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

A suite of black shales, calcareous shales and limestones, representing the Toarcian anoxic event (Jurassic, similar to 192 Ma), was collected from a I4-m length of core taken from the Belluno Trough, northern Italy, and examined in detail to assess variations in depositional environment during this interval. Geochemical and optical data indicate organic matter of algal and bacterial origin, with only trace contribution from terrestrial sources. Detailed biological marker data, interpreted with the aid of multivariate statistics, demonstrate fluctuations in primary producers and bacterial populations during black shale deposition. The onset of the anoxic event was not abrupt, but characterized by an apparent gradual development of anoxia and consequent enhanced organic-matter preservation. Different hopane biomarkers proved especially useful in recognizing preservation potential in the section, and correlate well with other ''preservation'' indicators. Increased oxygenation at the end of the event may have been associated with changes in primary productivity.