Paper No. 9
Presentation Time: 11:00 AM
A CR ISOTOPE PERSPECTIVE ON PROTEROZOIC REDOX EVOLUTION
PLANAVSKY, Noah, Geology and Geophysics, Yale University, New Haven, CT 06520, REINHARD, Christopher T., Division of Geological and Planetary Sciences, Caltech, Pasadena, CA 91125, WANG, Xiangli, University of Illinois at Urbana-Champaign, Urbana, IL 61820, FISCHER, Woodward W., Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, THOMPSON, Danielle, Earth Science, Carleton University, Ottawa, ON K1S 5B6, RAINBIRD, Robert, Natural Resources Canada, Geological Survey of Canada, Ottawa, ON K1A 0E8, JOHNSON, Thomas M., Geology, University of Illinois at Urbana-Champaign, Urbana, IL 61801 and LYONS, Timothy W., Department of Earth Sciences, University of California, Riverside, CA 92521, planavsky@gmail.com
The Cr isotope composition of sedimentary rocks is controlled by redox processes, with both theoretical and experimental studies indicate that Cr isotopes undergo limited fractionation during non-redox-dependent transformations. However, the oxidation and reduction of Cr produce large isotope fractionations—this provides a logical framework for using the Cr isotopic composition of ancient sedimentary rocks to track redox processes. Since the valence state of Cr in igneous rocks is exclusively Cr(III), the initial Cr reservoir would have been stable under reducing (or poorly oxidizing) conditions. Given that manganese (Mn) oxidation in terrestrial settings seems to be essential for a fully oxidative Cr cycle, Cr isotopes can be used to track atmospheric oxygenation.
To date, iron formation lithologies have been key in the construction of Cr isotope records. Archean iron formations typically record near crustal Cr isotope values, suggesting generally low atmospheric oxygen, which prevented significant Cr redox cycling in Earth surface environments. In contrast, there is far more variability in the late Neoproterozoic and Phanerozoic iron formation and ironstone Cr isotope record, providing a clear signal of significant Cr redox cycling. Surprisingly, we find that iron formation and ironstones deposited between 1.8 Ga and 0.8 Ga also have limited Cr isotope variability despite having significant authigenic Cr enrichments. Authigenic Cr is present in depositional phases as allochems (e.g., iron ooids). This observation of limited Cr isotope fractionations suggests a general lack of Cr redox cycling due to lower environmental oxygen concentrations for much of middle Proterozoic time. Further, we find a marked increasse in sedimentary Cr isotope varability (with δ53Cr values up to +2‰) in middle Neoproterozoic time, providing evidence for a preglacial (ca. 750 Ma) oxygenation event. This new Cr isotope work provides a platform to revisit the role that redox evolution played in controlling the onset of Neoproterzoic glacation and metazoan diversfacation.
