A LONG-TERM CARBON ISOTOPE RECORD ACROSS THE TRIASSIC-JURASSIC TRANSITION FROM ALASKA (Invited Presentation)

The Triassic-Jurassic boundary interval was a time of intense disturbances to marine communities and notably contains one of the largest extinctions of the Phanerozoic, the end-Triassic mass extinction (ETE, ~201 Ma). This interval is also characterized by major carbon cycle perturbations and potentially widespread oxygen deficiency within the oceans. While the cause(s) of extinction and environmental feedbacks are still debated, and under constrained, they are hypothesized to have been triggered by massive volcanism associated with the Central Atlantic Magmatic Province flood basalts. Whereas carbon isotope (δ¹³C) records immediately preceding the Triassic-Jurassic boundary (i.e. ETE interval) have been generated globally, there remains a lack of continuous, long-term records that capture the global carbon cycle prior to and well after the ETE. We seek to expand our understanding of the carbon cycle and environmental changes across this critical time interval by reconstructing a long-term δ¹³C record from a sedimentary section preserved in the Wrangell Mountains of Alaska that spans the Norian (Late Triassic) to Sinemurian (Early Jurassic) stages (~227–190.8 Ma).

The section studied at Grotto Creek represents distal facies of a mixed carbonate-silica ramp deposited on an isolated plateau at tropical latitudes in the open Panthalassan Ocean. This succession provides a unique window into open marine conditions and also holds the potential for excellent temporal control as it contains abundant ash layers throughout, as well as key ammonoid, bivalve, and conodont fossil occurrences providing robust biostratigraphic control. In our organic δ¹³C data, we observe distinctive excursions at the Norian/Rhaetian boundary. δ¹³C broadly increases higher in the Hettangian and decreases across the boundary into the Sinemurian. These findings are generally consistent with records generated elsewhere and provide key constraints to our understanding of local versus global signals preserved in the δ¹³C record. Thus, improving our ability to tie mechanistic triggers more precisely to the biotic crises surrounding the Triassic-Jurassic boundary interval.