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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.
