Thallium Isotopic Evidence for Widespread Late Devonian Marine Anoxia

The Late Devonian was a time of protracted faunal crisis and significant ecological reorganization accompanied by several intervals of widespread organic-rich deposition within globally-extensive epicontinental seas. Several black shale horizons deposited during this time (the Upper and Lower Kellwasser Events, UKE and LKE respectively) appear to be globally distributed and are associated with extinction pulses and perturbations to the global carbon and sulfur cycles. While the causes of these late Devonian events remain debated, general agreement exists that they likely involved factors such as: LIP emplacement driving global warming and cooling (glacial?); rapid sea-level fluctuations; and enhanced chemical weathering and nutrient delivery from the establishment and spread of complex terrestrial plant communities. These various forcings share the development of marine anoxia as an important likely feedback. Whether these Devonian anoxic episodes were principally localized in poorly-ventilated epeiric seas (like the modern Black Sea), or if they were more widely-developed and globally-significant (akin to the Mesozoic OAEs) remains an ongoing debate. To explore the local versus global nature of these events we analyzed the thallium stable isotopic composition (ɛ²⁰⁵Tl) of the Chattanooga Shale from an outcrop located in central Tennessee, where the unit spans the LKE, UKE and Frasnian-Fammenian boundary. We record a shift in ɛ²⁰⁵Tl to less negative values below the F-F boundary coincident with positive shifts in both δ¹³C and δ³⁴S. These results suggest a global reduction in Mn-oxide burial and an expansion of anoxia during the late Fransnian. Interestingly, ɛ²⁰⁵Tl values remain less negative well into the Fammenian. This pattern is very similar to what has been observed during the later Mesozoic OAEs providing strong evidence for a globally significant increase in marine anoxia during the Late Devonian which likely contributed to the faunal crisis.