USING PAIRED CARBON AND SULFUR ISOTOPES FROM CARBONATE ROCKS TO CONSTRAIN THE TIMING OF ANOXIA DURING THE LATE DEVONIAN IN THE GREAT BASIN REGION, USA: IMPLICATIONS FOR CHEMOSTRATIGRAPHIC CORRELATIONS

The Devonian period is known both for the proliferation of forests on land and as a time of several extinctions of marine life. These extinctions occurred during the Late Devonian across the Frasnian-Famennian boundary (FFB) and Devonian-Carboniferous boundary. Though the Late Devonian (FFB) mass extinction is one of the “big five” mass extinctions of the Phanerozoic, the main driver(s) are still not fully understood. Global marine ocean anoxia was likely an important driver as the occurrence of black shales and positive carbon isotope excursions (δ¹³C) occurred during extinction pulses. However, black shales are not globally present, and δ¹³C values are prone to alteration and may not be a reliable proxy for anoxia if used alone. Thus, δ¹³C trends should be coupled with other geochemical proxies for anoxia to pinpoint when anoxia occurred and how that compares to biodiversity trends.

In this study stable carbon and sulfur isotopes (δ³⁴S) – measured from carbonate associated sulfate (CAS) and pyrite – were analyzed from four bio-stratigraphically constrained sections in order to test whether evidence for anoxia occurs in the Great Basin region (western USA) across the FFB. Sulfur can be a valuable proxy to identify anoxic periods in carbonate rocks, especially in conjunction with δ¹³C trends, as both isotopic systems will be affected by anoxia as organic matter and pyrite burial rates increase. Because alteration processes affect each system differently, diagenetic overprints can be identified and not confused with secular changes in seawater values. In the four studied sections, evidence for anoxia is preserved, and in some sections may slightly post-date the FFB.

For example, at Bactrian Mountain positive excursions in δ³⁴S_CAS and δ³⁴S_pyrite (8‰ and 30‰, respectively) reach peak values above the FFB (mid-late triangularis conodont zone). However, at the Coyote Knolls west section, peak positive δ¹³C and δ³⁴S excursions appear to occur just above the FFB and the magnitude of the excursions are much smaller than at Bactrian Mountain (2‰ and 10‰ for δ³⁴S_CAS and δ³⁴S_pyrite, respectively). These trends indicate that anoxia occurred during the Late Devonian, but further work needs to be done to unravel why the magnitudes vary in our studied sections and why the peak excursions differ in timing between the studied sections.