HR: 08:45h
AN: V41D-04    [Abstracts]
TI: Modeling the Rise of Atmospheric Oxygen
AU: * Claire, M
EM: mclaire@astro.washington.edu
AF: University of Washington, Department of Astronomy, Seattle, WA 98505 United States
AU: Catling, D
EM: davidc@atmos.washington.edu
AF: University of Washington, Department of Atmospheric Sciences, Seattle, WA 98505 United States
AU: Zahnle, K
EM: Kevin.J.Zahnle@nasa.gov
AF: NASA Ames Research Center, Astrobiology and Space Research Directorate, Moffet Field, CA 94035 United States
AB: Abundant geological evidence shows that atmospheric O$_2$ rose from less than a few ppmv to at least a few parts per thousand at $\sim$2.4 Ga. This transition is important to understand because the increase in O$_2$ levels changed the course of biological evolution. Biomarkers show that the source of O$_2$, oxygenic photosynthesis, existed long before the rise of O$_2$. A theoretical understanding remains elusive for how oxygenic photosynthesis could have originated long before a detectable rise of O$_2$ and what controlled the timing of the O$_2$ increase. We describe a time-dependent biogeochemical model of redox fluxes between the atmosphere-ocean system and the solid Earth. The rate of change of the quantity of O$_2$ in the atmosphere is given by the difference in the O$_2$ source and sink fluxes. The source of O$_2$ is equivalent to burial flux of organic carbon, whereas the losses of O$_2$ are due to photochemical destruction (including reaction with reducing volcanic and metamorphic gases) and continental weathering. The oxidizing effect of the escape of hydrogen to space is also calculated. The biosphere in the model consists of a coupled photosynthetic-methanogenic system. In addition, the model includes parameterizations of changing solar luminosity and the greenhouse effect. Results show that before the rise of O$_2$, the atmosphere is redox-dominated by methane, even in the presence of photosynthetic O$_2$ fluxes comparable to those today. Hydrogen escape, associated with the decomposition of CH$_4$ in the upper atmosphere, irreversibly drives the Earth system to more oxidized conditions. We find that the oxic transition occurs when the flux of reduced species from volcanic and metamorphic gases drops below the flux of O$_2$ associated with organic carbon burial. A precipitous drop in CH$_4$ levels accompanies the transition, lowering global temperatures to potentially Snowball Earth levels. We find that the timing of the oxic transition is primarily affected by the amount of iron in the Earth's crust. If hydrogen escape is turned off in the model, the Earth remains stuck with a Titan-like methane-rich atmosphere and no atmospheric oxic transition occurs. Thus, the basic overall model behavior-an oxic transition accompanied by a decrease of methane-is a robust feature, given the overall character of the redox fluxes and unidirectional hydrogen escape.
DE: 5407 Atmospheres--evolution
DE: 0315 Biosphere/atmosphere interactions
DE: 0325 Evolution of the atmosphere
DE: 0330 Geochemical cycles
DE: 0400 Biogeosciences
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting