Evolution of the Proterozoic Earth System: Insights from the ∆17O Record of Sedimentary Sulfate Minerals

Peter William Crockford1, Justin A Hayles2, Galen P Halverson3, Andrey Bekker4, Robert Rainbird5 and Boswell A Wing3, (1)Carleton University, Department of Earth Sciences, Ottawa, ON, Canada, (2)Louisiana State Univ., Department of Geology & Geophysics, Baton Rouge, LA, United States, (3)McGill University, Earth and Planetary Sciences, Montreal, QC, Canada, (4)University of California Riverside, Earth and Planetary Sciences, Riverside, United States, (5)Geological Survey of Canada, Ottawa, ON, Canada
Abstract:
Triple oxygen isotope ratios (18O/16O and 17O/16O) are a powerful tool to tease out interconnections within the Surface Earth System, both today and throughout Earth’s history. This ability comes from the fact that stratospheric photochemistry imparts a negative ∆17O anomaly (∆17O = δ17O – 0.52×δ18O) to atmospheric oxygen whose magnitude is proportional to pCO2 levels and photosynthetic oxygen production. Atmospheric oxygen readily weathers continental sulfides and, as a result, the secular variations in atmospheric ∆17O values may be recorded in marine sulfate minerals (barite, gypsum and anhydrite).

The largest ∆17O anomalies found in the rock record are from peculiar barite layers that immediately post-date the 635 Ma Marinoan Snowball Event. While these anomalies have been interpreted to result from a weak post-glacial photosynthetic O2 flux, the balance of other evidence (e.g., Zn isotope records of near-modern post-glacial productivity) suggests that they instead reflect the elevated CO2 levels thought to be required to exit a snowball state. As this situation illustrates, the ∆17O record by itself does not provide a unique solution between production of the anomaly by stratospheric reactions and its destruction by global biospheric productivity. In the context of additional geological and geochemical constraints, however, a marine sulfate ∆17O record has the potential to provide new insights into paleoatmospheres, paleoclimates, and paleoproductivity.

We have produced new data (n ≈ 200) for Proterozoic evaporites that extend the sulfate ∆17O record from the Neoproterozoic to ~2.3 Ga. This data will be interpreted within our current understanding of Proterozoic Earth System Evolution on basinal to global scales and will address key questions that include: Were Paleoproterozoic glacial episodes terminated by elevated pCO2? Was the Great Oxidation Event accompanied by enhanced productivity? Does the lack of C isotope variability throughout the mid-Proterozoic “Boring Billion” reflect constant primary productivity? Did a balance between CO2 levels and solar luminosity maintain the temperate mid-Proterozoic climate?

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