Disentangling the Hettangian carbon isotope record: Implications for the aftermath of the end-Triassic mass extinction

Abstract

This study provides an organic carbon stable isotope (δ 13 C org) record calibrated with detailed ammonite biostratigraphy, following the end-Triassic biological crisis. Precise correlation between this crucial fossil group and the δ 13 C org record is key to understanding feedbacks between biological and environmental events following mass extinction. The latest Triassic and Hettangian δ 13 C org record shows several negative and positive excursions. The end-Triassic negative shift coinciding with the mass extinction interval is followed by a positive excursion in the earliest Hettangian Psiloceras spelae beds, which marks the onset of recovery in the marine ecosystem. This positive trend is interrupted by a second negative δ 13 C org excursion in the P. pacificum beds related to a minor ammonite extinction event. This pattern of the δ 13 C org curve culminates in the uppermost Hettangian Angulata Zone major positive excursion. This indicates that both the ecosystem and the carbon cycle remained in a state of perturbation for at least 2 Ma, although the recovery of some pelagic taxa already began at the base of Jurassic. The early and late Hettangian positive δ 13 C org excursions have been confused in several recent papers. Here, we show that during the Hettangian there are indeed two distinct positive δ 13 C org excursions. Phases of anoxia and further pulses of Central Atlantic Magmatic Province volcanism during the Hettangian might have inhibited the full recovery for that interval of time. The main Liasicus-Angulata organic positive CIE (carbon isotope excursion) during the Late Hettangian might be related to gradual decreasing of pCO 2 due to protracted high organic burial, and coincides with a second phase of recovery, as indicated by a pulse of ammonoid diversification. Copyright 2012 by the American Geophysical Union.