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 (MnO_x) 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 MnO_x. 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.

在过去的十年中，稳定的铬（Cr）同位素系统已广泛用作古氧代用品。普遍的观点是，Cr的氧化作用基本上仅限于陆地环境，因此Cr同位素系统非常适合跟踪大气的氧合作用。此外，尽管锰（III，IV）氧化物矿物质（MnO _x）通常被认为是不溶性Cr（III）氧化为Cr（VI）的主要天然氧化剂，但可能会分馏Cr同位素，但有相互矛盾的报道。 MnO _x分离Cr同位素的方向和幅度的文献。我们研究了海洋Mn（II）氧化细菌的实验室培养以及从切萨皮克湾季节性低氧带采集的海水样品中的天然微生物群落，研究了耦合的Mn（II）和Cr（III）氧化过程中的Cr同位素分馏，美国马里兰州。我们发现了与微生物Mn（II）氧化有关的快速海洋Cr（III）氧化的证据。我们还表明，Cr（III）最初部分氧化为Cr（VI）可以产生0.8±0.1‰的同位素分数。此外，鉴于Cr（VI）的还原和螯合到海洋沉积物中会导致较大的同位素分馏（> 1‰），因此在海洋环境中快速Cr氧化的证据与沉积记录中Cr同位素变化必须与陆上大气联系的观点相矛盾。氧化风化。