The Relationship Between the Geochemical Cycles of Carbon and Sulfur during the Neoproterozoic Interglacial Interval

During the middle Neoproterozoic (Cryogenian), geologic evidence supports at least two periods of extreme climate change, or snowball Earth events, the Sturtian (~710 Ma) and Marinoan (~635 Ma) glaciations. During these events it is hypothesized that glaciers extended into tropical latitudes at sea level, and that the entire ocean may have frozen over due to ice-albedo feedback. While numerous studies have focused on understanding the mechanisms for cooling at the beginning of the Cryogenian, and modeling the ice extent, limited focus has been placed on sediments deposited between the two snowball Earth events.. Sulfate levels may have been extremely low in the immediate aftermath of the Sturtian event for two reasons. First, if the hydrologic cycle were diminished during the glaciation, riverine flux of sulfate to the ocean would have been reduced. Second, as oceanic anoxia developed as a result of extensive ice cover, a substantial portion of the marine sulfate reservoir would have been removed via bacterial sulfate reduction and iron sulfide burial. As the Sturtian event ended and the hydrologic cycle was reinvigorated, marine sulfate levels should have increased through the interglacial interval. And, in low sulfate systems, sulfate concentrations strongly influence the efficiency of nutrient recycling and therefore primary production, by affecting phosphorus availability. Here, we have reconstructed the evolution of δ13Ccarbonate, δ34Spyrite, and at a lower resolution, δ34Ssulfate, as recorded in interglacial carbonates of the Abenab Subgroup (northern Namibia) in order to examine the relationship between the geochemical cycles of carbon and sulfur and to test the hypothesis that increasing sulfate concentrations in part drove elevated rates of surface water primary production and therefore enriched δ13Ccarbonate values. Our results indicate that sulfate levels were fluctuating through this time, and that these variations are related to changes in the C cycle.