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      Large mass-independent oxygen isotope fractionations in mid-Proterozoic sediments: Evidence for a low-oxygen atmosphere?

      Planavsky, Noah J.; Reinhard, Christopher T.; Isson, Terry T.; Ozaki, Kazumi; Crockford, Peter W.
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      Planavsky_Large mass-independent oxygen isoptope....pdf
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      DOI
       10.1089/ast.2019.2060
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      Planavsky, N. J., Reinhard, C. T., Isson, T. T., Ozaki, K., & Crockford, P. W. (2020). Large mass-independent oxygen isotope fractionations in mid-Proterozoic sediments: Evidence for a low-oxygen atmosphere? Astrobiology, 20(5), 628–636. https://doi.org/10.1089/ast.2019.2060
      Permanent Research Commons link: https://hdl.handle.net/10289/13753
      Abstract
      Earth's ocean-atmosphere system has undergone a dramatic but protracted increase in oxygen (O₂) abundance. This environmental transition ultimately paved the way for the rise of multicellular life and provides a blueprint for how a biosphere can transform a planetary surface. However, estimates of atmospheric oxygen levels for large intervals of Earth's history still vary by orders of magnitude-foremost for Earth's middle history. Historically, estimates of mid-Proterozoic (1.9-0.8 Ga) atmospheric oxygen levels are inferred based on the kinetics of reactions occurring in soils or in the oceans, rather than being directly tracked by atmospheric signatures. Rare oxygen isotope systematics-based on quantifying the rare oxygen isotope ¹⁷O in addition to the conventionally determined ¹⁶O and ¹⁸O-provide a means to track atmospheric isotopic signatures and thus potentially provide more direct estimates of atmospheric oxygen levels through time. Oxygen isotope signatures that deviate strongly from the expected mass-dependent relationship between ¹⁶O, ¹⁷O, and ¹⁸O develop during ozone formation, and these "mass-independent" signals can be transferred to the rock record during oxidation reactions in surface environments that involve atmospheric O₂. The magnitude of these signals is dependent upon 𝘱O₂, 𝘱CO₂, and the overall extent of biospheric productivity. Here, we use a stochastic approach to invert the mid-Proterozoic Δ¹⁷O record for a new estimate of atmospheric 𝘱O₂, leveraging explicit coupling of 𝘱O₂ and biospheric productivity in a biogeochemical Earth system model to refine the range of atmospheric 𝘱O₂ values that is consistent with a given observed Δ¹⁷O. Using this approach, we find new evidence that atmospheric oxygen levels were less than ∼1% of the present atmospheric level (PAL) for at least some intervals of the Proterozoic Eon.
      Date
      2020
      Type
      Journal Article
      Publisher
      Mary Ann LIebert
      Rights
      This is an author's accepted version of an article published in Astrobiology. © 2020 Mary Ann LIebert, Inc. Final publication is available from Mary Ann Liebert, Inc., publishers http://dx.doi.org/10.1089/ast.2019.2060
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