WAS ANOXIA THE MAIN DRIVER OF THE LATE DEVONIAN MASS EXTINCTION? NEW INSIGHTS USING PAIRED STABLE CARBON AND SULFUR ISOTOPES

The Late Devonian mass extinction is one of the “big five” mass extinctions of the Phanerozoic where ~40% of marine genera went extinct by the end of the Frasnian. The cause of this extinction is thought to be due to global anoxia based on the co-occurrence of positive carbon isotopic (δ¹³C) excursions and extinction pulses (known as the Kellwasser Event). It is thought that anoxic bottom waters increased organic burial rates and resulted in a positive δ¹³C excursion. Positive δ¹³C excursions, however, can be created via other processes, such as changes in weathering sources or alteration. Thus, if anoxia was the primary cause of the Devonian mass extinction (at the Frasnian-Famennian boundary; FFB), other proxy evidence for anoxia should also be preserved during this time in Earth history. Sulfur isotopes (δ³⁴S) are one such proxy where anoxia will cause pyrite burial rates to increase and produce a positive δ³⁴S excursion via the biologic fractionation effect associated with sulfate reduction and pyrite formation. Thus, if anoxia existed, both δ¹³C and δ³⁴S should increase together.

This study reports new δ¹³C and δ³⁴S trends measured from carbonate rocks (as carbonate-associated sulfate; CAS) and pyrite from three Late Devonian carbonate successions in the Great Basin region. In this study we test whether evidence for anoxia occurs before or at the FFB in the form of paired δ¹³C and δ³⁴S excursions. Two of the measured sections contain a positive δ¹³C carbonate excursion above the FFB that co-occurs with CAS and pyrite δ³⁴S excursions. The magnitude of these excursions varies between sections for CAS (δ³⁴S_CAS: 2–8‰) and pyrite (δ³⁴S_PY: 15–35‰), but both excursions postdate the main Frasnian extinction pulse. Petrographic study of these rocks shows that facies throughout the isotopic excursions exhibit near primary textures and early marine cements using cathodoluminescence microscopy. Because paired δ¹³C and δ³⁴S excursions are found above the FFB from these sections and postdate the FFB, this suggests that anoxia was not the main cause for the extinction and other driver(s) may have contributed to extinctions than previously thought.