WHERE IS THE LATE DEVONIAN ANOXIC KELLWASSER EVENT ACROSS THE FRASNIAN-FAMENNIAN BOUNDARY?: NEW INSIGHTS FROM THE GREAT BASIN REGION, USA

The Late Devonian mass extinction represents one of the five largest Phanerozoic mass extinctions, but the causes of this biotic crisis remain poorly understood. Widespread anoxia is thought to have played a crucial role in this mass extinction based in part on carbon isotopic data (δ¹³C). One or two positive δ¹³C excursions are reported at or below the Frasnian-Famennian Stage Boundary (FFB), which is known as the Kellwasser Event (KE) and is present in several sections around the globe. The KE is thought to represent a period of higher organic burial rates under anoxic conditions, also contributing to the high faunal turnover across the FFB. However, this notion relies on an indirect proxy for marine anoxia and is not strongly supported by additional indirect proxies for anoxia (e.g. sulfur isotopes; δ³⁴S). Thus it remains unclear whether anoxia was globally present across the FFB or whether these δ¹³C excursions are more reflective of local basin dynamics superimposed on a global signal.

Here we present new δ¹³C and δ³⁴S data from three successions in the Great Basin region (USA) to test whether geochemical evidence for anoxia is preserved across the FFB. Evidence for positive δ¹³C or δ³⁴S excursions across the FFB is absent in all three sections. Petrographic analysis of representative facies does not support a diagenetic overprint that erased primary geochemical signatures. Above the FFB in the upper Guillmette Fm. (Bactrian Mountain section, southern NV), however, δ¹³C and δ³⁴S increase in parallel from baseline values of ~ -0.7‰ and 28‰ to +0.7‰ and 35‰, respectively, until the top of the measured section (Late Palmatolepis triangularis conodont Zone). These parallel δ¹³C and δ³⁴S excursions could represent elevated burial rates of organic matter and pyrite, respectively, under anoxic conditions. These excursions occur immediately above a 7-m-thick quartz arenite, which we interpret to represent lowstand deposition followed by the incursion of anoxic waters into shelf environments during a transgressive sequence. Results from this study question the global anoxia model that the KE was the cause of the faunal crisis prior to the FFB. We suggest a slight refinement where anoxia near the FFB was diachronous on a global scale, which may also explain the protracted faunal turnover instead of a rapid mass extinction event.