SURFACE OXIDATION STATES AND THE ARCHEAN/PROTEROZOIC TRANSITION

Which determined the surface oxidation state of the present Earth: mantle changes, tectonics, biological innovation, or all three? Current thought ascribes the evolution of microbial diversity and its modulation of global biogeochemical cycles as being intimately linked with changes in the surface oxidation state and tectonic regime of the planet over geologic time. However, exploring for a chemical record of these changes is fraught with difficulty due to the ephemeral nature of the atmosphere, problems in interpreting chemical or morphological remnants of life and modifications to the record by metamorphic overprinting.

New insights can be gained regarding the rate function of p O₂ increases in the Precambrian by using a combined approach that couples mass-independently fractioned (MIF) sulfur isotopic signals expressed as Δ³³S or the deviation from a mass-dependent fractionation line for the 3-isotope system (³⁴S/³²S vs. ³³S/³²S) in sulfides, with the nitrogen isotopic record (expressed as d¹⁵N_AIR) of kerogens and a growing body of data for ammoniated phyllosilicates. These data are derived from a wide variety of Archean, Proterozoic and Phanerozoic sediments. It has been proposed that changes in mantle oxidation state could have played a part in delaying the rise of atmospheric oxygen until ca. 2.3–2.4 Ga. Secular changes in sedimentation style in concert with the stabilization of the continents and a relatively "late"(?) rise of cyanobacteria coincided with the progressive and irreversible oxidation of the surface zone in the Paleoproterozoic. This view is supported by existing sulfur and nitrogen data, observations for no significant changes in mantle f O₂ over geologic time, geochemical biomarker studies and molecular phylogenetic reconstructions.