CHASING THE LATE DEVONIAN KELLWASSER EVENT IN THE GREAT BASIN REGION, USA: WAS ANOXIA THE MAIN DRIVER OF THE LATE DEVONIAN MASS EXTINCTION?

The Late Devonian mass extinction is known as one of the “big five” mass extinctions in the Phanerozoic where the causes of this biotic crisis are generally attributed to global and persistent anoxia. Evidence for anoxia exists as widespread organic-rich black shale deposits, as well as one or two positive stable carbon isotopic (δ¹³C) excursions in both bulk carbonate (δ¹³C_carb) and organic matter (δ¹³C_org) records. These excursions are known as the Kellwasser events and have been documented around the globe. However, other geochemical proxies for anoxia have yet to be well documented. This includes global proxies in the form of stable sulfur isotopes (δ³⁴S) and proxies more representative of local conditions, such as I/(Ca+Mg) and Fe-speciation values. If anoxia was the main driver of this Late Devonian biotic crisis, particularly in nearshore environments, then these geochemical trends should indicate anoxia persisted prior to or during the mass extinction interval.

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 at the Frasnian-Famennian boundary (FFB). We identify two positive 1–2‰ δ¹³C_carb excursions above and below the FFB, which possibly represent the Kellwasser events, but only in the interval with the upper excursion does additional evidence for anoxia exist. Above the FFB a positive δ³⁴S exists in both the sulfate (as carbonate-associated sulfate; CAS) and pyrite records (~9‰ and ~35‰, respectively). Evidence of local water column anoxia exists in the form of ~0 I/(Ca+Mg) values starting before the δ¹³C and δ³⁴S excursions, coincident with preliminary Fe-speciation trends that suggest possibly ferruginous water conditions existed. Collectively, these geochemical trends suggest that both locally and globally that evidence for anoxia post-dates the FFB, suggesting that anoxia was likely not the main driver of the Late Devonian biotic crisis. These new observations suggest that other causes for this mass extinction be revisited for their potential in playing a larger role for biodiversity trends.