A hot climate on early Earth: implications to biospheric evolution

David W. Schwartzman, Department of Biology, Howard University

The case for a very warm Archean/early Proterozoic (50-70 deg C) (aside from the Huronian) now includes: 1) Sedimentary chert oxygen and silicon isotopic record from the same samples [1], evidence for nearly constant seawater O18/O16 ratio going back into the Archean. 2) Higher weathering intensities. 3) Stratified oceans apparently persisting until the mid Proterozoic. 4) The apparent absence of deeply-rooted mesophiles/psychrophiles on molecular phylogenetic trees. Organismal emergence times occur at surface temperatures corresponding to the maximum growth temperatures of their living models. (Atmospheric pO2 levels alone cannot explain the big delay in the appearance of the "Higher" Kingdoms). 5) High atmospheric pCO2 prior to the first evidence of atmospheric methane at 2.8 Gya, consistent with apparent requirements for Mn-bicarbonate cluster formation leading to oxygenic photosynthesis. 6) No apparent glaciations for over a billion years in mid-Proterozoic. 7) Progressive increase of biotic enhancement of weathering from the Archean to its present value emerges as a model result with plausible limits on continental growth and outgassing rates [2].

Implications to biotic and biospheric evolution include: 1) Temperature constraint on emergence of major organismal groups. 2) Atmospheric pO2 constraint on macroeucaryotes. 3) Atmospheric pCO2 constraint on cyanobacterial emergence, emergence of leaves (megaphylls) in Devonian. 4) A geophysiology of biospheric evolution, likewise on Earth-like planets around Sun-like stars.

References

[1] Knauth, L.P., 2005. Paleogeography, Paleoclimatology, Paleoecology, 219: 53-69. Robert, F. and M. Chaussidon, 2006, Nature 443, 969 – 972. [2] Schwartzman, D. Life, Temperature, and the Earth: The Self-Organizing Biosphere, (1999, 2002)