DECREASING WATER COLUMN DENITRIFICATION BEFORE THE END-TRIASSIC EXTINCTION? INSIGHT FROM STABLE NITROGEN ISOTOPES
δ13Corg values reveal an overall positive excursion from the Rhaetian to the Hettangian, punctuated by second-order negative excursion coincident with the end-Triassic extinction horizon (the so-called I-CIE, noted globally), indicating that the succession is well-preserved and can be easily correlated to others. δ15N values shift through the Rhaetian from ~9‰ (early –mid Rhaetian) to ~3‰ (late Rhaetian). Rhaetian values exhibit a 3‰ cyclicity within this trend, potentially related to Milankovitch cycles, given the radiometric age control available for the succession. δ15N values stabilize near 2.5‰ in association with the end-Triassic extinction and Triassic-Jurassic boundary, with little variability during the Hettangian (< ±0.5 ‰). Our preferred hypothesis is that the shift towards lighter nitrogen isotopic values and cyclicity observed during the Rhaetian reflects a dynamic nitrogen cycle and fluctuating water column denitrification through time, with progressively less denitrification through the Rhaetian. Carbon to nitrogen ratios and %TOC support this interpretation, as well as preliminary redox-sensitive trace metal analyses. Other sections from the end-Triassic also show similar responses. We will discuss possible mechanisms for these changes and their potential link to CAMP volcanism and the end-Triassic extinction.