RISE IN ABUNDANCE OF SCHIZOSPHAERELLA SPP. AFTER THE S-P EVENT (SINEMURIAN-PLIENSBACHIAN, EARLY JURASSIC, SOUTHERN ALPS)

The Sinemurian-Pliensbachian boundary Event (S-P Event, Early Jurassic) is a global negative δ13C excursion that has been found in wood and marine carbonates, both in deep-and shallow water sedimentary environments, and is thought to reflect the injection of large amounts of CO2 into the atmosphere-ocean system. The S-P Event coincided with major changes on carbonate platforms. On the Trento Platform, which was located in the northwestern Tethys, a major change from a microbialite-dominated carbonate factory to one dominated by skeletal grains occurred across the negative carbon-isotope perturbation. This change was attributed to a crisis of the factory possibly due to a wet climate phase associated with increased terrigenous input and the onset of meso-eutrophic conditions. If and how the S-P Event might have influenced deep- water marine environments was instead not investigated yet. A thick succession of cherty limestone that deposited in deep waters in the Lombardian Basin, west of the Trento Platform is exposed in the Tofino section, in the central Southern Alps. Investigation of these rocks shows that in the Sinemurian nannoplankton was scarcely diversified and rare, with most of the pelagic carbonate given by the “calcisphere” Schizosphaerella spp. Point counting and supervised image classification techniques were carried out on SEM images in order to estimate the percent contribution of carbonate represented by Schizosphaerella spp. to the bulk rock volume. Results show that this contribution was low (< 10%) until after the S-P Event, when the percentage of Schizosphaerella rose above the 40%. Similar increases in the abundance of calcispheres were documented after the Mid-Carnian and Early Toarcian global carbon cycle perturbations. In the case of the Toarcian, this was interpreted as the response to nutrification event and rise of atmospheric CO2 levels by accelerated planktonic biological and carbonate pumps. Therefore, results of the investigations of the Tofino section envisage a possible connection between the S-P Event perturbation of the carbon cycle, the crisis of shallow water carbonate factories and the sudden rise in pelagic carbonate production. In conclusion, data presented in this contribution may help shed light on one of the major changes that occurred in the Early Jurassic oceans, when the spread and diversification of calcareous nannoplankton brought it to progressively become a fundamental contributor to carbonate production in open ocean settings and a major actor in the global carbon cycle.