REMINERALIZATION AND DENITRIFICATION: NITROGEN AS A LOCAL REDOX INDICATOR ACROSS THE END-TRIASSIC MASS EXTINCTION

The latest Triassic to Early Jurassic represents an interval of extraordinary disturbance to marine ecosystems that culminated in the end-Triassic mass extinction (ETME), which was characterized by a notable loss of diversity as well as perturbations to biogeochemical cycles. The environmental changes surrounding the extinction remain a subject of discussion but are thought to be linked to the emplacement of the Central Atlantic Magmatic Province. Previous studies of the ETME have suggested a loss of oxygen leading up to, during and after the extinction horizon in sedimentary records from Canada, Germany, England, and Japan. The majority of these studies are spatially and temporally limited as they are generally restricted to epeiric seas and only focus on the interval immediately preceding and following the ETME.

To expand our knowledge of redox variation across the ETME, this study focuses on the Grotto Creek section (Alaska), which represents a deepwater, siliceous carbonate ramp in the ancient Panthalassa Ocean. This site provides a semi-continuous open ocean record of the Norian through Hettangian, encompassing the time leading up to and immediately following the ETME. We present nitrogen stable isotope values (δ¹⁵N) from this section to provide evidence for water column deoxygenation that occurred ~8 Ma prior to the extinction interval and persisted through the ETME. During episodes of oxygen depletion, the terminal electron acceptor in organic matter oxidation shifts from oxygen to nitrate, termed denitrification. Episodes of partial water column deoxygenation result in incomplete denitrification, subsequently leaving behind residual nitrate that carries a high δ¹⁵N value. In the case of the interval enveloping the ETME, we propose that a positive δ¹⁵N perturbation (3‰ increase) starting around the Norian-Rhaetian boundary (~209 Ma) reflects a transition from a widely oxygenated local water column to an increasingly oxygen-depleted water column that persisted through the ETME. The timing of this initial water column deoxygenation in the open Panthalassic Ocean provides key evidence for the deterioration of marine environments leading up to the ETME.