EXTREME NITROGEN LIMITATION COINCIDES WITH MARINE EXTINCTIONS DURING THE TRIASSIC-JURASSIC TRANSITION

The Kennecott Point and Frederick Island sections of Haida Gwai, British Columbia ere deposited on the flanks of an offshore volcanic arc in the Panthalassic ocean, spanning the Triassic-Jurassic transition, and thus are a valuable recorder of paleoenvironmental conditions in the world’s largest ocean basin during a major mass extinction event. Here we present a nitrogen isotope record ranging from the Norian through the Hettangian stages of the Triassic and Jurassic, and encompassing several biological disturbances including major and minor extinctions. During periods of carbon cycle stability, δ¹⁵N -AIR is generally in the range of 2-4 ‰, fairly typical for sedimentary organic matter in modern N limited environments. This is suggestive of an active nitrogen cycle featuring regeneration of organic N. δ¹⁵N values drop to ~0 ‰ or lower coinciding with three major biological events: the extinction of the previously abundant bivalve Monotis at the Norian-Rhaetian boundary, the main Tr-J mass extinction, and the onset of the Hettangian carbon and sulfur cycle disturbances observed by Williford et al. (2009). These rapid drops to δ¹⁵N values of < 0 ‰ suggest an increased reliance on nitrogen fixed from the atmosphere, likely due to increasing limitation of regenerated bioavailable nitrogen following enhanced denitrification in an anoxic environment, an extreme state in which organic productivity was likely dominated by prokaryotes. δ¹⁵N values show some correlation with the organic C:N ratio, which may indicate a terrestrial organic matter influence, however neither parameter corresponds with sedimentary facies, and the correlation might also result from changing primary producer community composition or differential degradation of organic matter due to fluctuating redox conditions. Such extreme states of nitrogen limitation, in which virtually all organic N needed to be fixed directly from the atmosphere, may have restricted primary productivity and delayed recovery from late Triassic ecosystem disturbances.