AN ORGANIC GEOCHEMICAL PROFILE OF THE TOARCIAN ANOXIC EVENT IN NORTHERN ITALY

A suite of black shales, calcareous shales and limestones, representin g the Toarcian anoxic event (Jurassic, similar to 192 Ma), was collect ed from a I4-m length of core taken from the Belluno Trough, northern Italy, and examined in detail to assess variations in depositional env ironment during this interval. Geochemical and optical data indicate o rganic matter of algal and bacterial origin, with only trace contribut ion from terrestrial sources. Detailed biological marker data, interpr eted with the aid of multivariate statistics, demonstrate fluctuations in primary producers and bacterial populations during black shale dep osition. The onset of the anoxic event was not abrupt, but characteriz ed by an apparent gradual development of anoxia and consequent enhance d organic-matter preservation. Different hopane biomarkers proved espe cially useful in recognizing preservation potential in the section, an d correlate well with other ''preservation'' indicators. Increased oxy genation at the end of the event may have been associated with changes in primary productivity.