A BRIEF PERIOD OF MARINE OXYGENATION DURING THE END-TRIASSIC MASS EXTINCTION – A THALLIUM ISOTOPE MODELLING APPROACH

The end-Triassic mass extinction (ETME) is associated with an expansion of marine anoxia, although the timing and extent of anoxia is debated. Thallium (Tl) isotopes are a novel and powerful geochemical proxy that can evaluate the early onset and extent of global marine deoxygenation by tracking the burial rate of Mn-oxide minerals. The Tl isotope record from Panthalassan and Tethyn successions containing the ETME have similar magnitudes, trends, and values. Both datasets show a gradual shift in Tl isotopes to less negative values suggesting marine deoxygenation occurs by the Late Rhaetian. This is followed by a rapid, negative excursion in Tl isotopes that indicating a transient shift to more oxic conditions after the onset of the extinction. These changes are followed in the early Hettangian by another shift to less negative values in Tl isotopes that reflects returned marine deoxygenation.

Here, we present a modelling study that examines the minimum amount of time and Mn-oxide burial required to produce the Tl isotope values that reflect a transient shift to more oxygenated marine conditions in the latest Triassic. Modelling results indicate an interval of ~1-5 kyr could account for the Tl isotope perturbation lasting throughout the latest Rhaetian. One way to drive these changes could be enhanced weathering of Mn coupled with a sea level transgression. However, the injection of aerosols such as SO₂, released during volcanism associated with the emplacement of the Central Atlantic Magmatic Province, likely played a larger role. These aerosols can drive global cooling, leading to an increase in marine oxygen solubility and global marine oxygenation. This short interval of increased oxic seafloor area is long enough to bury a significant amount of Mn-oxides that were previously liberated during widespread marine deoxygenation, resulting in the negative Tl isotope excursion. These results suggest low, but also highly variable global marine oxygenation across the ETME. The similarity between the ETME Tl isotope record to that of other mass extinction intervals, such as the end-Ordovician and end-Permian extinction events, highlights that similar drivers and instability in marine oxygenation likely played a role in driving extinction during these events.