Diagenetic and Hydrothermal Processes Produce Heavy Molybdenum Isotope Signatures in Pelagic Sediments of the North and South Pacific

Abstract Molybdenum isotopes (δ 98 Mo) have been used for more than two decades to investigate past‐ocean redox conditions due to the distinct behavior of this element in oxic and anoxic environments. The oxic sink is generally accepted to have a near‐constant δ 98 Mo value of −0.7‰, a value derived primarily from the δ 98 Mo signatures of Fe–Mn crusts and nodules. However, little information is available on the downcore evolution of δ 98 Mo in oxic pelagic sediments, which constitute the largest fraction of the oxic Mo sink. Here, we present δ 98 Mo authigenic data for two deep‐sea sites with oxygenated pore waters covering 90 Myr of sedimentation, one from the North Pacific Gyre and another from the South Pacific Gyre. In both records, sedimentary δ 98 Mo auth values systematically increase downcore from −0.5‰ at the sediment surface to a maximum of ∼+0.3‰. This shift cannot be explained by changes in the ocean's redox state but is likely related to the diagenetic conversion of Mn‐oxides into different mineral phases. Our findings indicate that the δ 98 Mo signal generated by adsorption at the sediment‐water interface is not preserved during burial, suggesting that the canonical −0.7‰ value of the oxic sink requires re‐evaluation. In the lower section of the South Pacific Site, we also identify two positive excursions reaching δ 98 Mo auth values as high as +1.7‰. This portion of the site was influenced by near‐field hydrothermal processes, and we propose that these large excursions reflect Mo adsorption onto Fe (oxyhydr)oxides.