PAIRED SULFUR ISOTOPE VALUES FROM THE MIDDLE ORDOVICIAN OF ARGENTINA: NEW CONSTRAINTS FOR A DUAL-RESERVOIR MODEL FOR SULFUR CYCLING

Changes in ocean circulation patterns resulting from global cooling in the Middle-Late Ordovician would have led to the disruption of greenhouse environmental conditions, upwelling of nutrient-rich deep water, and perturbations in oceanic nutrient budgets. Evidence for such oceanographic change is found worldwide in the C- and S-isotope records of marine carbonate in the Middle Ordovician as the Middle Darriwilian isotope carbon excursion (MDICE) and large changes to the marine sulfur cycle recorded in carbonate associated sulfate (CAS) and pyrite (PY). Measurements of δ¹³C values in Darriwilian strata from the Argentine Precordillera suggest differentiated surficial and deep-water reservoirs. This is consistent with previously reported δ³⁴S patterns observed in early Darriwilian strata suggesting growth of the marine sulfate reservoir and the persistence of deep-water anoxia. Progressive ventilation of deep-water environments should be reflected as a stepwise change in the δ³⁴S values of coeval pyrite and sulfate between deeper and shallower water environments.

Here we evaluate the geochemical consequences of Middle Ordovician ventilation with additional high-resolution δ³⁴S_CAS and δ³⁴S_PY data from the Las Chacritas, Las Aguaditas, and Gualcamayo formations of the Argentine Precordillera, that represent distinct, yet coeval depositional environments spanning from well mixed oxygenated waters to deeper-waters beneath the chemocline. We identify a gradient in δ³⁴S_CAS values between formations from near +25‰ in the Las Chacritas Formation to values near 0‰ in the Gualcamayo Formation. δ³⁴S_PY values are lower and more variable than coeval δ³⁴S_CAS in shallow water facies. δ³⁴S_PY values from the deepest water facies are around −12‰. Such S-isotope compositions are consistent with a dual-reservoir model for S-cycling, implying isotopically distinct shallow-water sulfate and deep-water sulfide reservoirs, and transient oxidation of the deep-water sulfide reservoir in the Darriwilian. S-isotope data from these formations recorded facies dependent δ³⁴S values that originated form the persistent marine chemocline during the Middle Ordovician, where the greatest extent of isotopic change occurs closest to the position of the marine chemocline.