PANIC! IN THE PANTHALASSIC: REDOX TRENDS LEADING UP TO THE END-TRIASSIC MASS EXTINCTION FROM EASTERN PANTHALASSA

The latest Triassic represents a time of major disturbance to marine ecosystems that culminated in a mass extinction: the end-Triassic mass extinction or ETME. This interval was characterized by enhanced diversity loss and perturbations to the environment and global biogeochemical cycles. Marine deoxygenation has been proposed as an extinction mechanism for the ETME, however the record of marine redox during the ETME remains poorly documented. Further, studies that have investigated changes in marine redox have focused on the interval of greatest loss to biological diversity while new evidence suggests marine ecosystems became progressively stressed well before the ETME. Therefore, the temporal links between changes in marine ecosystems and deoxygenation need further investigation. Here we present sedimentary nitrogen isotopes (δ¹⁵N) to characterize local nitrogen cycling and marine redox conditions preceding, during, and after the ETME at two locations in the eastern equatorial Panthalassa: Grotto Creek (GC), Alaska, and Ferguson Hill (FH), Nevada. GC represents deeper water deposition on a siliceous-carbonate ramp on the Wrangellia terrane, and FH represents a shallow-water carbonate ramp deposited on the western Pangean margin. At GC, a δ¹⁵N excursion of ~+3‰ initiates near the Norian–Rhaetian boundary (~209 Ma) and declines just after the ETME. At FH, which does not expose Norian and lower Rhaetian strata, we observe a δ¹⁵N fall from 5‰ in the uppermost Rhaetian to ~3‰ in the lowermost Hettangian. We propose that the δ¹⁵N perturbations found in both sections indicate deoxygenation and incomplete denitrification within the water column, subsequently leaving behind residual nitrate enriched in δ¹⁵N. Therefore, these isotopic signals reflect a transient shift from a widely oxygenated local water column to a deoxygenated one that persisted through the ETME in eastern equatorial Panthalassa. The GC data also indicate that deoxygenation initiated up to ~8 Ma prior to, and persisted through, the ETME and may be linked to the stressed ecosystem observed during this time. In summary, our data adds to a growing body of evidence of marine environmental deterioration before the ETME and serves to sharpen our understanding of this critical interval of Earth History.