Isotopically heavy molybdenum burial in oxic pelagic sediments and implications for paleo-redox reconstruction

Abstract

Molybdenum (Mo) concentrations and isotopic signatures (δ⁹⁸Mo) are useful proxies for reconstructing paleo-redox conditions and tracing material cycling in subduction systems. While the Mo isotope geochemistry of diverse geological settings has been extensively studied over the past two decades, the diagenetic effects on Mo behavior in pelagic oxic sediments remain poorly understood. Here, we investigate Mo cycling in Mn-rich pelagic sediments in an 847 cm long sediment core from the deep Pacific Ocean. We observe a systematic downcore decrease in Mo content (28.0 to 6.10 μg/g) together with increasing Mo isotope ratios (δ⁹⁸Mo: −0.52 ‰ to +1.32 ‰). Synchrotron X-ray absorption spectroscopy shows that the average oxidation state of Mn decreases with depth, which is consistent with the transformation of Mn minerals during sediment burial. In contrast to the canonical δ⁹⁸Mo (−0.7 ‰) of Fe-Mn nodules and crusts, the depth-integrated δ⁹⁸Mo value of the Mn-rich sediments is unusually high, likely due to recycling of isotopically light Mo. If our findings are representative for larger areas of the deep ocean, they could potentially challenge the conventional global Mo budget. A revised isotopically heavier oxic Mo sink in the global ocean’s Mo budget would imply a more extensive expansion of oceanic anoxia and/or reduction of the oxic sink during geological periods when seawater had lower δ⁹⁸Mo values.