[1] 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 redoxsensitive element. Manganese-rich hemipelagic sediments from the eastern tropical Pacific have a mean characteristic Mo isotope signature (d 98/95 Mo = À0.49 ± 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 (d 98/95 Mo = 1.64 ± 0.04%, 2 SDOM with n = 136). Both of these values are in contrast to highly sulfidic (>11 mM 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 d 98/95 Mo = $1.6% and Mn-rich sediments where d 98/95 Mo = approximately À0.5%, with a minor contribution from euxinic settings where H 2 S aq > 11 mM.

Experiments were carried out to determine molybdenum isotope fractionation associated with adsorption to pyrite. Results show that the Mo isotope composition of the aqueous solution becomes progressively heavier as Mo is adsorbed, with a Mo isotope fractionation as large as 2.9‰. This fractionation is larger than observed for typical anoxic continental margin marine sediments (e.g., ~0.7‰), suggesting that Mo adsorption to pyrite is not the dominant process operating in these environments. However, our adsorption results do suggest the possibility that anoxic fractionation processes could impart an isotope signature within the geologic record similar to the isotope fractionation observed under oxygenated conditions. Additional experiments were conducted at high (~100 µM) dissolved Mo concentrations, but at varying pH and ∑H 2 S concentrations, and in each case these experiments promoted Mo-sulfide precipitation. Even though the experiments were conducted under differing conditions and produced different amounts of precipitate, the results suggest a constant fractionation of ~0.9‰ associated with this Mo removal process. This fractionation is more consistent with that inferred for anoxic continental margin marine sediments, suggesting that a process similar to Mo-sulfide precipitation, rather than an adsorption process, may be responsible for the Mo isotope compositions observed in these environments. The findings of this study suggest that, regardless of the geochemical mechanism employed, sediment Mo sequestration under anoxic conditions may impart a significant isotopic fractionation relative to parent seawater.

Highlights: Elevated carbonate concentrations coincide with maximum lake level Negative excursions in both  13 C and  15 N coincide with maximum lake level Results imply changes in lake chemistry during lake level oscillation centered at 1879