The Western Antarctic Peninsula (WAP) is one of the fastest warming regions on Earth, with increasing freshwater input from melting glaciers and ice shelves. Although surface-layer freshening is well documented, the extent to which glacial meltwater influences subsurface waters remains poorly constrained. Here, we investigated the vertical distribution and origin of freshwater anomalies using hydrographic, isotopic (δ¹⁸O, δ²H), and major-ion data from three sites along the WAP: Cierva Cove, Petermann Island, and Paradise Bay. The data show consistent freshening below 50 m depth. Chloride dilution and isotope depletion define conservative mixing between local seawater and a strongly δ¹⁸O-depleted glacial meltwater endmember, with this signal extending to depths greater than 90 m in the more enclosed embayments. Estimated meltwater fractions at these depths are approximately 0.5 to 2%. Major-ion and halogen ratios also vary conservatively, supporting mixing between seawater and glacial meltwater rather than addition of a chemically distinct subsurface fluid. Despite limited vertical sampling, the deepest samples at each site remain consistent with the inferred surface-to-depth mixing relationships. These findings indicate that glacial meltwater can be stored well below the surface layer along parts of the WAP, likely through plume-driven neutral-buoyancy intrusions, lateral advection, and mixing. Recognising this subsurface meltwater reservoir is important for understanding local stratification and for improving representation of freshwater input in ocean models of the region.

Continental shelves host extensive offshore relatively fresh groundwater (ORFG) systems whose origin, distribution, and persistence are strongly influenced by paleo sea-level fluctuations; however, the paleo-hydrological controls governing their present-day salinity patterns remain poorly constrained. Here, we investigate the ORFG system offshore New Jersey, USA, by integrating high-resolution seismic profiles, borehole constraints, geochemical and isotopic data, and three-dimensional variable-density groundwater flow and solute transport modeling. Model results indicate that most ORFG was emplaced by topographically driven groundwater flow during sea-level lowstands over the past similar to 100 kyr. During Pleistocene lowstands, enhanced hydraulic gradients across the exposed continental shelf promoted widespread seaward groundwater flow, leading to the development of low-salinity groundwater over large areas. Subsequent marine transgression led to partial salinization through diffusive and density-driven mixing with seawater. Our results further demonstrate that cyclical flushing and re-salinification of shelf sediments over glacial-interglacial timescales are inherently asymmetric, favoring the long-term retention of freshened groundwater. Compared with conventional layered modeling approaches, the incorporation of depth-migrated seismic stratigraphy and geostatistical property modeling substantially improves the characterization of ORFG spatial heterogeneity and volume distribution. However, these results are based on a single representative geostatistical realization and therefore do not explicitly quantify stochastic uncertainty. Overall, this study highlights the long-term legacy of paleo-hydrological conditions on continental-shelf groundwater systems and provides a process-based perspective for interpreting ORFG occurrence, evolution, and vulnerability along passive continental margins.

Study region: This study deals with the isotopic composition of monsoon precipitation in Southern Oman, specifically in the Salalah coastal plain and the adjacent Dhofar Mountains. Study focus: Monsoon variability on the southern Arabian Peninsula has been described based on the oxygen isotope composition of various terrestrial archives, often on the millennial scale. However, the factors influencing the spatio-temporal variability of the oxygen isotope composition of today's monsoon precipitation have not yet been described for this region. Here, we present stable isotope ratios of oxygen and hydrogen for the Indian Summer Monsoon in the greater Salalah area. New hydrological insights for the region: The precipitation amount-weighted mean isotope values along an elevation transect show lighter signatures at higher elevation, but generally low deuterium excess values (d<10 parts per thousand). To trace the origin and (short-term) history of the precipitation system, we applied a 3-step model incorporating evaporation of seawater, condensation, and sub-cloud evaporation. Our modelling exercise indicates that the classic elevation effect is quasi-negligible, and that sub-cloud evaporation is likely to be the main driver behind the encountered isotope pattern. This pseudo-elevation effect implies that the precipitation is not depleted in heavy isotopes with increasing elevation, but rather enriched with decreasing elevation. This effect may prevail in other (semi-arid) areas as well.

Abstract Sediments in productive coastal regions are hotspots of hydrogen sulfide (H 2 S) production through microbial sulfate reduction. Under anoxic seafloor conditions, H 2 S can escape the sediment and accumulate in bottom waters, where elevated concentrations render the benthic environment toxic, posing ecological and economic risks. The central Kiel Bight in the southwestern Baltic Sea shows a strong seasonal water column stratification, with hypoxic to anoxic bottom waters in late summer. Euxinic conditions (H 2 S > 1 μ M) have been observed locally, but their extent and underlying controls are not fully understood. This study investigates spatial and temporal patterns of bottom water H 2 S in the central Kiel Bight using multi‐annual sediment and water column measurements combined with machine learning models. We find that euxinia affects ~ 27% of the 622 km 2 seafloor in late summer, with recurring H 2 S detections over the past decade highlighting its frequent occurrence. Sedimentary H 2 S production is enhanced in areas with high organic matter delivery, yet H 2 S production alone does not control its accumulation in bottom waters. Instead, the highest H 2 S levels are observed in channels and basins where long deep‐water residence times deplete oxidants, thereby facilitating H 2 S accumulation. Furthermore, our findings demonstrate that effective H 2 S monitoring requires sampling directly above the seafloor, such as collecting the supernatant water from sediment cores, because H 2 S is often restricted to the lowermost water layer undetected by conventional water column profiling. Adapted monitoring is crucial to predict future development of euxinia in the shallow Baltic Sea and other coastal environments.

The majority of the organic matter (OM) degradation on the seafloor occurs in coastal regions. Since oxygen (O2) becomes quickly depleted in the top sediments, most of the OM decomposition is driven by microbial sulfate reduction (SR) and fermentation, the latter generating molecular hydrogen (H2). If the H2 is not consumed by hydrogenotrophic microorganisms and accumulates in the sedimentary porewaters, OM degradation is hindered. Despite the importance of H2 scavenging microorganisms for OM mineralization, the knowledge on H2 oxidizers and their constraints in coastal marine sediments is still quite limited. Here we investigated the role of H2 oxidizers in top (2 to 5 cm, suboxic-sulfidic) and bottom (18 to 22 cm, sulfidic) coastal sediments from a location exposed to seasonal hypoxia in the SW Baltic Sea. We used sediments from April, May and August, representative of different seasons. We spiked respective sediment slurries with H2 and incubated them for up to 4 weeks under O2-free conditions. H2 consumption potential, methane production and shifts in bacterial and archaeal 16S rRNA gene amplicons (generated from RNA) were assessed over time. The seasonal variations in sedimentary community compositions and pore water geochemistry already gave distinct starting conditions for the H2 enrichments. Sediments exposed to near anoxic bottom water conditions favored a microbial starter community exhibiting the highest H2 oxidation potential. Most of the observed H2 oxidation potential appeared associated with hydrogenotrophic sulfate reducers. The putative involvement of massively enriched ANME in H2 cycling in May 18 to 22 cm sediment horizons is conspicuous. While the differences in the observed H2 oxidation potentials in the studied sediment slurries are likely related to the (season-depending) overall redox state of the sediments and interstitial waters, the influence of microbial interconnections could not be fully resolved and evaluated, demonstrating the need for further consumption- and community-based studies.

The burial of particulate organic carbon (POC) on continental margins represents a major long-term sink for atmospheric carbon. Hence, accurate descriptions of the benthic marine POC cycle are necessary for understanding the ocean's CO2 uptake capacity. Here, benthic POC cycling is examined in the Skagerrak basin, the largest depocenter for POC in the North Sea. POC burial fluxes, benthic dissolved inorganic carbon (DIC) fluxes, POC rain rates and burial efficiencies (CBE) are reported at six stations based on porewater, solid phase and in-situ benthic flux measurements. On average, 10.9 mmol m(-2) d(-1) of POC rains onto the seafloor, of which 6.6 mmol m(-2) d(-1) is recycled back into the water column as DIC and 4.3 mmol m(-2) d(-1) is permanently buried in the sediment, resulting in a CBE of 40%. The data indicate that the mass accumulation rate (MAR), faunal activity and the quality of the settling POC are key driving mechanisms for the benthic POC cycle. Three independent approaches are used to distinguish between the contributions of autochthonous and allochthonous POC to the total sedimentary carbon pool. On average, 18 +/- 6% of POC originates from local production, while 82 +/- 6% is transported laterally into the Skagerrak basin. Considering previously reported MARs, the temporal evolution of POC burial in the Skagerrak basin shows a substantial decline over the last similar to 100 years, indicating variability in the lateral POC input. Possible reasons for the POC burial decline are qualitatively discussed, including changes in sediment cycling and primary production in the North Sea.

During research expedition SO299 with the German RV Sonne, we discovered the first deep-sea hydrothermal vent system along the Tabar-Lihir-Tanga-Feni island chain in northeastern Papua New Guinea. The Karambusel vent field is hosted by a volcanic center on the western flank of Conical Seamount that formed ~ 89 ka ago. Karambusel is remarkable in that it hosts both a fossil high-temperature, gold-rich mineralization and an active low-temperature (< 51 °C) vent system precipitating arsenic-, antimony-, thallium-, and mercury-rich sulfide minerals. Chemosymbiotic fauna is associated with the vent system and we identified more endemic species than in previous studies on nearby seeps. Our study shows that the magmatic event at Karambusel likely triggered the epithermal mineralization at Karambusel and at the central summit of Conical Seamount. The current hydrothermal fluids originate from condensed magmatic vapor or connate fluids. Gas bubbles were observed at some vent sites and the proportion of methane in the gas phase exceeds that of any other hydrothermal vent system. The composition of the light hydrocarbons points towards a thermogenic origin. Karambusel is thus the first hybrid magmatic-hydrothermal vent and hydrocarbon seep system discovered globally which explains the highly endemic vent fauna as a consequence of the unique ecological niche.

Since the 19th century, the North Sea sediment system has been subject to a dynamic hydrographic regime and intense human alteration. The Skagerrak serves as the largest depocenter for suspended sediment originating from the North Sea. Thus, deposits in the Skagerrak provide a historical record of potential shifts in the sediment cycle of the North Sea. Despite the availability of mass accumulation rate (MAR) data in the Skagerrak, previous studies focused on steady-state reconstructions and little is known about how these rates may have changed over time. To address this knowledge gap, we present age-depth models based on the natural radionuclide 210Pb and the anthropogenic time markers 137Cs, fraction modern 14C (F14C) and mercury (Hg) to determine the MAR before and after the year 1963 at six stations in the deep Skagerrak basin between 434 and 677 m water depth. We applied 1963 as the boundary since this year is constrained by 137Cs and F14C peaks in the sediment cores due to atomic weapons testing and changes in sedimentary Hg contents. Our primary result reveals that the MAR in the deep Skagerrak basin decreased from 0.17 to 0.14 g cm- 2 yr- 1 averaged across the stations. We further simulate the effect of bioturbation on the solid phase profiles by applying a reaction transport model to the data, revealing that the decline in MAR is more pronounced when bioturbation is considered (from 0.17 to 0.09 g cm-2 yr- 1). Decreasing MARs in the Skagerrak basin indicate that the sediment system of the North Sea substantially changed over time. Possible reasons include a shift in the North Sea circulation pattern, enhanced sediment trapping in the Wadden Sea and reduced sediment inputs due to river damming, deepening of harbor channels and coastal protection. However, we stress that our data do not allow for a quantitative analysis of the major driving factors behind the temporal variability of sediment cycling. Hence, we recommend combining our results with information on the provenance of the Skagerrak deposits and integrating the Skagerrak data into largerscale physical models that consider non-steady state particle transport in the North Sea.

During the last glacial period, continents and surrounding shelves in high latitude regions of the Northern Hemisphere were covered by ice sheets. Their retreat during the late Pleistocene and Holocene resulted in isostatic adjustments of the previously glaciated landmass, which influenced post-glacial changes in relative sea level (RSL). Many questions, however, remain about the timing and impact of the ice retreat on the continental shelf environments and RSL after the Last Glacial Maximum, and of short-lived climatic events, such as the Younger Dryas. This study aims to reconstruct the deglaciation history and changes in RSL for the southern Gulf of St. Lawrence off Prince Edward Island on the eastern Canadian continental shelf for the past 14 ka, and to determine the influence of the Younger Dryas on the ice margin. Using information from sub-bottom profiles, sediment cores, and multibeam bathymetry, this study finds that most of the continental shelf was already flooded 13.6 ka ago, as evidenced by the presence of Bølling-Allerød marine sediments at a modern water depth of less than 50 m and ~15 km off the modern coastline. During the Younger Dryas cooling event, sedimentation rates increased from 0.1 to 1 cm a-1, likely as a consequence of readvancing ice masses. We observe an erosional truncation on top of the Younger Dryas sediment package, which presumably indicates a drop in RSL in the early Holocene. Based on our new data, we propose an updated RSL curve for the region that accounts for the presence of sea ice coverage rather than complete ice coverage as well as a geological model highlighting the sedimentation history over the past 14 ka and role of the Younger Dryas. The new paleo-environmental and RSL reconstructions shed light on the potential impact of short-lived climatic events at the former ice margin during deglaciation and reduce uncertainties for about past sea level changes.

Abstract Recent studies have proposed calcite and dunite as possible alkaline materials for enhanced benthic weathering in shallow depocenters of the Baltic Sea as a marine carbon dioxide removal strategy. In this study, insights on calcite and dunite weathering from laboratory incubations and long-term benthocosm experiments are combined with a numerical box-model to assess the carbon dioxide uptake potential of mineral addition to organic-rich sediments in the southwest Baltic Sea. The results reveal that calcite has an up to 10-fold higher carbon dioxide uptake efficiency and is therefore the preferable material for enhanced benthic weathering as a marine carbon dioxide removal method, with costs per tonne of sequestered carbon dioxide ranging between 82 and 462 euro for calcite while reaching 558–1920 euro for dunite. These findings could be applicable to other areas in the Baltic Sea and also globally to sediments in the wider coastal shelf with similar geochemical properties.

Sediment resuspension of blue carbon ecosystems (e.g., seagrass beds) and muddy sediments exposes buried particulate organic carbon to oxygenated waters and remineralization, potentially enhancing carbon dioxide fluxes. However, the kinetics of carbon degradation under oxic and anoxic conditions are poorly constrained. We report the results of incubation experiments with sediments from Kiel Bight to simulate sediment resuspension events induced by natural and anthropogenic resuspension in this area. A numerical model determined that oxic carbon remineralization rates were up to two-fold higher than those under anoxic conditions. A coupled sediment-water column model demonstrated that pyrite oxidation, rather than carbon oxidation, has the potential to induce large carbon dioxide emissions to the atmosphere following anthropogenic sediment disturbance by trawling. Upscaling to muddy areas of Kiel Bight suggests an annual emission of up to similar to 14 k tonnes of carbon dioxide per year. Pyrite oxidation may contribute to a weakening of the carbon shelf pump and a reduction of anthropogenic carbon dioxide uptake.

Since industrial times, human and natural processes have affected the sediment system of the North Sea. As a substantial proportion of the suspended sediment in the North Sea is ultimately deposited in the Skagerrak, it offers a representative archive for reconstructing the temporal variability of the North Sea sediment system. However, little is known about how sedimentation rates in the Skagerrak may have changed over time. In this study, we present high-resolution age-depth models based on the natural radionuclide 210Pb and the anthropogenic time markers 137Cs, 14C and mercury to determine average sedimentation rates before and after the year 1963 at six stations in the Skagerrak. This year was selected because its age-depth relationship was clearly reflected by peak activities or concentrations in the sedimentary data of the time markers. The main result of this study is a consistent decrease in sedimentation rates at all stations. On average, sedimentation rates decreased from 0.36 to 0.15 cm yr-1, suggesting a substantial alteration of the North Sea sediment system. We tentatively discuss possible driving factors including a shift in the North Sea circulation pattern, increased sediment deposition in the Wadden Sea, and reduced sediment inputs into the North Sea due to coastal protection and river damming. In terms of the overall North Sea sediment cycle, these processes may outweigh the effects of sediment resuspension by human activities and storm events, as well as temperature, humidity and sea level rise caused by climate change.

Carbonate margins form a significant portion of the world's coastlines, contain substantial freshwater resources, and play a vital role in global hydrogeological processes. These regions are promising reservoirs for offshore freshened groundwater (OFG), a potential resource for coastal and island populations. Mapping OFG within continental margins using marine controlled-source electromagnetic (CSEM) data relies on electrical resistivity as a proxy, requiring sophisticated inversion techniques. Given the ambiguity in deriving discrete resistivity distributions from CSEM data, understanding uncertainty is essential for reliable OFG inference. The conventionally used two-dimensional deterministic inversion provides a best-fit solution but does not assess resistivity uncertainties, limiting OFG characterization. To address this, we apply trans-dimensional Bayesian inversion on marine CSEM data from a semi-arid carbonate setting off eastern Gozo (Maltese Islands, Mediterranean Sea). Here, we integrate deterministic and trans-dimensional inversion results with seismic reflection data to identify two distinct, continuous resistivity anomalies within the Lower Coralline Limestone formation. The first, shallower resistive body starts similar to 4 km from the coast, appearing across all the CSEM profiles at 210-250 m below sea-level, with resistivity increasing landward. This anomaly may suggest an OFG body. The second, deeper anomaly starts at 350-400 m below sea-level and extends deeper. Whether it represents a second OFG unit or geological changes remains uncertain. Our findings offer new insights into resistivity distributions within carbonate margins, highlighting their OFG potential and the value of trans-dimensional sampling. This study augments CSEM research, underlining the need to extend coastal hydrogeological studies offshore for improved environmental conservation and resource management.

Comparing directly measured soil temperatures with noble gas recharge temperatures (NGTs) inferred from noble gas concentrations indicates that the infiltrating soil water equilibrates with soil air near the soil surface during the rainy season. Therefore, NGTs of groundwater recently recharged by the Indian Summer Monsoon (ISM) in the Dhofar Mountains in southern Oman reflect the soil temperatures of the 3-month period and do not represent an annual mean. This finding highlights the need to account for seasonality when interpreting NGT data in regions with pronounced dry and wet seasons. We extend the observations from the southern flank of the Dhofar Mountains to three wells situated on the northern flank of the Dhofar Mountains. Two of these wells yield water of Holocene age that was recharged by the monsoon, their NGT signals are therefore classified as seasonal. The NGT calculated from a third well for recharge conditions during the Last Glacial Maximum (LGM), when the ISM was absent, is approximately 3 degrees C lower than that of the two Holocene wells. The lower LGM noble gas temperature corresponds well with the lower annual Sea Surface Temperature (SST) in the nearby Arabian Sea. NGTs from published studies from northern Oman are 1-3 degrees C higher when compared with our data of the same period in the southern Oman. We explain this regional difference of reconstructed temperatures for the LGM and Holocene groundwater with a more continental climatic influence on the infiltration conditions further to the north. The published NGTs from northern Oman show a large temperature difference between the late Holocene and the LGM. In view of our finding of seasonal NGT signals under monsoonal climate, part of this difference may reflect a change in the precipitation regime rather than in air temperature.

Although similar to 20% of global carbonate production occurs on extra-tropical carbonate depositional systems, our understanding of these environments still lags behind that of tropical ones. The Maltese shelf in the central Mediterranean offers an opportunity to study in situ facies distribution and the factors controlling it in a light-dominated setting. The investigated region of the Maltese shelf visually exhibits three main depositional environments: seagrass meadows, sand flats and rhodolith and maerl beds. While visually distinctive, the grain composition of the sediments does not provide a clear differentiation of the three environments but rather a gradient. This gradient is marked by increasing grain size with water depth, a transition from green to red calcareous algae and an increase in the fraction of low magnesium calcite of total carboantes. While some of these features can be explained by changes in light availability, other factors are also in play. Baffling by seafloor vegetation and currents, storms and internal waves inducing sediment reworking appear to play important roles in governing the sediment texture and composition across the Maltese shelf. The role of seagrass meadows in regulating production and accumulation rates of carbonates appears to be of greater importance in Mediterranean C-type carbonate factories than in southern Atlantic ones and this could be an important marker to identify them in the geological record.

Cruise AL575 is fully embedded in the project GEOSTOR, which is part of the DAM mission “Marine Carbon Sinks in Decarbonization Pathways”. The investigations contribute to the assessment of the mechanisms and potential risks of CO2 leakage along boreholes drilled through the sedimentary overburden above subseafloor storage formations in the North Sea. A total of 50 abandoned wells were investigated in the British sector of the North Sea with the ship's single- and multi-beam echosounder systems, and at 4 selected boreholes showing hydroacoustic gas release the gases, water column and sediments were sampled and gas fluxes were quantified during ROV dives. In addition, the 9 abandoned wells in the GEOSTOR study area in the central part of the German North Sea sector were surveyed hydroacoustically. Biogenic methane from shallow gas accumulations in the upper 1000 m of the sediment strata is emitted at the abandoned wells. It rises to the seafloor along a migration pathway introduced by the drilled borehole. The cruise allowed to enlarge the dataset on gas leakage at abandoned wells collected during previous North Sea cruises and increase our understanding of the underlying mechanism that serves as an analog for CO2 leakage from storage units perforated by old wells. (Alkor-Berichte AL575)

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 210Pb 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 210Pb rain rates. The spatial distribution of MAR is based on the predicted porosity and existing sedimentation rate data. Our findings reveal highest MAR and 210Pb rain rates to occur in two parallel belt structures that align with the general circulation pattern in the Skagerrak. While high 210Pb 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 210Pb rain rate of 4.7 · 1014 dpm yr-1. By comparing atmospheric to total 210Pb rain rates, we further estimate that 24% of the 210Pb 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 close this imbalance. However, we further hypothesize that previous estimates of suspended sediment inputs into the North Sea might have been underestimated, considering recently revised and elevated estimates on coastal erosion rates in the surrounding region of the North Sea.