Biogeochemical constraints on the Triassic‐Jurassic boundary carbon cycle event

Abstract

The end‐Triassic mass extinctions represent one of the five most severe biotic crises in Earth history, yet remain one of the most enigmatic. Ongoing debate concerns the environmental effects of the Central Atlantic Magmatic Province (CAMP) eruptions and their linkage with the mass extinction event across the Triassic‐Jurassic boundary. There is conflicting paleo‐evidence for changes in atmospheric p CO 2 during the extrusion of the CAMP basalts. Studies on sediments from European and Pacific localities have, however, identified a substantial negative isotopic anomaly (up to −3.5‰) across the TR‐J boundary, providing an important indicator of changes in the operation of the ancient global carbon cycle. We sought to explain the paleo‐evidence by utilizing a carbon cycle model for the “hothouse” world of the end‐Triassic that emphasizes the chemical weathering of silicate and carbonate rocks and the ocean carbonate chemistry. We find that volcanic CO 2 outgassing fails to fully account for either a sufficient rise in atmospheric p CO 2(indicated by the stomata of fossil leaves) or the sedimentary isotopic fingerprint. Instead, the scenario that best fits all of the geologic evidence is a positive feedback loop in which warming, due to a buildup of volcanically derived CO 2, triggers destabilization of seafloor methane hydrates and the catastrophic release of CH 4[Pálfy et al., 2001]. We calculate that this carbon cycle perturbation was huge, involving the release of ∼8000–9000 Gt C as CO 2 during the CAMP basaltic eruptions and ∼5000 Gt C as CH 4. In the model the initial isotopic excursion is assumed to take place over ∼70 kyr, while complete reequilibration of the ocean‐atmosphere system with respect to CO 2 is accomplished over 700–1000 kyr. Our results thus provide a preliminary theoretical explanation for the bioevents, estimated p CO 2 changes, and isotopic excursions observed in marine and continental sediments at this time.