Schwartzman's Case for a Hot Archean Climate

The Case for a Hot Archean Climate and its Implications to the History of the Biosphere

Author:

Schwartzman

Abstract:

The case for a much warmer climate on the early Earth than now is presented. The oxygen isotope record in sedimentary chert and the compelling case for a near constant isotopic oxygen composition of seawater over geologic time support thermophilic surface temperatures prevailing in the Archean, with some support for hot conditions lasting until about 1.5 billion years ago, aside from lower temperatures including glacial episodes at 2.1-2.4 Ga and possibly an earlier one at 2.9 Ga. Other evidence includes the following: 1) Melting temperatures of proteins resurrected from sequences inferred from robust molecular phylogenies give paleotemperatures at emergence consistent with a very warm early climate. 2) High atmospheric pCO2 levels in the Archean are consistent with high climatic temperatures near the triple point of primary iron minerals in banded iron formations, the formation of Mn-bicarbonate clusters leading to oxygenic photosynthesis and generally higher weathering intensities on land. These higher weathering intensities would not have occurred if seafloor weathering dominated the carbon sink, pulling down the temperature, hence this empirical evidence supports a hot climate and high carbon dioxide levels. 3) The inferred viscosity of seawater at 2.7 Ga is consistent with a hot Archean climate. 5) A cold Archean is hard to explain taking into account the higher outgassing rates of carbon dioxide, significantly smaller land areas and weaker biotic enhancement of weathering than present in the context of the long-term carbon cycle, taking into account the fainter Archean sun in climate modeling. This evidence points to an important conclusion regarding biological evolution, namely to the critical role of a temperature constraint holding back the emergence of major organismal groups, starting with phototrophs, culminating with metazoans in the latest Precambrian.

This is not a great paper but attacks each of the arguments against a hot Archean climate one by one.  I do not believe he did a great job.  He completely skips addressing the Francevillian Biota.  He also just dismisses the  Huronian and Pongola Glaciations. he also seems very comfortable with writing off huge portions of geological time in broad sweeps, stating the boring billion was homogenous in the ocean structure and behavior.  Finally, he dismisses the Young Faint Sun Paradox through saying the additional CO2 would be sufficient to make up for the decreased solar insolation while not talking about the hypothesized higher cloud coverage (and albedo) than in the present.  etc.

Personal take: the Archean is likely hotter than the present.  I doubt it was 70 C.