4 Special Session: A Life in Earth History from Tectonics to Climate I: The Scientific Legacy of Paul F. Hoffman

Sunday, 27 October 2013
8:00 AM - 12:00 PM
Mile High Ballroom 4EF

A CR ISOTOPE PERSPECTIVE ON PROTEROZOIC REDOX EVOLUTION

Paper number: 4-11
Program: Special Sessions
Time: 11:00 AM – 11:15 AM

Authors

Abstract

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.