EARLY JURASSIC ENVIRONMENTAL CHANGE (HETTANGIAN - PLIENSBACHIAN): NEW EVIDENCE FROM A MULTI-PROXI RECORD FROM THE DORSET SECTION (GB)

The end Triassic mass-extinction event (eTME ∼200.4 Ma ago) has been recognized as one of the five major Phanerozoic extinction events with 40–73 % loss in biodiversity. It caused important floral and faunal turnover on land, major extinction in the marine realm and important geochemical perturbations. The carbon isotope records show a double negative shift (initial and main CIE) during this time. The initial CIE coincides with the eTME whereas the main CIE was recorded afterwards, within the Hettangian, demonstrating that Earth's biosphere did not fully recover for several million years. Indeed, widespread shallow marine anoxia, palynological variations, and climatic change were identified in the aftermath of the eTME. Furthermore, subsequent carbon isotope excursions have been recorded in the Sinemurian interval (obtusum zone, early-late Sinemurian; Sinemurian-Pliensbachian boundary). However, the mecanisms and environmental impact associated with these perturbations are still not completely understood.

In order to better describe and explain the eTME recovery and the Sinemurian events, we have performed a high resolution (440 samples) multi-proxy geochemical study of the Dorset section spanning from the Hettangian (Planorbis zone) to the Pliensbachian (Margaritatus zone). The stable carbon isotopes recorded in organic matter and carbonate allows to trace the major carbon cycle perturbations in great detail (Hettangian main-CIE; obtusum zone – early late Sinemurian; Sinemurian-Pliensbachian boundary). The associated climatic variations, paleooceanographic change, water oxygenation conditions and productivity will be discussed in this presentation using carbon and oxygen isotopes, bulk-rock and clay mineralogy, the alteration index (CIA), organic matter content (TOC), hydrogen index (HI), phosphorus content and the composition of major and trace elements.