Deep Ocean Sediments Research Articles

Paleo-atmospheric CO2 reconstructions from deep-ocean sediments

Paleo-atmospheric CO2 reconstructions from deep-ocean sediments

Predicting particle catchment areas of deep-ocean sediment traps using machine learning

Abstract. The ocean's biological carbon pump plays a major role in climate and biogeochemical cycles. Photosynthesis at the surface produces particles that are exported to the deep ocean by gravity. Sediment traps, which measure deep-carbon fluxes, help to quantify the carbon stored by this process. However, it is challenging to precisely identify the surface origin of particles trapped thousands of meters deep due to the influence of ocean circulation on the sinking path of carbon. In this study, we conducted a series of numerical Lagrangian experiments in the Porcupine Abyssal Plain region of the North Atlantic and developed a machine learning approach to predict the surface origin of particles trapped in a deep-ocean sediment trap. Our numerical experiments support the predictive performance of the machine learning approach, and surface conditions appear to provide valuable information for accurately predicting the source area, suggesting a potential application with satellite data. We also identify factors that potentially affect prediction efficiency, and we show that the best predictions are associated with low kinetic energy and the presence of mesoscale eddies above the trap. This new tool could provide a better link between satellite-derived sea surface observations and deep-ocean sediment trap measurements, ultimately improving our understanding of the biological-carbon-pump mechanism.

Late Pleistocene charcoal-rich sediments in the Puerto Rico Trench, possible remnants of gigantic wildfires in North-Eastern South America

An 18,000 km2 area of the Guyana Shield of South America, known as the Gran Sabana, is characterized by savannah vegetation that contrasts strongly with surrounding rain forests. Its origin has been linked to multiple episodes of forest fires. In this paper, we report a deposit encountered in two piston cores sampled during the CASEIS marine cruise, at 6000 m-depth at the southern entrance of the Puerto Rico Trench. The existence of this deposit call into question our understanding of the evolution of the Gran Sabana. We sampled its upper ∼60 cm, which comprises leaves and wood fragments, seeds, and charcoal, intermixed with siliciclastic sediment of igneous-metamorphic continental provenance. Radiocarbon dates of the vegetal fragments and charcoal range between 30 and 23 kyr BP. We propose that these deep ocean charcoal-rich sediments, located 2500 km offshore from the Orinoco Delta, may be remnants of gigantic forest fires of the Guyana Shield. We infer that this material was eroded during an extreme regional rainfall event, transported down rivers during one or more episodes to the Orinoco delta, and then travelled offshore via a deep turbiditic submarine system flowing on the Atlantic seafloor. It finally reached the Puerto Rico Trench, forming what we term, the Baracuda Trench Debrite. While published paleoclimatic analyses of lacustrine sediments have suggested that the Gran Sabana originated during episodes of wildfire ∼12.5 kyr BP ago, radiocarbon dating of Baracuda Trench Debrite suggests the occurrence of earlier fires in this region, leading us to re-evaluate the age of the Gran Sabana. These fires occurred during the low glacial maximum (LGM) and were likely promoted by climate change.

CO2 Storage in Deep Oceanic Sediments in the form of Hydrates: Energy Evaluation and Advantages Related to the Use of N2-Containing Mixtures

Permanent storage in suitable geological sites and/or deep aquifers is emerging as the most concrete and effective solution to mitigate its increasing concentration in the atmosphere. This article experimentally investigated its storage in deep marine environments in the form of hydrates. Gas hydrates were formed into a small-scale reactor, designed to reproduce marine seafloors. Hydrates were formed with pure carbon dioxide and with CO2-based gaseous mixture containing nitrogen at different concentrations, equal, respectively, to 30, 50 and 60 vol%. The results obtained for each mixture were then compared to each other. In particular, the quantity of hydrates formed was evaluated as a function of the thermodynamic conditions selected for the experiments. The energy spent for the process, calculated by considering the cooling and compression phases, was calculated for the unit quantity of hydrates formed and for the unit quantity of carbon dioxide stored. Finally, the energy requirements for gas cooling and for gas compression were calculated separately in order to comprehend the contribution of the single process for each mixture.

Identification of DDT+ in Deep Ocean Sediment and Biota in the Southern California Bight

Identification of DDT+ in Deep Ocean Sediment and Biota in the Southern California Bight

Quantification and isotopic characterization of benzene polycarboxylic acids (BPCA)-derived black carbon in deep oceanic sediments: Towards assessing pyrogenic inputs from marine sources

Methodologies based on benzene polycarboxylic acids (BPCA) selectively target the polymeric aromatic fraction of black carbon (BC) and are considered adequate to quantify pyrogenic inputs in environmental samples such as soils, lakes, and marine dissolved organic carbon. However, the usefulness of these methodologies to quantify BPCA-derived BC in deep-sea sediments has not been fully evaluated. In this manuscript we describe and validate a procedure to quantify BPCAs in deep oceanic sediments with very low organic carbon content. The resulting analytical procedure has produced reproducible quantitative data for BPCAs over a period of 10 months (coefficient of variation, CV = 6.4 − 6.6%). The stable carbon isotopes (δ13C) of BC_BPCA have been characterized using an LC Isolink™-irMS system with an accuracy better than 0.5‰. The quantitative and isotopic composition of several marine sediments has been characterized to investigate the relative contributions of marine/diagenetic and continental/pyrogenic sources to the BC accumulated in oceanic sediments from different contexts, ranging from upwelling systems to remote oceanic locations. Overall, a significant fraction of the sedimentary BC is of marine origin and should be considered in inventories of pyrogenic materials accumulated in the world oceans. However, the continental/pyrogenic sources can be largely dominant in marine settings with large inputs of pyrogenic materials.

Controls on terrigenous sediment supply to northwestern South China Sea based on a sediment trap record at Xisha Trough

The South China Sea (SCS) is the largest marginal sea in the western Pacific and comprises an ideal natural laboratory for studying deep-sea processes. Previous investigations in the region have primarily focused on surface currents and to a lesser extent on intermediate currents, while observations of bottom currents are scarce, hindering our understanding of deep ocean sediment transport and deposition mechanisms. Here, we carried out grain size measurement and end-member modelling analysis (EMMA) on samples collected by a sediment trap at a water depth of 1500 m in the Xisha Trough in the northwestern South China Sea. Grain size data shows that terrigenous material has particle sizes ranging from 0.3 μm to 745.9 μm and a peak at 4–11 μm. The EMMA result suggests that finer sediment accounting for an average of 48.2% of the sediment represents material transported by deep water currents (End-member 1; EM1), and this is primary sediment source in the region. Coarser sediment contributing to 32.9% of the sediment is the second most significant source (EM3), representing the deposits of surface currents influenced by monsoons and deposited close to the source. A third sediment type (EM2) constituting approximately 18.9% of the sediment exhibits a wide range of grain sizes and rapid changes, indicating transport in the bottom nepheloid layer triggered by anticyclonic eddies. Our study identifies three primary sediment transport mechanisms to the northwestern SCS, bridging present-day observations with paleoceanography and providing a crucial basis for future research on paleoceanographic reconstructions.

Diversity and enzymatic, biosurfactant and phytotoxic activities of culturable Ascomycota fungi present in marine sediments obtained near the South Shetland Islands, maritime Antarctica.

We studied the culturable fungal community recovered from deep marine sediments in themaritime Antarctic, and assessed their capabilities to produce exoenzymes, emulsifiers and metabolites with phytotoxic activity. Sixty-eight Ascomycota fungal isolates were recovered and identified. The most abundant taxon recovered was the yeast Meyerozyma guilliermondii, followed by the filamentous fungi Penicillium chrysogenum, P. cf. palitans, Pseudeurotium cf. bakeri, Thelebolus balaustiformis, Antarctomyces psychrotrophicus and Cladosporium sp. Diversity indices displayed low values overall, with the highest values obtained at shallow depth, decreasing to the deepest location sampled. Only M. guilliermondii and P. cf. palitans were detected in the sediments at all depths sampled, and were the most abundanttaxa at all sample sites. The most abundant enzymes detected were proteases, followed by invertases, cellulases, lipases, carrageenases, agarases, pectinases and esterases. Four isolates showed good biosurfactant activity, particularly the endemic species A. psychrotrophicus. Twenty-four isolates of P. cf. palitans displayed strong phytotoxic activities against the models Lactuca sativa and Allium schoenoprasum. The cultivable fungi recovered demonstrated good biosynthetic activity in the production of hydrolytic exoenzymes, biosurfactant molecules and metabolites with phytotoxic activity, reinforcing the importance of documenting the taxonomic, ecological and biotechnological properties of fungi present in deep oceanic sediments of the Southern Ocean.

Did a Kilonova Set Off in Our Galactic Backyard 3.5 Myr ago?

The recent detection of the live isotopes 60Fe and 244Pu in deep ocean sediments dating back to the past 3–4 Myr poses a serious challenge to the identification of their production site(s). While 60Fe is usually attributed to standard core-collapse supernovae, actinides are r-process nucleosynthesis yields, which are believed to be synthesized in rare events, such as special classes of supernovae or binary mergers involving at least one neutron star. Previous works concluded that a single binary neutron star merger cannot explain the observed isotopic ratio. In this work, we consider a set of numerical simulations of binary neutron star mergers producing long-lived massive remnants expelling both dynamical and spiral-wave wind ejecta. The latter, due to a stronger neutrino irradiation, also produce iron-group elements. Assuming that large-scale mixing is inefficient before the fading of the kilonova remnant and that the spiral-wave wind is sustained over a 100–200 ms timescale, the ejecta emitted at mid-high latitudes provide a 244Pu over 60Fe ratio compatible with observations. The merger could have happened 80–150 pc away from the Earth and between 3.5 and 4.5 Myr ago. We also compute expected isotopic ratios for eight other live radioactive nuclides showing that the proposed binary neutron star merger scenario is distinguishable from other scenarios proposed in the literature.

Temporal changes of 137Cs concentrations in the Far Eastern Seas: partitioning of 137Cs between overlying waters and sediments

Deep-ocean sediments, similarly to seawater, are important reservoirs of 137Cs, an anthropogenic radionuclide with a relatively long half-live found in the Earth system. To better understand the geochemical behaviour of 137Cs in the ocean, we examined the temporal changes of 137Cs activity concentrations in the overlying waters and in sediments from the Far Eastern Seas (Sea of Japan, SOJ, and Okhotsk Sea, OS) during the period of 1998–2021. The 137Cs activity levels showed exponential changes during the observed period. The decay-corrected change rates of 137Cs in deep waters of SOJ exhibited a slow increase, while 137Cs levels in seawater and sediment in OS decreased gradually. This reflects a topographical difference, as SOJ is a semi-closed sea, whereas OS receives continuously inflow of subarctic waters. It was confirmed that 137Cs released after the Fukushima Dai-ichi Nuclear Power Plant accident was rapidly transported into the deep waters of the SOJ. To elucidate the transfer processes of 137Cs from seawater to sediment, we discussed the temporal changes of the partition coefficients (Kd) of 137Cs between the overlying water and the surface sediment. In shallow areas (< 1500 m water depth), Kd values were almost constant within the sampling periods, although the temporal changes in the Kd values occurred in deeper waters (> 2500 m depth). The Kd values increased with increasing depth, which may reflect a pressure effect as a possible mechanism. These findings suggest that chemical processes may be important factors controlling the transport of 137Cs between seawater and sediment, although more complicated phenomena occurred in deep waters and sediments of the SOJ (> 3000 m depth).

High variability and exceptionally low thermal conductivities in nearshore sediments: a case study from the Eckernförde Bay

Heat flow measurements are a standard technique in Geophysics both onshore and offshore. Recently, such measurements became increasingly important in shallow waters. The increasing amount of offshore power installations makes it necessary to have a good knowledge about the subsurface heat flow and the thermal properties of the sediments to optimize the construction of the necessary powerlines. While the thermal properties are well studied for deep ocean sediments, only few published data exist for nearshore sediments. In this study, we investigate the sediment temperatures and thermal conductivities of nearshore sediments in the German part of the Baltic Sea. The shallow sediment temperatures reflect the interplay of the response to the seasonal cycle in connection with the sediments’ thermal conductivity. We find thermal conductivity values ranging from 0.67 to 3.34 W/(m*K) for the sediments down to ~ 4.2 m below seafloor. This variability exceeds that of conservative estimates widely used for coastal sediments and is also much higher than the variability found in the deep oceans. Sandy sediments show thermal conductivities larger than 1 W/(m*K) whereas organic-rich muds have lower values (< 1 W/(m*K)). Furthermore, the thermal conductivities seem to decrease with increasing free gas content in the sediment. The latter needs to be confirmed by further investigations.

Evaluating liquid CO2 hydrate formation kinetics, morphology, and stability in oceanic sediments on a lab scale using top injection

Storing captured CO2 into the deep oceanic sediments as CO2 hydrates offers a potential approach to reducing atmospheric carbon emissions. However, two major challenges are slow CO2 hydrate formation kinetics and lack of an understanding of CO2 hydrate stability dynamics. To counter these challenges, the conversion of high-pressure liquid CO2 (LCO2) to CO2 hydrates has been experimentally explored [with and without a hydrate kinetic promoter L-tryptophan] in a lab-scale reactor embedded with an artificial seabed. The stability of these CO2 hydrates was also evaluated by submerging them in the aqueous phase (mimicking hydrostatic pressure of fluid) at thermodynamic conditions found at an oceanic depth of 1 km [3–4 °C, 10 MPa] for 14 days. In-situ high-pressure Raman spectroscopy was used to estimate the real-time solubility of CO2 in the aqueous medium, and an image processing thresholding technique was used for detecting CO2 hydrate formation/dissociation patterns in sediments during the stability testing.The lab-scale experimental results indicate that in comparison to gaseous CO2, the liquid CO2 forms hydrates much faster [60 times] in the sediments. These CO2 hydrates exhibited decent stability over 14 days when submerged in an aqueous medium [3–4 °C, 10 MPa]. At the end of the stability test, hydrates were dissociated by thermal stimulation to assess the CO2 hydrates inside the sediments qualitatively. A significant decrease in the height of the sand bed [6.0 cm to 5.5 cm] and the evolution of CO2 across the sediments confirmed the presence of a good number of hydrates inside the sediments.

Depth-dependent Variations in Elemental and Mineral Distribution in the Deep Oceanic Floor Sediments (WP21GPC04) near the Mariana Trench in the Western Pacific Ocean

Depth-dependent Variations in Elemental and Mineral Distribution in the Deep Oceanic Floor Sediments (WP21GPC04) near the Mariana Trench in the Western Pacific Ocean

Dispersal and Distribution of Thermophilic Endospores in Deep-Sea Ecosystems

The distance-decay relationship is a central concept in biogeography and spatial biodiversity, describing how two distinct entities decrease in similarity as the distance between them increases. The decay of community similarity with geographical distance is driven by multiple factors, such as gene drift, environmental selection, and the accumulation of mutations over time. While the distance-decay relationship has been recognized for several decades, there are certain circumstances where the biogeographical patterns of certain species and communities cannot be predicted by this relationship. The example addressed in this project is the case of thermophilic endospore-forming bacteria found in permanently cold deep ocean sediments. Thermophilic endospores (thermospores) are routinely found on the deep ocean floor, a permanently cold environment that does not support their metabolic activity (Hubert et al. 2009). Thermospores are metabolically dormant states developed by some thermophilic bacteria having optimal growth temperatures between 40°C and 70°C. There is evidence that these heat-loving bacteria originate from the deep subsurface and are transported upward to the deep ocean via geological features of the oceanic crust including geofluid fluxes in high-temperature axial systems and natural hydrocarbon seeps (Gittins et al. 2022). Due to the ability of endospores to stay viable for thousands of years and resist a wide range of physicochemical stressors, they can disperse over long distances while remaining unaffected by changing factors such as selection, drift, or mutation (Gittins et al. 2022; Fig. 1). Consequently, these thermospores have the potential to challenge the distance-decay relationship and exhibit unique biogeographical patterns. This work quantifies endospores in sediment cores at and around hydrocarbon seeps using cores from deep-sea Scotian Slope sediments. Given the challenge of quantifying specific groups of endospores (e.g., thermophiles but not mesophiles), high-temperature germination assays that allow tracking of an exponential increase in signal as spores in marine sediment samples germinate and grow can be used. Distinct exponential increases can be attributed to different populations of germinated thermospores in the post-germination growth phase and are being monitored through measurements of sulfate reduction rates (using radiolabelled 35SO4), strain-specific quantitative PCR (qPCR), and fluorescence in-situ hybridization (FISH) (Rezende et al. 2017). These measurements revealed growth dynamics enabling an estimation of the initial cell numbers using exponential functions. These approaches are being applied to samples obtained via push coring using a remotely operated vehicle at different distances from deep sea hydrocarbon seeps, to test for the presence of abundance gradients of different thermospores. Current results already show a difference in thermospore abundance between sites at different distances from the seep. Quantitative tracking of the dispersal of thermospores by oceanic currents in the deep sea provides an excellent opportunity to investigate their distribution and potential to colonize more remote habitats, improving the understanding of microbial biodiversity and biogeography on our planet. By investigating thermospores and their interactions with the geophysical features of the deep sea and deep subsurface, this study aims to challenge the canonical idea that speciation occurs with distance and presents new perspectives on the theme of microbial biogeography. The evidence supporting the subsurface origin of thermophilic endospores, along with the proposed model for their dispersal and distribution, will contribute toward improving an integrated understanding of ecology and geology and their intersection in the microbial realm.

Investigating High-Pressure Liquid CO2 Hydrate Formation, Dissociation Kinetics, and Morphology in Brine and Freshwater Static Systems

Carbon capture and storage [CCS] is crucial for mitigating CO2 emissions. One of the potential CCS concepts is to compress and store the captured CO2 into deep oceanic sediments as gas hydrates. However, seawater is highly saline [brine], which may impair the formation/dissociation kinetics and storage of CO2 hydrates. Therefore, it is essential to understand the liquid CO2 [LCO2] hydrate formation and dissociation kinetics in static brine systems. In this experimental study, we have examined the formation/dissociation kinetics and morphology of high-pressure LCO2 hydrates in brine using a static [unstirred] high-pressure crystallizer at deep oceanic [1 km] thermodynamic conditions [10 MPa, 1–2 °C]. The results are compared with [unstirred/stirred] freshwater systems with/without hydrate promoters. Three key stages have been identified in the experiments: nucleation [stage 1], LCO2-hydrate-brine film formation [stage 2], and LCO2-hydrate-brine film breakage [stage 3]. In the absence of stirring, the formation of the LCO2-hydrate-brine film resists the mass transfer of LCO2 into the brine, and most likely, the volume expansion during hydrate formation causes the LCO2-hydrate-brine film to break. New hydrate morphological growth patterns have been identified. It was estimated that the hydrate conversion in the freshwater system was higher [27.5% (±3.04%) in 21.1 (±1.26) h] compared to the brine system [25.0% in 24.2 (±0.58) h]. LCO2 hydrates dissociate faster in brine [1.7 (±0.14) h] compared to the freshwater system [5.7 (±1.77) h]. Finally, the presence of the eco-friendly hydrate promoter 500 ppm l-tryptophan can delay the dissociation process.

Fossil coccolith morphological attributes as a new proxy for deep ocean carbonate chemistry

Abstract. Understanding the variations in past ocean carbonate chemistry is critical to elucidating the role of the oceans in balancing the global carbon cycle. The fossil shells from marine calcifiers present in the sedimentary record are widely applied as past ocean carbon cycle proxies. However, the interpretation of these records can be challenging due to the complex physiological and ecological response to the carbonate system during an organisms' life cycle and the potential for preservation at the seafloor. Here we present a new dissolution proxy based on the morphological attributes of coccolithophores from the Noëlaerhabdaceae family (Emiliania huxleyi &gt; 2 µm, and small Gephyrocapsa spp.). To evaluate the influences of coccolithophore calcification and coccolith preservation on fossil morphology, we measured morphological attributes, mass, length, thickness, and shape factor (ks) of coccoliths in a laboratory dissolution experiment and surface sediment samples from the South China Sea. The coccolith morphological data in surface sediments were also analyzed with environment settings, namely surface temperature, nutrients, pH, chlorophyll a concentration, and carbonate saturation of bottom water by a redundancy analysis. Statistical analysis indicates that carbonate saturation of the deep ocean explains the highest proportion of variation in the morphological data instead of the environmental variables of the surface ocean. Moreover, the dissolution trajectory in the ks vs. length of coccoliths is comparable between natural samples and laboratory dissolution experiments, emphasizing the importance of carbonate saturation on fossil coccolith morphology. However, the mean ks alone cannot fully explain the main variations observed in our work. We propose that the normalized ks variation (σ/ks), which is the ratio between the standard deviation of ks (σ) and the mean ks, could reflect different degrees of dissolution and size-selective dissolution, influenced by the assemblage composition. Applied together with the σ/ks ratio, the ks factor of fossil coccoliths in deep ocean sediments could be a potential proxy for a quantitative reconstruction of past carbonate dissolution dynamics.

Search for Novel Halophilic and Halotolerant Producers of Antimicrobial Compounds in Various Extreme Ecosystems

Abstract—The recent increase of antibiotic resistance in pathogenic microorganisms stimulated interest in the search for new antimicrobial compounds and their producers. Among the latter, halophilic microorganisms are considered to be the most promising group, since actinomycetes, producers of the majority of the known clinically important classes of antibiotics, are widely represented in this group. The present work reports the results of the search with three different approaches for new antimicrobial compounds in halophilic and halotolerant microorganisms inhabiting three different types of extreme ecosystems. Metagenomic analysis of microbial communities of subsurface thermal mineral waters revealed biosynthetic clusters of putative antimicrobial compounds, which belong to bacteria and archaea of uncultured lineages. Enrichment cultures with antimicrobial activity were obtained from the cold, deep oceanic sediments; analysis of their phylogenetic diversity resulted in identification of the potential producers of antimicrobial compounds as the members of class Desulfuromonadia. Finally, antimicrobial activity of a new species of soil natronophilic streptomycetes, Streptomyces sp. ACA25, was characterized; it was active only against gram-positive bacteria. The genome of this organism was sequenced, and the pathways for biosynthesis of polypeptide, polyketide, and beta-lactam antibiotics were identified. Active fractions of the ACA25 culture, containing antimicrobial compounds of polyketide and beta-lactam nature, were obtained. The active polyketide was identified as rosamycin, an antibiotic of the macrolide structural group. However, the fact that it did not inhibit bacterial translation highlighted structural differences between the new polyketide and rosamycin.

Estimating Coccolithophore PIC:POC Based on Coccosphere and Coccolith Geometry

AbstractThe marine biological calcification and photosynthesis, which can produce particulate inorganic and organic carbon (PIC and POC), have the opposite effects on seawater pCO2. Coccolithophores are a kind of marine unicellular algae with both of the two biological processes, and PIC and POC productions of them can shape the water column rain ratio as a dominant driver for Earth's carbon cycle. Thus, the changes in ancient coccolithophore PIC:POC can be important for the paleoceanographic and paleoclimatic studies of carbon cycle modeling. However, ancient coccolithophore PIC:POC is poorly constrained because of the occasional occurrences of intact coccospheres in deep ocean sediments, as detached coccoliths are commonly the remnants of fossilized coccolithophores. Here, we carry out the biometric analysis of coccosphere and coccolith from the living cells of Emiliania huxleyi and Gephyrocapsa oceanica in the South China Sea, and confirm a significant correlationship between their PIC:POC and lateral coccolith aspect ratio (ARL). ARL here is defined by the ratio of mean coccolith thickness with respect to coccolith length. A linear regression is given, (R2 = 0.59, n = 121), for the reconstruction of ancient Noelaerhabdaceae coccolithophore PIC:POC based on individual coccoliths in marine sediments. Based on this equation, we reconstruct ancient Noelaerhabdaceae coccolithophore PIC:POC since 14 million years ago (Ma) using published coccolith data, which reveal a long‐term decrease in PIC:POC from 7 to 4 Ma. We suggest that such a change in coccolithophore physiology may be induced by a simultaneous long‐term decline in seawater calcium concentration.

Vertical distribution of pH in the top ~10 m of deep-ocean sediments: Analysis of a unique dataset

We analyze, for the first time in the oceanographic literature, pH over the top ~10 m of the sediment (down to 11.9 m) in a deep-sea environment, together with the oxidation/reduction potential and concentrations of solid organic carbon (OC) and CaCO3. A total of 1157 sediment cores were collected from years 2000 to 2011 over &amp;gt;300,000 km2 in the South China Sea, at water depths up to 3702 m. We found that there were marked downward pH increases in the upper 2 m of the sediment (first 20-40 ka, corresponding to the geochemically active period). In deeper, older sediment (up to 200 ka), pH was generally less variable with depth but not uniform, and solid OC may have been consumed down to ≥10 m depth. This reflected interactions between in situ geochemical diagenetic processes, which tended to create vertical variations, and vertical diffusion of ions, which tended to even out vertical variability. In other words, there were slow diagenetic geochemical processes in the sediment layer below 2 m, and the effects of these in situ processes were partly offset by vertical diffusion. Overall, our study identified a previously unknown consistent pH difference between the upper 2 m of the sediment and the underlying layer down to ≥10 m, and suggested combinations of geochemical diagenetic processes and vertical diffusion of ions in the porewater to explain it. These results provide a framework for further studies of pH in the top multi-meter layer of the sediment in the World Ocean.

A global synthesis of high-resolution stable isotope data from benthic foraminifera of the last deglaciation

We present the first version of the Ocean Circulation and Carbon Cycling (OC3) working group database, of oxygen and carbon stable isotope ratios from benthic foraminifera in deep ocean sediment cores from the Last Glacial Maximum (LGM, 23-19 ky) to the Holocene (<10 ky) with a particular focus on the early last deglaciation (19-15 ky BP). It includes 287 globally distributed coring sites, with metadata, isotopic and chronostratigraphic information, and age models. A quality check was performed for all data and age models, and sites with at least millennial resolution were preferred. Deep water mass structure as well as differences between the early deglaciation and LGM are captured by the data, even though its coverage is still sparse in many regions. We find high correlations among time series calculated with different age models at sites that allow such analysis. The database provides a useful dynamical approach to map physical and biogeochemical changes of the ocean throughout the last deglaciation.