A U and Cr Isotope Record of Archean Oxygen Levels

Abstract:

The exact timing of the emergence of oxygenic photosynthesis remains poorly constrained and estimates of ocean-atmosphere oxygen levels in Earth's early history are only defined in very broad strokes. Uranium (U) and chromium (Cr) isotopes are emerging paleoredox proxies that can move forward our understanding of Archean surface oxygen levels. We will present a new suite of sedimentary U and Cr isotope data and a new framework to interpret the existing Archean record. Archean samples display essentially the same range of U isotope variability as modern sedimentary rocks, indicating the formation of soluble U(VI) in weathering environments during the Archean. This reaction requires surface oxygen levels well above those characteristic of a prebiotic atmosphere—by several orders of magnitude. Cr isotope variability is the Archean, with limited exceptions is muted. However, rare cases of Cr isotope variability require surface oxygen levels above those required by the U isotope record and above the maximum atmospheric oxygen threshold linked to ‘atmospheric rare S isotope anomalies’. These results provide evidence for intermittent surface oxygenation preceding the Great Oxidation Event and suggest the possibility of significantly variable oxygen levels at Earth's surface during the Archean.