A coupled atmosphere-ecosystem model of the early Archean Earth

A coupled photochemical-ecosystem model has been developed to simulate the early Archean biosphere. The model incorporates kinetic and nutrient limitations 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, H-2-, 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 H-2, CO, S-8, and H2S through the atmosphere-ocean interface and by the upwelling rate of Fe2+ from the deep oceans. For reasonable estimates of the supply rates of these compounds, we find that the biogenic CH4 flux should have ranged from approximately (1)/(3) 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 favour the higher CH4 concentrations. Calculated NPP is lower than in the modern oceans by a factor of at least 40. In our model, H-2-based metabolism is moderately more productive than Fe2+-based metabolism, with S-based metabolism being considerably less productive. Internal recycling of sulphur within the surface ocean could conceivably raise rates of sulphur metabolism by a factor of 10 higher than the values predicted by our model. Although explicit climate calculations have not been performed here, our results are consistent with the idea that the Archean climate was warm, and possibly very hot. Some or most of our ecosystem scenarios are consistent with the carbon isotope record, depending on how that record is interpreted. If the conventional view is correct and organic carbon burial accounted for approximately 20% of total carbon burial during the Archean, then only two of our phototroph-based model ecosystems are plausible. However, if a recent alternative analysis is correct and only approximately 0-10% of total buried carbon was organic, then essentially all of our anaerobic ecosystems are plausible. A better understanding of both the geochemical and the biological records is needed to better constrain our models.