Isotopic evidence for massive oxidation of organic matter following the Great Oxidation Event

The stable isotope record of marine carbon indicates that the Proterozoic Eon began and ended with extreme fluctuations in the carbon cycle. In both the Paleoproterozoic [2500 to 1600 million years ago (Ma)] and Neoproterozoic (1000 to 542 Ma), extended intervals of anomalously high carbon isotope ratios (delta C-13) indicate high rates of organic matter burial and release of oxygen to the atmosphere; in the Neoproterozoic, the high delta C-13 interval was punctuated by abrupt swings to low delta C-13, indicating massive oxidation of organic matter. We report a Paleoproterozoic negative delta C-13 excursion that is similar in magnitude and apparent duration to the Neoproterozoic anomaly. This Shunga-Francevillian anomaly may reflect intense oxidative weathering of rocks as the result of the initial establishment of an oxygen-rich atmosphere.