GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 222-1
Presentation Time: 8:00 AM


HENDERSON, Miles A., Geology Program, The University of Texas of the Permian Basin, 4901. E. University, Odessa, TX 79762, SZYNKIEWICZ, Anna, Earth and Planetary Sciences, The University of Tennessee, 602 Strong Hall, 1621 Cumberland Ave, Knoxville, TN 37996, GOMEZ, Fernando J., LAC-Laboratorio de Analisis de Cuencas, CICTERRA-Centro de Investigaciones en Ciencias de la Tierra, FFEFyN-Universidad Nacional de Cordoba, Velez Sarsfield 1611 2do Piso Of 7, Cordoba, X5016GCA, Argentina and KAH, Linda C., Department of Earth and Planetary Sciences, The University of Tennessee, 602 Strong Hall, 1621 Cumberland Ave, Knoxville, TN 37996

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 δ34SCAS and δ34SPY 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 δ34SCAS values between formations from near +25‰ in the Las Chacritas Formation to values near 0‰ in the Gualcamayo Formation. δ34SPY values are lower and more variable than coeval δ34SCAS in shallow water facies. δ34SPY 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.