BIOGEOCHEMISTRY OF THE TRIASSIC-JURASSIC BOUNDARY

New biostratigraphic and stable carbon isotope data are presented from Triassic-Jurassic (Tr-J) boundary sections at Kennecott Point and Frederick Island, Queen Charlotte Islands (QCI), Canada, Muller Canyon, Nevada, USA, and Marokopa, New Zealand. These boundary sections show a consistent isotopic trend indicative of several major perturbations to the carbon cycle: one negative excursion in δ¹³C_org of 2 ‰ at the boundary and one positive excursion of 3 to 5 ‰ following the boundary. The post-Tr-J boundary positive excursion is especially prominent in boundary sections from QCI, where the high organic content of the black shales makes the rocks suitable for a survey of biomarkers. If an extinction mechanism similar to that for the Permian-Triassic extinction was in operation during Tr-J time, there may be molecular fossil evidence for both microbial expansion and photic zone anoxia or euxinia preserved in Tr-J rocks. Litho-, bio- and chemostratigraphy from these boundary localities do not support a single impact cause for the late Triassic extinctions, although impact events such as the Manicouagan may well have exerted significant stress on a biosphere still recovering from the Permian-Triassic mass extinction. The data support the idea of a severely degraded late Triassic environment persisting for millions of years, possibly caused by global warming induced by Central Atlantic Magmatic Province volcanism. Warming and sea level change may have led to destabilization of seafloor methane hydrates and runaway greenhouse conditions.