DI022-06
Secular oxidation of the mantle decreased the proportion of reducing gases in volcanic emissions and could trigger the atmospheric oxygenation

Monday, 14 December 2020: 19:25
Virtual
Shintaro Kadoya, University of Washington Seattle Campus, Earth and Space Sciences, Seattle, WA, United States, David C Catling, University of Washington, Earth and Space Sciences, Seattle, WA, United States, Robert William Nicklas, Scripps Institution of Oceanography, Geoscience Research Division, La Jolla, MD, United States, Igor S Puchtel, University of Maryland College Park, Department of Geology, College Park, MD, United States and Ariel D Anbar, Arizona State University, School of Earth and Space Exploration and School of Molecular Sciences, Tempe, AZ, United States
Recorded Presentation
Abstract:
Atmospheric O2 first increased ~2.4 Ga, marking the onset of the Great Oxidation Event (GOE). However, oxygenic photosynthesis likely began several hundred million years earlier. Previous studies attributed this delay to a decrease in O2 sinks rather than an increase in O2 sources. However, the actual mechanism for the decrease in O2 sinks is still uncertain. One possibility is a secular oxidation of the mantle, which decreased the proportion of reducing gases in volcanic emissions. Although the existence of secular mantle oxidation has been long dismissed, recently two new high-resolution studies revealed that the oxygen fugacity of the mantle increased by ~ 1.3 log10 units between 3.5 and 2.4 Ga (Aulbach & Stagno, 2016; Nicklas et al. 2019).

We investigate the evolution of volcanic gas composition using these new data demonstrating secular mantle oxidation. We also estimate the evolution of O2 production from photosynthesis using carbon isotope data. Then, we quantitatively evaluate the possible timing of atmospheric oxygenation. We find that secular mantle oxidation results in a decrease in the fraction of reducing gases in volcanic emissions, which is equivalent to a decrease in kinetically efficient O2 sinks. Additionally, O2 production changes relatively little from the Archean to Proterozoic. Hence, Archean O2 sinks overwhelmed O2 sources, i.e., the atmosphere was anoxic, before 2.5 Ga. After 2.5 Ga, the calculated probability that O2 sources overwhelmed O2 sinks. i.e., that the atmosphere was oxic, is larger than 95%. Thus, secular oxidation of the mantle could have triggered atmospheric oxygenation.

The oxidation of the mantle could result from homogenizing an initially redox-heterogeneous mantle and/or from hydrogen escape to space. While the first mechanism is dependent on the depth of a primordial magma ocean and is likely Earth-specific, the latter mechanism could be universal among terrestrial planets.

The rise of atmospheric O2 enabled biological evolution of more complex aerobes. If, as our work suggests, the oxidation of the mantle was ultimately important for atmospheric oxygenation, then solid Earth processes may have set the tempo of biological evolution.

Refs:

  • Aulbach & Stagno (2016) Geology 44, 751–754.
  • Nicklas et al. (2019) GCA 250, 49–75.
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