Oxidative forcing of global climate change: A biogeochemical record across the oldest Paleoproterozoic ice age in North America

Carbon isotope compositions of organic matter in fine-grained siliciclastic units deposited above and below glacial diamictite at the base of the ca. 2.45-2.22 Ga Huronian Supergroup in Ontario, Canada were studied to constrain relationships between profound fluctuations in the exogenic carbon cycle and dramatic climate changes at the beginning of the Proterozoic Eon. In both drill core and outcrop sections the organic matter preserved in proximal lithofacies, dominated by coarse-grained sand and silt, are enriched in ¹³C relative to distal lithofacies, dominated by argillites. In the drill core, sand-dominated lithofacies of the McKim Formation beneath the glacial diamictite of the Ramsay Lake Formation have a narrow range of δ¹³C values (- 28.4 to - 26.0‰ V-PDB), but organic matter in argillite-dominated lithofacies of the outcrop section ∼ 40 km to the southeast is somewhat more ¹³C-depleted with values ranging from - 34.5 to - 26.4‰. Similarly, sand-dominated lithofacies of the Pecors Formation above the glacial diamictite in the drill core section with δ¹³C values of ca. - 28‰ are notably ¹³C-enriched relative to argillite-dominated lithofacies, which record values as low as - 40.5‰. The sand-dominated lithofacies of the Pecors Formation in the outcrop sections have δ¹³C compositions ranging from - 34.4 to - 27.9‰. The isotopic differences appear to be unrelated to organic carbon abundances, so we suggest that these are controlled by environmental differences in proximal and distal settings. The strong ¹³C-depletion in the organic-lean McKim and Pecors argillites, especially in the drill core section of the Pecors Formation, is consistent with significant biological methane production and oxidative recycling by methanotrophs both before and after the ice age in shallow-water environments stratified with respect to oxygen. The rise of atmospheric oxygen and subsequent enhanced biogeochemical methane cycling in shallow-water settings likely contributed to unstable climate conditions during the Paleoproterozoic glacial epoch.