DIAGENETIC δ34SCAS VARIABILITY IN METEORICALLY ALTERED DOLOSTONES: A CASE STUDY FROM THE UPPER ORDOVICIAN OF THE GREAT BASIN

Carbon and sulfur isotope chemostratigraphy have provided important data sets for constraining environmental changes that accompanied the end Ordovician (Hirnantian) mass extinction and glaciation. Carbonate successions from low paleolatitudes have been particularly valuable sources of integrated geochemical, paleontological, and sedimentological data. Positive excursions in carbon isotope ratios from carbonate and organic carbon are well established for the Hirnantian, as is a coeval positive excursion in sulfur isotope ratios from pyrite. However, there is no parallel excursion in sulfur isotope ratios of carbonate-associated sulfate (δ³⁴S_CAS) in well-preserved limestones from Anticosti Island; rather, δ³⁴S_CAS maintains relatively stable values (~25‰) through the Hirnantian.

Using the Anticosti data as a “least-altered” Hirnantian δ³⁴S_CAS benchmark, we test the fidelity of the δ³⁴S_CAS signal in strata that have undergone significant post-depositional alteration. We have recently used δ¹³C chemostratigraphy and sequence stratigraphy to identify Hirnantian strata in the Ely Springs Dolostone, a meteorically altered shallow water carbonate exposed in the Great Basin, USA. Here we present δ³⁴S_CAS data from two of these Upper Ordovician sections; the section at South Egan Range occupies a mid-shelf location, whereas the deeper section at Pancake Range represents the shelf-slope margin. Carbonate-associated sulfate concentrations are low (<120 ppm), as is typical of meteorically altered carbonates. Like the Anticosti limestones, the dolostones do not host a δ³⁴S_CAS excursion in the Hirnantian, and a ~5‰ scatter persists around the long-term secular trends. However there is a ~10‰ offset between the two sections. Isotope ratios of the shelf edge deposits are ~23‰, similar to those of well-preserved limestones, whereas the mid-shelf deposits fluctuate around ~33‰. Mid-shelf sediment at South Egan Range may have incorporated pore water sulfate enriched in ³⁴S through microbial sulfate reduction as a result of limited mixing with the open ocean during the sea level lowstand associated with the Hirnantian glacial maximum. These results suggest that while dolostones have the potential to preserve primary δ ³⁴S_CAS signals, diagenetic overprints may vary across a sedimentary basin.