OPEN-OCEAN NITROGEN LIMITATION PRECEDED THE END-TRIASSIC MASS EXTINCTION: EVIDENCE FROM HAIDA GWAII, BRITISH COLUMBIA

The end-Triassic mass extinction is widely viewed as a “greenhouse” crisis, related to rapid atmospheric warming and changes in ocean circulation following the eruption of the Central Atlantic Magmatic Province. This study examines how the marine nitrogen cycle responded to these changes, and the extent to which the immediate mass extinction interval represents a perturbation in nitrogen cycling from the “baseline” of the Late Triassic world, which had already been free of permanent polar ice for over 50 million years. The Kennecott Point section on Haida Gwaii, deposited in the open Panthalassic Ocean, can be used as a test case to compare the relationship between bulk-sediment nitrogen isotopic composition and estimates of marine productivity, before and after the end-Triassic crisis interval. We observed two major negative excursions in nitrogen isotopes, characterized by δ¹⁵N values ≤0 ‰, corresponding with periods of diminished productivity. These episodes likely represent enhanced nitrogen fixation in the surface ocean, as a result of N limitation and suppressed vertical mixing, and their correspondence with low productivity or export suggests that nitrogen availability limited productivity following the development of stable density stratification. The first of these events occurs at the Norian-Rhaetian stage boundary, and corresponds to the extinction of monotid bivalves. The second occurs in the latest Rhaetian, as a precursor to the main end-Triassic extinction event, and is associated with a decreased abundance of heteromorph ammonoids. The subsequent shoaling of euxinic waters into the surface ocean, as evidenced by biomarkers studies of the Kennecott Point section, was fatal for most Triassic fauna; however this transition corresponds with a return to positive nitrogen isotope values and a recovery in organic carbon accumulation, suggesting that the introduction of regenerated NH₄⁺ to the photic zone may have allowed productivity to recover among planktonic anaerobes and prasinophytes able to tolerate photic-zone euxinia.