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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 zoneAdsorption of hydrothermally derived Mo onto Fe‐Mn (oxyhydr)oxide results in extremely low δ98Mo values in the solid phasePotential postdepositional effects need to be assessed when using Mo‐based proxies for paleoredox reconstruction [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]