CARBON AND SULFUR ISOTOPES: PAIRING PROXIES FROM CARBONATE ROCKS TO CONSTRAIN THE TIMING OF ANOXIA DURING THE LATE DEVONIAN IN THE GREAT BASIN REGION, USA

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 five bio-stratigraphically constrained sections 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 separated from secular changes in seawater values. In the five studied sections, evidence for anoxia is preserved, with some displaying variability in timing and magnitude.

For example, positive excursions across the FFB in δ³⁴S_CAS and δ³⁴S_pyrite (2‰ and 10‰, respectively) at the deeper facies Coyote Knolls west section appear muted when compared to the excursions recorded at Bactrian Mountain (8‰ and 30‰, respectively). Though models exist that show depth as an important mechanism of excursion preservation and variability, these models do not fully capture the trend across the five studied sections. While recorded δ¹³C and δ³⁴S excursions indicate that anoxia occurred during the Late Devonian, further work is needed to pick apart why the magnitudes of excursions vary and to what degree global signals are constrained.