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Abstract

The Paleoproterozoic Lomagundi δ13Ccarb excursion between 2.22 and 2.06 Ga corresponds to the most significant period of atmospheric oxygenation and is referred to as the “Great Oxidation Event” (GOE). Large δ13Ccarb excursions are often followed by periods of accelerated biological evolution, suggesting that rocks deposited immediately after 2.06 Ga might preserve changes in environmental chemistry and microbial communities after the GOE. The poorly studied rocks of the Belcher, Nastapoka, and Richmond Gulf Groups, along the northwestern margin of the Superior Craton, Canada, have been dated between 2.03 and 1.870 Ga (Chandler and Parrish, 1989; Hamilton et al., 2009), and thus represent ideal rocks to investigate the evolution of microbial communities and of the carbon cycle in a higher redox state. These Paleoproterozoic successions contain exceptionally well-preserved carbonates with abundant and diverse stromatolites. The Belcher Group preserves three successive transgressions of a drowning carbonate platform between two major volcanic episodes (Ricketts and Donaldson, 1989). Progressive transitions from green/gray to red argillites/mudstones and the development of beds of carbonate concretions indicate a progressive oxidation of fine-grained marine sedimentary rocks and organic matter near the end of the second and third megacycles (Tukarak and Costello-Laddie Fms.). Data from the McLeary and Kasegalik Fms. in the Belcher Group show uniform δ13Ccarb values around +0.1‰ and δ13Corg values between -26.1‰ and -22.1‰. Current work is focused on completing the carbon isotope chemostratigraphies to establish geochemical correlations between the entire Belcher Group and the correlative Richmond Gulf and Nastapoka Groups.