A framework for understanding Mo isotope records of Archean and Paleoproterozoic Fe- and Mn-rich sedimentary rocks: Insights from modern marine hydrothermal Fe-Mn oxides

Abstract Molybdenum isotopic compositions (δ98/95Mo) of Archean and Paleoproterozoic Fe- and Mn-rich sedimentary rocks have been used to investigate local accumulations of O2 in an O2-lean ocean. Previous studies interpret that these δ98/95Mo values would be a representation of the global minimum for δ98/95Mo of contemporaneous seawater and would, therefore, link to global paleoredox conditions. Here, we present new δ98/95Mo data on modern marine hydrothermal Fe-Mn oxides, for a wide range of Fe/Mn ratios, from five hydrothermal systems. Samples composed mainly of Fe oxides (Fe/Mn > 10) show positive values (δ98/95Mo ∼ +0.7‰), whereas those containing substantial amounts of Mn oxides (Fe/Mn