PALEOPROTEROZOIC EARTH: THE RISE OF ATMOSPHERIC O2 AND THE DECLINE OF CH4

The evidence for an initial rise of atmospheric O₂ at ~2.3 Ga is now very strong (1,2). The preservation of mass-independently fractionated sulfur isotopes in sediments requires that pO₂ was < 10^-5 PAL (times the Present Atmospheric Level) prior to this time (3). Its most probable value was near 10^-13 PAL, in agreement with earlier predictions based on theoretical models (4,5). The initial rise of O₂ was essentially a titration process. Before 2.3 Ga, production of reduced gases (mostly H₂) from volcanism overwhelmed the net production of O₂ from photosynthesis followed by organic carbon burial; after that time, the reverse was true. This resulted in a step-function increase in O₂ from ~10^-13 PAL to somewhere between 10^-2 and 0.5 PAL.

The rise of O₂ should have had a pronounced effect on climate if methane was an important contributor to the atmospheric greenhouse effect in the Archean/early Paleoproterozoic. Climate model calculations (6) show that a CH₄ concentration of 1000 ppmv could have kept the Late Archean climate warm even if atmospheric CO₂ levels were no higher than today. Coupled photochemical/climate/ecosystem calculations (7,8) show that such CH₄ concentrations are to be expected. Organic haze begins to form, however, at CH₄/CO₂ ratios >1 (9), and this would have cooled the climate by producing an anti-greenhouse effect. The Late Archean climate may therefore have stabilized at a point where the CH₄ and CO₂ concentrations were equal and the planet was covered by an optically thin organic haze (9). The rise of O₂ at 2.3 Ga destroyed this methane greenhouse, thereby triggering the Huronian glaciations.

References: 1. Holland, H. D. In Early Life on Earth, S. Bengtson, ed., New York, Columbia Univ. Press, p. 237 (1994). 2. Farquhar, J. et al. Science 289, 756 (2000). 3. Pavlov, A.A. and Kasting, J.F. Astrobiology 2, 27 (2002). 4. Walker, J. C. G. et al. In J. W. Schopf, ed., Earth's Earliest Biosphere: Its Origin and Evolution, Princeton, NJ, Princeton Univ. Press, p. 260 (1983). 5. Kasting, J. F. Science 259, 920 (1993). 6. Pavlov, A.A. et al. J. Geophys. Res. 105, 11,981 (2000). 7. Kasting, J. F. et al. Orig. Life Evol. Biosph. 31, 271 (2001). 8. Kharecha, P. et al. Geobiology (submitted). 9. Pavlov, A.A. et al. J. Geophys. Res. 106, 23,267 (2001).