Thus, using coral SST proxy records from different reef zones combined with in situ observations will improve conservation programs that are developed to monitor and predict potential thermal stress on coral reefs.

Description: A bottom-simulating reflector (BSR) occurs west of Svalbard in water depths exceeding 600 m, indicating that gas hydrate occurrence in marine sediments is more widespread in this region than anywhere else on the eastern North Atlantic margin. Regional BSR mapping shows the presence of hydrate and free gas in several areas, with the largest area located north of the Knipovich Ridge, a slow-spreading ridge segment of the Mid Atlantic Ridge system. Here, heat flow is high (up to 330 mW m-2), increasing towards the ridge axis. The coinciding maxima in across-margin BSR width and heat flow suggest that the Knipovich Ridge influenced methane generation in this area. This is supported by recent finds of thermogenic methane at cold seeps north of the ridge termination. To evaluate the source rock potential on the western Svalbard margin, we applied 1D petroleum system modeling at three sites. The modeling shows that temperature and burial conditions near the ridge were sufficient to produce hydrocarbons. The bulk petroleum mass produced since the Eocene is at least 5 kt and could be as high as ~0.2 Mt. Most likely, source rocks are Miocene organic-rich sediments and a potential Eocene source rock that may exist in the area if early rifting created sufficiently deep depocenters. Thermogenic methane production could thus explain the more widespread presence of gas hydrates north of the Knipovich Ridge. The presence of microbial methane on the upper continental slope and shelf indicates that the origin of methane on the Svalbard margin varies spatially.

Description: Hydrothermal convection at mid-ocean ridges links the ocean's long-term chemical evolution to solid earth processes, forms hydrothermal ore deposits, and sustains the unique chemosynthetic vent fauna. Yet the depth extent of hydrothermal cooling and the inseparably connected question of how the lower crust accretes remain poorly constrained. Here based on coupled models of crustal accretion and hydrothermal circulation, we provide new insights into which modes of lower crust formation and hydrothermal cooling are thermally viable and most consistent with observations at fast-spreading ridges. We integrate numerical models with observations of melt lens depth, thermal structure, and melt fraction. Models matching all these observations always require a deep crustal-scale hydrothermal flow component and less than 50% of the lower crust crystallizing in situ.

Description: Marine carbonate chemistry measurements have been carried out annually since 2009 during UK research cruises along the Extended Ellett Line (EEL), a hydrographic transect in the northeast Atlantic Ocean. The EEL intersects several water masses that are key to the global thermohaline circulation, and therefore the cruises sample a region in which it is critical to monitor secular physical and biogeochemical changes. We have combined results from these EEL cruises with existing quality-controlled observational data syntheses to produce a hydrographic time series for the EEL from 1981 to 2013. This reveals multidecadal increases in dissolved inorganic carbon (DIC) throughout the water column, with a near-surface maximum rate of 1.800.45 mu molkg(-1)yr(-1). Anthropogenic CO2 accumulation was assessed, using simultaneous changes in apparent oxygen utilization (AOU) and total alkalinity (TA) as proxies for the biogeochemical processes that influence DIC. The stable carbon isotope composition of DIC (C-13(DIC)) was also determined and used as an independent test of our method. We calculated a volume-integrated anthropogenic CO2 accumulation rate of 2.80.4mgCm(-3)yr(-1) along the EEL, which is about double the global mean. The anthropogenic CO2 component accounts for only 316% of the total DIC increase. The remainder is derived from increased organic matter remineralization, which we attribute to the lateral redistribution of water masses that accompanies subpolar gyre contraction. Output from a general circulation ecosystem model demonstrates that spatiotemporal heterogeneity in the observations has not significantly biased our multidecadal rate of change calculations and indicates that the EEL observations have been tracking distal changes in the surrounding North Atlantic and Nordic Seas.

Description: The formation of a subsurface anticyclonic eddy in the Peru-Chile Undercurrent (PCUC) in January and February 2013 is investigated using a multi-platform four-dimensional observational approach. Research vessel, multiple glider and mooring-based measurements were conducted in the Peruvian upwelling regime near 12°30'S. The dataset consists of 〉 10000 glider profiles and repeated vessel-based hydrography and velocity transects. It allows a detailed description of the eddy formation and its impact on the near-coastal salinity, oxygen and nutrient distributions. In early January, a strong PCUC with maximum poleward velocities of ∼ 0.25 m/s at 100 to 200 m depth was observed. Starting on January 20 a subsurface anticyclonic eddy developed in the PCUC downstream of a topographic bend, suggesting flow separation as the eddy formation mechanism. The eddy core waters exhibited oxygen concentrations 〈 1μmol/kg, an elevated nitrogen-deficit of ∼ 17μmol/l and potential vorticity close to zero, which seemed to originate from the bottom boundary layer of the continental slope. The eddy-induced across-shelf velocities resulted in an elevated exchange of water masses between the upper continental slope and the open ocean. Small scale salinity and oxygen structures were formed by along-isopycnal stirring and indications of eddy-driven oxygen ventilation of the upper oxygen minimum zone were observed. It is concluded that mesoscale stirring of solutes and the offshore transport of eddy core properties could provide an important coastal open-ocean exchange mechanism with potentially large implications for nutrient budgets and biogeochemical cycling in the oxygen minimum zone off Peru.

Description: Export of brine-enriched water from Siberian shelves is thought to be a key parameter in maintaining the Arctic Halocline, which isolates the fresh and cold surface water from the warm Atlantic water and thus prevent dramatic change in the Arctic sea-ice thermodynamic. In this study, we used five years of oxygen isotope and hydrological summer surveys to better understand the factors controlling the brine inventory and distribution over the Laptev Sea shelf. The inventory was maximal in 2011 and 2007 and minimal in 2010. The brine inventory interannual variations are coherent with the winter Arctic Oscillation index that was maximal in 2011 and 2007 and minimal in 2010, which is known to modulate Arctic winds and sea-ice export pattern. While we should remain cautious since our record is limited to 5-years, our results suggest that the combined effect of the Arctic Oscillation and of the Arctic Dipole is the main factor controlling the annual variations in the inventory of brine-enriched waters from the Laptev Sea shelf between 2007 and 2011, especially during extreme negative Arctic Oscillation and Arctic Dipole conditions as in 2010.

Description: This chapter investigates the particular challenges for joint inversion in the context of hydrocarbon exploration. It reviews the current literature and introduces interesting approaches that have been developed in the context of hydrocarbon exploration and could potentially be used in other application areas as well. Joint inversion methods incorporating petro physical models of the reservoir can directly yield quantities of interest such as porosity and permeability, but have to be carefully tuned to the area under investigation. The chapter presents two detailed joint inversion case studies, one for subsalt imaging and one for sub basalt imaging. It also covers emulation of the forward calculation, a concept that can be applied generally in joint inversion, but has only been applied in hydrocarbon exploration. For a successful joint inversion, it is necessary to invert each dataset, construct different coupling approaches, and evaluate the impact on the final results.

Description: Here we present the first record of Sr/Ca variability in a massive Porites lutea coral from the Lakshadweep Archipelago, Arabian Sea. The annual mean sea surface temperature (SST) in this region and the surrounding areas has increased steadily in the recent past. During some major El Nino events, SSTs are even higher, imposing additional thermal-stress on corals, episodically leading to coral bleaching. We infer from the coral-Sr/Ca record (1981-2008) that during some of these events high and persistent SSTs lead to a dampening of the temperature signal in coral-Sr/Ca, impairing the coral's ability to record full scale warming. Thus, coral-Sr/Ca may provide a history of past El Nino Southern-Oscillation (ENSO) induced thermal-stress episodes, which are a recurrent feature also seen in cross-spectral analysis between coral-Sr/Ca and the Nino3.4 index. Despite the impact of episodical thermal-stress during major El Nino events, our coral proxy faithfully records the seasonal monsoon-induced summer cooling on the order of approximate to 2.3 degrees C. Calibration of coral-Sr/Ca with instrumental grid-SST data shows significant correlation to regional SST and monsoon variability. Hence, massive Porites corals of this region are highly valuable archives for reconstructing long-term changes in SST, strongly influenced by monsoon variability on seasonal scales. More importantly, our data show that this site with increasing SST is an ideal location for testing the future effects of the projected anthropogenic SST increase on coral reefs that are already under thermal-stress worldwide.

Description: A multiyear mooring record (2007–2014) and satellite imagery highlight the strong temperature variability and unique hydrographic nature of the Laptev Sea. This Arctic shelf is a key region for river discharge and sea ice formation and export and includes submarine permafrost and methane deposits, which emphasizes the need to understand the thermal variability near the seafloor. Recent years were characterized by early ice retreat and a warming near-shore environment. However, warming was not observed on the deeper shelf until year-round under-ice measurements recorded unprecedented warm near-bottom waters of +0.6°C in winter 2012/2013, just after the Arctic sea ice extent featured a record minimum. In the Laptev Sea, early ice retreat in 2012 combined with Lena River heat and solar radiation produced anomalously warm summer surface waters, which were vertically mixed, trapped in the pycnocline, and subsequently transferred toward the bottom until the water column cooled when brine rejection eroded stratification.

Description: We present geological observations and geochemical data for the youngest volcanic features on the slow-spreading Mid-Atlantic Ridge at 8°48'S that shows seismic evidence for a thickened crust and excess magma formation. Young lava flows with high sonar reflectivity cover about 14 km2 in the axial rift and were probably erupted from two axial volcanic ridges each of about 3 km in length. Three different lava units occur along an about 11 km long portion of the ridge, and lavas from the northern axial volcanic ridge differ from those of the southern axial volcanic ridge and surrounding lava flows. Basalts from the axial rift flanks and from a pillow mound within the young flows are more incompatible element depleted than those from the young volcanic field. Lavas from this volcanic area have 226Ra-230Th disequilibria model ages of 1,000 and 4,000 years whereas the older lavas from the rift flank and the pillow mound, but also some of the lava field, are older than 8,000 years. Glasses from the northern and southern ends of the southern lava unit indicate up to 100°C cooler magma temperatures than in the center and increased assimilation of hydrothermally altered material. The compositional heterogeneity on a scale of 3 km suggests small magma batches rising vertically from the mantle to the surface without significant lateral flow and mixing. The observations on the 8°48'S lava field support the model of low frequency eruptions from single ascending magma batches that has been developed for slow-spreading ridges.

Description: A coupled ocean biogeochemistry-circulation model is used to investigate the impact of observed past and anticipated future wind changes in the southern hemisphere on the oxygen minimum zone in the tropical Pacific. We consider the industrial period until the end of the 21st century and distinguish effects due to a strengthening of the westerlies from effects of a southward shift of the westerlies that is accompanied by a poleward expansion of the tropical trade winds. Our model results show that a strengthening of the westerlies counteracts part of the warming-induced decline in the global marine oxygen inventory. A poleward shift of the trade-westerlies boundary, however, triggers a significant decrease of oxygen in the tropical oxygen minimum zone. In a business-as-usual CO2 emission scenario, the poleward shift of the trade-westerlies boundary and warming-induced increase in stratification contribute equally to the expansion of suboxic waters in the tropical Pacific.

Description: We attempt to assess the Holocene surface-subsurface seawater density gradient on millennial time scale based on the reconstruction of potential density (σθ) by combining data from dinoflagellate cyst assemblages and planktic foraminiferal (Neogloboquadrina pachyderma (s)) stable oxygen isotopes (δ18Oc). Following several calibration exercises, the likeliness of favorable seasonal preconditioning to open ocean convection is evaluated. The reconstructed σθ values reveal unfavorable conditions for vertical convection in the western Nordic Seas prior to ~7–6.5 ka B.P., with a westward increase and persistence of surface water buoyancy. Active overturning became more likely after 6.5 ka B.P. as suggested by a reduced and recurrently inverted vertical σθ gradient, while intermittent eastward spreading of lower density surface waters continued to modulate the area of potential overturning. Despite some reservation regarding the accuracy of the σθ values reconstructed, the documentation of relative changes of σθ gradients through time and space is suggested as a helpful tool for the appraisal of past overturning likeliness.

Description: A Department of Energy collaboration aims to make climate model development faster and more efficient by creating a prototype of a system for testing model components.

Description: The impact of increasing anthropogenic atmospheric nitrogen deposition on marine biogeochemistry is uncertain. We performed simulations to quantify its effect on nitrogen cycling and marine productivity in a global 3-D ocean biogeochemistry model. Nitrogen fixation provides an efficient feedback by decreasing immediately to deposition, whereas water column denitrification increases more gradually in the slowly expanding oxygen deficient zones. Counterintuitively, nitrogen deposition near oxygen deficient zones causes a net loss of marine nitrogen due to the stoichiometry of denitrification. In our idealized atmospheric deposition simulations that only account for nitrogen cycle perturbations, these combined stabilizing feedbacks largely compensate deposition and suppress the increase in global marine productivity to 〈2%, in contrast to a simulation that neglects nitrogen cycle feedbacks that predicts an increase of 〉15%. Our study emphasizes including the dynamic response of nitrogen fixation and denitrification to atmospheric nitrogen deposition to predict future changes of the marine nitrogen cycle and productivity.

Description: Marine-terminating glaciers play a critical role in controlling Greenland's ice sheet mass balance. Their frontal margins interact vigorously with the ocean, but our understanding of this interaction is limited, in part, by a lack of bathymetry data. Here we present a multibeam echo sounding survey of 14 glacial fjords in the Uummannaq and Vaigat fjords, west Greenland, which extends from the continental shelf to the glacier fronts. The data reveal valleys with shallow sills, overdeepenings (〉 1300 m) from glacial erosion, and seafloor depths 100-1000 m deeper than in existing charts. Where fjords are deep enough, we detect the pervasive presence of warm, salty Atlantic Water (AW) (〉 2.5 degrees C) with high melt potential, but we also find numerous glaciers grounded on shallow (〈 200 m) sills, standing in cold (〈 1 degrees C) waters in otherwise deep fjords, i.e., with reduced melt potential. Bathymetric observations extending to the glacier fronts are critical to understand the glacier evolution.

Description: The current generation of Earth system models that participate in the Coupled Model Intercomparison Project phase 5 (CMIP5) does not, on average, produce a strengthened Northern Hemisphere (NH) polar vortex after large tropical volcanic eruptions as suggested by observational records. Here we investigate the impact of volcanic eruptions on the NH winter stratosphere with an ensemble of 20 model simulations of the Max Planck Institute Earth system model. We compare the dynamical impact in simulations of the very large 1815 Tambora eruption with the averaged dynamical response to the two largest eruptions of the CMIP5 historical simulations (the 1883 Krakatau and the 1991 Pinatubo eruptions). We find that for both the Tambora and the averaged Krakatau-Pinatubo eruptions the radiative perturbation only weakly affects the polar vortex directly. The position of the maximum temperature anomaly gradient is located at approximately 30°N, where we obtain significant westerly zonal wind anomalies between 10hPa and 30hPa. Under the very strong forcing of the Tambora eruption, the NH polar vortex is significantly strengthened because the subtropical westerly wind anomalies are sufficiently strong to robustly alter the propagation of planetary waves. The average response to the eruptions of Krakatau and Pinatubo reveals a slight strengthening of the polar vortex, but individual ensemble members differ substantially, indicating that internal variability plays a dominant role. For the Tambora eruption the ensemble variability of the zonal mean temperature and zonal wind anomalies during midwinter and late winter is significantly reduced compared to the volcanically unperturbed period.

Description: Sustainable Ocean Development — A Perspective from Former, Current and Future Kiel Marine Scientists; New York, 28–30 September 2015

Description: We examine the simulated Atlantic Multidecadal Variability (AMV) in a model that includes a correction for a longstanding problem with climate models, namely the misplacement of the North Atlantic Current. The corrected model shows that in the warm AMV phase, heat is lost by the ocean in the northwestern part of the basin and gained by the ocean to the east, suggesting an advective transfer of heat by the mid-latitude westerlies. The basin wide response is consistent with a role for cloud feedback and is in broad agreement with estimates from observations, but is poorly represented in the uncorrected model. The corrected model is then used to show that the ocean/atmosphere heat transfer is influenced by low frequency variability in the overlying atmosphere. We also argue that changing ocean heat transport is an essential feature of our results.

Description: The submarine Istanbul-Silivri fault segment, within 15km of Istanbul, is the only portion of the North Anatolian Fault that has not ruptured in the last 250years. We report first results of a seafloor acoustic ranging experiment to quantify current horizontal deformation along this segment and assess whether the segment is creeping aseismically or accumulating stress to be released in a future event. Ten transponders were installed to monitor length variations along 15 baselines. A joint least squares inversion for across-fault baseline changes, accounting for sound speed drift at each transponder, precludes fault displacement rates larger than a few millimeters per year during the 6month observation period. Forward modeling shows that the data better fit a locked state or a very moderate surface creep-less than 6mm/yr compared to a far-field slip rate of over 20mm/yr-suggesting that the fault segment is currently accumulating stress.

Description: About 50 Gt of carbon is fixed photosynthetically by surface ocean phytoplankton communities every year. Part of this organic matter is reprocessed within the plankton community to form aggregates which eventually sink and export carbon into the deep ocean. The fraction of organic matter leaving the surface ocean is partly dependent on aggregate sinking velocity which accelerates with increasing aggregate size and density, where the latter is controlled by ballast load and aggregate porosity. In May 2011, we moored nine 25 m deep mesocosms in a Norwegian fjord to assess on a daily basis how plankton community structure affects material properties and sinking velocities of aggregates (Ø 80–400 µm) collected in the mesocosms' sediment traps. We noted that sinking velocity was not necessarily accelerated by opal ballast during diatom blooms, which could be due to relatively high porosity of these rather fresh aggregates. Furthermore, estimated aggregate porosity (Pestimated) decreased as the picoautotroph (0.2–2 µm) fraction of the phytoplankton biomass increased. Thus, picoautotroph-dominated communities may be indicative for food webs promoting a high degree of aggregate repackaging with potential for accelerated sinking. Blooms of the coccolithophore Emiliania huxleyi revealed that cell concentrations of ~1500 cells/mL accelerate sinking by about 35–40%, which we estimate (by one-dimensional modeling) to elevate organic matter transfer efficiency through the mesopelagic from 14 to 24%. Our results indicate that sinking velocities are influenced by the complex interplay between the availability of ballast minerals and aggregate packaging; both of which are controlled by plankton community structure.

Description: The causes of natural variation in alkalinity in the North Pacific surface ocean need to be investigated to understand the carbon cycle and to improve predictive algorithms. We used GLODAPv2 to test hypotheses on the causes of three longitudinal phenomena in Alk*, a tracer of calcium carbonate cycling. These phenomena are (a) an increase from east to west between 45°N and 55°N, (b) an increase from west to east between 25°N and 40°N, and (c) a minor increase from west to east in the equatorial upwelling region. Between 45°N and 55°N, Alk* is higher on the western than on the eastern side, and this is associated with denser isopycnals with higher Alk* lying at shallower depths. Between 25°N and 40°N, upwelling along the North American continental shelf causes higher Alk* in the east. Along the equator, a strong east-west trend was not observed, even though the upwelling on the eastern side of the basin is more intense, because the water brought to the surface is not high in Alk*. We created two algorithms to predict alkalinity, one for the entire Pacific Ocean north of 30°S and one for the eastern margin. The Pacific Ocean algorithm is more accurate than the commonly used algorithm published by Lee et al. (2006), of similar accuracy to the best previously published algorithm by Sasse et al. (2013), and is less biased with longitude than other algorithms in the subpolar North Pacific. Our eastern margin algorithm is more accurate than previously published algorithms.

Description: We studied the tephra inventory of 18 deep sea drill sites from six DSDP/ODP legs (Legs 84, 138, 170, 202, 205, 206) and two IODP legs (Legs 334 and 344) offshore the southern Central American Volcanic Arc (CAVA). Eight drill sites are located on the incoming Cocos plate and ten drill sites on the continental slope of the Caribbean plate. In total we examined ∼840 ash-bearing horizons and identified ∼650 of these as primary ash beds of which 430 originated from the CAVA. Correlations of ash beds were established between marine cores and with terrestrial tephra deposits, using major and trace element glass compositions with respect to relative stratigraphic order. As a prerequisite for marine-terrestrial correlations we present a new geochemical data set for significant Neogene and Quaternary Costa Rican tephras. Moreover, new Ar/Ar ages for marine tephras have been determined and marine ash beds are also dated using the pelagic sedimentation rates. The resulting correlations and provenance analyses build a tephrochronostratigraphic framework for Costa Rica and Nicaragua that covers the last 〉8 Myr. We define 39 correlations of marine ash beds to specific tephra formations in Costa Rica and Nicaragua; from the 4.15 Ma Lower Sandillal Ignimbrite to the 3.5 ka Rincón de la Vieja Tephra from Costa Rica, as well as another 32 widely distributed tephra layers for which their specific region of origin along Costa Rica and Nicaragua can be constrained.

Description: The ocean contributes to regulating atmospheric CO2 levels, partly via variability in the fraction of primary production (PP) which is exported out of the surface layer (i.e., the e ratio). Southern Ocean studies have found that contrary to global-scale analyses, an inverse relationship exists between e ratio and PP. This relationship remains unexplained, with potential hypotheses being (i) large export of dissolved organic carbon (DOC) in high PP areas, (ii) strong surface microbial recycling in high PP regions, and/or (iii) grazing-mediated export that varies inversely with PP. We find that the export of DOC has a limited influence in setting the negative e ratio/PP relationship. However, we observed that at sites with low PP and high e ratios, zooplankton-mediated export is large and surface microbial abundance low suggesting that both are important drivers of the magnitude of the e ratio in the Southern Ocean.

Description: We find that summer methane (CH4) release from seabed sediments west of Svalbard substantially increases CH4 concentrations in the ocean but has limited influence on the atmospheric CH4 levels. Our conclusion stems from complementary measurements at the seafloor, in the ocean, and in the atmosphere from land-based, ship and aircraft platforms during a summer campaign in 2014. We detected high concentrations of dissolved CH4 in the ocean above the seafloor with a sharp decrease above the pycnocline. Model approaches taking potential CH4 emissions from both dissolved and bubble-released CH4 from a larger region into account reveal a maximum flux compatible with the observed atmospheric CH4 mixing ratios of 2.4–3.8 nmol m−2 s−1. This is too low to have an impact on the atmospheric summer CH4 budget in the year 2014. Long-term ocean observatories may shed light on the complex variations of Arctic CH4 cycles throughout the year.

Description: We used an earth system model of intermediate complexity to study the effects of Solar Radiation Management (SRM) by sea spray geoengineering on ocean biogeochemistry. SRM slightly decreased global ocean net primary productivity (NPP) relative to the control run. The lower temperatures in the SRM run decreased NPP directly but also indirectly increased NPP in some regions due to changes in nutrient availability resulting from changes in ocean stratification and circulation. Reduced light availability had a minor effect on global total NPP but a major regional effect near the nutrient rich upwelling region off the coast of Peru, where light availability is the main limiting factor for phytoplankton growth in our model. Unused nutrients from regions with decreased NPP also fueled NPP elsewhere. In the context of RCP4.5 simulation used here, SRM decreased ocean carbon uptake due to changes in atmospheric CO2 concentrations, seawater chemistry, NPP, temperature, and ocean circulation.

Description: Despite a clear correlation of alkenone unsaturation and sea surface temperatures (SST) throughout most parts of the ocean, scatter of the regression for various calibration equations has been shown to increase significantly at low SSTs. In this study, we combine previously published (n = 101) and new (n = 51) surface sediment data from the northern North Atlantic to constrain uncertainties of alkenone paleothermometry at low SSTs and to discuss possible sources of the increased scatter in the regression. The correlation between alkenone unsaturation and SSTs is strongest, in particular at the cold end (SSTs 〈 10°C), when the tetra-unsaturated alkenones (C37:4) are included in the unsaturation index (expressed as inline image) and regressed against spring-summer temperature. Surface ocean salinity and sea ice cover are not correlated with inline image per se. However, samples located in regions of permanent winter sea ice cover exhibit a significant warm bias. Deviation from the linear regression is posited to be related to a number of additional non-exclusive factors, such as advection of allochthonous material, local temperature stratification, and uncertainty in the absolute age of surface sediment samples assumed to be equivalent to modern conditions. We conclude that alkenone unsaturation allows accurate reconstruction of SST records from many regions of the North Atlantic if the factors confounding alkenone paleothermometry detailed here can be excluded.

Description: The Peruvian upwelling system encompasses the most intense and shallowest oxygen minimum zone (OMZ) in the ocean. This system shows pronounced submesoscale activity like filaments and fronts. We carried out glider-based observations off Peru during austral summer 2013 to investigate whether submesoscale frontal processes ventilate the Peruvian OMZ. We present observational evidence for the subduction of highly oxygenated surface water in a submesoscale cold filament. The subduction event ventilates the oxycline but does not reach OMZ core waters. In a regional submesoscale-permitting model we study the pathways of newly upwelled water. About 50% of upwelled virtual floats are subducted below the mixed layer within 5 days emphasizing a hitherto unrecognized importance of subduction for the ventilation of the Peruvian oxycline.

Description: During Integrated Ocean Drilling Program Expedition 311 five boreholes were drilled across the accretionary prism of the northern Cascadia subduction zone. Logging-while-drilling borehole images are utilized to determine breakout orientations to define maximum horizontal compressive stress orientations. Additionally, wireline logging data at two of these sites and from Site 889 of Ocean Drilling Program Leg 146 are used to define breakouts from differences in the aperture of caliper arms. At most sites, the maximum horizontal compressive stress SHmax is margin-normal, consistent with plate convergence. Deviations from this trend reflect local structural perturbations. Our results do not constrain stress magnitudes. If the margin-normal compressional stress is greater than the vertical stress, the margin-normal SHmax direction we observe may reflect current locking of a velocity-weakening shallow megathrust and thus potential for trench-breaching, tsunamigenic rupture in a future megathrust earthquake.

Description: The present study proposes a novel method that merges satellite ocean color bio-optical products with Argo temperature-salinity profiles to infer the vertical distribution of the particulate backscattering coefficient (bbp). This neural network-based method (SOCA-BBP for Satellite Ocean-Color merged with Argo data to infer the vertical distribution of the Particulate Backscattering coefficient) uses three main input components: (1) satellite-based surface estimates of bbp and chlorophyll a concentration matched up in space and time with (2) depth-resolved physical properties derived from temperature-salinity profiles measured by Argo profiling floats and (3) the day of the year of the considered satellite-Argo matchup. The neural network is trained and validated using a database including 4725 simultaneous profiles of temperature-salinity and bio-optical properties collected by Bio-Argo floats, with concomitant satellite-derived products. The Bio-Argo profiles are representative of the global open-ocean in terms of oceanographic conditions, making the proposed method applicable to most open-ocean environments. SOCA-BBP is validated using 20% of the entire database (global error of 21%). We present additional validation results based on two other independent data sets acquired (1) by four Bio-Argo floats deployed in major oceanic basins, not represented in the database used to train the method; and (2) during an AMT (Atlantic Meridional Transect) field cruise in 2009. These validation tests based on two fully independent data sets indicate the robustness of the predicted vertical distribution of bbp. To illustrate the potential of the method, we merged monthly climatological Argo profiles with ocean color products to produce a depth-resolved climatology of bbp for the global ocean.

Description: New records of stable silicon isotope signatures (δ30Si) together with concentrations of biogenic opal and organic carbon from the central (9° S) and northern (5° S) Peruvian margin reveal changes in diatom productivity and nutrient utilization during the past 20,000 years. The findings are based on a new approach using the difference between the δ30Si signatures of small (11-32μm) and large (〉150μm) diatom fractions (Δ30Sicoscino-bSi) in combination with the variance in diatom assemblages for reconstruction of past upwelling intensity. Combination of our records with two previously published records from the southern upwelling area off Peru (12-15° S) shows a general decoupling of the environmental conditions at the central and southern shelf mainly caused by a northward shift of the main upwelling cell from its modern position (12-15° S) towards 9° S during Termination 1. At this time only moderate upwelling intensity and productivity levels prevailed between 9° S and 12° S interpreted by a more northerly position of Southern Westerly Winds and the South Pacific Subtropical High. Furthermore, a marked decrease in productivity at 12-15° S during Heinrich Stadial 1 coincided with enhanced biogenic opal production in the Eastern Equatorial Pacific, which was induced by a southward shift of the Intertropical Convergence zone and enhanced northeasterly trade winds. Modern conditions were only established at the onset of the Holocene. Past changes in preformed δ30Si signatures of subsurface waters reaching the Peruvian Upwelling System did not significantly affect the preserved δ30Si signatures.

Description: The response of eddy kinetic energy (EKE) to the strengthening of Southern Hemisphere winds occurring since the 1950s is investigated with a global ocean model having a resolution of 1/12° in the Antarctic Circumpolar Current domain. The simulations expose regional differences in the relative importance of stochastic and wind-related contributions to inter-annual EKE changes. In the Pacific and Indian sectors the model captures the EKE variability observed since 1993 and confirms previous hypotheses of a lagged response to regional wind stress anomalies. Here, the multi-decadal trend in wind stress is reflected in an increase in EKE typically exceeding 5 cm2 sec-2 decade-1. In the western Atlantic EKE variability is mostly stochastic, is weakly correlated with wind fluctuations, and its multi-decadal trends are close to zero. The non-uniform distribution of wind-related changes in the eddy activity could affect the regional patterns of ocean circulation and biogeochemical responses to future climate change.

Description: Hyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been inferred to delimit the western extent of the continent-ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160 km long wide-angle seismic profile (Western Extension 1). Travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the S reflector. The S reflector correlates with the 6.0–7.0 km s−1 velocity contours, corresponding to peridotite serpentinization of 60–30%, respectively. West of the Peridotite Ridge, shallow and sparse Moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer, which increases in thickness from ~0.5 km at ~20 km west of the Peridotite Ridge to ~1.5 km, 35 km further west. P wave velocities increase smoothly and rapidly below top basement, to a depth of 2.8–3.5 km, with an average velocity gradient of 1.0 s−1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth.

Description: Interest in stratospheric aerosol and its role in climate have increased over the last decade due to the observed increase in stratospheric aerosol since 2000 and the potential for changes in the sulfur cycle induced by climate change. This review provides an overview about the advances in stratospheric aerosol research since the last comprehensive assessment of stratospheric aerosol was published in 2006. A crucial development since 2006 is the substantial improvement in the agreement between in situ and space-based inferences of stratospheric aerosol properties during volcanically quiescent periods. Furthermore, new measurement systems and techniques, both in situ and space based, have been developed for measuring physical aerosol properties with greater accuracy and for characterizing aerosol composition. However, these changes induce challenges to constructing a long-term stratospheric aerosol climatology. Currently, changes in stratospheric aerosol levels less than 20% cannot be confidently quantified. The volcanic signals tend to mask any nonvolcanically driven change, making them difficult to understand. While the role of carbonyl sulfide as a substantial and relatively constant source of stratospheric sulfur has been confirmed by new observations and model simulations, large uncertainties remain with respect to the contribution from anthropogenic sulfur dioxide emissions. New evidence has been provided that stratospheric aerosol can also contain small amounts of nonsulfate matter such as black carbon and organics. Chemistry-climate models have substantially increased in quantity and sophistication. In many models the implementation of stratospheric aerosol processes is coupled to radiation and/or stratospheric chemistry modules to account for relevant feedback processes

Description: The ice albedo feedback is one of the key factors of accelerated temperature increase in the high northern latitudes under global warming. This study assesses climate impacts and risks of idealized Arctic Ocean albedo modification (AOAM), a proposed climate engineering method, during transient cli- mate change simulations with varying representative concentration pathway (RCP) scenarios. We find no potential for reversing trends in all assessed Arctic climate metrics under increasing atmospheric CO2 con- centrations. AOAM only yields an initial offset during the first years after implementation. Nevertheless, sea ice loss can be delayed by 25(60) years in the RCP8.5(RCP4.5) scenario and the delayed thawing of perma- frost soils in the AOAM simulations prevents up to 40(32) Pg of carbon from being released by 2100. AOAM initially dampens the decline of the Atlantic Meridional Overturning and delays the onset of open ocean deep convection in the Nordic Seas under the RCP scenarios. Both these processes cause a subsurface warming signal in the AOAM simulations relative to the default RCP simulations with the potential to desta- bilize Arctic marine gas hydrates. Furthermore, in 2100, the RCP8.5 AOAM simulation diverts more from the 2005–2015 reference state in many climate metrics than the RCP4.5 simulation without AOAM. Considering the demonstrated risks, we conclude that concerning longer time scales, reductions in emissions remain the safest and most effective way to prevent severe changes in the Arctic.

Description: The temporal dynamics of the concentrations of nitrate (N), phosphate (P), and the N:P ratio in the upper water column (200-600m) of the Mediterranean (MED) Sea were investigated using observational data (~123,100 data points) collected between 1985 and 2014. The studied variables were found to evolve similarly in the western and eastern MED Sea. In both basins, the N concentration increased during the first part of the observational period (1985-1998), and the temporal trend of N was broadly consistent with the history of riverine and atmospheric nitrogen input from populated areas in Europe, with a lag period of 20years. In subsequent years, the N concentration was high and relatively constant between 1998 and 2005, after which N decreased gradually, although the decreasing trend was indistinct in the western basin. In particular, the trend of constant then declining N after 1998 is consistent with the history of pollutant nitrogen emissions from the European continent, allowing a 20 year lag following the introduction of regulation of pollutant nitrogen in the 1970s. The three-phase temporal transition in P in both basins was more consistent with the riverine phosphorus input, with a lag period of 20years. Our analysis indicates that the recent dynamics of N and P in the upper MED Sea has been sensitive to the dynamics of anthropogenic nitrogen and phosphorus input from atmospheric deposition and rivers

Description: Part of the kinetic energy that maintains ocean circulation cascades down to small scales until it is dissipated through mixing. While most steps of this downward energy cascade are well understood, an observational gap exists at horizontal scales of 103-101 m that prevents characterizing a key step in the chain: the transition from anisotropic internal wave motions to isotropic turbulence. Here we show that this observational gap can be covered using high-resolution multichannel seismic (HR-MCS) data. Spectral analysis of acoustic reflectors imaged in the Alboran Sea thermocline shows that this transition is likely caused by shear instabilities. In particular, we show that the averaged horizontal wavenumber spectra of the reflectors vertical displacements display three subranges that reproduce theoretical spectral slopes of internal waves [λx 〉 100 m], Kelvin-Helmholtz-type shear instabilities [100 m 〉 λx 〉 33 m], and turbulence [λx 〈 33 m], indicating that the whole chain of events is occurring continuously and simultaneously in the surveyed area.

Description: If unstructured meshes are refined to locally represent eddy dynamics in ocean circulation models, a practical question arises on how to vary the resolution and where to deploy the refinement. We propose to use the observed sea surface height variability as the refinement criterion. We explore the utility of this method (i) in a suite of idealized experiments simulating a wind-driven double gyre flow in a stratified circular basin and (ii) in simulations of global ocean circulation performed with FESOM. Two practical approaches of mesh refinement are compared. In the first approach the uniform refinement is confined within the areas where the observed variability exceeds a given threshold. In the second one the refinement varies linearly following the observed variability. The resolution is fixed in time. For the double gyre case it is shown that the variability obtained in a high-resolution reference run can be well captured on variable-resolution meshes if they are refined where the variability is high and additionally upstream the jet separation point. The second approach of mesh refinement proves to be more beneficial in terms of improvement downstream the midlatitude jet. Similarly, in global ocean simulations the mesh refinement based on the observed variability helps the model to simulate high variability at correct locations. The refinement also leads to a reduced bias in the upper-ocean temperature

Description: A three-dimensional, process-based model of the ocean's carbon and nitrogen cycles, including 13C and 15N isotopes, is used to explore effects of idealized changes in the soft-tissue biological pump. Results are presented from one preindustrial control run (piCtrl) and six simulations of the Last Glacial Maximum (LGM) with increasing values of the spatially constant maximum phytoplankton growth rate μmax, which accelerates biological nutrient utilization mimicking iron fertilization. The default LGM simulation, without increasing μmax and with a shallower and weaker Atlantic Meridional Overturning Circulation and increased sea ice cover, leads to 280 Pg more respired organic carbon (Corg) storage in the deep ocean with respect to piCtrl. Dissolved oxygen concentrations in the colder glacial thermocline increase, which reduces water column denitrification and, with delay, nitrogen fixation, thus increasing the ocean's fixed nitrogen inventory and decreasing δ15NNO3 almost everywhere. This simulation already fits sediment reconstructions of carbon and nitrogen isotopes relatively well, but it overestimates deep ocean δ13CDIC and underestimates δ15NNO3 at high latitudes. Increasing μmax enhances Corg and lowers deep ocean δ13CDIC, improving the agreement with sediment data. In the model's Antarctic and North Pacific Oceans modest increases in μmax result in higher δ15NNO3 due to enhanced local nutrient utilization, improving the agreement with reconstructions there. Models with moderately increased μmax fit both isotope data best, whereas large increases in nutrient utilization are inconsistent with nitrogen isotopes although they still fit the carbon isotopes reasonably well. The best fitting models reproduce major features of the glacial δ13CDIC, δ15N, and oxygen reconstructions while simulating increased Corg by 510–670 Pg compared with the preindustrial ocean. These results are consistent with the idea that the soft-tissue pump was more efficient during the LGM. Both circulation and biological nutrient utilization could contribute. However, these conclusions are preliminary given our idealized experiments, which do not consider changes in benthic denitrification and spatially inhomogenous changes in aeolian iron fluxes. The analysis illustrates interactions between the carbon and nitrogen cycles as well as the complementary constraints provided by their isotopes. Whereas carbon isotopes are sensitive to circulation changes and indicate well the three-dimensional Corg distribution, nitrogen isotopes are more sensitive to biological nutrient utilization.

Description: We show that inflows of oxygenated waters into sulfidic layers have a strong impact on biogeochemical transformation at oxic/anoxic transition zones. Taking the pelagic methane dynamics in the Gotland Basin as an example, we performed our studies when one of the largest inflows ever recorded entered the Baltic Sea in March 2015. An inflowing gravity current transported oxic waters into the sulfidic deep layers and freshly generated a near-bottom secondary redox interface. At the upper slope, where the inflowing water masses were vigorously turbulent and the main and secondary redox interfaces in close contact to each other, methane oxidation rates inside the transition zone were found to be higher compared to the weakly turbulent basin interior. At the main redox interface in the basin center, lateral intrusions of oxygenated waters into intermediate water depth may have stimulated the growth of the methanotrophic community and their activity.

Description: Eruptive activity at Turrialba volcano (Costa Rica) has escalated significantly since 2014, causing airport and school closures in the capital city of San José. Whether or not new magma is involved in the current unrest seems probable but remains a matter of debate as ash deposits are dominated by hydrothermal material. Here, we use high frequency gas monitoring to track the behavior of the volcano between 2014 and 2015, and to decipher magmatic vs. hydrothermal contributions to the eruptions. Pulses of deeply-derived CO2-rich gas (CO2/Stotal 〉 4.5) precede explosive activity, providing a clear precursor to eruptive periods that occurs up to two weeks before eruptions, which are accompanied by shallowly derived sulfur-rich magmatic gas emissions. Degassing modeling suggests that the deep magmatic reservoir is ~8-10 km deep, whereas the shallow magmatic gas source is at ~3-5 km. Two cycles of degassing and eruption are observed, each attributed to pulses of magma ascending through the deep reservoir to shallow crustal levels. The magmatic degassing signals were overprinted by a fluid contribution from the shallow hydrothermal system, modifying the gas compositions, contributing volatiles to the emissions, and reflecting complex processes of scrubbing, displacement, and volatilization. H2S/SO2 varies over two orders of magnitude through the monitoring period and demonstrates that the first eruptive episode involved hydrothermal gases whereas the second did not. Massive degassing ( 〉3000 T/day SO2 and H2S/SO2 〉 1) followed, suggesting boiling off of the hydrothermal system. The gas emissions show a remarkable shift to purely magmatic composition (H2S/SO2 〈 0.05) during the second eruptive period, reflecting the depletion of the hydrothermal system or the establishment of high temperature conduits bypassing remnant hydrothermal reservoirs, and the transition from phreatic to phreatomagmatic eruptive activity.

Description: The North Atlantic is an important basin for the global ocean's uptake of anthropogenic and natural carbon dioxide (CO2), but the mechanisms controlling this carbon flux are not fully understood. The air-sea flux of CO2, F, is the product of a gas transfer velocity, k, the air-sea CO2 concentration gradient, ΔpCO2, and the temperature- and salinity-dependent solubility coefficient, α. k is difficult to constrain, representing the dominant uncertainty in F on short (instantaneous to interannual) timescales. Previous work shows that in the North Atlantic, ΔpCO2 and k both contribute significantly to interannual F variability but that k is unimportant for multidecadal variability. On some timescale between interannual and multidecadal, gas transfer velocity variability and its associated uncertainty become negligible. Here we quantify this critical timescale for the first time. Using an ocean model, we determine the importance of k, ΔpCO2, and α on a range of timescales. On interannual and shorter timescales, both ΔpCO2 and k are important controls on F. In contrast, pentadal to multidecadal North Atlantic flux variability is driven almost entirely by ΔpCO2; k contributes less than 25%. Finally, we explore how accurately one can estimate North Atlantic F without a knowledge of nonseasonal k variability, finding it possible for interannual and longer timescales. These findings suggest that continued efforts to better constrain gas transfer velocities are necessary to quantify interannual variability in the North Atlantic carbon sink. However, uncertainty in k variability is unlikely to limit the accuracy of estimates of longer-term flux variability.

Description: Data assimilation was recently suggested to smooth out the sharp gradients that characterize the tropopause inversion layer (TIL) in systems that did not assimilate TIL-resolving observations. We investigate whether this effect is present in the ERA-Interim reanalysis and the European Centre for Medium-Range Weather Forecasts (ECMWF) operational forecast system (which assimilate high-resolution observations) by analyzing the 4D-Var increments and how the TIL is represented in their data assimilation systems. For comparison, we also diagnose the TIL from high-resolution GPS radio occultation temperature profiles from the COSMIC satellite mission, degraded to the same vertical resolution as ERA-Interim and ECMWF operational analyses. Our results show that more recent reanalysis and forecast systems improve the representation of the TIL, updating the earlier hypothesis. However, the TIL in ERA-Interim and ECMWF operational analyses is still weaker and farther away from the tropopause than GPS radio occultation observations of the same vertical resolution.

Description: Three active-source seismic refraction profiles are integrated with morphological and potential field data to place the first regional constraints on the structure of the Kermadec subduction zone. These observations are used to test contrasting tectonic models for an along-strike transition in margin structure previously known as the 32°S boundary. We use residual bathymetry to constrain the geometry of this boundary and propose the name Central Kermadec Discontinuity (CKD). North of the CKD, the buried Tonga Ridge occupies the forearc with VP 6.5–7.3 km s-1 and residual free-air gravity anomalies constrain its latitudinal extent (north of 30.5°S), width (110 ± 20 km) and strike (~005° south of 25°S). South of the CKD the forearc is structurally homogeneous down-dip with VP 5.7–7.3 km s-1. In the Havre Trough backarc, crustal thickness south of the CKD is 8-9 km, which is up-to 4 km thinner than the northern Havre Trough and at least 1 km thinner than the southern Havre Trough. We suggest that the Eocene arc did not extend along the current length of the Tonga-Kermadec trench. The Eocene arc was originally connected to the Three Kings Ridge and the CKD was likely formed during separation and easterly translation of an Eocene arc substrate during the early Oligocene. We suggest that the first-order crustal thickness variations along the Kermadec arc were inherited from before the Neogene and reflect Mesozoic crustal structure, the Cenozoic evolution of the Tonga-Kermadec-Hikurangi margin and along-strike variations in the duration of arc volcanism.

Description: Observational studies of Arctic stratospheric final warmings have shown that tropical/subtropical air masses can be advected to high latitudes and remain confined within a long-lived “frozen-in” anticyclone (FrIAC) for several months. It was suggested that the frequency of FrIACs may have increased since 2000 and that their interannual variability may be modulated by (i) the occurrence of major stratospheric warmings (mSSWs) in the preceding winter and (ii) the phase of the quasi-biennial oscillation (QBO). In this study, we tested these observational-based hypotheses for the first time using a chemistry climate model. Three 145 year sensitivity experiments were performed with the National Center of Atmospheric Research's Community Earth System Model (CESM): one control experiment including only natural variability, one with an extreme greenhouse gas emission scenario, and one without the QBO in the tropical stratosphere. In comparison with reanalysis, the model simulates a realistic frequency and characteristics of FrIACs, which occur under an abrupt and early winter-to-summer stratospheric circulation transition, driven by enhanced planetary wave activity. Furthermore, the model results support the suggestion that the development of FrIACs is favored by an easterly QBO in the middle stratosphere and by the absence of mSSWs during the preceding winter. The lower stratospheric persistence of background dynamical state anomalies induced by deep mSSWs leads to less favorable conditions for planetary waves to enter the high-latitude stratosphere in April, which in turn decreases the probability of FrIAC development. Our model results do not suggest that climate change conditions (RCP8.5 scenario) influence FrIAC occurrences.

Description: We present an integrated approach of the seismic structure and activity along the offshore SW Hellenic subduction from combined observations of marine and land seismic stations. Our imaging of the slab top topography from teleseismic receiver function analysis at ocean bottom seismometers supports a trenchward continuation of the along-dip slab faults beneath the Peloponnesus. We further show that their morphostructural control accounts for the backstepping of the thrust contact of the Mediterranean Ridge accretionary wedge over the upper plate. Local seismic activity offshore SW Peloponnesus constrained by ocean bottom seismometer observations reveals a correlation with specific features of the forearc: the Matapan Troughs. We study the Mw6.8 14.02.2008 interplate earthquake offshore SW Peloponnesus and show that its nucleation, rupture zone, and aftershocks sequence are confined to one slab panel between two adjacent along-dip faults and are thus controlled by not only the offshore slab top segmentation but also the upper plate sea-bottom morphology.

Description: Seafloor seepage of hydrocarbon-bearing fluids has been identified in a number of marine forearcs. However, temporal variations in seep activity and the structural and tectonic parameters that control the seepage often remain poorly constrained. Subduction-zone earthquakes for example, are often discussed to trigger seafloor seepage but causal links that go beyond theoretical considerations have not yet been fully established. This is mainly due to the inaccessibility of offshore epicentral areas, the infrequent occurrence of large earthquakes, and challenges associated with offshore monitoring of seepage over large areas and sufficient time periods. Here, we report visual, geochemical, geophysical, and modelling results and observations from the Concepción Methane Seep Area (offshore Central Chile) located in the rupture area of the 2010 Mw. 8.8 Maule earthquake. High methane concentrations in the oceanic water column and a shallow sub-bottom depth of sulfate penetration indicate active methane seepage. The stable carbon isotope signature of the methane and hydrocarbon composition of the released gas indicate a mixture of shallow-sourced biogenic gas and a deeper sourced thermogenic component. Pristine fissures and fractures observed at the seafloor together with seismically imaged large faults in the marine forearc may represent effective pathways for methane migration. Upper-plate fault activity with hydraulic fracturing and dilation is in line with increased normal Coulomb stress during large plate-boundary earthquakes, as exemplarily modelled for the 2010 earthquake. On a global perspective our results point out the possible role of recurring large subduction-zone earthquakes in driving hydrocarbon seepage from marine forearcs over long timescales.

Description: Paleoceanographical studies of Marine Isotope Stage (MIS) 11 have revealed higher-than-present sea surface temperatures (SSTs) in the North Atlantic and in parts of the Arctic but lower-than-present SSTs in the Nordic Seas, the main throughflow area of warm water into the Arctic Ocean. We resolve this contradiction by complementing SST data based on planktic foraminiferal abundances with surface salinity changes using hydrogen isotopic compositions of alkenones in a core from the central Nordic Seas. The data indicate the prevalence of a relatively cold, low-salinity, surface water layer in the Nordic Seas during most of MIS 11. In spite of the low-density surface layer, which was kept buoyant by continuous melting of surrounding glaciers, warmer Atlantic water was still propagating northward at the subsurface thus maintaining meridional overturning circulation. This study can help to better constrain the impact of continuous melting of Greenland and Arctic ice on high-latitude ocean circulation and climate.

Description: We investigate the individual and joint decadal variability of Southern Ocean state quantities, such as the strength of the Ross and Weddell Gyres, Drake Passage transport, and sea ice area, using the National Institute of Water and Atmospheric Research UK Chemistry and Aerosols (NIWA-UKCA) model and CMIP5 models. Variability in these quantities is stimulated by strong deep reaching convective events in the Southern Ocean, which produce an Antarctic Bottom Water-like water mass and affect the large-scale meridional density structure in the Southern Ocean. An increase in the (near) surface stratification, due to freshwater forcing, can be a precondition for subsequent strong convection activity. The combination of enhanced-gyre driven sea ice and freshwater export, as well as ongoing subsurface heat accumulation, lead to a time lag between changes in oceanic freshwater and heat content. This causes an ongoing weakening of the stratification until sudden strong mixing events emerge and the heat is released to the atmosphere. We find that strong convection reduces sea ice cover, weakens the subpolar gyres, increases the meridional density gradient and subsequently results in a positive Drake Passage transport anomaly. Results of available CMIP5 models confirm that variability in sea ice, Drake Passage transport, and the Weddell Gyre strength is enhanced if models show strong open ocean convective events. Consistent relationships between convection, sea ice, Drake Passage transport, and Ross Gyre strength variability are evident in most models, whether or not they host open ocean convection.

Description: The upwelling area off North-West Africa is characterized by high export production, high nitrate and low oxygen concentration in bottom waters. The underlying sediment consists of sands that cover most of the continental shelf. Due to their permeability sands allow for fast advective pore water transport and can exhibit high rates of nitrogen (N) loss via denitrification as reported for anthropogenically eutrophied regions. However, N loss from sands underlying naturally eutrophied waters is not well studied, and in particular, N loss from the North-West African shelf is poorly constrained. During two research cruises in April/May 2010/2011, sediment was sampled along the North-West African shelf and volumetric denitrification rates were measured in sediment layers down to 8 cm depth using slurry incubations with 15N-labeled nitrate. Areal N loss was calculated by integrating volumetric rates down to the nitrate penetration depth derived from pore water profiles. Areal N loss was neither correlated with water depth nor with bottom water concentrations of nitrate and oxygen but was strongly dependent on sediment grain size and permeability. The derived empirical relation between benthic N loss and grains size suggests that pore water advection is an important regulating parameter for benthic denitrification in sands and further allowed extrapolating rates to an area of 53,000 km2 using detailed sediment maps. Denitrification from this region amounts to 995 kt yr-1 (average 3.6 mmol m-2 d-1) which is 4 times higher than previous estimates based on diffusive pore water transport. Sandy sediments cover 50-60% of the continental shelf and thus may contribute significantly to the global benthic N loss.

Description: Landslides are common features in the vicinity of volcanic islands. In this contribution, we investigate landslides emplacement and dynamics around the volcanic island of Martinique based on the first scientific drilling of such deposits. The evolution of the active Montagne Pelée volcano on this island has been marked by three major flank-collapses that removed much of the western flank of the volcano. Subaerial collapse volumes vary from 2 to 25 km3 and debris avalanches flowed into the Grenada Basin. High-resolution seismic data (AGUADOMAR-1999, CARAVAL-2002, and GWADASEIS-2009) is combined with new drill cores that penetrate up to 430 m through the three submarine landslide deposits previously associated to the aerial flank-collapses (Site U1399, Site U1400, Site U1401, IODP Expedition 340, Joides Resolution, March–April 2012). This combined geophysical and core data provide an improved understanding of landslide processes offshore a volcanic island. The integrated analysis shows a large submarine landslide deposit, without debris avalanche deposits coming from the volcano, comprising up to 300 km3 of remobilized seafloor sediment that extends for 70 km away from the coast and covers an area of 2100 km2. Our new data suggest that the aerial debris avalanche deposit enter the sea but stop at the base of submarine flank. We propose a new model dealing with seafloor sediment failures and landslide propagation mechanisms, triggered by volcanic flank-collapse events affecting Montagne Pelée volcano. Newly recognized landslide deposits occur deeper in the stratigraphy, suggesting the recurrence of large-scale mass-wasting processes offshore the island and thus, the necessity to better assess the associated tsunami hazards in the region.

Description: Ocean acidification and decreasing seawater saturation state with respect to calcium carbonate (CaCO3) minerals have raised concerns about the consequences to marine organisms that build CaCO3 structures. A large proportion of benthic marine calcifiers incorporate Mg2+ into their skeletons (Mg-calcite), which, in general, reduces mineral stability. The relative vulnerability of some marine calcifiers to ocean acidification appears linked to the relative solubility of their shell or skeletal mineralogy, although some organisms have sophisticated mechanisms for constructing and maintaining their CaCO3 structures causing deviation from this dependence. Nevertheless, few studies consider seawater saturation state with respect to the actual Mg-calcite mineralogy (ΩMg-x) of a species when evaluating the effect of ocean acidification on that species. Here, a global dataset of skeletal mole % MgCO3 of benthic calcifiers and in situ environmental conditions spanning a depth range of 0 m (subtidal/neritic) to 5600 m (abyssal) was assembled to calculate in situ ΩMg-x. This analysis shows that 24% of the studied benthic calcifiers currently experience seawater mineral undersaturation (ΩMg-x 〈 1). As a result of ongoing anthropogenic ocean acidification over the next 200 to 3000 years, the predicted decrease in seawater mineral saturation will expose approximately 57% of all studied benthic calcifying species to seawater undersaturation. These observations reveal a surprisingly high proportion of benthic marine calcifiers exposed to seawater that is undersaturated with respect to their skeletal mineralogy, underscoring the importance of using species-specific seawater mineral saturation states when investigating the impact of CO2-induced ocean acidification on benthic marine calcification.

Description: An extreme cold sea surface temperature event occurred in the Atlantic cold tongue region in boreal summer 2009. It was preceded by a strong negative Atlantic meridional mode event associated with north-westerly wind anomalies along the equator from March to May. Although classical equatorial wave dynamics suggest that westerly wind anomalies should be followed by a warming in the eastern equatorial Atlantic, an abrupt cooling took place. In the literature two mechanisms—meridional advection of subsurface temperature anomalies and planetary wave reflection—are discussed as potential causes of such an event. Here, for the first time we use in situ measurements in addition to satellite and reanalysis products to investigate the contribution of both mechanisms to the 2009 cold event. Our results suggest that meridional advection is less important in cold events than in corresponding warm events, and, in particular, did not cause the 2009 cold event. Argo float data confirm previous findings that planetary wave reflection contributed to the onset of the 2009 cold event. Additionally, our analysis suggests that higher baroclinic modes were involved.

Description: Water mass exchange between the Arctic Ocean and the Norwegian-Greenland Seas has played an important role for the Atlantic thermohaline circulation and Northern Hemisphere climate. We reconstruct past water mass mixing and erosional inputs from the radiogenic isotope compositions of neodymium (Nd), lead (Pb), and strontium (Sr) at Ocean Drilling Program site 911 (leg 151) from 906 m water depth on Yermak Plateau in the Fram Strait over the past 5.2 Myr. The isotopic compositions of past bottom waters were extracted from authigenic oxyhydroxide coatings of the bulk sediments. Neodymium isotope signatures obtained from surface sediments agree well with present-day deepwater εNd signature of −11.0 ± 0.2. Prior to 2.7 Ma the Nd and Pb isotope compositions of the bottom waters only show small variations indicative of a consistent influence of Atlantic waters. Since the major intensification of the Northern Hemisphere Glaciation at 2.7 Ma the seawater Nd isotope composition has varied more pronouncedly due to changes in weathering inputs related to the waxing and waning of the ice sheets on Svalbard, the Barents Sea, and the Eurasian shelf, due to changes in water mass exchange and due to the increasing supply of ice-rafted debris (IRD) originating from the Arctic Ocean. The seawater Pb isotope record also exhibits a higher short-term variability after 2.7 Ma, but there is also a trend toward more radiogenic values, which reflects a combination of changes in input sources and enhanced incongruent weathering inputs of Pb released from freshly eroded old continental rocks.

Description: Silica is an essential element for marine life and plays a key role in the biogeochemistry of the ocean. Glacial activity stimulates rock weathering, generating dissolved silica that is exported to coastal areas along with meltwater. The magnitude of the dissolved silica export from large glacial areas such as the Greenland Ice Sheet is presently poorly quantified and not accounted for in global budgets. Here we present data from two fjord systems adjacent to the Greenland Ice Sheet which reveal a large export of dissolved silica by glacial meltwater relative to other macronutrients. Upscaled to the entire Greenland Ice Sheet, the export of dissolved silica equals 22 ± 10 Gmol Si yr−1. When the silicate-rich meltwater mixes with upwelled deep water, either inside or outside Greenland's fjords, primary production takes place at increased silicate to nitrate ratios. This likely stimulates the growth of diatoms relative to other phytoplankton groups.

Description: Much of modern climate science fails to consider millennium-scale processes, many of which may prove to be important for predicting the climate trajectory in the shorter term.

Description: Long-term monitoring over one year revealed high temporal variability of gas emissions at a cold seep in 1250 m water depth offshore Vancouver Island, British Columbia. Data from the North East Pacific Time series Underwater Networked Experiment observatory operated by Ocean Networks Canada were used. The site is equipped with a 260 kHz Imagenex sonar collecting hourly data, conductivity-temperature-depth sensors, bottom pressure recorders, current meter, and an ocean bottom seismograph. This enables correlation of the data and analyzing trigger mechanisms and regulating criteria of gas discharge activity. Three periods of gas emission activity were observed: (a) short activity phases of few hours lasting several months, (b) alternating activity and inactivity of up to several day-long phases each, and (c) a period of several weeks of permanent activity. These periods can neither be explained by oceanographic conditions nor initiated by earthquakes. However, we found a clear correlation of gas emission with bottom pressure changes controlled by tides. Gas bubbles start emanating during decreasing tidal pressure. Tidally induced pressure changes also influence the subbottom fluid system by shifting the methane solubility resulting in exsolution of gas during falling tides. These pressure changes affect the equilibrium of forces allowing free gas in sediments to emanate into the water column at decreased hydrostatic load. We propose a model for the fluid system at the seep, fueled by a constant sub-surface methane flux and a frequent tidally controlled discharge of gas bubbles into the ocean, transferable to other gas emission sites in the world's oceans.

Description: The postseismic deformation captured with continuous Global Positioning System (cGPS) monitoring following many recent mega-thrust events has been shown to be a signal composed of two dominant processes: afterslip on the plate interface and viscoelastic relaxation of the continental and oceanic mantles in response to the coseismic stress perturbation. Following the south-central Chile 2010 Maule Mw 8.8 earthquake, the time series from the regional cGPS network show a distinct curvature in the pathway of the horizontal motion that is not easily fit by a stationary decaying pattern of afterslip in combination with viscoelastic relaxation. Here we show that with realistic assumptions about the long-term decay of the afterslip signal, the postseismic signal can be decomposed into three first-order contributing processes: plate interface re-locking, plate interface afterslip, and mantle viscoelastic relaxation. From our analyses we conclude that the plate interface recovers its interseismic locking state rapidly (model space ranges between an instant recovery and a period of 1 year); a finding that supports laboratory experimental evidence as well as some recent studies of aftershocks and postseismic surface deformation. Furthermore, re-locking is the main cause of the curvature in the cGPS signal, and this study presents a plausible range of geodetic re-locking rates following a megathrust earthquake.

Description: Reading the sediment record in terms of past climates is challenging since linking climate change to the associated responses of sedimentary systems is not always straightforward. Here we analyze the erosional response of landscapes on the Tibetan Plateau to interglacial climate forcing. Using the theory of dynamical systems on Holocene time series of geochemical proxies, we derive a sedimentary response model that accurately simulates observed proxy variation in three lake records. The model suggests that millennial variations in sediment composition reflect a self‐organization of landscapes in response to abrupt climate change between 11.6 and 11.9 ka BP. The self‐organization is characterized by oscillations in sediment supply emerging from a feedback between physical and chemical erosion processes, with estimated response times between 3,000 to 18,000 years depending on catchment topography. The implications of our findings emphasize the need for landscape response models to decipher the paleoclimatic code in continental sediment records.

Description: Extensive field campaigns in the Mauritanian upwelling region between 2005 and 2016 provide the database for analyzing the seasonal variability of the eastern boundary circulation (EBC) and associated water mass distribution at 18°N. The data set includes shipboard upper ocean current, hydrographic, and oxygen measurements from nine research cruises conducted during upwelling (December to April) and relaxation (May to July) seasons. During the upwelling season, the EBC closely resembles a classical eastern boundary current regime, with a poleward undercurrent flowing beneath an equatorward coastal jet. In contrast, elevated poleward flow exceeding 30 cm/s and extending from the surface down to 250-m depth is observed during the relaxation season. The pronounced seasonal variability of the across-shore structure of the EBC can be related to local wind forcing and is in general agreement with Sverdrup balance. The EBC transport is correlated to the wind stress curl leading the transport by 7 days. The short lead time suggests a fast response of locally forced waves adjusting the EBC to wind forcing. The seasonal and vertical water mass distribution is presented based on hydrographic observations. The meridional oxygen distribution and corresponding water mass partitioning into South and North Atlantic Central Water masses reveal a possible northerly ventilation pathway in the deeper layers of the central water stratum. Our results suggest that the poleward surface flow and the poleward undercurrent both are a consequence of the cyclonic wind stress curl forcing and thus propose to name it the Mauritania Current.

Description: The South China Sea (SCS) is the largest semienclosed marginal sea in the western Pacific (WP) and connects to the west Pacific through the Luzon Strait (LU). In this study, we use the observation of transient tracer chlorofluorocarbon‐12 (CFC‐12) to calculate the ventilation time scales of the SCS, LU, and WP. The CFC‐12 and oxygen data are used together to identify the sandwiched structure vertically of the flows across the LU. The CFC‐12 and oxygen distributions reveal a pronounced decrease westward across the LU and a slight decrease southward in the transport of the SCS. The mean age gradient of the salinity minimum (Smin) water between the WP and the northern SCS could be a consequence of intensive mixing and entrainment of the inflow water from the WP. An expected difference in age between the LU and SSCS is verified to reflect the transit time for the given water layers in the SCS. Thus, a mean transit time of 77 ± 20 years is estimated for the intermediate water in the SCS interior.

Description: The ocean is estimated to contribute up to ~20% of global fluxes of atmospheric nitrous oxide (N2O), an important greenhouse gas and ozone depletion agent. Marine oxygen minimum zones contribute disproportionately to this flux. To further understand the partition of nitrification and denitrification and their environmental controls on marine N2O fluxes, we report new relationships between oxygen concentration and rates of N2O production from nitrification and denitrification directly measured with 15N tracers in the Eastern Tropical Pacific. Highest N2O production rates occurred near the oxic‐anoxic interface, where there is strong potential for N2O efflux to the atmosphere. The dominant N2O source in oxygen minimum zones was nitrate reduction, the rates of which were 1 to 2 orders of magnitude higher than those of ammonium oxidation. The presence of oxygen significantly inhibited the production of N2O from both nitrification and denitrification. These experimental data provide new constraints to a multicomponent global ocean biogeochemical model, which yielded annual oceanic N2O efflux of 1.7–4.4 Tg‐N (median 2.8 Tg‐N, 1 Tg = 1012 g), with denitrification contributing 20% to the oceanic flux. Thus, denitrification should be viewed as a net N2O production pathway in the marine environment.

Description: Meter‐scale AUV bathymetric mapping and ROV sampling of the entire 47 km‐long Alarcon Rise between the Pescadero and Tamayo transforms show that the shallowest inflated portion of the segment hosts all four active hydrothermal vent fields and the youngest, hottest, and highest effusion rate lava flows. This shallowest inflated part is located ∼1/3 of the way between the Tamayo and Pescadero transforms and is paved by a 16 km2 channelized flow that erupted from 9 km of en echelon fissures and is larger than historic flows on the East Pacific Rise or on the Gorda and Juan de Fuca Ridges. Starting ∼5 km south of the Pescadero transform, 6.5 km of the Alarcon Rise is characterized by faulted ridges and domes of fractionated lavas ranging from basaltic andesite to rhyolite with up to 77.3 wt % SiO2. These are the first known rhyolites from the submarine global mid‐ocean ridge system. Silicic lavas range from 〉11.7 ka, to as young as 1.1 ka. A basalt‐to‐basaltic andesite sequence and an andesite‐to‐dacite‐to‐rhyolite sequence are consistent with crystal fractionation but some intermediate basaltic andesite and andesite formed by mixing basalt with dacite or rhyolite. Magmatism occurred along the bounding Tamayo and Pescadero transforms as extensive channelized flows. The flows erupted from ring faults surrounding uplifted sediment hills inferred to overlie sills. The transforms are transtensional to accommodate magma migration from the adjacent Alarcon Rise.