HR: 09:15h
AN: PP21A-06    [Abstracts]
TI: A Very Weakly Reducing Atmosphere Prior to the Rise in Atmospheric Oxygen
AU: * Claire, M
EM: mclaire@astro.washington.edu
AF: University of Washington, Department of Astronomy Box 351580, Seattle, WA 98195, United States
AU: Catling, D
EM: David.Catling@bristol.ac.uk
AF: University of Bristol, Department of Earth Sceinces Wills Memorial Building, Bristol, BS8 1RJ, United Kingdom
AU: Zahnle, K
EM: Kevin.J.Zahnle@nasa.gov
AF: NASA Ames Research Center, Space Science Division Mail Stop 245-3, Moffett Field, CA 94035, United States
AB: We describe the photochemistry of stable anoxic atmospheres in response to declining fluxes of reducing gases. The goal is to understand the atmospheric consequences of increases in the oxidation state of the surface ocean prior to the Great Oxidation Event, when Earth's atmosphere first transitioned from weakly reducing to weakly oxidizing on a global scale approximately 2.4 billion years ago (Ga). Drill cores of the 2.5 Ga Mt. McRae shale reveal evidence for free oxygen availability in Earth's surface ocean, and perhaps require the presence of a trace amount of surficial free oxygen to induce oxidative weathering of the continents (Anbar et al., 2007 ; Kaufman et al., 2007). Simultaneous co-detection of a mass-independent sulfur isotope (MIF S) signal indicates generally reducing conditions must have existed throughout the troposphere (Zahnle et al., 2006) at this time. Using a simple box model, we had previously predicted (Claire et al., 2006) that such an atmosphere might exist as a result of likely evolutionary models of planetary fluxes. This work will use detailed photochemical models to investigate the vertical dependence of methane photolysis, OH production, SO₂ photolysis, and S₈ deposition in very weakly reducing atmospheres that may have proceeded the Great Oxidation Event. Anbar et al."A Whiff of Oxygen Before the Great Oxidation Event?" Science, 2007, in press Claire et al."Biogeochemical modelling of the rise in atmospheric oxygen." Geobiology, 4, 239-269, 2006. Kaufman et al."Late Archean biospheric oxygenation and atmospheric evolution." Science, 2007, in press. Zahnle et al."The loss of mass-independent fractionation in sulfur due to a Paleoproterozoic collapse of atmospheric methane." Geobiology, 4, 271-283, 2006.
DE: 0325 Evolution of the atmosphere (1610, 8125)
DE: 1022 Composition of the hydrosphere
DE: 4912 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4805)
DE: 5210 Planetary atmospheres, clouds, and hazes (0343)
DE: 5225 Early environment of Earth
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting