Molybdenum behavior during early diagenesis: Insights from Mo isotopes

We present molybdenum isotope and concentration data from 14 sites in the eastern Pacific from the central California to the Peru margin. The environments studied have been chosen to represent a broad range in oxidation‐reduction (redox) potential, which provide a framework for the behavior of this redox‐sensitive element. Manganese‐rich hemipelagic sediments from the eastern tropical Pacific have a mean characteristic Mo isotope signature ( δ 98/95 Mo = −0.49 ± 0.04‰, two times the standard deviation of the mean (2 SDOM) with n = 14) that reflects fractionation between ocean water and authigenic Mo associated with Mn oxides. Authigenic Fe‐Mo‐S deposits from reducing continental margin settings also have a characteristic Mo isotopic signature ( δ 98/95 Mo = 1.64 ± 0.04‰, 2 SDOM with n = 136). Both of these values are in contrast to highly sulfidic (>11 μ M H 2 S aq ) restricted basin environments, which contain Mo isotope values analytically indistinguishable from seawater. In terms of the Mo isotope composition, the modern oceanic Mo sink is dominated by continental margin “type” environments where δ 98/95 Mo = ∼1.6‰ and Mn‐rich sediments where δ 98/95 Mo = approximately −0.5‰, with a minor contribution from euxinic settings where H 2 S aq > 11 μ M.