Is the Redox state of the Archean Atmosphere Constrained?

Abstract

ABSTRACT The nature of the Archean atmosphere has significant ramifications for ore-forming processes, and it is widely held that it was more reducing than at present. The case for a more reducing atmosphere (i.e., without ferric iron) during part of the Archean relies on evidence from inferred detrital uraninite and pyrite, the lesser volume of redbeds before 2 Ga, and possible altered paleosols. In favor of an oxidizing atmosphere similar to Present and with stable ferric iron are the inferred pisolites on unconformity surfaces in the Witwatersrand basin, some redbeds older than 2 Ga, and several Archean sulfate-bearing assemblages. Carbon and sulfur isotopes might favor an oxidizing atmosphere, as might the positive correlation between carbon and clay in clastic sediments, but much other evidence is either poorly supported, or nondiagnostic of atmosphere conditions. Inferred detrital pyrite, uraninite, and gold, and inferred paleosols also appear to be nondiagnostic, given the evidence for widespread alteration.