HR: 0830h
AN: PP21B-1168    [PDF]
TI: High Methane Abundance in the Archean and Proterozoic Atmosphere. Why CO2 is not Enough.
AU: * Pavlov, A A
EM: pavlov@lasp.colorado.edu
AF: Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, Duane Physics Building 392 UCB, Boulder, CO 80309-0399 United States
AU: Kasting, J F
EM: kasting@essc.psu.edu
AF: Department of Geosciences, The Pennsylvania State University, Deike Building, University Park, PA 16802 United States
AU: Toon, O B
EM: btoon@lasp.colorado.edu
AF: Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, Duane Physics Building 392 UCB, Boulder, CO 80309-0399 United States
AU: Pollard, D
EM: pollard@essc.psu.edu
AF: Earth System Science Center, The Pennsylvania State University, 2217 Earth& Engr Sciences, University Park, PA 16802 United States
AU: Tian, F
EM: Teddy.Tian@lasp.colorado.edu
AF: Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, Duane Physics Building 392 UCB, Boulder, CO 80309-0399 United States
AB: Decreased solar luminosity (Gough, 1981) and multiple lines of geologic evidence in favor of a "liquid" ocean in the Archean set a puzzle known as "Faint Young Sun" paradox. For several decades, elevated atmospheric CO2 levels were considered to be the most self-consistent solution for the warm Archean climate (Walker et al., 1977; Kasting et al., 1993). However, to offset a ~25% decreased solar luminosity (at ~3.5 Gyr ago) and keep the mean global surface temperature at ~288K, CO2 should have been at a steady-state concentration of about 0.3 bars. At such high levels CO2 would condense (Mellon, 1996) in the Earth's polar regions (as it does on Mars today) and no longer could be considered as the only "stabilizer" of the Archean climate. Lack of siderite in paleosols (Rye et al., 1995) and lack of glaciations in Archean/Proterozoic also does not support large CO2 concentrations and pure CO2 greenhouse in the Precambrian. Climate simulations (Pavlov et al., 2000) show that 100-1000 ppm of methane would be sufficient to maintain warm climate under decreased solar luminosity without invoking huge CO2 levels. Therefore, the key question is how to maintain such high CH4 levels. In the anoxic Archean environment (Pavlov \& Kasting, 2002), the lifetime of methane molecule would be long ~10000 years. Previous photochemical calculations show that to maintain the "steady-state" 1000 ppm of CH4, the methane flux into Archean atmosphere should have been close to the present day biogenic methane flux (Pavlov et al., 2001) which is debatable. However, previous calculations assumed a high ("diffusion-limited") rate of hydrogen loss to space. If atmosphere was anoxic, hydrogen should have been lost at much (5-100 times) slower rate (Tian et al., 2003). Here we demonstrate that 100-1000 ppm could be maintained with much smaller methane flux in the hydrogen-rich Archean atmosphere. In the oxygenated Proterozoic atmosphere the lifetime of methane becomes much shorter. However, the biogenic flux from the oxygen/sulfate-poor Proterozoic ocean could have been even higher than the present total biogenic flux. The methane abundance in the oxygenated atmosphere is a non-linear function of methane source because methane molecules destroy their major sink - OH radicals (Prather, 1996). We showed (Pavlov et al., 2003) that ~100 ppm of methane in Proterozoic could be maintained with only 7-10 times increased present biogenic flux. We conclude that methane was abundant throughout Archean and Proterozoic and most likely was responsible for lack of glaciations in the Precambrian.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0325 Evolution of the atmosphere
DE: 0343 Planetary atmospheres (5405, 5407, 5409, 5704, 5705, 5707)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting