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Project Progress

Persistent deep ocean anoxia and relatively low terrestrial weathering rates that prevailed during Neoproterozoic “Snowball” episodes led to significant perturbations of the sulfur cycle, as evidenced by some of the largest and most rapid sulfur isotope variations known for the past 750 million years. These variations have been documented in marine strata from three continents. We have also presented the first comprehensive evidence that indicates low oceanic sulfate concentrations (perhaps 10% of modern) for most of the late Meso- to Neoproterozoic. As a result of this conclusion, we provided a new model for Neoproterozoic atmospheric chemistry and paleoclimate. As the result of very low oceanic sulfate concentrations, methane production rates and flux to the atmosphere were much higher than at present, enabling methane to prevail as an important greenhouse gas at that time. Thus, as long as sulfate concentrations remained low, most organic matter oxidation was accomplished by methanogenesis and global climate remained relatively warm. However, increasing oxygenation of the atmosphere and ocean in the late Proterozoic caused sulfate concentrations to rise, promoting sulfate reduction and decreasing rates of methanogenesis. This led to an abrupt decrease in atmospheric methane concentration and rapid cooling to cause the first “Snowball Earth” event. “Molar tooth structure”, and unusual fabric found in Proterozoic carbonate rocks, disappeared after about 750 Ma. We interpret this texture as resulting from shallow methanogenesis, methane saturation and bubbling of gas out of sediments as the result of little or no bacterial sulfate reduction. Nearly all organic matter was degraded under oxic conditions in shallow water environments or by a consortium of bacteria favoring methanogenesis.