Search

Carbon dioxide removal from the atmosphere and storage over long times scales in terrestrial and marine reservoirs is urgently needed to limit global warming and for sustainable management of the global carbon cycle. Ocean alkalinity enhancement by the artificial addition of carbonate minerals to the seafloor has been proposed as a method to sequester atmospheric CO2 and store it in the ocean as dissolved bicarbonate. Here, a reaction-transport model is used to scrutinize the efficacy of calcite addition and dissolution at a well-studied site in the southwestern Baltic Sea – a brackish coastal water body in northern Europe. We find that most calcite is simply buried without dissolution under moderate addition rates. Applying the model to other sites in the Baltic Sea suggests that dissolution rates and efficiencies are higher in areas with low salinity and undersaturated bottom waters. A simple box model predicts a tentative net CO2 uptake rate from the atmosphere of 3.2 megatonnes of carbon dioxide per year for the wider Baltic Sea after continually adding calcite to muddy sediments for 10 years. More robust estimates now require validation by field studies.

We document an apparent downward displacement of the Matuyama‐Brunhes magnetic reversal by ∼20 m at Scotia Sea International Ocean Discovery Program Site U1538 (Pirie Basin) by comparison with the well‐defined paleomagnetic record at nearby Site U1537 (Dove Basin). Detailed stratigraphic correlation between the two sites is possible due to similar lithologic variations. However, the two sites have distinctly different porewater geochemistry. Notably, Site U1538 indicates a greater demand for electron acceptors to oxidize organic carbon and Fe 2+ enrichment below the depth of SO 4 2− depletion. Magnetic parameters indicate enrichment of an authigenic magnetic mineral with strong remanence properties around the depth of SO 4 2− depletion (∼46 m at Site U1538) relative to magnetic parameters at correlative depths at Site U1537. Fe 2+ enrichment below the depth of SO 4 2− depletion is not predicted based on the energetically favorable order of electron acceptors for microbial respiration but is documented here and in other depositional settings. This indicates Fe 2+ production exceeds the production of H 2 S by SO 4 2− reduction, providing a geochemical environment that favors the production and preservation of ferrimagnetic remanence‐bearing iron sulfides over paramagnetic pyrite and, thus, a mechanism for deep chemical remanent magnetization acquisition at depths of tens of meters. The influence of authigenic ferrimagnetic iron sulfides on paleomagnetic signals can be difficult to demonstrate with magnetic properties alone; therefore, this finding has implications for evaluating the fidelity of magnetostratigraphic records with complementary geochemical data. Such situations should be considered in other depositional environments with similar porewater Fe 2+ accumulation below the SO 4 2− reduction depth. Plain Language Summary Sediment on the seafloor is an archive of Earth history, including past climate variations, ice sheet dynamics, and geomagnetic field variatio

The reaction-transport model determines changes in sediment biogeochemistry and benthic fluxes at the study site (Boknis Eck) following the artificial addition of calcite to the seafloor as a potential carbon dioxide removal strategy. The code is written in MATHEMATICA v12 software (https://www.wolfram.com/mathematica/). No input files are needed to run the model. The model was built in the framework of the RETAKE project funded by the German Federal Ministry of Education and Research, Grant No. 03F0895, Project RETAKE, DAM Mission “Marine carbon sinks in decarbonization pathways” (CDRmare).

The research cruise M182 (MOSES Eddy-III) concludes a trilogy of research cruises conducted in the frame of the Helmholtz Association's Modular Observation Solutions for Earth Systems (MOSES) infrastructure, aiming at a comprehensive multi- and interdisciplinary study of ocean eddies in the northern tropical Atlantic. As an integral part of the BMBF-funded project "Role of eddies in the carbon pump of eastern upwelling systems - Demonstration Case Canary Current System" (REEBUS), launched in 2019, this study focuses on investigating the multi- layered influence of eddies on the lateral transport of biogeochemical properties and the carbon pump within deep-sea ecosystems. Ocean eddies have a significant impact on the physical, biogeochemical and biological aspects of coastal upwelling regions. Given the profound socio- economic impact of coastal upwelling areas, REEBUS takes a multidisciplinary approach to investigate the influence of ocean eddies on these systems. This expedition is motivated by the objective of acquiring novel insights into the functional dynamics of eddies, specifically elucidating their role in the CO2 source and sink functions inherent to eastern boundary upwelling systems. Furthermore, our investigative efforts encompassed a thorough examination of the biological carbon pump within both the designated upwelling areas and their associated oligotrophic regions. The synthesis of data from all REEBUS cruises, coupled with the inclusion of data from prior expeditions (MOSES Eddy-I and II) within this geographical region, promises to yield a high scientific potential.

Sediment fluxes to the seafloor govern the fate of elements and compounds in the ocean and serve as a prerequisite for research on elemental cycling, benthic processes and sediment management strategies. To quantify these fluxes over seafloor areas, it is necessary to scale up sediment mass accumulation rates (MAR) obtained from multiple sample stations. Conventional methods for spatial upscaling involve averaging of data or spatial interpolation. However, these approaches may not be sufficiently precise to account for spatial variations of MAR, leading to poorly constrained regional sediment budgets. Here, we utilize a machine learning approach to scale up porosity and 210 Pb data from 145 and 65 stations, respectively, in the Skagerrak. The models predict the spatial distributions by considering several predictor variables that are assumed to control porosity and 210 Pb rain rates. The spatial distribution of MAR is based on the predicted porosity and existing sedimentation rate data. Our findings reveal highest MAR and 210 Pb rain rates to occur in two parallel belt structures that align with the general circulation pattern in the Skagerrak. While high 210 Pb rain rates occur in intermediate water depths, the belt of high MAR is situated closer to the coastlines due to lower porosities at shallow water depths. Based on the spatial distributions, we calculate a total MAR of 34.7 Mt yr -1 and a 210 Pb rain rate of 4.7 · 10 14 dpm yr -1 . By comparing atmospheric to total 210 Pb rain rates, we further estimate that 24% of the 210 Pb originates from the local atmospheric input, with the remaining 76% being transported laterally into the Skagerrak. The updated MAR in the Skagerrak is combined with literature data on other major sediment sources and sinks to present a tentative sediment budget for the North Sea, which reveals an imbalance with sediment outputs exceeding the inputs. Substantial uncertainties in the revised Skagerrak MAR and the literature data might clo

The greenhouse gas methane (CH4) contributed to a warm climate that maintained liquid water and sustained Earth’s habitability in the Precambrian despite the faint young sun. The viability of methanogenesis (ME) in ferruginous environments, however, is debated, as iron reduction can potentially outcompete ME as a pathway of organic carbon remineralization (OCR). Here, we document that ME is a dominant OCR process in Brownie Lake, Minnesota (midwestern United States), which is a ferruginous (iron-rich, sulfate-poor) and meromictic (stratified with permanent anoxic bottom waters) system. We report ME accounting for ≥90% and >9% ± 7% of the anaerobic OCR in the water column and sediments, respectively, and an overall particulate organic carbon loading to CH4 conversion efficiency of ≥18% ± 7% in the anoxic zone of Brownie Lake. Our results, along with previous reports from ferruginous systems, suggest that even under low primary productivity in Precambrian oceans, the efficient conversion of organic carbon would have enabled marine CH4 to play a major role in early Earth’s biogeochemical evolution.

The impact of oxygen on the preservation of organic matter in marine surface sediments is still controversial. We revisited this long-standing debate by determining the burial efficiency of sedimentary organic matter in the Black Sea, the largest anoxic and euxinic basin in the modern ocean. Surface sediments were sampled in the Danube paleodelta on the northwestern margin of the Black Sea at 420–1550 m water depth. Steady-state modeling of solid species (particulate organic carbon and nitrogen) and solutes (ammonium, sulfate, and total alkalinity) in sediments was performed to quantify rates of mass accumulation, particulate organic matter (POM) degradation, and POM burial. We develop a novel analytical model to quantify these rates applying an inverse modelling approach to down core data accounting for molecular diffusion, sediment burial and compaction. Our model results indicate that 56.7 ± 6.6 % of the particulate organic matter deposited in the study area is not degraded in surface sediments but accumulates below 10 cm sediment depth. This burial efficiency is substantially higher than those previously derived for seafloor areas underlying oxygenated bottom waters. Hence, our study confirms previous studies showing that euxinic bottom water conditions promote the preservation of particulate organic matter in marine sediments.

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

Ocean alkalinity enhancement (OAE) is an emerging strategy that aims to mitigate climate change by increasing the alkalinity of seawater. This approach involves increasing the alkalinity of the ocean to enhance its capacity to absorb and store carbon dioxide (CO2) from the atmosphere. This chapter presents an overview of the technical aspects associated with the full range of OAE methods being pursued and discusses implications for undertaking research on these approaches. Various methods have been developed to implement OAE, including the direct injection of alkaline liquid into the surface ocean; dispersal of alkaline particles from ships, platforms, or pipes; the addition of minerals to coastal environments; and the electrochemical removal of acid from seawater. Each method has its advantages and challenges, such as scalability, cost effectiveness, and potential environmental impacts. The choice of technique may depend on factors such as regional oceanographic conditions, alkalinity source availability, and engineering feasibility. This chapter considers electrochemical methods, the accelerated weathering of limestone, ocean liming, the creation of hydrated carbonates, and the addition of minerals to coastal environments. In each case, the technical aspects of the technologies are considered, and implications for best-practice research are drawn. The environmental and social impacts of OAE will likely depend on the specific technology and the local context in which it is deployed. Therefore, it is essential that the technical feasibility of OAE is undertaken in parallel with, and informed by, wider impact assessments. While OAE shows promise as a potential climate change mitigation strategy, it is essential to acknowledge its limitations and uncertainties. Further research and development are needed to understand the long-term effects, optimize techniques, and address potential unintended consequences. OAE should be viewed as complementary to extensive emission r

An essential prerequisite for the implementation of ocean alkalinity enhancement (OAE) applications is their environmental safety. Only if it can be ensured that ecosystem health and ecosystem services are not at risk will the implementation of OAE move forward. Public opinion on OAE strategies will depend first and foremost on reliable evidence that no harm will be done to marine ecosystems, and licensing authorities will demand measurable criteria against which environmental sustainability can be determined. In this context mesocosm experiments represent a highly valuable tool in determining the safe operating space of OAE applications. By combining biological complexity with controllability and replication, they provide an ideal OAE test bed and a critical stepping stone towards field applications. Mesocosm approaches can also be helpful in testing the efficacy, efficiency and permanence of OAE applications. This chapter outlines strengths and weaknesses of mesocosm approaches, illustrates mesocosm facilities and suitable experimental designs presently employed in OAE research, describes critical steps in mesocosm operation, and discusses possible approaches for alkalinity manipulation and monitoring. Building on a general treatise on each of these aspects, the chapter describes pelagic and benthic mesocosm approaches separately, given their inherent differences. The chapter concludes with recommendations for best practices in OAE-related mesocosm research.

The balance between degradation and preservation of sedimentary organic carbon (OC) is important for global carbon and oxygen cycles 1 . The relative importance of different mechanisms and environmental conditions contributing to marine sedimentary OC preservation, however, remains unclear 2–8 . Simple organic molecules can be geopolymerized into recalcitrant forms by means of the Maillard reaction 5 , although reaction kinetics at marine sedimentary temperatures are thought to be slow 9,10 . More recent work in terrestrial systems suggests that the reaction can be catalysed by manganese minerals 11–13 , but the potential for the promotion of geopolymerized OC formation at marine sedimentary temperatures is uncertain. Here we present incubation experiments and find that iron and manganese ions and minerals abiotically catalyse the Maillard reaction by up to two orders of magnitude at temperatures relevant to continental margins where most preservation occurs 4 . Furthermore, the chemical signature of the reaction products closely resembles dissolved and total OC found in continental margin sediments globally. With the aid of a pore-water model 14 , we estimate that iron- and manganese-catalysed transformation of simple organic molecules into complex macromolecules might generate on the order of approximately 4.1 Tg C yr −1 for preservation in marine sediments. In the context of perhaps only about 63 Tg C yr −1 variation in sedimentary organic preservation over the past 300 million years 6 , we propose that variable iron and manganese inputs to the ocean could exert a substantial but hitherto unexplored impact on global OC preservation over geological time.

Ocean alkalinity enhancement (OAE) is an emerging strategy that aims to mitigate climate change by increasing the alkalinity of seawater. This approach involves increasing the alkalinity of the ocean to enhance its capacity to absorb and store carbon dioxide (CO2) from the atmosphere. This chapter presents an overview of the technical aspects associated with the full range of OAE methods being pursued and discusses implications for undertaking research on these approaches. Various methods have been developed to implement OAE, including the direct injection of alkaline liquid into the surface ocean; dispersal of alkaline particles from ships, platforms, or pipes; the addition of minerals to coastal environments; and the electrochemical removal of acid from seawater. Each method has its advantages and challenges, such as scalability, cost effectiveness, and potential environmental impacts. The choice of technique may depend on factors such as regional oceanographic conditions, alkalinity source availability, and engineering feasibility. This chapter considers electrochemical methods, the accelerated weathering of limestone, ocean liming, the creation of hydrated carbonates, and the addition of minerals to coastal environments. In each case, the technical aspects of the technologies are considered, and implications for best-practice research are drawn. The environmental and social impacts of OAE will likely depend on the specific technology and the local context in which it is deployed. Therefore, it is essential that the technical feasibility of OAE is undertaken in parallel with, and informed by, wider impact assessments. While OAE shows promise as a potential climate change mitigation strategy, it is essential to acknowledge its limitations and uncertainties. Further research and development are needed to understand the long-term effects, optimize techniques, and address potential unintended consequences. OAE should be viewed as complementary to extensive emission r

A new box model is employed to simulate the oxygen-dependent cycling of nutrients in the Peruvian oxygen minimum zone (OMZ). Model results and data for the present state of the OMZ indicate that dissolved iron is the limiting nutrient for primary production and is provided by the release of dissolved ferrous iron from shelf and slope sediments. Most of the removal of reactive nitrogen occurs by anaerobic oxidation of ammonium where ammonium is delivered by aerobic organic nitrogen degradation. Model experiments simulating the effects of ocean deoxygenation and warming show that the productivity of the Peruvian OMZ will increase due to the enhanced release of dissolved iron from shelf and slope sediments. A positive feedback loop rooted in the oxygen-dependent benthic iron release amplifies, both, the productivity rise and oxygen decline in ambient bottom waters. Hence, a 1% decline in oxygen supply reduces oxygen concentrations in sub-surface waters of the continental margin by 22%. The trend towards enhanced productivity and amplified deoxygenation will continue until further phytoplankton growth is limited by the loss of reactive nitrogen. Under nitrogen-limitation, the redox state of the OMZ is stabilized by negative feedbacks. A further increase in productivity and transition to sulfidic conditions is only possible if the rate of nitrogen fixation increases drastically under anoxic conditions. Such a transition would lead to a wide-spread accumulation of toxic sulfide with detrimental consequences for fishery yields in the Peruvian OMZ that currently provides a significant fraction of the global fish catch.

High alkalinity values observed in coastal seas promote the uptake of CO2 from the atmosphere. However, the alkalinity budget of coastal areas and marginal seas is poorly understood, even though some of the recently observed alkalinity enhancement can be ascribed to riverine fluxes and anaerobic processes in shelf sediments. Here, we investigate the alkalinity budget of the Baltic Sea to identify previously unrecognized alkalinity sources. We quantify the generation of alkalinity and dissolved calcium (Ca) in this marginal sea applying simple mass balance calculations. Using this approach, we identify alkalinity and Ca sources of approximately 324 Gmol yr-1 and 122 Gmol yr-1, respectively, that cannot be ascribed to the riverine input. The magnitude of the Ca source suggests that a major fraction of the excess alkalinity (244 Gmol yr-1) is induced by the dissolution of calcium carbonate (CaCO3). A review of available field data shows that carbonate-bearing rocks at the coast and the seabed of the Baltic Sea are rapidly eroded and may provide sufficient CaCO3 to close the Ca budget. Hence, dissolution of eroded CaCO3 is the most likely source for the Ca enrichment observed in Baltic Sea water. This hypothesis is supported by mass accumulation rates of sediments derived from radioisotope data that are evaluated to derive a basin-wide rate of mud to muddy sand accumulation at the bottom of the Baltic Sea. The resulting value (139 Tg yr-1) exceeds current estimates of riverine particle fluxes into the Baltic Sea by more than one order of magnitude and confirms that rates of till erosion are sufficiently high to account for the Ca and most of the alkalinity excess in Baltic Sea water. Finally, we show that deliberate addition of CaCO3 to sediments deposited in the Baltic Sea could neutralize significant amounts of CO2 and help to achieve net-zero greenhouse gas emissions in the Baltic region.

Highlights • An artificial CO2 release demonstrated MMV techniques for offshore CCS. • Detection of leakage was demonstrated using acoustic, chemical and physical approaches. • Attribution of leakage was proved possible using artificial and natural tracer compounds. • Leakage quantification was possible using approaches not previously applied to CCS studies. • Non-catastrophic leaks were detected at levels below those that would cause environmental harm. Carbon capture and storage is a key mitigation strategy proposed for keeping the global temperature rise below 1.5 °C. Offshore storage can provide up to 13% of the global CO2 reduction required to achieve the Intergovernmental Panel on Climate Change goals. The public must be assured that potential leakages from storage reservoirs can be detected and that therefore the CO2 is safely contained. We conducted a controlled release of 675 kg CO2 within sediments at 120 m water depth, to simulate a leak and test novel detection, quantification and attribution approaches. We show that even at a very low release rate (6 kg day−1), CO2 can be detected within sediments and in the water column. Alongside detection we show the fluxes of both dissolved and gaseous CO2 can be quantified. The CO2 source was verified using natural and added tracers. The experiment demonstrates that existing technologies and techniques can detect, attribute and quantify any escape of CO2 from sub-seabed reservoirs as required for public assurance, regulatory oversight and emissions trading schemes.

Carbon capture and storage (CCS) is a key technology to reduce carbon dioxide (CO2) emissions from industrial processes in a feasible, substantial, and timely manner. For geological CO2 storage to be safe, reliable, and accepted by society, robust strategies for CO2 leakage detection, quantification and management are crucial. The STEMM-CCS (Strategies for Environmental Monitoring of Marine Carbon Capture and Storage) project aimed to provide techniques and understanding to enable and inform cost-effective monitoring of CCS sites in the marine environment. A controlled CO2 release experiment was carried out in the central North Sea, designed to mimic an unintended emission of CO2 from a subsurface CO2 storage site to the seafloor. A total of 675 kg of CO2 were released into the shallow sediments (~3 m below seafloor), at flow rates between 6 and 143 kg/d. A combination of novel techniques, adapted versions of existing techniques, and well-proven standard techniques were used to detect, characterise and quantify gaseous and dissolved CO2 in the sediments and the overlying seawater. This paper provides an overview of this ambitious field experiment. We describe the preparatory work prior to the release experiment, the experimental layout and procedures, the methods tested, and summarise the main results and the lessons learnt.

The AL561 cruise was conducted in the framework of the project APOC (“Anthropogenic impacts on Particulate Organic Carbon cycling in the North Sea”). This collaborative project between GEOMAR, AWI, HEREON, UHH, and BUND is to understand how particulate organic carbon (POC) cycling contributes to carbon sequestration in the North Sea and how this ecosystem service is compromised and interlinked with global change and a range of human pressures include fisheries (pelagic fisheries, bottom trawling), resource extraction (sand mining), sediment management (dredging and disposal of dredged sediments) and eutrophication. The main aim of the sampling activity during AL561 cruise was to recover undisturbed sediment from high accumulation sites in the Skagerrak/Kattegat and to subsample sediment/porewater at high resolution in order to investigate sedimentation transport processes, origin of sediment/POC and mineralization processes over the last 100- 200 years. Moreover, the actual processes of sedimentation and POC degradation in the water column and benthic layer will be addressed by sampling with CTD and Lander devices. In total 9 hydroacoustic surveys (59 profiles), 4 Gravity Corer, 7 Multicorer, 3 Lander and 4 CTD stations were successfully conducted during the AL561 cruise.

According to many prognostic scenarios by the Intergovernmental Panel on Climate Change (IPCC), a scaling-up of carbon dioxide (CO2) capture and storage (CCS) by several orders-of-magnitude is necessary to meet the target of ≤2 °C global warming by 2100 relative to preindustrial levels. Since a large fraction of the predicted CO2 storage capacity lies offshore, there is a pressing need to develop field-tested methods to detect and quantify potential leaks in the marine environment. Here, we combine field measurements with numerical models to determine the flow rate of a controlled release of CO2 in a shallow marine setting at about 119 m water depth in the North Sea. In this experiment, CO2 was injected into the sediment at 3 m depth at 143 kg d-1. The new leakage monitoring tool predicts that 91 kg d-1 of CO2 escaped across the seafloor, and that 51 kg d-1 of CO2 were retained in the sediment, in agreement with independent field estimates. The new approach relies mostly on field data collected from ship-deployed technology (towed sensors, Acoustic Doppler current profiler—ADCP), which makes it a promising tool to monitor existing and upcoming offshore CO2 storage sites and to detect and quantify potential CO2 leakage.

Iron is a key micronutrient controlling phytoplankton growth in vast regions of the global ocean. Despite its importance, uncertainties remain high regarding external iron source fluxes and internal cycling on a global scale. In this study, we used a global dissolved iron dataset, including GEOTRACES measurements, to constrain source and scavenging fluxes in the marine iron component of a global ocean biogeochemical model. Our model simulations tested three key uncertainties: source inputs of atmospheric soluble iron deposition (varying from 1.4–3.4 Gmol/yr), reductive sedimentary iron release (14–117 Gmol/yr), and compared a variable ligand parameterization to a constant distribution. In each simulation, scavenging rates were tuned to reproduce the observed global mean iron inventory for consistency. The variable ligand parameterization improved the global model-data misfit the most, suggesting that heterotrophic bacteria are an important source of ligands to the ocean. Model simulations containing high source fluxes of atmospheric soluble iron deposition (3.4 Gmol/yr) and reductive sedimentary iron release (114 Gmol/yr) further improved the model most notably in the surface ocean. High scavenging rates were then required to maintain the iron inventory resulting in relatively short surface and global ocean residence times of 0.83 and 7.5 years, respectively. The model simulates a tight spatial coupling between source inputs and scavenging rates, which may be too strong due to underrepresented ligands near source inputs, contributing to large uncertainties when constraining individual fluxes with dissolved iron concentrations. Model biases remain high and are discussed to help improve global marine iron cycle models.

Carbon capture and storage (CCS) is a key technology to reduce carbon dioxide (CO2) emissions from industrial processes in a feasible, substantial, and timely manner. For geological CO2 storage to be safe, reliable, and accepted by society, robust strategies for CO2 leakage detection, quantification and management are crucial. The STEMM-CCS (Strategies for Environmental Monitoring of Marine Carbon Capture and Storage) project aimed to provide techniques and understanding to enable and inform cost-effective monitoring of CCS sites in the marine environment. A controlled CO2 release experiment was carried out in the central North Sea, designed to mimic an unintended emission of CO2 from a subsurface CO2 storage site to the seafloor. A total of 675 kg of CO2 were released into the shallow sediments (~3 m 49 below seafloor), at flow rates between 6 and 143 kg/d. A combination of novel techniques, adapted versions of existing techniques, and well-proven standard techniques were used to detect, characterise and quantify gaseous and dissolved CO2 in the sediments and the overlying seawater. This paper provides an overview of this ambitious field experiment. We describe the preparatory work prior to the release experiment, the experimental layout and procedures, the methods tested, and summarise the main results and the lessons learnt.

An extensive data set of biogenic silica (BSi) fluxes is presented for the Peruvian oxygen minimum zone (OMZ) at 11ºS and 12ºS. Each transect extends from the shelf to the upper slope (∼1000 m) and dissects the permanently anoxic waters between ∼200 – 500m water depth. BSi burial (2100 mmol m‐2 yr‐1) and recycling fluxes (3300 mmol m‐2 yr‐1) were highest on the shelf with mean preservation efficiencies (34±15%) that exceed the global mean of 10 – 20%. BSi preservation was highest on the inner shelf (up to 56%), decreasing to 7% and 12% under anoxic waters and below the OMZ, respectively. The data suggest that the main control on BSi preservation is the rate at which reactive BSi is transported away from undersaturated surface sediments by burial and bioturbation to the underlying saturated sediment layers where BSi dissolution is thermodynamically and/or kinetically inhibited. BSi burial across the entire Peruvian margin between 3ºS to 15ºS and down to 1000m water depth is estimated to be 0.1 – 0.2 Tmol yr‐1; equivalent to 2 – 7% of total burial on continental margins. Existing global data permit a simple relationship between BSi rain rate to the seafloor and the accumulation of unaltered BSi, giving the possibility to reconstruct rain rates and primary production from the sediment archive in addition to benthic Si turnover in global models.

An understanding of how the coupled cycles of carbon, iron and sulfur in sediments respond to environmental change throughout Earth history requires the reconstruction of biogeochemical processes over a range of spatial and temporal scales. In this study, sediment cores from the southwestern Black Sea were analyzed to gain insight into past changes in biogeochemical processes with particular focus on the cycling of dissolved organic carbon (DOC). The sediment consists of Late Pleistocene deposits of iron oxide-rich and organic-poor lacustrine sediments, a Holocene sapropel layer deposited after the inflow of saline Mediterranean seawater about 9300 yr BP, and overlying recent marine sediments. The porewaters displayed high concentrations of DOC, acetate, dissolved iron and an extended depth interval over which sulfate and methane were both present. The historical fluctuations of the fluxes of carbon, sulfur and iron species at the seafloor that led to these present-day geochemical profiles, and which cannot be easily interpreted from the measured data alone, were hindcasted with a reaction-transport model. The model suggests that the inflow of Mediterranean seawater impacted the rain rate and reactivity of organic matter reaching the sediments, which shifted the sedimentary redox regimes throughout the Holocene that now are reflected on different lithology units. Organic matter in the sapropel layer is apparently the main source of modern-day accumulations of DOC and acetate, both of which probably sustained subsurface microbial activity throughout the post-glacial period. The ratio between DOC and dissolved inorganic carbon (DIC) flux to the bottom water decreased from ∼40% before the inflow of Mediterranean water to ∼2% at the present day. We suggest that the coexistence of methanogenesis and sulfate reduction was associated with sulfate-reducing bacteria and methanogens sharing common substrates of acetate and lactate and utilizing non-competitive substrates such

From 2008 through 2019, a comprehensive research project, SFB 754, Climate - Biogeochemistry Interactions in the Tropical Ocean, was funded by the German Research Foundation to investigate the climate-biogeochemistry interactions in the tropical ocean with a particular emphasis on the processes determining the oxygen distribution. During three 4-year long funding phases, a consortium of more than 150 scientists conducted or participated in 34 major research cruises and collected a wealth of physical, biological, chemical, and meteorological data. A common data policy agreed upon at the initiation of the project provided the basis for the open publication of all data. Here we provide an inventory of this unique data set and briefly summarize the various data acquisition and processing methods used.

Highlights • A biogeochemical baseline of sediment geochemistry at potential offshore CCS sites. • Diagnostic indicators of CO2 leakage based on stoichiometry of porewater chemistry. • Porewater chemistry is modified by reverse weathering processes at Goldeneye site. Abstract Injection of carbon dioxide (CO2) into subseafloor reservoirs is gaining traction as a strategy for mitigating anthropogenic CO2 emissions to the atmosphere. Yet, potential leakage, migration and dissolution of externally-supplied CO2 from such reservoirs are a cause for concern. The potential impact of CO2 leakage on the biogeochemistry of sediments and overlying waters in the North Sea was studied during a controlled subsurface CO2 release experiment in 2019 at a potential carbon capture and storage site (Goldeneye). This study describes the natural (unperturbed) biogeochemistry of sediments. They are classified as muddy sand to sandy mud with low organic carbon content (∼0.6 %). Distributions of dissolved inorganic carbon (DIC) and total alkalinity (TA) in sediment porewaters are reported in addition to in situ benthic fluxes of dissolved nutrients and oxygen between the sediments and the overlying water. Oxygen fluxes into the sediment, measured using benthic chambers and eddy covariance, were 6.18 ± 0.58 and 5.73 ± 2.03 mmol m−2 d-1, respectively. Diagnostic indicators are discussed that could be used to detect CO2 enrichment of sediments due to reservoir leakage at CCS sites. These include the ratio TA and ammonium to sulfate in sediment porewaters, benthic fluxes and chloride-normalized cation distributions. These indicators currently suggest that the organic carbon at Goldeneye has an oxidation state below zero and is mainly degraded via sulfate reduction. Carbonate precipitation is apparently negligible, whereas decreases in Mg2+ and K+ point toward ongoing alteration of lithogenic sediments by reverse weathering processes.

Summary The major goal of the RV METEOR cruise M156 to Cape Verdian waters and the Mauritanian upwelling area off West Africa was to contribute to a better quantitative understanding of the effects of mesoscale eddies on CO2 source/sink mechanisms and the biological carbon pump in eastern boundary upwelling areas as well as their effects to the oligotrophic periphery including the deep-sea floor. The cruise M156 (MOSES Eddy Study I) was conducted within the framework of the BMBF funded REEBUS project (Role of Eddies in the Carbon Pump of Eastern Boundary Upwelling Systems) by a consortium of physical, biological (benthic microbiology, bacterial plankton, protists) and biogeochemical oceanographers. Specific aims were i. the quantification of solute and particle fluxes within and at the periphery of eddies; ii. to determine the turnover of carbon species, air-sea gas exchange of CO2, iii. the determination of the protistan and bacterial plankton community structures in the surface layers of an eddy, and iv. to quantify the magnitude and variability of material fluxes to the seabed and turnover in the sediment underneath the eddy passage. To achieve these aims, the cruise had two major observing strategies: i. an intense benthic/pelagic program along the zonal eddy passage at 18°N. Along this corridor ranging from 24°20’ to 16°30’W, five benthic/pelagic stations (E1 to E5) in different water depths and distances from the Mauritanian coast were performed. The motivation for this survey has been to resolve zonal gradients in pelagic element cycling as well as of organic matter degradation and burial in the seabed, which in turn could potentially be linked with changes in eddy induced primary- and export production. ii. the detailed investigation of an individual eddy to investigate physical, biogeochemical and biological processes on meso- to submeso-scales (100km to 10m). Satellite data analysis was performed before and during the cruise to identify a suitable eddy fro

Sediments in oxygen-depleted marine environments can be an important sink or source of bio-essential trace metals in the ocean. However, the key mechanisms controlling the release from or burial of trace metals in sediments are not exactly understood. Here, we investigate the benthic biogeochemical cycling of Fe and Cd in the oxygen minimum zone off Peru. We combine bottom water profiles, pore water profiles, as well as benthic fluxes determined from pore water profiles and in-situ from benthic chamber incubations along a depth transect at 12° S. In agreement with previous studies, both concentration-depth profiles and in-situ benthic fluxes indicate a Fe release from sediments into bottom waters. Diffusive Fe fluxes and Fe fluxes from benthic chamber incubations are roughly consistent (0.3–17.1 mmol m−2 y−1), indicating that diffusion is the main transport mechanism of dissolved Fe across the sediment-water interface. The occurrence of mats of sulfur oxidizing bacteria on the seafloor represents an important control on the spatial distribution of Fe fluxes by regulating hydrogen sulfide (H2S) concentrations and, potentially, Fe sulfide precipitation within the surface sediment. Removal of dissolved Fe after its release to anoxic bottom waters is rapid in the first 4 m away from the seafloor (half-life < 3 min) which hints to oxidative removal by nitrite or interaction with particles in the benthic boundary layer. Benthic flux estimates of Cd are indicative of a flux into the sediment within the oxygen minimum zone. Fluxes from benthic chamber incubations (up to 22.6 µmol m−2 y−1) exceed the diffusive fluxes (< 1 µmol m−2 y−1) by a factor > 25, indicating that downward diffusion of Cd across the sediment-water interface is of subordinate importance for Cd removal from benthic chambers. As Cd removal in benthic chambers co-varies with H2S concentrations in the pore water of surface sediments, we argue that Cd removal is mediated by precipitation of CdS within the cha