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.
- atmospheric oxygen