DRAMATIC DECREASE OF THE PELAGIC CARBONATE PRODUCTION BY NANNOPLANKTON ACROSS THE EARLY TOARCIAN ANOXIC EVENT

The early Toarcian anoxic event (early Jurassic, ~183Myr) coincides with a major biological crisis of marine ecosystems leading to the almost complete disappearance of shallow-water carbonate factories. A global perturbation of the carbon cycle is testified by an important negative excursion of the carbon stable isotopes (E. exaratum ammonite Subzone). At that time, the only pelagic carbonate producer was represented by nannoplankton, namely coccolithophorids and Schizosphaerella, a probable calcareous dinoflagellate. These unicellular algae did account for photosynthetic primary production as well. The quantification of spatial distribution of nannoplankton productivity in the Tethys ocean during the anoxic event is therefore crucial for the understanding of the role of primary and carbonate production in the mechanisms of carbon sequestration from oceanic surface waters and its eventual export to the sedimentary reservoirs. Coccolith and Schizosphaerella absolute abundance were quantified in different sections located within the Jurassic western Tethys. Nannofossil abundance was then corrected by the accumulation rate calculated for each studied site. The size of Schizosphaerella was measured in two sections. The results of this work show that nannofossil abundance and size significantly decrease during the early Toarcian anoxic event. The decrease in pelagic carbonate production, however, did not occur synchronously with the demise of shallow-water carbonate platforms, but was delayed of about 600kyr. Absolute abundance of nannofossils significantly varies from a site to another suggesting a variable nannoplankton production in different areas within the western Tethys. The highest absolute abundance measured in this study corresponds to the low productivity areas in modern Oceans. This is consistent with the fact that nannoplankton was in the early stage of its evolution during the early Jurassic; abundance seems to increase exponentially though Mesozoic.