SEARCHING FOR THE LATE DEVONIAN KELLWASSER EVENT IN THE GREAT BASIN REGION, USA: WAS ANOXIA THE MAIN DRIVER OF THE LATE DEVONIAN MASS EXTINCTION? (Invited Presentation)

The Late Devonian mass extinction is known as one of the Phanerozoic “big five” mass extinctions. The causes of this biotic crisis are widely thought to be caused by global and persistent anoxia. Evidence for anoxia exists as widespread organic-rich black shale deposits, a lack of bioturbation and ichnofabrics diagnostic of anoxic conditions, the occurrence of pyrite framboids, and positive stable carbon isotopic (δ¹³C) excursions in both bulk carbonate (δ¹³C_carb) and organic matter (δ¹³C_org) records. This anoxic event occurs across the Frasnian-Famennian Boundary (FFB), which is recognized as the Kellwasser event, has been documented around the globe. Despite the widespread evidence of anoxia during this time, other geochemical proxies for anoxia have yet to be well documented. Other proxies used to identify anoxic conditions, such as stable sulfur isotopes (δ³⁴S), I/(Ca+Mg) ratios, and Fe-speciation data, should all indicate anoxia existed prior to or during the mass extinction interval if anoxia was the main driver of this Late Devonian biotic crisis.

In this study we sampled four Late Devonian carbonate successions from the Great Basin region (eastern NV and western UT) to test whether geochemical evidence for anoxia predates the biotic crisis across the FFB. We identify two positive 1–2‰ δ¹³C_carb excursions above and below the FFB, which possibly represent the Kellwasser event. However, only at the upper excursion do multiple lines of evidence suggest anoxia occurred. Above the FFB a positive δ³⁴S excursion is preserved in both the sulfate (as carbonate-associated sulfate) and pyrite records (~9‰ and ~35‰, respectively). Near zero I/(Ca+Mg) ratios are also preserved immediately before the upper excursion, suggesting that locally waters were anoxic before the global event was recorded in δ¹³C and δ³⁴S records. Collectively, these geochemical trends indicate that both locally and globally evidence for anoxia post-dates the FFB, challenging the notion that anoxia was the main driver of the Late Devonian biotic crisis. These new observations require a re-evaluation of the causes of this mass extinction and considering whether other causes played a larger role than previously thought.