ALBERT

Description: The intraplate Hawaiian-Emperor Seamount Chain has long been considered a hotspot track generated by the motion of the Pacific plate over a deep mantle plume, and an ideal feature therefore for studies of volcanic structure, magma supply, plume-crust interaction, flexural loading, and upper mantle rheology. Despite their importance as a major component of the chain, the Emperor Seamounts have been relatively little studied. In this paper, we present the results of an active-source wide-angle reflection and refraction experiment conducted along an ocean-bottom-seismograph (OBS) line oriented perpendicular to the seamount chain, crossing Jimmu guyot. The tomographic P wave velocity model, using ∼20,000 travel times from 26 OBSs, suggests that there is a high-velocity (〉6.0 km/s) intrusive core within the edifice, and the extrusive-to-intrusive ratio is estimated to be ∼2.5, indicating that Jimmu was built mainly by extrusive processes. The total volume for magmatic material above the top of the oceanic crust is ∼5.3 × 104 km3, and the related volume flux is ∼0.96 m3/s during the formation of Jimmu. Under volcanic loading, the ∼5.3-km-thick oceanic crust is depressed by ∼3.8 km over a broad region. Using the standard relationships between Vp and density, the velocity model is verified by gravity modeling, and plate flexure modeling indicates an effective elastic thickness (Te) of ∼14 km. Finally, we find no evidence for large-scale magmatic underplating beneath the pre-existing crust.

Description: Primary consumers in aquatic ecosystems are frequently limited by the quality of their food, often expressed as phytoplankton elemental and biochemical composition. However, the effects of these food quality indicators vary across studies, and we lack an integrated understanding of how elemental (e.g. nitrogen, phosphorus) and biochemical (e.g. fatty acid, sterol) limitations interactively influence aquatic food webs. Here, we present the results of a meta-analysis using 〉100 experimental studies, confirming that limitation by N, P, fatty acids, and sterols all have significant negative effects on zooplankton performance. However, effects varied by grazer response (growth vs. reproduction), specific manipulation, and across taxa. While P limitation had greater effects on zooplankton growth than fatty acids overall, P and fatty acid limitation had equal effects on reproduction. Furthermore, we show that: nutrient co-limitation in zooplankton is strong; effects of essential fatty acid limitation depend on P availability; indirect effects induced by P limitation exceed direct effects of mineral P limitation; and effects of nutrient amendments using laboratory phytoplankton isolates exceed those using natural field communities. Our meta-analysis reconciles contrasting views about the role of various food quality indicators, and their interactions, for zooplankton performance, and provides a mechanistic understanding of trophic transfer in aquatic environments.

Description: Pronounced glacial and interglacial climate cycles characterized northern ecosystems during the Pleistocene. Our understanding of the resultant community transformations and past ecological interactions strongly depends on the taxa found in fossil assemblages. Here, we present a shotgun metagenomic analysis of sedimentary ancient DNA (sedaDNA) to infer past ecosystem-wide biotic composition (from viruses to megaherbivores) from the Middle and Late Pleistocene at the Batagay megaslump, East Siberia. The shotgun DNA records of past vegetation composition largely agree with pollen and plant metabarcoding data from the same samples. Interglacial ecosystems at Batagay attributed to Marine Isotope Stage (MIS) 17 and MIS 7 were characterized by forested vegetation (Pinus, Betula, Alnus) and open grassland. The microbial and fungal communities indicate strong activity related to soil decomposition, especially during MIS17. The local landscape likely featured more open, herb-dominated areas, and the vegetation mosaic supported birds and small omnivorous mammals. Parts of the area were intermittently/partially flooded as suggested by the presence of water-dependent taxa. During MIS 3, the sampled ecosystems are identified as cold-temperate, periodically flooded grassland. Diverse megafauna (Mammuthus, Equus, Coelodonta) coexisted with small mammals (rodents). The MIS 2 ecosystems existed under harsher conditions, as suggested by the presence of cold-adapted herbaceous taxa. Typical Pleistocene megafauna still inhabited the area. The new approach, in which shotgun sequencing is supported by metabarcoding and pollen data, enables the investigation of community composition changes across a broad range of taxonomic groups and inferences about trophic interactions and aspects of soil microbial ecology.

Description: Although submarine landslides have been studied for decades, a persistent challenge is the integration of diverse geoscientific datasets to characterise failure processes. We present a core-log-seismic integration study of the Tuaheni Landslide Complex to investigate intact sediments beneath the undeformed seafloor as well as post-failure landslide deposits. Beneath the undeformed seafloor are coherent reflections underlain by a weakly-reflective and chaotic seismic unit. This chaotic unit is characterised by variable shear strength that correlates with density fluctuations. The basal shear zone of the Tuaheni landslide likely exploited one (or more) of the low shear strength intervals. Within landslide deposits is a widespread “Intra-debris Reflector”, previously interpreted as the landslide’s basal shear zone. This reflector is a subtle impedance drop around the boundary between upper and lower landslide units. However, there is no pronounced shear strength change across this horizon. Rather, there is a pronounced reduction in shear strength ∼10-15 m above the Intra-debris Reflector that presumably represents an induced weak layer that developed during failure. Free gas accumulates beneath some regions of the landslide and is widespread deeper in the sedimentary sequence, suggesting that free gas may have played a role in pre-conditioning the slope to failure. Additional pre-conditioning or failure triggers could have been seismic shaking and associated transient fluid pressure. Our study underscores the importance of detailed core-log-seismic integration approaches for investigating basal shear zone development in submarine landslides.

Description: Phytoplankton stand at the base of the marine food-web, and play a major role in global carbon cycling. Rising CO2 levels and temperatures are expected to enhance growth and alter carbon:nutrient stoichiometry of marine phytoplankton, with possible consequences for the functioning of marine food-webs and the oceanic carbon pump. To date, however, the consistency of phytoplankton stoichiometric responses remains unclear. We therefore performed a meta-analysis on data from experimental studies on stoichiometric responses of marine phytoplankton to elevated pCO2 and 3–5° warming under nutrient replete and limited conditions. Our results demonstrate that elevated pCO2 increased overall phytoplankton C:N (by 4%) and C:P (by 9%) molar ratios under nutrient replete conditions, as well as phytoplankton growth rates (by 6%). Nutrient limitation amplified the CO2 effect on C:N and C:P ratios, with increases to 27% and 17%, respectively. In contrast to elevated pCO2, warming did not consistently alter phytoplankton elemental composition. This could be attributed to species- and study-specific increases and decreases in stoichiometry in response to warming. While our observed moderate CO2-driven changes in stoichiometry are not likely to drive marked changes in food web functioning, they are in the same order of magnitude as current and projected estimations of oceanic carbon export. Therefore, our results may indicate a stoichiometric compensation mechanism for reduced oceanic carbon export due to declining primary production in the near future

Description: In this study, we used stable isotopes of oxygen (δ18O), deuterium (δD), and dissolved inorganic carbon (δ13CDIC) in combination with temperature, salinity, oxygen and nutrient concentrations to characterize the coastal (71-78 °W) and an oceanic (82-98 °W) water masses (SAAW-Subantarctic Surface Water; STW-Subtropical Water; ESSW-Equatorial Subsurface water; AAIW-Antarctic Intermediate Water; PDW-Pacific Deep Water) of the Southeast Pacific (SEP). The results show that δ18O and δD can be used to differentiate between SAAW-STW, SAAW-ESSW and ESSW-AAIW. δ13CDIC signatures can be used to differentiate between STW-ESSW (oceanic section), SAAW-ESSW, ESSW-AAIW and AAIW-PDW. Compared with the oceanic section, our new coastal section highlights differences in both the chemistry and geometry of water masses above 1000 m. Previous paleoceanographic studies using marine sediments from the SEP continental margin used the present-day hydrological oceanic transect to compare against, as the coastal section was not sufficiently characterized. We suggest that our new results of the coastal section should be used for past characterizations of the SEP water masses that are usually based on continental margin sediment samples.

Description: Closure of the Central American Seaway (CAS) and hydrology of the Caribbean Sea triggered Northern Hemisphere Glaciation and played an important role in the Pliocene to modern-day climate re-establishing the deep and surface ocean currents. New data on Mn/Ca obtained with femtosecond laser ablation inductively coupled plasma mass spectrometry on well-preserved tests of the epibenthic foraminifer Cibicidoides wuellerstorfi and infaunal C. mundulus contribute to the interpretation of paleoenvironmental conditions of the Caribbean Sea between 5.2 and 2.2 Ma (million years) across the closure of the CAS. Hydrothermal activity at the Lesser Antilles may be a primary source of Mn in the well-oxygenated Plio-Pleistocene Caribbean Sea. Incorporation of Mn in the benthic foraminifer shell carbonate is assumed to be affected by surface ocean nutrient cycling, and may hence be an indicator of paleoproductivity. Key Points - Femtosecond-laser ablation inductively coupled plasma mass spectrometry provides a new approach on distinguishing Mn of the ontogenetic shell calcite from Mn of the authigenic coatings - Ontogenetic Mn within the foraminifer shell calcite may result from the regional nutrient cycle - Mn in the deep eastern Caribbean Sea may mainly derive from hydrothermal sources along the Antilles Island Arc

Description: Shifts in microbial communities and their functioning in response to environmental change result from contemporary interspecific and intraspecific diversity changes. Interspecific changes are driven by ecological shifts in species composition, while intraspecific changes are here assumed to be dominated by evolutionary shifts in genotype frequency. Quantifying the relative contributions of interspecific and intraspecific diversity shifts to community change thus addresses the essential, yet understudied question as to how important ecological and evolutionary contributions are to total community changes. This debate is to date practically constrained by (a) a lack of studies integrating across organizational levels and (b) a mismatch between data requirements of existing partitioning metrics and the feasibility to collect such data, especially in microscopic organisms like phytoplankton. We experimentally assessed the relative ecological and evolutionary contributions to total phytoplankton community changes using a new design and validated its functionality by comparisons to established partitioning metrics. We used a community of coexisting Emiliania huxleyi and Chaetoceros affinis with initially nine genotypes each. First, we exposed the community to elevated CO2 concentration for 80 days (~50 generations) to induce interspecific and intraspecific diversity changes and a total abundance change. Second, we independently manipulated the induced interspecific and intraspecific diversity changes in an assay to quantify the corresponding ecological and evolutionary contributions to the total change. Third, we applied existing partitioning metrics to our experimental data and compared the outcomes. Total phytoplankton abundance declined to one-fifth in the high CO2 exposed community compared to ambient conditions. Consistently across all applied partitioning metrics, the abundance decline could predominantly be explained by ecological shifts and to a low extent by evolutionary changes. We discuss potential consequences of the observed community changes on ecosystem functioning. Furthermore, we explain that the low evolutionary contributions likely resulted of intraspecific diversity changes that occurred irrespectively of CO2. We discuss how the assay could be upscaled to more realistic settings, including more species and drivers. Overall, the presented calculations of eco-evolutionary contributions to phytoplankton community changes constitute another important step towards understanding future phytoplankton shifts, and eco-evolutionary dynamics in general.

Description: The ongoing development of the Global Carbon Project (GCP) global methane (CH4) budget shows a continuation of increasing CH4 emissions and CH4 accumulation in the atmosphere during 2000–2017. Here, we decompose the global budget into 19 regions (18 land and 1 oceanic) and five key source sectors to spatially attribute the observed global trends. A comparison of top-down (TD) (atmospheric and transport model-based) and bottom-up (BU) (inventory- and process model-based) CH4 emission estimates demonstrates robust temporal trends with CH4 emissions increasing in 16 of the 19 regions. Five regions—China, Southeast Asia, USA, South Asia, and Brazil—account for 〉40% of the global total emissions (their anthropogenic and natural sources together totaling 〉270 Tg CH4 yr−1 in 2008–2017). Two of these regions, China and South Asia, emit predominantly anthropogenic emissions (〉75%) and together emit more than 25% of global anthropogenic emissions. China and the Middle East show the largest increases in total emission rates over the 2000 to 2017 period with regional emissions increasing by 〉20%. In contrast, Europe and Korea and Japan show a steady decline in CH4 emission rates, with total emissions decreasing by ~10% between 2000 and 2017. Coal mining, waste (predominantly solid waste disposal) and livestock (especially enteric fermentation) are dominant drivers of observed emissions increases while declines appear driven by a combination of waste and fossil emission reductions. As such, together these sectors present the greatest risks of further increasing the atmospheric CH4 burden and the greatest opportunities for greenhouse gas abatement.

Description: Present estimates of the biogeochemical cycles of calcium, strontium and potassium in the ocean reveal large imbalances between known input and output fluxes. Using pore fluid, incubation and solid sediment data from North Pacific multi-corer cores we show that, contrary to the common paradigm, the top centimetres of abyssal sediments can be an active site of authigenic precipitation of clay minerals. In this region, clay authigenesis is the dominant sink for potassium and strontium and consumes nearly all calcium released from benthic dissolution of calcium carbonates. These observations support the idea that clay authigenesis occurring over broad regions of the world ocean may be a major buffer for ocean chemistry on the time scale of the ocean overturning circulation, and key to the long-term stability of Earth’s climate. Key Points North Pacific red clay sediments are a sink for marine calcium, strontium and potassium Authigenic formation of clay minerals is prevalent in pelagic sediments throughout the North Pacific The main mechanism for clay formation is recrystallisation of aluminosilicates, neoformation can occur in biogenic silica rich sediments