Methane Greenhouses and Anti-Greenhouses During the Archean Era

Details Methane Greenhouses and Anti-Greenhouses During the Archean Era Methane Greenhouses and Anti-Greenhouses During the Archean Era

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.b52c..03k&link_type=abstract

American Geophysical Union, Fall Meeting 2002, abstract #B52C-03

Mathematics

Logic

0315 Biosphere/Atmosphere Interactions, 0325 Evolution Of The Atmosphere, 0330 Geochemical Cycles, 0400 Biogeosciences, 1030 Geochemical Cycles (0330)

Scientific paper

Climate and life are coupled today through the biogeochemical carbon cycle, but they may have been even more tightly coupled in the distant past when atmospheric O2 levels were lower. The finding of mass-independently fractionated S isotopes in Archean rocks confirms that pO2 was very low, probably <10-13 times the present level, prior to 2.3 Ga (1). The Sun was also some 20 percent less luminous at this time (2). High CO2 levels were initially proposed to solve this `faint young Sun problem' (3); however, these levels are in conflict in data from paleosols (4). CH4 is an alternative greenhouse gas which could have kept the Archean climate warm if present at concentrations of 0.01-0.1 percent by volume (5). The primary source of methane is biological. CH4 is produced by methanogenic bacteria that today live in anaerobic environments such as the intestines of ruminants and the water-logged soils underlying rice paddies. During the Archean, however, methanogens should have been widespread, and the methane they produced would have had a long photochemical lifetimes, around 10,000 years (6). Most methanogens are thermophiles or hyperthermophiles, and those which are more thermophilic have shorter doubling times than those that prefer cooler temperatures. This suggests that a positive feedback loop may have existed, whereby methanogens warmed the climate by releasing CH4, which in turn promoted the proliferation of faster-growing methanogens. This positive feedback would have been halted, however, once the ratio of CH4 to CO2 in the atmosphere exceeded unity. At this point, polymerization of CH4 by solar UV radiation would have caused the formation of an organic haze layer similar to that observed today on Titan. Such a haze layer would have cooled the climate by creating an `anti-greenhouse effect.' This creates an overall negative feedback loop that may have been responsible for maintaining a stable Archean climate. The rise of O2 at 2.3 Ga disrupted this equilibrium and may well have triggered widespread, possibly Snowball, Huronian glaciation. References: 1) Farquhar, J., Bao, H. and Thiemans, M. Science 289, 756-758 (2000). 2) Gough, D. O. Solar Phys. 74, 21-34 (1981). 3) Walker, J. C. G., Hays, P. B. and Kasting, J. F. J. Geophys. Res. 86, 9776-9782 (1981). 4) Pavlov, A. A., Kasting, J. F., Brown, L. L., Rages, K. A. and Freedman, R. J. Geophys. Res. 105, 11,981-11,990 (2000). 5) Pavlov, A. A., Kasting, J. F. and Brown, L. L. J. Geophys. Res. 106, 23,267-23,287 (2001). 6) Rye, R., Kuo, P. H. and Holland, H. D. Nature 378, 603-605 (1995).

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