HR: 13:45h
AN: PP22B-02 INVITED     [PDF]
TI: Methane Greenhouses and Anti-Greenhouses During the Precambrian
AU: * Kasting, J F
EM: kasting@essc.psu.edu
AF: Department of Geosciences Penn State University, 443 Deike, University Park, PA 16802 United States
AU: Pavlov, A A
EM: pavlov@lasp.colorado.edu
AF: LASP University of Colorado, 392 UCB, Boulder, CO 80309-0392 United States
AB: Methane was arguably an important greenhouse gas during the Archean and early Paleoproterozoic Eras, prior to the rise of O2 at ~2.3 Ga (1,2). Atmospheric CH4 concentrations of 1000 ppmv or more are predicted, assuming that methanogenic bacteria evolved early and that they were widely distributed in the prevailing anaerobic biosphere (2,3). Indeed, previous models (1-3) may have underestimated both the concentration of CH4 and the greenhouse effect itself at this time. Knauth and Lowe (4) have used oxygen isotopes in cherts to argue that the mean surface temperature at 3.2-3.5 Ga was between 55oC and 85oC. The Sun is thought to have been some 23 percent dimmer at that time (5), so this would have required substantial greenhouse warming by both CH4 and CO2. Further constraints are imposed by the formation of hydrocarbon haze, and an accompanying "anti-greenhouse effect", if the CH4 concentration exceeds that of CO2 (2). Thus, only certain combinations of CH4 and CO2 are possible. The rise of O2 at 2.3 Ga destabilized the methane greenhouse, leading to widespread (possibly global) glaciation (1,6). The next 1.5 billion years were warm, however, as evidenced by the absence of glacial deposits and the continuing high temperatures implied by O isotopes in cherts (7). CH4 levels could have remained relatively high during this time, 20-100 ppmv, as a consequence of low concentrations of dissolved O2 and sulfate in the deep oceans, and increased recycling of organic matter by fermentation and methanogenesis (8,9). An increase in either O2 or dissolved sulfate at around 0.75 Ga may have once again decreased atmospheric CH4 levels and triggered the widespread Neoproterozoic glaciations. \\ References: 1. Pavlov, A. A., Kasting, J. F., Brown, L. L., Rages, K. A. \& Freedman, R. J. Geophys. Res. 105, 11,981-11,990 (2000). 2. Pavlov, A. A., Kasting, J. F. \& Brown, L. L. J. Geophys. Res. 106, 23,267-23,287 (2001). 3. Kasting, J. F., Pavlov, A. A. \& Siefert, J. L. Origins of Life Evol. Biosph. 31, 271-285 (2001). 4. Knauth, L. P. \& Lowe, D. R. GSA Bull. 115, 566-580 (2003). 5. Gough, D. O. Solar Phys. 74, 21-34 (1981). 6. Evans, D. A., Beukes, N. J. \& Kirshvink, J. L. Nature 386, 262-266 (1997). 7. Knauth, L. P. NASA Astrobiol. Inst. Spring Meeting Abstracts (Tucson, AZ, Feb., 2003). 8. Pavlov, A. A., Hurtgen, M. T., Kasting, J. F. \& Arthur, M. A. Geology 31, 87-90 (2003). 9. Canfield, D. E. Nature 396, 450-453 (1998).
DE: 0315 Biosphere/atmosphere interactions
DE: 0325 Evolution of the atmosphere
DE: 9619 Precambrian
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting