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

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?