HR: 13:40h
AN: U52B-01    [PDF]
TI: Carbon Dioxide Cycling and a Methane Greenhouse on Ancient Earth
AU: * Zahnle, K
EM: kzahnle@mail.arc.nasa.gov
AF: NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035 United States
AU: Sleep, N
EM: norm@pangea.stanford.edu
AF: Stanford University, Dept. Of Geophysics, Stanford, CA 94305 United States
AB: The continental cycle of silicate weathering and metamorphism dynamically buffers atmospheric CO$_2$ and climate. Feedback is provided by the strong temperature dependence of silicate weathering. Hydrothermal alteration of oceanic basalts can also dynamically buffer CO$_2$. The oceanic cycle links the surface to the mantle. Feedback is provided by the dependence of carbonatization on dissolved carbonates in sea-water. The oceanic cycle has no thermostat. Currently the continental cycle is more important, but early in Earth's history, especially if heat flow were higher than it is now, more vigorous plate tectonics would have made the oceanic cycle dominant. We find that CO$_2$ greenhouses thick enough to defeat the faint early Sun are implausible and that, if no other greenhouse gases are invoked, very cold climates are expected for much of the Proterozoic and the Archaean. This prediction for the most part flies in the face of evidence that early Earth was warm. We favor biogenic methane as the chief sup-plement to CO$_2$. High methane abundances (hundred to thousands of ppm) are expected given methane's current biological source and an anoxic atmosphere. Methane is a likely primary product of the advent of oxygenic photosynthesis. To first ap-proximation methane and oxygen enter the atmosphere in stochiometric proportions. Mostly the methane and oxygen react with each other, but a small fraction of the oxygen reacts with rocks or with volcanic or metamorphic gases. This it is methane that accumulates first. High methane abundances in turn affect the oxidation state of the Earth because they facilitate hydrogen escape. Possible consequences of the irreversible oxidation of the Earth include the oxygenation of the atmosphere.
DE: 0325 Evolution of the atmosphere
DE: 0330 Geochemical cycles
DE: 0400 Biogeosciences
DE: 1729 Planetology
DE: 8125 Evolution of the Earth
SC: U
MN: 2003 Fall Meeting