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Methane mixing ratios of 100-300 ppm in the Proterozoic atmosphere (0.75-2.3 Ga) would have been sufficient to offset the climatic effects of the faint early sun and maintain the warm climate during those ϳ1.5 b.y. The major argument against this type of the atmosphere is the short atmospheric oxidation time of methane after the first oxygenation event ca. 2.3 Ga. Here we argue that the net methane flux from the oxygen-poor Proterozoic ocean could have been 10-20 times higher than the present total biological methane flux. We demonstrate that increased methane production would have been sufficient to maintain methane concentrations at 100-300 ppm, which would keep the surface warm throughout the Proterozoic without invoking high CO 2 levels (although the CO 2 abundance could have been higher as well). A second oxygenation event at the end of the Proterozoic would have resulted in a decrease of methane flux and could have caused the first Neoproterozoic ''snowball'' glaciation.

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Global carbon cycle perturbations throughout Earth history are frequently linked to changing paleogeography, glaciation, ocean oxygenation, and biological innovation. A pronounced carbonate carbon-isotope excursion during the Ediacaran Period (635 to 542 million years ago), accompanied by invariant or decoupled organic carbon-isotope values, has been explained with a model that relies on a large oceanic reservoir of organic carbon. We present carbonate and organic matter carbon-isotope data that demonstrate no decoupling from approximately 820 to 760 million years ago and complete decoupling between the Sturtian and Marinoan glacial events of the Cryogenian Period (approximately 720 to 635 million years ago). Growth of the organic carbon pool may be related to iron-rich and sulfate-poor deep-ocean conditions facilitated by an increase in the Fe:S ratio of the riverine flux after Sturtian glacial removal of a long-lived continental regolith.

Significant variability in ␦ 34 S pyrite values in Neoproterozoic sedimentary rocks has been attributed to the evolution of nonphotosynthetic sulfide-oxidizing bacteria and the advent of sulfur disproportionation reactions in response to Earth's evolving redox chemistry. We analyzed trace sulfate in carbonates from South Australia and Namibia and reconstructed the sulfur isotope evolution of seawater sulfate. Comparison of our ␦ 34 S sulfate record with published ␦ 34 S pyrite data from the same or equivalent successions indicates that ␦ 34 S sulfate ؊ ␦ 34 S pyrite (⌬ 34 S) rose gradually through the second half of the Neoproterozoic and fluctuated coincident with episodes of glaciation, but did not exceed 46‰ before ca. 580 Ma. Large variability in ␦ 34 S pyrite in the Neoproterozoic can be explained as a consequence of low sulfate concentrations and rapidly fluctuating ␦ 34 S sulfate in seawater rather than the onset of sulfur disproportionation reactions mediated by nonphotosynthetic sulfide-oxidizing bacteria.

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