| Abstract: | Assuming steady state of carbon dioxide levels in a “pressure-cooker” atmosphere/ocean system (10–20 bars, near 100°C) produced by a land weathering sink and volcanic source (BLAG model), an abiotic Earth model for 3.8 Ga requires present biotic enhancements of weathering to be on the order of 100 or greater, consistent with the limit inferred from experimental and field studies. Using a plausible ratio of the present biotic enhancement (from higher plants) to enhancements produced by microbial activity alone, along with models for continental growth and outgassing rates consistent with geologic evidence, we find that computed surface temperatures hover near 20°C over geologic time, slowly decreasing to present, after a rapid initial decline as a result of microbial colonization of land. Results are consistent with the first possibility for glaciation in the late Archean/early Proterozoic. Useful modeling of climatic evolution, taking into account biotic enhancement of weathering, can now apparently be extended into the Precambrian, assuming operation of the carbonate-silicate buffer. |
