Coupled Changes in Sulfur and Carbon Isotopes Preceding the Sturtian Glaciation of the Neoproterozoic

Abstract:

The relationship between the carbon and sulfur cycles in the Neoproterozoic has proven to be complex. Unusually high δ¹³C_carbonate persists throughout this period, however large negative excursions precede both the Sturtian (~710 Ma) and the Marinoan (635 Ma) glaciations, the mechanisms of which are still debated. Previous data shows that during the interglacial interval, sulfate concentrations were very low and abnormally enriched δ³⁴S_pyrite values inversely correlate with shifts in δ¹³C_carbonate. However, very little carbonate-associated sulfate (CAS) could be extracted from these sections. To better understand the relationship between the global carbon and sulfur cycles during this tumultuous time, high-resolution coupled records of δ¹³C_carbonate and δ³⁴S_sulfate are needed. Here we present paired δ³⁴S_sulfate and δ³⁴S_pyrite data from carbonates of the Coates Lake Group (Mackenzie Mountains, Canada), which represent deposition preceding the Sturtian glaciation, to further explore the connection between the Neoproterozoic sulfur and carbon cycles, and to gain insight into the possible mechanisms driving major perturbations in the carbon isotope record. Leading into the glaciation, δ¹³C_carbonate increases from -9.5‰ to 8‰ over approximately 200 meters of section, then abruptly drops to ~2‰ over 30 meters. Both sulfate and pyrite isotopes track these changes, indicating that as organic carbon burial is steadily increasing and driving δ¹³C_carbonate more positive, pyrite burial is increasing as well. Δ³⁴S decreases as δ¹³C_carbonate and δ³⁴S increase, providing further evidence that more organic carbon and pyrite are being buried, leading to a decrease in oceanic sulfate concentrations. Δ³⁴S then increases as δ¹³C_carbonate decreases, indicating a positive flux to the sulfate reservoir prior to the Sturtian glaciation. This relationship between carbon and sulfur differs significantly from what is seen during the interglacial interval, and represents one of the earliest occurrences of positively coupled changes between carbon and sulfur isotopes in the Neoproterozoic.