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

摘要

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 C-13 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 delta C-13 values (-28.4 to -26.0 parts per thousand VPDB), but organic matter in argillite-dominated lithofacies of the outcrop section similar to 40 km to the southeast is somewhat more C-13-depleted with values ranging from -34.5 to -26.4 parts per thousand. Similarly, sand-dominated lithofacies of the Pecors Formation above the glacial diamictite in the drill core section with delta C-13 values of ca. -28 parts per thousand are notably C-13-enriched relative to argillite-dominated lithofacies, which record values as low as -40.5 parts per thousand. The sand-dominated lithofacies of the Pecors Formation in the outcrop sections have delta C-13 compositions ranging from -34.4 to -27.9 parts per thousand. 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 C-13-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. (c) 2007 Elsevier B.V. All rights reserved.