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.
The focus of the proposal was on the geological context and the origin of the 17O-depelted barite in Marinoan cap carbonates in South China where the distinct, anomalous 17O depletion was first discovered. The large magnitude of sulfate 17O depletion at the end of Marinoan glaciation at 635 million years ago suggests an extremely high pCO2 atmosphere at that time, constituting therefore the strongest supporting evidence for the "snowball" Earth hypothesis. Intellectual Merit Before the project, we only knew the occurrence but not the sedimentary context of the mineral phases that bear the 17O anomaly. Without the stratigraphic information and its spatial variability, the nature, duration, and magnitude of this mysterious atmospheric event could not be further explored. After the project was funded, we have learned a great deal on the Marinoan 17O depletion (MOSD) event, as demonstrated by the four published papers by our group so far. In brief, major findings are 1) The record of the MOSD event starts at the top of the cap dolostone overlying the Marinoan glacial diamictites. It did not start synchronously with the cap dolostone deposition. 2) The MOSD event lasted between 0 to 1 million years, a rather confidently determined duration for a geological event occurred at 635 million years ago. 3) The 17O depleted signature came from continental weathering and is recorded in barite minerals. The anomalies are not seen in carbonate associated sulfate in the post-Marinoan carbonate sequences. 4) Independent global atmosphere-biosphere dynamic model supports the less-than-1 million-year duration as well as the magnitude of the 17O depletion of the MOSD event. Publications (* denotes graduate students or postdoctoral researcher): Cao, Xiaobin* and Bao, Huiming, 2013, Dynamic model constraints on oxygen-17 depletion in atmospheric O2 after a snowball Earth. PNAS, July 2013; www.pnas.org/cgi/doi/10.1073/pnas.1302972110. Bryan A. Killingsworth*, Justin A. Hayles*, Chuanming Zhou, and Huiming Bao, 2013, Sedimentary Constraints on the Duration of the Marinoan Oxygen-17 Depletion (MOSD) Event. PNAS, Feb. 6, 2013; www.pnas.org/cgi/doi/10.1073/pnas.1213154110. Peng, Yongbo*, Bao, Huiming, Zhou, Chuanming, Yuan, Xunlai, and Luo, Taiyi, 2013, Oxygen isotope composition of meltwater from a Neoproterozoic glaciation in South China. Geology, 41 (3), 367-370. Peng, Yongbo*, Bao, Huiming, Zhou, Chuanming, and Yuan, Xunlai, 2011, 17O-depleted barite from two Marinoan cap dolostone sections, South China, Earth and Planetary Science Letters, 305, 21-31. Broader Impacts 1) Trained three PhD students: Dr. Yongbo Peng (graduated), Bryan Killingsworth, and Justin Hayles. All did fieldwork in South China and have their papers published in the subject area. 2) Trained one Master’s student (Dustin Boyd). 3) Trained one postdoctoral researcher Dr. Xiaobin Cao in modeling. 4) Trained in laboratory undergraduate student Miley Jackson, a Chemistry major from University of Louisiana at Lafayette under Louisiana Biomedical Research Network (LBRN) Summer Research Program, and Edward Lo, a freshman at LSU. 5) Incorporated "snowball" Earth hypothesis in a large introductory geology course Geol1003 (Earth System History) and exposed to more than 700 undergraduate students so far. 6) Offered a public lecture titled "Earth at a time of a runaway ice age" in Saturday Science at LSU series for local high school students (>150 students) and at LSU Science Club whose members are LSU faculty from diverse departments. 7) Worked with Ashley Berthelot at LSU University Relation on a piece that introduce our new findings on "snowball" Earth to various media outlets (e.g. www.sciencedaily.com/releases/2013/02/130228155626.htm).
