COUPLED ORGANIC AND CARBONATE δ13C RECORDS OF THE LATE TRIASSIC AND EARLY JURASSIC IN NORTHERN ITALY: IMPLICATIONS FOR CARBON CYCLING DURING THE AFTERMATH OF THE END-TRIASSIC MASS EXTINCTION

A large protracted positive carbon isotope excursion has been observed in the lowermost Jurassic following the end-Triassic mass extinction. However, the lack of paired records from carbonate rocks (δ¹³C_carb) and organic carbon (δ¹³C_org) and limited biostratigraphic constraints leave open the possibility that variations in δ¹³C_carb and δ¹³C_org are not correlative and do not represent a shift in the δ¹³C of the global carbon pool. Consequently, the long term carbon cycle behavior following the end-Triassic mass extinction remains incompletely understood.

Here we present the first extended, coupled δ¹³C_carb and δ¹³C_org records of the uppermost Triassic and lowermost Jurassic from stratigraphic sections in the Lombardy Basin of northern Italy. The large positive excursion previously observed in the carbonates also occurs in the organics from the same samples, but with a smaller magnitude. Because few post-depositional mechanisms affect the isotopic composition of C_org and C_carb in similar ways, the correspondence of the two curves presents strong support for a primary origin for the large positive isotopic excursion. The more muted response of the organics is consistent with variation in the fractionation between carbonates and organic carbon, mixing of contemporaneous organic matter with an input of extrabasinal organic carbon of a constant isotopic composition, or some combination of the two. In either case, the occurrence of the positive excursion in multiple locations globally in both carbonates and organic matter is best explained by a change in the isotopic value of the global carbon reservoir. The elevated δ¹³C values and increased magnitude of the difference between the carbonates and organics is consistent with the predicted biogeochemical consequences of heightened pCO₂. The coincidence of the extinction and carbon cycle disturbance with emplacement of the Central Atlantic Magmatic Province suggests that volatiles derived from its emplacement were the likely source of the perturbation.