SUPERCONTINENTS, GLACIATION AND ATMOSPHERIC OXIDATION: AN HOLISTIC HYPOTHESIS

The first widespread (global?) glaciation (2.4-2.2 Ga) and the “Great Oxidation” (2.3-2.0 Ga) are well known Paleoproterozoic events. Also well understood is the negative feedback between atmospheric carbon dioxide, organic carbon, and oxygen (i.e. photosynthesis and decay), with removal of buried organic carbon during subduction resulting in increased atmospheric oxygen. The first global glaciation and the onset of oxidation at ~ 2.3 Ga fit symmetrically in a Neoarchean-Paleoproterozoic time scale defined by global superplume events affecting supercontinents at ~ 2.7 (“Kenorland”) and 1.9 Ga (“Laurentia” and one unnamed, comprising present-day southern cratons). The consumption of buried sea-floor organic carbon during subduction and supercontinent assembly would have been countered by increased plume-related atmospheric carbon dioxide production (and concomitant enhanced marine carbonate and black shale deposition), thus maintaining greenhouse paleo-atmospheres before and after global cooling/oxidation events. The first giant carbonate platforms at ~ 2.6 Ga, and extensive continental crust by ~ 2.5 Ga (~ 80% of modern volumes) would have increased global carbon dioxide drawdown through deposition of carbonate rocks and weathering of high-freeboard continents. Allied to relative global tectonic quiescence in the period between the two supercontinent/superplume events, this drawdown could explain widespread glaciation on elevated continental areas, at the correct latitudes. However, oxidation of the atmosphere would have required more active tectonism to remove buried carbon in subducted sediments, and hence occurred in the early stages of the ~ 2.0 Ga supercontinent assemblies, when the rate of net oxygen accumulation exceeded carbon dioxide generated at arcs and mid-ocean ridges. As so often in Earth history, the ~ 2.3-2.0 Ga oxidation event thus reflects the interplay of major Earth system processes operating at variable rates, and would logically have been followed by a return to a greenhouse environment by ~1.9 Ga.