The Neoproterozoic was a major turning point in Earth's surficial history, recording several widespread glaciations, the first appearance of complex metazoan life, and a major increase in atmospheric oxygen. Marine redox proxies have resulted in many different estimates of both the timing and magnitude of the increase in free oxygen, although the consensus has been that it occurred following the Marinoan glaciation, the second globally recorded "snowball Earth" event. A critically understudied rock type of the Neoproterozoic is iron formation associated with the Sturtian (first) glaciation. Samples from the <716 Ma Rapitan iron formation were analysed for their Re concentrations and Mo isotopic composition to refine the redox history of its depositional basin. Rhenium concentrations and Re/Mo ratios are consistently low throughout the bottom and middle of the iron formation, reflecting ferruginous to oxic basinal conditions, but samples from the uppermost jasper layers of the iron formation show significantly higher Re concentrations and Re/Mo ratios, indicating that iron formation deposition was terminated by a shift towards a sulfidic water column. Similarly, the delta Mo-98 values are close to 0.0 parts per thousand throughout most of the iron formation, but rise to similar to+0.7 parts per thousand near the top of the section. The delta Mo-98 from samples of ferruginous to oxic basinal conditions are the product of adsorption to hematite, indicating that the Neoproterozoic open ocean may have had a delta Mo-98 of similar to 1.8 parts per thousand. Together with the now well-established lack of a positive Eu anomaly in Neoproterozoic iron formations, these results suggest that the ocean was predominantly oxygenated at 700 Ma. (C) 2013 Elsevier B.V. All rights reserved.
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