EXPANSIONS OF OCEANIC ANOXIA FOLLOWING THE END–TRIASSIC MASS EXTINCTION REVEALED BY TL ISOTOPES

Global warming and expansion of oceanic anoxia have been invoked as potential mechanisms behind the end-Triassic mass extinction. Most studies exploring the link between environmental change and mass extinction have focused on the narrow interval of mass extinction, leaving the interval leading up to the event and its aftermath out of their scope. Moreover, most studies have focused on the Tethys region, leaving the much larger Panthalassan Ocean understudied.

Here, we present a long-term record of geochemical proxies for oceanic anoxia (Fe speciation, trace metal concentrations, and thallium isotopes) from tropical Panthalassa. We studied the Grotto Creek section, Southcentral Alaska, which exposes the upper middle Norian to Pliensbachian McCarthy Formation. The siliceous mudstones of the McCarthy Formation were deposited on the deepwater outer ramp off a seamount in tropical Panthalassa.

Our Fe speciation data and trace metal concentrations indicate dominantly anoxic conditions at the studied location, enabling the use of Tl isotopes to elucidate global redox conditions. The Tl isotope composition suggests oxic conditions through the late Norian followed by a slow perturbation to anoxic values during the late Rhaetian and a rapid return to oxic values at the onset of the mass extinction, followed by a second rapid return to anoxic values in the early Hettangian. Redox-sensitive trace metal concentrations are relatively enriched throughout the Late Triassic and increase after the onset of the mass extinction, then return to relatively enriched values throughout the Hettangian. This suggests a global expansion in anoxic seafloor likely related to a prolonged extinction interval and heterogenous biotic response.