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Project Progress

Pushker Kharecha, James Kasting, and Janet Siefert have written a paper, currently submitted to Geobiology, that describes a coupled model of the anaerobic, early Archean biosphere, prior to the origin of oxygenic photosynthesis. The model includes organisms that metabolize using H₂, H₂S, and Fe⁺⁺ as reductants. A primary goal was to estimate the production rate of methane. We find that for reasonable assumptions about hydrogen outgassing rates and the hydrogen escape rate to space, CH₄ production should have been within a factor of 3 of the modern value, ~500 Tg CH₄/yr. In the low-O₂ Archean atmosphere, this would have been enough to produce CH₄ concentrations of 1000 ppmv, or higher if the escape rate was slower than assumed. Such CH₄ concentrations could have made a major contribution to Earth’s greenhouse effect, helping to keep the climate warm despite reduced solar luminosity at that time. Prior to the origin of anoxygenic photosynthesis, primary production by H₂-using methanogens could have produced extremely high CH₄ concentrations, several percent or more. Greenhouse warming might then have produced a hot early Earth, ~70°C, perhaps explaining the observed predominance of hyperthermophiles near the base of the rRNA tree.

More recently, a graduate student, Irene Schneider, and an undergraduate, Patrick Kasting, have derived a new set of near-IR absorption coefficients for CH₄. This should allow us to improve our estimates of the greenhouse effect of CH₄-rich atmospheres.

Also, climate modeler David Pollard and I have a paper in press in JGR concerning Snowball Earth. In it, we argue that “thin-ice” solutions like the ones proposed by Chris McKay in GRL (2000) are plausible, even likely, in the tropics. This may explain how photosynthetic algae and other light-dependent organisms survived the Neoproterozoic glaciations.