Oxidative forcing of global climate change: A biogeochemical record across the oldest Paleoproterozoic ice age in North America ☆ Andrey Bekker a, ⁎ , Alan J. Kaufman b a Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd., NW, Washington, DC 20015, USA b Department of Geology, University of Maryland at College Park, College Park MD 20742, USA Received 13 November 2006; received in revised form 30 March 2007; accepted 2 April 2007 Available online 11 April 2007 Editor: H. Elderfield Abstract 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 13 C 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 δ 13 C 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 13 C- 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 δ 13 C values of ca. - 28‰ are notably 13 C-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 δ 13 C 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 13 C- 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. © 2007 Elsevier B.V. All rights reserved. Keywords: Paleoproterozoic; glaciation; methane; oxygen; carbon isotope excursions 1. Introduction Geochemical and sedimentological evidence suggests that atmospheric oxygen rose at the beginning and the end of the Proterozoic Eon [1–4] in broad association with prolonged glacial epochs. In both intervals, geochemical models have suggested that climate change was linked to Earth and Planetary Science Letters 258 (2007) 486 – 499 www.elsevier.com/locate/epsl ☆ Authors equally contributed to the paper and are listed in alpha- betical order. ⁎ Corresponding author. Tel.: +1 202 478 7974; fax: +1 202 478 8901. E-mail address: a.bekker@gl.ciw.edu (A. Bekker). 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.04.009