CARBON CYCLE DYNAMICS AND ENVIRONMENTAL CHANGE AT THE TRIASSIC-JURASSIC BOUNDARY

Here we present new carbon isotope and lithologic data from expanded, fossiliferous, shallow-marine carbonate sections spanning the Triassic-Jurassic boundary and early Jurassic recovery period. Over 1650 samples were collected and analyzed from seven separate localities in Northern and Southern Italy. In all studied sections, we observe a positive δ¹³C excursion (up to +6‰) beginning above the mass extinction horizon, with values declining and stabilizing near +2‰ late in the Hettangian. In previous studies, this isotope excursion has been interpreted to reflect a shift in the of the δ¹³C global ocean-atmosphere carbon pool. However, several observations suggest a strong contribution from local and regional factors: the magnitude of the excursion varies substantially among sections; δ¹³C values are closely associated with lithofacies; and the excursion is entirely absent from recently published bulk organic δ¹³C data. Together these observations suggest the δ¹³C excursion in carbonate rocks may reflect a substantial δ¹³C gradient in dissolved inorganic carbon across the western Tethys, with the most ¹³C-enriched waters in northern Italy. Compositional variation may account for differences in secular variation of δ¹³C in studies of bulk organic carbon. Thus, δ¹³C variation in the earliest Jurassic likely reflects global environmental disturbance, likely associated with the release of volatiles from CAMP volcanism, but the carbon isotope expression of these events is strongly associated with regional processes.