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

Abstract

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 Isotope Fingerprinting of Microbial Sulfate Reduction in Seep Carbonate Rocks

“…For seep carbonate rocks dominated by aragonite, precipitation occurred near the sediment-seawater interface, hence recording information from both ambient bottom waters and sedimentary pore water (e.g., Hu et al, 2014;Lin et al, 2021). In marine sediments, Mo can be hosted in different phases, most notably in Fe-Mn oxide, organic matter, and iron sulfides (e.g., Dahl et al, 2016;Jia et al, 2023a;Nan et al, 2023;Phillips & Xu, 2021). Therefore, the δ 98 Mo composition of bulk sediment or bulk authigenic carbonate samples integrates the sum of the δ 98 Mo signature of each of these Mo-bearing phases.…”