期刊
GEOCHIMICA ET COSMOCHIMICA ACTA
卷 297, 期 -, 页码 101-119出版社
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2021.01.008
关键词
Chromium oxidation; Manganese oxidation; Chromium isotope fractionation; Paleoredox proxy; Manganese oxide
资金
- National Aeronautics and Space Administration through the NASA Astrobiology Institute [NNA15BB03A]
- Chemical Oceanography program [NSF OCE-1558738, NSF OCE-1558692]
Stable chromium isotopes have been used as a paleoredox proxy to track atmospheric oxygenation, with previous belief that Cr oxidation is limited to terrestrial settings. However, recent research shows rapid marine Cr(III) oxidation linked to microbial Mn(II) oxidation, challenging the traditional view. Initial partial oxidation of Cr(III) to Cr(VI) can cause a small isotope fractionation, calling for a reevaluation of the link between Cr isotope values and atmospheric oxygenation.
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 lowoxygen 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. (C) 2021 The Author(s). Published by Elsevier Ltd.
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