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Guillaume Paris, IFP (France)
Annachiara Bartolini, Muséum d'Histoire Naturelle (France)
Silvia Gardin, Pierre & Marie Curie University (France)
Valérie Beaumont, IFP (France)
Kevin Page, University of Plymouth (United Kingdom)
Jean Guex, IGP
Quartier UNIL-Dorigny (Switzerland)
Marie-Emilie Clemence, Pierre & Marie Curie University (France)
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The end of Triassic represents one of the "big five" extinctions of the Phanerozoic. The mass extinction seems to be coeval with the Central Altantic Magmatic Province (CAMP) volcanic activity, a major sea-level regression and a high perturbation of the global carbon cycle. Across the Triassic-Jurassic transition, two distinct negative carbon isotope excursions (CIEs) in marine sediments have been recorded. In terms of ecological impact on the biosphere, the global release of volcanic CO2, associated to a global warming, is frequently proposed as linking CAMP and the T/J mass extinction. It has been proposed that a methane release would have amplified the greenhouse effect in combination with high concentrations of atmospheric CO2. The effect of the end-Triassic CO2 maximum would have led to a temporary undersaturation of carbonates within the world's oceans, affecting organisms with calcitic and aragonitic skeletons. Alternatively, the environmental stress might have had been generated by repeated releases of volcanic heavy metals and SO2 emissions. Volcanic activity would also lead to reduced light and subsequent cooling, as well as the acidification of sea water which might have had an impact on photosynthetic primary productivity and carbonate production, with a consequent major disturbance of the carbon pump and hence marine ecosystems.
In order to better understand the scenario connecting volcanism with the biotic turnover, we performed a high-resolution synecological analysis between benthic foraminifera and calcareous nannofossils, combined with geochemical and sedimentological analyses from sections in the Tiefengraben (Austrian Alps) and the West Somerset coast (SW England). Our goal was to understand biotic carbon pump performance before, during and after the mass extinction and to see how these correspond to changes in seawater chemistry, relative sea-level change and CIEs.
Our results shows a complex history of the water column dynamic consist of three main phases. 1) The beginning of the global release of volcanic CO2 and SO2 associated with cooling and temporary undersaturation of carbonates within the world's oceans, led to environmental deterioration at the sea water surface. 2) A 'disappearance interval' occurred coincident with the first CIE corresponding to a carbonate crisis and the bloom of a benthic opportunistic fauna. This event closely predates the maximum amplification of the supposed seawater acidification. It represents the local expression of the globally identified paroxystic phase of the biotic crisis and a strong disturbance of the biological carbon pump at the end of the Triassic. 3) The end of the crisis was marked by a repopulation event characterized by major long-term greenhouse effects at the base of the Hettangian, with a succession of alternating dysaerobic and anoxic phases characterizing the development of trophic conditions and favouring increased primary productivity and the deposition of black shales.
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