The impact of postdepositional alteration on iron- and molybdenum-based redox proxies

Abstract Ratios of (un)reactive iron species, authigenic molybdenum contents (Moauth), and isotope compositions (?98Moauth) in sedimentary rocks are geochemical proxies that widely used to reconstruct past marine redox states, which have been calibrated modern settings covering oxic euxinic conditions. However, syn- postdepositional processes can result alterations ambiguities proxy-derived signals challenge the validity paleoreconstructions. We present new data from organic-rich sediments two oxygen minimum zone Gulf California Peruvian margin. The results show Mo is fully immobilized shortly after deposition by reaction with hydrogen sulfide (H2S) produced during organoclastic sulfate reduction. Thus, any H2S deeper sediment (e.g., reduction coupled anaerobic methane oxidation) leaves initially deposited concentrations ?98Mo signatures unaltered, supports robustness Mo-based proxies. In contrast, Fe speciation reveal continued pyritization due constant exposure minerals H2S. Importantly, both bound oxides carbonates (highly reactive Fe) also poorly sheet silicates) undergo early diagenesis. This process generates Fe-based proxy falsely imply ferruginous or