The Toarcian black shale event in northern Italy

Abstract

During the Early Toarcian (Jurassic; c. 192 Ma), organic-rich sediments were deposited in many areas of Europe. This paper reports the major findings of the first part of an organic geochemical investigation of the Toarcian Oceanic Anoxic Event (OAE). Thirty-two black shales and associated lithologies were studied from seven localities in the Southern Alps of northern Italy. Toarcian organic-rich sediments (up to 4.3% TOC) were deposited in various palaeoceanographic settings within the Lombardy, Belluno and Julian Basins, and on the Trento Plateau. Results of biomarker analyses indicates that in all sections, mixed contributions from algae and bacteria dominate a variable, but generally minor, terrigenous input. The preservation of significant quantities of marine-derived organic matter and the typically well-developed sedimentary lamination suggest that the black shales accumulated under oxygen-deficient conditions in all palaeoceanographic settings. Molecular evidence suggests that the algal-derived components of the sedimentary organic matter is partially degraded in many of the basinal samples. This observation is here interpreted as being the result of aerobic decomposition of algal debris during transport through the water column. Other lines of evidence indicate that oxygen depletion intensified during the later stages of the Toarcian OAE, with enhanced preservation of authochthonous organic matter. In the light of these results, the preferred model for the accumulation of black shales on the Tethyan southern margin incorporates anoxic basin bottom waters, overlain by a relatively well-oxygenated water column.