REDOX FLUCTUATIONS ACROSS THE END TRIASSIC MASS EXTINCTION: TL ISOTOPIC EVIDENCE FOR EXPANSIONS IN GLOBAL ANOXIA

The end-Triassic mass extinction (ETME) is one of the “big five” mass extinction events recorded in the geologic record and is associated with the emplacement of the central Atlantic magmatic province (CAMP). Volcanism and increased greenhouse gas emissions associated with CAMP is thought to have led to a cascade of negative environmental impacts, notably rapid global warming and oceanic anoxia. Most studies examining the extent of anoxia have narrowly focused on the ETME and are commonly within the Tethys, with few assessments of the redox conditions within Panthalassa. The Grotto Creek section of the McCarthy formation in Alaska’s Wrangell mountains is a nearly complete and continuous upper-Triassic to lower-Jurassic sedimentary succession deposited within the tropical latitudes of Panthalassa. Importantly there is excellent local biostratigraphic age controls and new absolute age dates.

We present a suite of local and global redox proxy data from black shales. This long-term record of the redox history surrounding the ETME and Triassic-Jurassic boundary uses Fe speciation and trace metal concentrations to constrain local redox conditions, and thallium (Tl) isotopic composition measurements to evaluate the global redox state. Because Tl strongly responds to fluxes in Mn-oxide burial near the oxic-anoxic boundary and has a residence time shorter than ocean-mixing, it records changes in global marine (de)oxygenation. Evidence from Fe speciation suggests stable locally reducing and anoxic conditions in Grotto Creek, thus Tl isotopes can be used to evaluate the global redox state. Prior to the onset of the ETME, the Tl isotope composition suggest more oxic conditions before a slow perturbation to anoxic values and a rapid return to more oxic values at the extinction onset, and another rapid perturbation to reducing values in the early Hettangian. Concentrations of redox sensitive trace metals are enriched but less than typical reducing values, suggesting global drawdown as a result of increased anoxic seafloor area which sequesters trace metals more efficiently. These results suggest widespread reducing conditions throughout the ETME which may have inhibited primary producers due to limited trace metal ability, while Tl suggests rapid redox transitions at the extinction interval.