CARBON ISOTOPES AND THE BIOTIC RESPONSE TO MASS EXTINCTION

The Mesozoic record of biodiversity commences and ends with major mass extinctions and also contains a number of events of various magnitudes that punctuate the era. These events are, in general, better documented in the marine realm, and they all are associated with marked perturbations in δ¹³C, a proxy that fundamentally reflects the nature carbon cycling. Although in terms of overall trends, the records associated with these events represent either positive or negative excursions, in detail each extinction event records a unique pattern of dynamics within the carbon cycle. Based on early work, the trends in δ¹³C appear to be effective predictors of not only when the major phase or step of an extinction occurs - generally associated with the most pronounced isotopic shift – but also the duration of the recovery and repopulation intervals. To investigate this prediction more fully, this study aims to compare and contrast not only the records associated with the three larger mass extinctions– the Permo-Triassic, Triassic-Jurassic, and Cretaceous-Paleogene events – but also on the smaller bioevents documented for Mesozoic. For these various mass extinctions, existing δ¹³C records were compiled and compared to records of the biotic responses representing as broad a geographic and taxonomic breadth as possible. Initial results suggest that the most pronounced extinction step is associated with the first, major isotopic perturbation and that the duration of the recovery and repopulation intervals are closely tied to the length of time required for the perturbations to wane and return to ‘background’ levels. Furthermore, they suggest that: 1) the nature of carbon cycling as recorded in δ¹³C variation is an effective predictor of the biotic responses to large-scale biotic disruptions; 2) the structure of each event is unique suggesting that at least the cascades of environmental effects, if not the causal mechanism(s), are unique to each event; and 3) in general, there is little correlation between the magnitude of the extinction event and the rate of biotic recovery. Finally, it remains to be determined to what extent δ¹³C serves as a direct proxy for the causal mechanisms or whether it is a reflection of the biotic changes themselves.