A LONG-TERM RECORD OF THE LATE TRIASSIC THROUGH EARLY JURASSIC CARBON CYCLE

The latest Triassic to earliest Jurassic was a time of several disturbances to marine ecosystems and notably contains one of the largest extinctions of the Phanerozoic, the end-Triassic mass extinction or ETME. This interval is also characterized by perturbations to the carbon cycle recorded as carbon isotope (δ¹³C) excursions in the stratigraphic record. However, debate surrounds whether these features reflect local or global changes in the carbon cycle as there are distinct stratigraphic and geographic biases in published records. Specifically, while the δ¹³C record around ETME has been heavily interrogated, less focus has been on the longer-term record outside of this interval. Further, most studies have focused on the δ¹³C records from successions deposited in the Tethys Ocean. Thus the drivers behind the changes in the δ¹³C record around the ETME remain an open question.

In order to address these outstanding issues, we have reconstructed a δ¹³C record from Norian (Late Triassic) to Pliensbachian (Early Jurassic) stages (~227–190.8 Ma) from a sedimentary section preserved at Grotto Creek in the Wrangell Mountains of Alaska. This thick (more than 500 meters) succession represents distal facies of a mixed carbonate-siliceous ramp deposited off of an island arc at tropical latitudes in the open Panthalassan Ocean. It also has robust temporal control for our δ¹³C record with key biostratigraphic occurrences and abundant ash layers throughout the succession. In our organic δ¹³C data, we note negative excursions at the Norian/Rhaetian boundary, three across the ETME and Triassic-Jurassic boundary interval, and one in the lower Sinemurian. The presence of these excursions from the Norian/Rhaetian and Triassic-Jurassic Boundary in the Grotto succession suggests they represent global perturbations to the carbon cycle, in contrast to recent reports to the contrary. While the excursions near the ETME have been linked to outgassing from the Central Atlantic Magmatic Province, the driver of the Norian/Rhaetian and Sinemurian excursions remain an open question though their apparent global nature narrows the potential drivers. Overall, these findings provide key constraints to our understanding of local versus global signals preserved in the δ¹³C record and improves our ability to tie mechanistic triggers more precisely to the biotic crises surrounding the ETME.