Spatial and temporal heterogeneity of the marine nitrogen cycle during the end-Triassic mass extinction

The end-Triassic mass extinction (ETME) marks a pivotal event in Earth's history, characterized by major environmental changes in both marine and terrestrial settings and significant perturbations in the carbon and nitrogen biogeochemical cycles alongside extinction events. Here we employ high-resolution organic carbon isotopes (δ¹³C_org), nitrogen isotopes from bulk samples (δ¹⁵N_bulk), total organic carbon (TOC) and nitrogen content (TN), complemented by carbon (δ¹³C_kerogen) and nitrogen isotopes (δ¹⁵N_kerogen) of kerogen extracts from the Kuhjoch section in Austria in order to investigate the interplay between marine redox state, nitrogen cycling, and the biotic crisis across the Triassic–Jurassic boundary. Our results reveal a significant positive shift (∼3 ‰) in δ¹⁵N_bulk values, indicating a perturbed marine nitrogen cycle and expansion of the oxygen minimum zone prior to the ETME. The δ¹⁵N profiles suggest a transition from a nitrate-limited ocean dominated by nitrogen fixation to a post-extinction ocean with increased proportion of assimilation of NO₃⁻ undergoing non-quantitative denitrification. We also examine the spatial and temporal heterogeneity of the marine nitrogen cycle from different paleoenvironmental settings across the Triassic–Jurassic transition. Bioavailable nitrogen (NO₃⁻ and NH₄⁺) limitation prevailed at some localities before and during the ETME. However, the development of N limitation was not synchronous across different locations: it emerged before the ETME in some European basins and intensified after the ETME on Panthalassan shelves. The early Hettangian saw an expansion of euxinic waters into the photic zone and shoaling of the chemocline. Enhanced continental weathering and deep-water upwelling increased nutrient supply, thereby alleviating N limitation. Our new observations point to an unstable and stratified marine environment during the Triassic–Jurassic transition, and suggest that nitrogen bioavailability and redox conditions may be key factors for the devastation of marine ecosystems.