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Journal of the Geological Society; 2004; v. 161; issue.4; p. 685-693;
DOI: 10.1144/0016-764903-074
© 2004 Geological Society of London

Original Article

Phytoplankton evidence for the timing and correlation of palaeoceanographical changes during the early Toarcian oceanic anoxic event (Early Jurassic)

Emanuela Mattioli1, Bernard Pittet1, RaffaellaBucefalo Palliani2, Hans-Joachim RÖhl3, Annette Schmid-RÖhl3 & Elena Morettini4

1 1Université Claude Bernard Lyon 1, UMR 5125, Paléoenvironnements et Paléobiosphère, UFR Sciences de la Terre, 2 rue Dubois, 69622 Villeurbanne, France (e-mail: mattioli@univ-lyon1.fr)
2 2Department of Earth Sciences, Perugia University, Piazza Università, 06123 Perugia, Italy
3 3Hornschuchstrasse, 5, 72074 Tübingen, Germany
4 4Shell International Exploration and Production, Volmerlaan 8, Netherlands

The causes and duration of the early Toarcian anoxic event are controversial. Integration of data from calcareous and organic phytoplankton provides a biochronological framework that allows precise correlations across the western Tethys. In particular, the Carinolithus superbus nannofossil Zone can be used to correlate the levels enriched in organic matter and the related {delta}13C negative excursion. Although a variable duration is given in the literature for this negative excursion, it is likely that it lasted between 520 and 650 ka. Increased atmospheric pCO2, related to excess volcanic emissions (magmatic activity in the Karoo and Ferrar Provinces), had an impact on climate and ocean chemistry, and marked the inception of a biotic crisis affecting many organisms. The beginning of the crisis within shallow carbonate platforms, documented at southern latitudes, predates the levels enriched in organic matter. Dinoflagellate cysts experienced a decrease in abundance in the C. superbus Zone, until they temporarily disappeared. The nannoplankton crisis was twofold: a decrease in size and low calcified specimens are observed in addition to a drastic decrease in absolute abundance. The increased atmospheric pCO2, as a result of the magmatic activity and temporarily amplified by transient methane release, could have been the trigger for the biocalcification crisis, which first affected the probably more reactive neritic system, and eventually the nannoplankton community.

Key Words: Early Toarcian • anoxic event • nannoplankton • microplankton • calcification




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