Marine microbial Mn(II) oxidation mediates Cr(III) oxidation and isotope fractionation

Abstract The stable chromium (Cr) isotope system has been used extensively over the past decade as a paleoredox proxy. The prevailing view has been that Cr oxidation is essentially limited to terrestrial settings and that the Cr isotope system is thus ideally suited to track atmospheric oxygenation. Further, although manganese(III,IV) oxide minerals (MnOx) are generally accepted to be the primary naturally occurring oxidants for the oxidation of insoluble Cr(III) to Cr(VI) and may fractionate Cr isotopes, there are conflicting reports in the literature regarding the direction and magnitude of Cr isotope fractionation by MnOx. We investigated Cr isotope fractionation during coupled Mn(II) and Cr(III) oxidation by laboratory cultures of marine Mn(II)-oxidizing bacteria and by natural microbial communities in seawater samples collected from the seasonally low-oxygen zone of the Chesapeake Bay, Maryland, USA. We found evidence for rapid marine Cr(III) oxidation linked to microbial Mn(II) oxidation. We also show that the initial partial oxidation of Cr(III) to Cr(VI) can produce an isotope fractionation of 0.8 ± 0.1‰. Further, given that Cr(VI) reduction and sequestration into marine sediments can lead to large isotopic fractionations (>1‰), evidence for rapid Cr oxidation in marine environments contradicts the idea that Cr isotope variations in the sedimentary record must be linked to subaerial oxidative weathering. As oxygen can accumulate in the surface ocean even under an anoxic atmosphere, these results call for a reevaluation of how we can link Cr isotope values to atmospheric oxygenation.