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 < 10−1) generally exhibit negative values (δ98/95Mo ∼ −0.8 ‰). These δ98/95Mo values are consistent with isotopic fractionations due to the adsorption of seawater Mo onto Fe and Mn oxides, respectively. The aforementioned positive and negative values are connected by a positive correlation between δ98/95Mo values and the Fe/Mn ratios of samples with Fe/Mn ∼ 10−1–101. The positive correlation can be explained by the mixing of δ98/95Mo in Fe and Mn oxides. Based on these data, we propose that the measured δ98/95Mo trends for the modern hydrothermal Fe-Mn oxides can be reproduced using a simple mass-balance calculation with both modern seawater δ98/95Mo and Mo isotopic fractionations due to the adsorption of Mo onto Fe and Mn oxides. By applying this mass-balance calculation to published Fe/Mn and δ98/95Mo data on ancient Fe- and Mn-rich sedimentary rocks, we estimate Archean and Paleoproterozoic seawater δ98/95Mo and their deviations (Δ98/95Mo) from modern seawater δ98/95Mo. The long-term evolution of seawater δ98/95Mo inferred as a result suggests extensive deposition of Fe and Mn oxides at ∼2.3–2.2 Ga and an expansion of euxinic conditions at ∼1.9 Ga, which are broadly consistent with the proposed redox evolution of the ocean–atmosphere system during the Archean and Paleoproterozoic.