A COMPLEX MOSAIC OF SULFUR ISOTOPE RECORDS THROUGH THE LATE ORDOVICIAN MASS EXTINCTION IN THE GREAT BASIN AND ANTICOSTI ISLAND

A suite of dramatic environmental, climatic, and geologic changes unfolded in association with the Late Ordovician mass extinction, including glaciation, marine anoxia, and volcanism. Stable isotope records of carbonate associated sulfate (δ³⁴S_CAS) and sedimentary pyrite (δ³⁴S_pyr) have provided a basis for interpretations of such changes. However, uncertainties in the degree to which these data reflect local vs. global conditions and the effects of depositional systems and early diagenesis complicate interpretations of the sulfur isotope record. Here we present δ³⁴S_CAS and δ³⁴S_pyr records from four sections of the Ely Springs Dolostone across the Great Basin and four limestone sections across Anticosti Island in order to document the variability inherent in these systems.

In the Great Basin, upper Katian dolostones are characterized by consistent baseline δ³⁴S_CAS values. These baseline values range from +20‰ at South Egan Range to +30‰ at Barn Hills. A large magnitude positive δ³⁴S_CAS excursion occurs in Hirnantian strata at both South Egan and Silver Island ranges, broadly synchronous with the positive HICE excursion (Hirnantian isotopic carbon excursion). δ³⁴S_pyr tracks the δ³⁴S_CAS excursion tightly at Silver Island (r² = 0.52), but not at South Egan (r² = 0.22). Cherry Creek Range lacks a δ³⁴S excursion at the HICE. None of the studied sections contain the pre-Hirnantian negative δ³⁴S_CAS excursion previously reported in the deep-water limestones at Monitor Range.

On western Anticosti Island, limestones of the Katian Vaureal Formation record a negative excursion in δ³⁴S_CAS similar to that observed at Monitor Range, declining from +50‰ to +20‰ over 30 meters. However, viewed in the full context of δ³⁴S_CAS record of the exposed upper Ordovician strata, that excursion is the descending limb of a positive excursion that departed from a +30‰ baseline. In the east, upper Katian limestones are characterized by swings in δ³⁴S_CAS of >20‰ over several meters of stratigraphy, with no stable baseline. The Hirnantian Laframboise Member at the eastern sections hosts a prominent decline in Δ³⁴S (= δ³⁴S_CAS - δ³⁴S_pyr) driven by a positive δ³⁴S_pyr excursion.

The lack of a consistent chemostratigraphic signal within and between sedimentary basins highlights the importance of local depositional and early diagenetic processes in shaping the archive of deep time δ³⁴S.