Controls on Atmospheric O2: The Anoxic Archean and the Suboxic Proterozoic

Abstract:

Geochemists have now reached consensus that the Archean atmosphere was mostly anoxic, that a Great Oxidation Event (GOE) occurred at around 2.5 Ga, and that the ensuing Proterozoic atmosphere was consistently oxidized [1,2]. Evidence for this broad-scale change in atmospheric composition comes from a variety of sources, most importantly from multiple sulfur isotopes [3,4]. The details of both the Archean and Proterozoic environments remain controversial, however, as does the underlying cause of the GOE. Evidence of ‘whiffs’ of oxygen during the Archean [5] now extend back as far as 3.0 Ga, based on Cr isotopes [6]. This suggests that O₂ was being produced by cyanobacteria well before the GOE and that the timing of this event may have been determined by secular changes in O₂ sinks. Catling et al. [7] emphasized escape of hydrogen to space, coupled with progressive oxidation of the continents and a concomitant decrease in the flux of reduced gases from metamorphism. But hydrogen produced by serpentinization of seafloor could also have been a controlling factor [8]. Higher mantle temperatures during the Archean should have resulted in thicker, more mafic seafloor and higher H₂ production; decreasing mantle temperatures during the Proterozoic should have led to seafloor more like that of today and a corresponding decrease in H₂ production, perhaps by enough to trigger the GOE. Once the atmosphere became generally oxidizing, it apparently remained that way during the rest of Earth’s history. But O₂ levels in the mid-Proterozoic could have been as low at 10^-3 times the Present Atmospheric Level (PAL) [9]. The evidence, once again, is based on Cr isotopes. Possible mechanisms for maintaining such a ‘suboxic’ Proterozoic atmosphere will be discussed. Refs: 1. H. D. Holland, Geochim. Cosmochim. Acta 66, 3811 (2002). 2. H. D. Holland, Philosophical Transactions of the Royal Society B-Biological Sciences 361, 903 (Jun 29, 2006). 3. J. Farquhar, H. Bao, M. Thiemans, Science 289, 756 (2000). 4. C. T. Reinhard, N. J. Planavsky, T. W. Lyons, Nature 497, 100 (May, 2013). 5. A. D. Anbar et al., Science 317, 1903 (Sep, 2007). 6. S. A. Crowe et al., Nature 501, 535 (Sep, 2013). 7. D. C. Catling, K. J. Zahnle, C. P. McKay, Science 293, 839 (2001). 8. J. F. Kasting, Chem. Geol. 362, 13 (2013). 9. N. J. Planavsky et al., Science 346, 635 (Oct, 2014).