DECREASING WATER COLUMN DENITRIFICATION BEFORE THE END-TRIASSIC EXTINCTION? INSIGHT FROM STABLE NITROGEN ISOTOPES

Although changes in carbon isotopes coincident with the end-Triassic extinction are well established, the precise causes of those shifts and their relationship to the Central Atlantic Magmatic Province (CAMP) and the extinction remain enigmatic. Here, we report stable organic and inorganic carbon (δ¹³C_org , δ¹³C_carb) and nitrogen (δ¹⁵N) isotope measurements and preliminary trace metal data from the Levanto section in Northern Peru to explore changes in biogeochemical cycling across the end-Triassic extinction and emplacement of CAMP. Age dating of the section enables a robust understanding of chronology, and minimal lithologic and depositional changes through the section make Levanto an ideal study site for this interval.

δ¹³C_org 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.