CARBON ISOTOPE ANOMALY AND OTHER GEOCHEMICAL CHANGES AT THE TRIASSIC/JURASSIC BOUNDARY: IMPLICATIONS FOR EXTINCTION SCENARIOS

Although most major extinction events are associated with pronounced isotopic excursions, this has not been established for the end-Triassic. Here we report the most complete stable istope record of the end-Triassic event known so far, which suggests that a significant negative carbon isotopic anomaly does exist near the Triassic/Jurassic (Tr/J) boundary.

The Csõvár section in Hungary contains the Tr/J boundary in a sparsely fossiliferous marine limestone sequence that was deposited in proximal and distal toe-of-slope and basinal environments. The Tr/J boundary is defined by ammonoid and conodont biostratigraphy. We sampled 35 m of strata across the Tr/J transition for geochemical analyses.

The section is poor in organic carbon and contains well preserved, immature kerogen of marine origin. The carbon and oxygen isotopic composition of 53 bulk carbonate samples fluctuates throughout the section but major diagenetic effects were only detected in 7 samples. The most striking feature is a negative d¹³C_carb spike of –4‰ immediately below the system boundary. A smaller negative anomaly is also present in the Rhaetian. Correlative shifts occur in the d¹³C_org record, although with slight delay and somewhat smaller magnitude; a TOC minimum is also observed in the topmost Triassic. Trends in the d¹⁸O_carb curve are less clear but there is some evidence for a d¹⁸O_carb minimum, implying significant warming, that corresponds to the main carbon isotope anomaly.

Our results suggest that, similarly to other mass extinctions, the end-Triassic event was also associated with environmental perturbations and changes in the global carbon cycle. The negative d¹³C anomalies can be ascribed to a decline in primary productivity, although alternative explanations (e.g. invoking climatically induced methane hydrate dissociation) cannot be ruled out. It appears that the main isotope anomaly near the Tr/J boundary was not the result of a single event, but rather a culmination of a period of instability in the Earth systems, perhaps triggered by large-volume volcanism in the Central Atlantic Magmatic Province.