Marine Deoxygenation Predates the End-Triassic Mass Extinction Within the Equatorial Panthalassa and its Influence on Marine Ecosystems Before the Biotic Crisis

The end-Triassic Mass Extinction (ETME) was one of the "Big 5" mass extinctions of the Phanerozoic and is thought to have been caused by a series of environmental changes triggered by the emplacement of the Central Atlantic Magmatic Province (CAMP). While the overall driver of the ETME is well-accepted, the specific roles of subsequent environmental changes in driving the extinction remain unresolved due to both spatial and temporal biases within existing geochemical records. Additionally, recent studies suggest that environmental deterioration may have preceded the emplacement of both CAMP and the ETME. Here, we present nitrogen isotope (δ15N) and iron speciation data from a sedimentary succession located in Grotto Creek, Alaska in order track changes in the marine nitrogen cycle and redox across the upper Norian Stage of the Triassic through lower Hettangain Stage of the Jurassic (~215 to 198 million years ago). The geologic succession at Grotto Creek represents deeper water marine deposition in eastern equatorial Panthalassa, likely capturing regional oceanic redox trends. δ15N records from Grotto Creek show a pronounced positive excursion of +3‰ that initiates in the upper Norian, peaks near the Norian-Rhaetian boundary (NRB; ~208.5), and declines through the ETME and into the Hettangian. Throughout this interval, iron speciation data show persistent anoxic conditions and occasional euxinia occurred in the local bottom waters. We propose that the positive δ15N and iron speciation data from Grotto Creek reflect a progressive deoxygenation of the upper water column with the expansion of the equatorial oxygen minimum zone (OMZ). Specifically, the δ15N excursion reflects a shift in the dominant processing of bioavailable nitrogen from uptake and nitrification under oxic conditions to incomplete denitrification followed by complete denitrification as deoxygenation progressed. Records from existing study sites in other regions of the Panthalassa reflect similar redox fluctuations and support that this deoxygenation was a regional phenomenon. Other recent studies of the Late Norian and Rhaetian also document declines in biodiversity that predate the ETME, as well as, carbon isotope excursions, likely reflecting perturbations to the carbon cycle. Together, this evidence suggests that increasingly low-oxygen conditions initiated at least ~8 Ma before the emplacement of CAMP and likely paved the way for the ETME.