Postdepositional Behavior of Molybdenum in Deep Sediments and Implications for Paleoredox Reconstruction

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. 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 parts per thousand. 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. 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 delta 98Mo values in the solid phasePotential postdepositional effects need to be assessed when using Mo-based proxies for paleoredox reconstruction