SHIFTING BIOGEOCHEMICAL DYNAMICS DURING THE LATE TRIASSIC AND EARLY JURASSIC: INSIGHT FROM STABLE CARBON AND NITROGEN RECORDS

The stable carbon isotope composition of marine carbonate (δ¹³C_carb) and organic carbon (δ¹³C_org) changed during the end-Triassic mass extinction and the emplacement of the Central Atlantic Magmatic Province (CAMP). Interpretations of the C isotope records across the Triassic-Jurassic boundary vary widely, particularly in terms of what these records tell us about CAMP and associated C cycle change. Here, we report δ¹³C_org and δ¹³C_carb paired with stable N isotope (δ¹⁵N) measurements from the Levanto section (Northern Peru) to evaluate possible changes in biogeochemical cycling across the end-Triassic extinction and emplacement of CAMP. Our ~four million year record is the first high-resolution δ¹⁵N study spanning much of the Rhaetian (latest Triassic) through much of the Hettangian (earliest Jurassic). U-Pb ash bed dating and ammonite biostratigraphy (e.g. Guex et al., 2012; Wotzlaw et al., 2014) enable assessment of changes in isotope measurements through time and global correlation, respectively.

δ¹⁵N values shift systematically through the Rhaetian at the Levanto section, from ~9‰(early –mid Rhaetian) to ~3‰ (late Rhaetian). Rhaetian values exhibit cyclicity within this trend, with a variability of about 3‰. We speculate these changes may be related to Milankovitch cycles. δ¹⁵N values stabilize near 2.5‰ in association with the end-Triassic extinction and Triassic-Jurassic boundary, with very little variability seen through the Hettangian (< ±0.5 ‰). The shift from Milankovitch-driven cycling towards low variability and accompanying shift from more positive to low δ¹⁵N values could be explained by several potential hypotheses related to the cycling of organic matter, paleo-redox, and nutrient dynamics leading up to the end-Triassic mass extinction. We will discuss possible scenarios (e.g. change in nutrient supply, water column oxygenation) leading to these changes and the plausibility of each in light of other data (e.g. C:N, %TOC) in an attempt to better understand changes in biogeochemical cycling prior to the end-Triassic mass extinction.