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Published August 21, 2010 | Published
Journal Article Open

Anomalous negative excursion of carbon isotope in organic carbon after the last Paleoproterozoic glaciation in North America


Early Paleoproterozoic time (2.5–2.0 Ga) spanned a critical phase in Earth's history, characterized by repeated glaciations and an increase in atmospheric oxygen (the Great Oxidation Event (GOE)). Following the last and most intense glaciation of this period, marine carbonates record a large positive excursion of δ^(13)C value (termed the "Lomagundi event") between about 2.2 and 2.1 Ga coinciding with the global appearances of red beds and sulfates, which suggest an accumulation of high levels of atmospheric oxygen. Here we report the discovery of large negative excursions of δ^(13)C in organic matter (down to −55‰) from quartzose sandstones (of the Marquette Range and the Huronian Supergroups, North America) intermediate in age between the last Paleoproterozoic glaciation and the possible onset of the Lomagundi event. The negative excursion is concomitant with the appearance of intensely weathered quartzose sandstones, which may represent hot and humid conditions. There are some interpretations that potentially explain the negative excursions: (1) redeposition of older ^(13)C-depleted kerogen, (2) later post-depositional infiltration of oil, (3) active methane productions by methanogens in shallow-marine environments, or (4) dissociation of methane hydrate. If the latter two were the case, they would provide clues for understanding the environmental change connecting the intense glaciation and an increase in oxygen.

Additional Information

© 2010 American Geophysical Union. Received 4 May 2010; accepted 1 July 2010; published 21 August 2010. Y.S. thanks J. Marmo and Yukio Isozaki for fruitful discussions; H. Sakuma, Yuko Isozaki, H. Hasegawa, K. Shiroya, H. Yoshida, and H. Nakayama for technical help with XRF and XRD analyses; and T. Hirai, S. Kimura, M. Hailstone, and A. Pace for helping to collect rock samples. This study is partly supported by Grants in Aid from Japan Society for the Promotion of Science (14403004 and 18340128), the 21 century COE Program at Department of Earth and Planetary Science, University of Tokyo, the Mitsubishi foundation, and the Agouron Institute.

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