HR: 1340h
AN: PP13A-1032    [Abstracts]
TI: Negative Sulfate O-17 Anomalies and an Extraordinary Temporal Spike at the Immediate Aftermath of Marinoan "Snowball" Earth
AU: * Bao, H
EM: bao@lsu.edu
AF: Louisiana State University, Department of Geology & Geophysics, E235 Howe-Russell Geoscience Complex, Baton Rouge, LA 70803, United States
AU: Lyons, J
EM: jimlyons@ucla.edu
AF: University of California, Los Angeles, Institute of Geophysics and Planetary Physics, Department of Earth and Space Sciences, Los Angeles, CA 90095, United States
AU: Zhou, C
EM: cmzhou@nigpas.ac.cn
AF: Chinese Academy of Sciences, State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Nanjing, 210008, China
AB: Understanding the composition of the atmosphere over geologic time is critical to understanding Earth system history, as the atmosphere is closely linked to the lithosphere, hydrosphere, and biosphere. While much of the history of lithosphere and hydrosphere is contained in rock and mineral records, corresponding information about the atmosphere is scarce and elusive due to the lack of direct records. Geologists have used sedimentary minerals, fossils, and geochemical models to place constraints on the concentrations of CO₂, O₂, or CH₄ in the past. We report that the triple oxygen isotope composition of sulfate from ancient evaporites and barites exhibits variable negative ¹⁷O anomalies over the last 750 million years. We argue that these anomalies track those of atmospheric O₂ and in turn reflect the partial pressure of CO₂ (pCO₂) in the past via a stratospheric O₃-CO₂-O₂ photochemical reaction network. Our results suggest that pCO₂ was much higher in Early Cambrian than in younger era, agreeing with previous modeling results. Most significantly, the ¹⁷O anomalies of barites in Marinoan (~ 635 million years ago) cap carbonates display a distinct negative spike (down to -0.70‰), suggesting that pCO₂ was still at ~ 30 times of modern level by the time barite was precipitating in the cap carbonate sequences, which strongly supports the Neoproterozoic "snowball" Earth hypothesis. Consistently more negative  ¹⁷O values for the lower barite bed than for the upper one at multiple sites at Baizhu, Hubei Province, South China attest probably a rapid CO₂ drawdown. While the sulfate ¹⁷O record does not have the sensitivity to detect atmospheric pCO₂ changes between glacial and interglacial times, it can be most useful in evaluating extreme atmospheric conditions (e.g. pCO₂ and pO₂) and their dynamic changes that have occurred in Earth history, including "snowball" Earth, impact events, or atmospheric conditions in a much earlier Earth system.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1030 Geochemical cycles (0330)
DE: 1605 Abrupt/rapid climate change (4901, 8408)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting