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Elevated dissolved iron concentrations in the methanic zone are typical geochemical signatures of rapidly accumulating marine sediments. These sediments are often characterized by co-burial of iron oxides with recalcitrant aromatic organic matter of terrigenous origin. Thus far, iron oxides are predicted to either impede organic matter degradation, aiding its preservation, or identified to enhance organic carbon oxidation via direct electron transfer. Here, we investigated the effect of various iron oxide phases with differing crystallinity (magnetite, hematite, and lepidocrocite) during microbial degradation of the aromatic model compound benzoate in methanic sediments. In slurry incubations with magnetite or hematite, concurrent iron reduction, and methanogenesis were stimulated during accelerated benzoate degradation with methanogenesis as the dominant electron sink. In contrast, with lepidocrocite, benzoate degradation, and methanogenesis were inhibited. These observations were reproducible in sediment-free enrichments, even after five successive transfers. Genes involved in the complete degradation of benzoate were identified in multiple metagenome assembled genomes. Four previously unknown benzoate degraders of the genera Thermincola (Peptococcaceae, Firmicutes) , Dethiobacter (Syntrophomonadaceae, Firmicutes), Deltaproteobacteria bacteria SG8_13 (Desulfosarcinaceae, Deltaproteobacteria), and Melioribacter (Melioribacteraceae, Chlorobi) were identified from the marine sediment-derived enrichments. Scanning electron microscopy (SEM) and catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) images showed the ability of microorganisms to colonize and concurrently reduce magnetite likely stimulated by the observed methanogenic benzoate degradation. These findings explain the possible contribution of organoclastic reduction of iron oxides to the elevated dissolved Fe 2+ pool typically observed in methanic zones of rapidly accumulating coastal and continental margin sediments.

Molybdenum (Mo) is a trace element sensitive to oceanic redox conditions. The fidelity of sedimentary Mo as a paleoredox proxy of coeval seawater depends on the extent of Mo remobilization during postdepositional processes. Here we present the Mo content and isotope profiles for deep sediments from the Nankai Trough, Japan. The Mo signature suggests that these sediments have experienced extensive early diagenesis and hydrothermal alteration at depth. Iron (Fe)‐manganese (Mn) (oxyhydr)oxide alteration combined with Mo thiolation leads to a more than twenty‐fold enrichment of Mo within the sulfate reduction zone. Hydrothermal fluids and Mo adsorption onto Fe‐Mn (oxyhydr)oxides cause extremely negative Mo‐isotope values at the underthrust zone. These postdepositional Mo signals might be misinterpreted as expanded anoxia in the water column. Our findings highlight the importance of constraining postdepositional effects on Mo‐based proxies during paleoredox reconstruction. Plain Language Summary Molybdenum (Mo) serves as a proxy for marine paleoredox reconstruction, offering valuable insights into how the oceanic oxygen level evolves with Earth's climate. The reliability of the Mo proxy depends on how much Mo is transferred in and out of the sediments after deposition. In this study, we investigate the Mo content and Mo‐isotope composition of sediments, along with porewater geochemistry, from the International Ocean Discovery Program (IODP) Site C0023 down to 1,200 m below seafloor. We find that post‐depositional remobilization of Mo leads to Mo enrichments in sulfidic intervals of the sediment column. By contrast, at the underthrust hydrothermal zone, we suggest that Mo from hydrothermal fluids is mainly adsorbed onto mineral oxides resulting in the Mo‐isotope value to as low as −1.59‰. These Mo signals deviate from their primary values during deposition, but share some similarities with those derived from a range of marine redox conditions. As such, future studies need to evaluate the Mo behavior after burial before using this proxy for paleoredox reconstruction. Key Points Fe‐Mn (oxyhydr)oxide alteration combined with Mo thiolation leads to a more than 20‐fold enrichment of Mo within the sulfate‐methane transition zone Adsorption of hydrothermally derived Mo onto Fe‐Mn (oxyhydr)oxide results in extremely low δ98Mo values in the solid phase Potential postdepositional effects need to be assessed when using Mo‐based proxies for paleoredox reconstruction

Dissimilatory iron reduction (DIR) is suggested to be one of the earliest forms of microbial respiration. It plays an important role in the biogeochemical cycling of iron in modern and ancient sediments. Since microbial iron cycling is typically accompanied by iron isotope fractionation, stable iron isotopes are used as tracer for biological activity. Here we present iron isotope data for dissolved and sequentially extracted sedimentary iron pools from deep and hot subseafloor sediments retrieved in the Nankai Trough off Japan. Dissolved iron (Fe(II) aq ) is isotopically light throughout the ferruginous sediment interval but some samples have exceptionally light isotope values. Such light values have never been reported in natural marine environments and cannot be solely attributed to DIR. We show that the light isotope values are best explained by a Rayleigh distillation model where Fe(II) aq is continuously removed from the pore water by adsorption onto iron (oxyhydr)oxide surfaces. While the microbially mediated Fe(II) aq release has ceased due to an increase in temperature beyond the threshold of mesophilic microorganisms, the abiotic adsorptive Fe(II) aq removal continued, leading to uniquely light isotope values. These findings have important implications for the interpretation of dissolved iron isotope data especially in deep subseafloor sediments.

The area around the Antarctic Peninsula (AP) is facing rapid climatic and environmental changes, with so far unknown impacts on the benthic microbial communities of the continental shelves. In this study, we investigated the impact of contrasting sea ice cover on microbial community compositions in surface sediments from five stations along the eastern shelf of the AP using 16S ribosomal RNA (rRNA) gene sequencing. Redox conditions in sediments with long ice-free periods are characterized by a prevailing ferruginous zone, whereas a comparatively broad upper oxic zone is present at the heavily ice-covered station. Low ice cover stations were highly dominated by microbial communities of Desulfobacterota (mostly Sva1033, Desulfobacteria, and Desulfobulbia), Myxococcota, and Sva0485, whereas Gammaproteobacteria, Alphaproteobacteria, Bacteroidota, and NB1-j prevail at the heavy ice cover station. In the ferruginous zone, Sva1033 was the dominant member of Desulfuromonadales for all stations and, along with eleven other taxa, showed significant positive correlations with dissolved Fe concentrations, suggesting a significant role in iron reduction or an ecological relationship with iron reducers. Our results indicate that sea ice cover and its effect on organic carbon fluxes are the major drivers for changes in benthic microbial communities, favoring potential iron reducers at stations with increased organic matter fluxes.

The study investigates the in-situ strength of sediments across a plate boundary décollement using drilling parameters recorded when a 1180-m-deep borehole was established during International Ocean Discovery Program (IODP) Expedition 370, Temperature-Limit of the Deep Biosphere off Muroto (T-Limit). Information of the in-situ strength of the shallow portion in/around a plate boundary fault zone is critical for understanding the development of accretionary prisms and of the décollement itself. Studies using seismic reflection surveys and scientific ocean drillings have recently revealed the existence of high pore pressure zones around frontal accretionary prisms, which may reduce the effective strength of the sediments. A direct measurement of in-situ strength by experiments, however, has not been executed due to the difficulty in estimating in-situ stress conditions. In this study, we derived a depth profile for the in-situ strength of a frontal accretionary prism across a décollement from drilling parameters using the recently established equivalent strength (EST) method. At site C0023, the toe of the accretionary prism area off Cape Muroto, Japan, the EST gradually increases with depth but undergoes a sudden change at ~ 800 mbsf, corresponding to the top of the subducting sediment. At this depth, directly below the décollement zone, the EST decreases from ~ 10 to 2 MPa, with a change in the baseline. This mechanically weak zone in the subducting sediments extends over 250 m (~ 800–1050 mbsf), corresponding to the zone where the fluid influx was discovered, and high-fluid pressure was suggested by previous seismic imaging observations. Although the origin of the fluids or absolute values of the strength remain unclear, our investigations support previous studies suggesting that elevated pore pressure beneath the décollement weakens the subducting sediments.

The burial of organic matter (OM) within fine-grained continental shelf sediments represents one of the major long-term sinks of carbon. We investigated the key factors controlling organic carbon burial in sediments of the North Sea by using the Helgoland Mud Area (HMA) as a natural test field. The HMA represents the most significant depocentre of fine-grained and organic-rich sediments in the German Bight (SE North Sea). We examined factors including sedimentation and accumulation rate, sediment-mixing rate, grain size, total organic carbon (TOC) content, and aerobic remineralisation rate. Highest sedimentation rates (SRs) of up to ∼ 4.5 mm yr−1 and average TOC contents of 2 wt % were found in the southern part of the HMA, which is under the influence of the Elbe River outflow, reaching organic carbon burial efficiencies of >65 %. Sedimentation rates 4 times lower and the lowest TOC contents (0.7 wt %–1.0 wt %) were found in the shallow eastern part of the research area, with the lowest organic carbon burial efficiencies being 30 %. High sedimentation rates are known to limit oxygen exposure time, thereby enhancing OM preservation. Our data support this finding, demonstrating and confirming that sedimentation rate is the key factor determining organic carbon burial efficiency (OC BE) and long-term sedimentary carbon storage. In the southern part of the HMA, close to the outflow of the Elbe River, the OM being degraded is primarily of terrigenous origin, while, in the central and northern parts of the HMA, a mixture of marine and terrigenous OM is remineralised. At the sites dominated by the degradation of marine organic matter, as found in the western and northwestern HMA, the organic carbon burial efficiency is lower and fluctuates around 55 %. The burial efficiency of OM is highest in sedimentary habitats characterised by high sedimentation rates and OM of terrigenous sources. Sediment-mixing rates were highest in the northwestern HMA, where the highest bottom-trawling activity is also reported. The comparison of sites similar in depositional characteristics but different in bottom-trawling intensity suggests that, in the area of intense bottom trawling in the northwestern HMA, the sequestration of OM is reduced by around 30 %. The annual burial flux of organic carbon in the HMA amounts to an average of 22.5 g C m−2 yr−1. Considering the strong tidal currents in the shallow HMA, the burial flux is exceptionally high and even compares with those reported for the deeper Skagerrak and Norwegian Trough (∼ 10 to 66 g C m−2 yr−1), which are the main depocentres for fine-grained and organic-rich sediments in the North Sea. For the entire HMA, the total annual organic carbon accumulation amounts to 0.011 Tg C yr−1. These findings highlight the importance of depocentres for fine-grained sediments as important carbon sinks: while the area of the HMA represents only 0.09 % of the North Sea, it stores 0.76 % of the total annual accumulated organic carbon in this shelf sea area.

The low δ56Fe values of dissolved iron liberated by microbial iron reduction are characteristic of many shallow subsurface sediments and – if not significantly changed within the oxic sediment layer – the related benthic Fe fluxes into the water column. Here, we decipher whether stable Fe isotope signatures in pore water and the respective solid-phase sediment samples are also useful for unraveling the processes driving Fe liberation in deeper methanic sediments. We investigated the fine-grained deposits of the Helgoland mud area, North Sea, where Fe reduction in the methanic subsurface sediments was previously suggested to be coupled to methanogenic fermentation of organic matter and anaerobic methane oxidation. In the evaluated subsurface sediments, a combination of iron isotope geochemistry with reactive transport modeling for the deeper methanic sediments hints at a combination of processes affecting the stable isotope composition of dissolved iron. However, the dominant process releasing Fe at depth does not seem to lead to notable iron isotope fraction. Under the assumption that iron reducing microbes generally prefer isotopically light iron, the deep Fe reduction in this setting appears to be “pseudo-abiotic”: if fermentation is the main reason for Fe release at depth, the fermenting bacteria transfer electrons directly or indirectly to Fe(III), but our data do not indicate notable related isotopic fractionation. Our findings strongly contribute to the debate on the pathway for deep Fe2+ release by showing that the main underlying process is mechanistically different to the microbial Fe reduction dominating in the shallow sediments and encourages future studies to focus on the fermentative degradation of organic matter as a source of dissolved iron in methanic sediments.

Chemolithoautotrophic Hydrogenovibrio are ubiquitous and abundant at hydrothermal vents. They can oxidize sulfur, hydrogen, or iron, but none are known to use all three energy sources. This ability though would be advantageous in vents hallmarked by highly dynamic environmental conditions. We isolated three Hydrogenovibrio strains from vents along the Indian Ridge, which grow on all three electron donors. We present transcriptomic data from strains grown on iron, hydrogen, or thiosulfate with respective oxidation and autotrophic carbon dioxide (CO2) fixation rates, RubisCO activity, SEM, and EDX. Maximum estimates of one strain’s oxidation potential were 10, 24, and 952 mmol for iron, hydrogen, and thiosulfate oxidation and 0.3, 1, and 84 mmol CO2 fixation, respectively, per vent per hour indicating their relevance for element cycling in-situ. Several genes were up- or downregulated depending on the inorganic electron donor provided. Although no known genes of iron-oxidation were detected, upregulated transcripts suggested iron-acquisition and so far unknown iron-oxidation-pathways.

Assessing frequency and extent of mass movement at continental margins is crucial to evaluate risks for offshore constructions and coastal areas. A multidisciplinary approach including geophysical, sedimentological, geotechnical, and geochemical methods was applied to investigate multistage mass transport deposits (MTDs) off Uruguay, on top of which no surficial hemipelagic drape was detected based on echosounder data. Nonsteady state pore water conditions are evidenced by a distinct gradient change in the sulfate (SO42−) profile at 2.8 m depth. A sharp sedimentological contact at 2.43 m coincides with an abrupt downward increase in shear strength from ∼10 to >20 kPa. This boundary is interpreted as a paleosurface (and top of an older MTD) that has recently been covered by a sediment package during a younger landslide event. This youngest MTD supposedly originated from an upslope position and carried its initial pore water signature downward. The kink in the SO42− profile ∼35 cm below the sedimentological and geotechnical contact indicates that bioirrigation affected the paleosurface before deposition of the youngest MTD. Based on modeling of the diffusive re‐equilibration of SO42− the age of the most recent MTD is estimated to be <30 years. The mass movement was possibly related to an earthquake in 1988 (∼70 km southwest of the core location). Probabilistic slope stability back analysis of general landslide structures in the study area reveals that slope failure initiation requires additional ground accelerations. Therefore, we consider the earthquake as a reasonable trigger if additional weakening processes (e.g., erosion by previous retrogressive failure events or excess pore pressures) preconditioned the slope for failure. Our study reveals the necessity of multidisciplinary approaches to accurately recognize and date recent slope failures in complex settings such as the investigated area. Key Points Pore water profiles can be used to identify and date recent mass transport deposits Multidisciplinary studies are needed to assess the complexity of slide deposits Recent slope failure off Uruguay might be related to an earthquake in 1988

Unraveling present-day environmental changes and their link to the biogeochemical cycling of elements, primary productivity, and climate requires identifying the driving forces behind such changes in the past. Marine sediments store information about past environmental and depositional conditions. They consist of particles that are formed in the ocean or derived from the continents and may thus record the conditions in the water column and/or the terrestrial input into the ocean for thousands or millions of years. Diagenetic overprinting of the sediments in different redox zones may alter or even wipe out these primary signals. However, authigenic mineral enrichments and pore water profiles can shed light on the geochemical conditions in the sediment and ultimately on the factors the redox zonation is determined by (sedimentation rate, the upward flux of methane, bioirrigation etc.). The present study focuses on the diagenetic cycling of barium (Ba) and phosphorus (P) in sediments of the Black Sea and the continental margin off Uruguay and Argentina. Furthermore, the value of pore water profile shapes for assessing the occurrence and timing of recent mass-transport deposits (MTDs) at the northwestern rim of the Argentine Basin is investigated. In the open ocean, barite (BaSO4) typically forms in the water column during the decay and settling of organic matter. Barium can therefore be used as a proxy for primary productivity and was as such frequently applied for Cretaceous Oceanic Anoxic Event (OAE) successions - sediments that deposited during widespread oxygen depletion in the water column. The present-day Black Sea shows a stratified, mainly anoxic water column and therefore is considered as modern analog to ocean basins during the OAEs. Pore water profiles and the sediment composition at two sites in the northwestern Black Sea indicate an intense redistribution of Ba at the sulfate-methane transition zone (SMTZ) and the precipitation of authigenic barite at ~1.5-2 m depth. Below the SMTZ, barite is dissolved due to undersaturation. The primary (possibly biogenic) Ba signal is completely erased in deeper sediments below the SMTZ, which is evidence for the limitation of Ba as productivity proxy in settings that are characterized by a shallow SMTZ. The enhanced preservation of organic material due to anoxia in the water column leads to high organic matter concentrations in the sediment and fuels the generation of methane. High diffusive upward fluxes of methane, in turn, result in a shallow SMTZ and a low preservation of barite. Barite particles in the Black Sea surface sediment show shapes that differ from typical biogenic or marine barite. Oversaturation of the bottom water with respect to barite possibly leads to an additional flux of particulate Ba into the sediment. Furthermore, barite precipitation in anoxic waters likely is diminished by the enhanced preservation of organic matter. Thus, even if the Ba-signal would record water column processes, it could not directly be linked to primary productivity. The Black Sea, a freshwater lake during the Pleistocene, was flooded with seawater in the Holocene. Consequently, it was subject to drastic salinity changes during the past ~9 kyrs. In response to increasing salinity and sulfate concentrations in the bottom water, the SMTZ migrated downward to its present depth at ~2 m in the study area. Since barite enrichments are preserved in sulfate-bearing sediments, relict authigenic enrichments of this mineral were assumed to trace the downward movement of the SMTZ. However, transport and reaction modeling considering sedimentation rate, diffusion of sulfate into the sediment, a constant diffusive methane flux from deep sediments, and anaerobic oxidation of methane as a sink for both species revealed that the sediments that were potentially affected by diagenetic barite formation during the downward movement of the SMTZ are at present located below the SMTZ. Thus, they were subject to BaSO4 dissolution and the Ba signal does not provide information with respect to this drastic salinity increase anymore. Sediment cores and pore water profiles of sites at the continental margin off Uruguay and Argentina were examined in order to identify deposits that show transient pore water conditions as a result of recent mass movements. Whereas a linear decrease of sulfate with sediment depth usually relates to steady state pore water conditions, kink- and concaveshaped sulfate profiles may be indicative for submarine landslides. Previous studies on this subject mostly lacked sediment echosounder surveys and/or clear sedimentological evidence for the coupling of non-linear sulfate profiles to mass movements. Furthermore, it remained unclear, how profiles disturbed by mass movements can be discriminated from those related to any other perturbing processes, such as advection or sudden increases of the upward flux of methane. The integration of sediment echosounder, sedimentological, geotechnical, and pore water data from several sites at the continental margin was carried out for the first time in this study. The data reveal that in this very dynamic depositional setting, non-steady state pore water profiles are often, but not exclusively related to mass movements. At sites where mass movement events likely caused the observed non-linearity of the pore water profiles, the diffusive re-equilibration of the sulfate profile was simulated applying a transport and reaction model. In this way, the ages of the MTDs were estimated. At one site, the geochemical simulation revealed an age of the MTD of less than 30 years, which suggested a coupling to a weak earthquake that hit the region in 1988. Limit Equilibrium slope stability analysis revealed that the 1988 earthquake was indeed a likely trigger for slope failure in the investigated area. Sediments at continental margin settings, such as the area off Uruguay and Argentina, receive high inputs of P either adsorbed onto Fe (oxyhydr)oxides, incorporated into organic matter, fish debris or associated with terrigenous material. Previous studies by other authors showed that during early diagenesis, P is not only released into the pore water close to the sediment surface but also at and above the SMTZ. Diagenetic precipitation of apatite and vivianite was suggested to represent a major and irrevocable sink for pore water phosphate. We investigated pore water, the bulk sediment composition, and P and Fe fractions at three stations at the continental slope off Uruguay and Argentina and observed that even below the SMTZ, phosphate is liberated into the pore water. We relate this phosphate release in deep sediments to the reduction of deeply buried Fe (oxyhydr)oxides that passed the SMTZ rapidly and without severe alteration due to extremely high sedimentation rates during the Pleistocene. The process of this deep iron reduction is not yet understood, but apparently it has a major impact on the P cycling at the investigated sites. The formation of authigenic apatite likely plays a role in the investigated sediment intervals. However, with the extraction method applied, we were not able to distinguish between biogenic and authigenic apatite. The pore water profiles reveal that vivianite precipitation does not represent a major sink for dissolved phosphate and iron.Besides the sedimentation rates, the retention potential of the sediment determines the shape of the phosphate profiles. Highest dissolved phosphate concentrations of up to 450 μM are found at a site with high carbonate concentrations in the sediment. The carbonate dilutes the portion of reactive Fe particles and thus diminishes the retention potential of the sediment with respect to phosphate. A third important factor for the redistribution of P (and Fe) in continental margin settings is the extent of the sulfidic zone or rather the time a respective sediment horizon is exposed to sulfidic conditions. A stable SMTZ with a broad sulfidic zone results in a very effective transformation of Fe (oxyhydr)oxides into Fe sulfides and thus prevents the Fe (oxyhydr)oxide reduction within the methanic zone. Compared to nutrient cycling and benthic processes, there has been little research to element redistributions in deeper sediments. This thesis contributes to a better understanding of post-depositional alterations of Ba and P records and to more reliable interpretations of respective fossil records.