Measuring terrestrial Precambrian atmospheric oxygen levels: An isotopic approach
David Auerbach Colwyn1, Noah Planavsky
2, Christopher T. Reinhard
3, J Barry Maynard
4, Axel Hofmann
5, Nadezhda Alfimova
6, Xiangli Shaun Wang
7, Bleuenn Gueguen
7 and Dan Asael
8, (1)Yale University, Geology & Geophysics, New Haven, CT, United States, (2)Yale University, Department of Earth and Planetary Sciences, New Haven, CT, United States, (3)Georgia Institute of Technology Main Campus, School of Earth and Atmospheric Sciences, Atlanta, United States, (4)University of Cincinnati, Department of Geology, Cincinnati, OH, United States, (5)University of Johannesburg, Auckland Park, South Africa, (6)Russian Academy of Sciences, Institute of Precambrian Geology and Geochronology, St. Petersburg, Russia, (7)Yale University, New Haven, CT, United States, (8)Yale University, Dept. of Geology and Geolphysics, New Haven, CT, United States
Contact First Author: David Auerbach Colwyn; david.colwyn@colorado.edu
Previously Published Material: Part (<50%) of the Cr isotope data have been presented at the 2014 AGU Fall Meeting.
English Abstract:
The history of atmospheric oxygen levels through Earth history has been the topic of intense interest since at least the 1960s (Cloud, 1968). By 2000, a two-step model had become strongly established in Precambrian research in which oxygen is hypothesized to have remained relatively constant except for large, geologically rapid, unidirectional increases in O
2 in the early Paleoproterozoic (the “Great Oxidation Event”) and the Neoproterozoic. Work in the last decade has begun to add detail to this simple view, discriminating between different O
2 levels rather than simply assessing its presence or absence (e.g., Lyons et al., 2014). While typically less common or complete than marine records, terrestrial records such as paleosols form in direct contact with the atmosphere and thus provide direct records of atmospheric composition (e.g., Rye and Holland, 1998).
Uranium and chromium are redox-sensitive isotope systems that are thought to behave relatively simply in terrestrial soils. Uranium fractionation occurs at O2 levels above ~10-5 times present atmospheric levels (PAL) (Partin et al., 2013), while chromium responds above ~10-3 PAL (Crowe et al., 2013). Here we present paired U and Cr isotope measurements on a set of globally distributed paleosols that span the time interval from Mesoarchean (≥3.0 Ga) to Mesoproterozoic (1.1 Ga). These paleosols are considered to have excellent preservation based on textural, mineralogical, and chemical examination. By pairing these two isotope systems, we construct a quantitatively constrained history of oxygen levels, rather than simply assessing “almost none” versus “more than almost none.” The record shows, perhaps surprisingly, that O2 levels remained below ~10-5 PAL until at least 2.45 Ga, just before the GOE. However, other records (e.g., Anbar et al., 2007; Crowe et al., 2013) suggest that higher levels of O2 were present at least locally prior to the GOE for perhaps several hundred million years. Reconciling these results with the rapid mixing time of the atmosphere will be a future challenge, and will inform our understanding of the critical transitions in Earth history occurring during the late Archean and Proterozoic, including the Huronian glaciation, the expansion of oxidative photosynthesis, and the appearance of eukaryotes.
