| A coupled atmosphere-ecosystem model has been developed to simulate the early Archean biosphere. The model incorporates kinetic and nutrient limits on biological productivity, along with constraints imposed by metabolic thermodynamics. We have used this model to predict the biogenic CH4 flux and net primary productivity (NPP) of the marine biosphere prior to the advent of oxygenic photosynthesis. Organisms considered include chemotrophic and organotrophic methanogens, H2-, H2S-, and Fe-using anoxygenic phototrophs, S-reducing bacteria, CO-using acetogens, and fermentative bacteria.; CH4 production and NPP in our model are limited by the downward flux of H2, CO, S8, and H2S through the atmosphere-ocean interface and by the upwelling rate of Fe2+ from the deep oceans. For reasonable estimates of the supply of these compounds, we find that the biogenic CH4 flux should have ranged from ∼⅓ to 2.5 times the modern CH4 flux. In the anoxic Archean atmosphere, this would have produced CH4 concentrations of 100 ppmv to as much as 35,000 ppmv (3.5%), depending on the rate at which hydrogen escaped to space. Recent calculations indicating that hydrogen escape was slow favor the higher CH4 concentrations. Calculated NPP is lower than in the modern oceans by a factor of at least 40. In our model, metabolism based on H 2 and Fe is about equally productive, with S-based metabolism being considerably less productive. Internal recycling of sulfur within the surface ocean, neglected here, could conceivably raise rates of sulfur metabolism to much higher values.; Although explicit calculations of the methane greenhouse effect are not performed here, our model results are consistent with the idea that the early Archean mean surface temperature could have been very high, perhaps as high as the 55-85°C estimate obtained from oxygen isotopes in 3.3-Ga cherts. The climate could have been particularly warm if methanogens evolved before anoxygenic phototrophs, as this would have maximized the ratio of CH 4 production to organic carbon burial. CH4 concentrations and surface temperatures should have declined once phototrophs evolved because increased primary productivity and organic carbon burial would have drawn down total atmospheric hydrogen mixing ratios. CH4 concentrations may have increased again (and the climate warmed) in the late Archean following the origin of oxygenic photosynthesis because primary productivity would no longer have been constrained by the supply of reductants. A better understanding of the geologic record is needed to test these climate scenarios. |
