GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 115-7
Presentation Time: 10:00 AM


YAGER, Joyce A.1, BERELSON, William M.1, CORSETTI, Frank A.1, WEST, A. Joshua1, ROSAS, Silvia2 and BOTTJER, David J.1, (1)Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, (2)Departamento de Ingenieria, Pontifica Universidad Catolica del Peru, Av. Universitaria 1801, San Miguel, Lima, 32, Peru,

The stable carbon isotope composition of marine carbonate (δ13Ccarb) and organic carbon (δ13Corg) 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 δ13Corg and δ13Ccarb 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.