The origin of 17O-depleted barite in Neoproterozoic cap carbonates in South China

Louisiana State University, Baton Rouge LA

Investigators

Abstract

Intellectual merit: Bao and colleagues recently discovered anomalous depletions of the stable oxygen isotope, 17O, in sulfate oxygen in the rock record. These enigmatic signals are distinctly large in sulfate associated with Marinoan (~ 635 Myrs old) glaciation around the world. Particularly, barite minerals in the basal Doushantuo Formation in South China exhibit variable magnitudes of the 17O anomaly. Since 1980s, many compounds on Earth have been found to possess triple oxygen isotope compositions that deviate from the terrestrial fractionation line. All of these previously reported anomalies are positive, i.e. the 17O is anomalously enriched, due to different degrees of inheritance of ozone?s positive 17O anomaly. In the PI's recent publications (Nature 2008 and Science 2009) he proposed that the newly discovered anomalous 17O depletion came from atmospheric O2, as a result of stratospheric O2−CO2−O3 chemistry in which the exchange between O3 and CO2 results in an anomalous 17O depletion in O2. Other variables being the same, increasing pCO2 can increase the magnitude of the negative 17O anomaly for O2. The large magnitude of sulfate 17O depletion at the end of Marinoan glaciation suggests probably an extremely high pCO2 atmosphere at that time, constituting therefore a strong support for the ?snowball Earth? hypothesis. This proposal will focus on the geological context and the origin of the 17O-depelted barite in Marinoan cap carbonates in South China where the distinctly negative 17O anomaly was first discovered. The study is urgent because it helps constrain the timing and origin of the 17O anomaly at the aftermath of a global glaciation and the result will be critical to interpreting the meaning of barite?s 17O anomalies and their environmental implications. Broader Impacts: By understanding how this study fits into a broader and new scientific problem, graduate students will be trained in acquiring and synthesizing multidisciplinary data including field geology, stratigraphy, petrography, stable isotope geochemistry, atmosphere-biosphere interactions, and Earth history. Students will gain valuable experience working overseas with international colleagues and diversify their own ?portfolio? of scientific collaboration and funding source. The project also provides an active and exciting research platform for teaching and recruiting in a large introduction course and for outreach effort through community activities. The results may ultimately provide a tracer for elevated levels of carbon dioxide in the atmosphere.