EXPLORATION OF MARINE DEOXYGENATION USING CYCLING OF CARBON AND THALLIUM DURING THE LATE DEVONIAN EXTINCTIONS ACROSS THE APPALACHIAN BASIN

The Frasnian-Famennian transition (358.9-372.2 Ma) records one of numerous environmental and biotic crises within the Late Devonian (383-359 Ma). This extinction event appears to be coincident with several environmental changes: marine transgressions, ocean acidification, marine deoxygenation, global cooling and warming. Also found in this interval are positive carbon isotope excursions (δ¹³C_org) in the marine record that likely indicate large perturbations in the global carbon cycle. Significant efforts have been made to study the cause of the extinctions and one of the commonly invoked drivers is widespread marine deoxygenation. However, gaps remain in our understanding of the extent of marine deoxygenation during these events and their relationship to the extinctions.

This study seeks to better constrain the extent of global deoxygenation across the Frasnian-Famennian transition through the application of the novel thallium isotope (ε²⁰⁵Tl) proxy and the more widely used iron speciation proxy. Specifically, we investigate these geochemical records from series of sections of the Ohio and Chattanooga shales that represent a depositional transect of proximal to distal within the Appalachian Basin of North America. The iron speciation tracks local redox at each site and determines whether the shale was deposited under an anoxic water column; a prerequisite for the capture of marine ε²⁰⁵Tl. Thallium isotopes track the global rate precipitation and burial of Mn oxides in the ocean that are linked to the levels of global marine oxygenation. This process results in a large fractionation of thallium isotopes and less negative marine ε²⁰⁵Tl. Thus far, iron speciation at our location closest to the open ocean and near the Cumberland Sill indicates local deposition under an euxinic water column. ε²⁰⁵Tl from this site reflects a decline in marine oxygenation with an increase in ε²⁰⁵Tl coincident with the positive shift in δ¹³C_org near the Frasnian-Famennian boundary. Continued work will explore the potential spatial variation in Tl isotope record through the investigation of additional sections across the Appalachian Basin. However, these preliminary results suggest the deoxygenation was coincident with carbon cycle perturbation reflecting environmental disturbance that led to the devastation of marine biodiversity during this time.