TIMING OF THE TOARCIAN OCEANIC ANOXIC EVENT AND MASS EXTINCTION: EVIDENCE FROM EUROPE AND TIBET

The Early Jurassic (early Toarcian Stage) was marked by the emplacement of a large igneous province in the Karoo-Ferrar region, global warming, C cycle perturbations, and a oceanic anoxic event (OAE). This interval also witnessed the most severe marine mass extinction in the 135 myr interval between the start of the Jurassic and the end of the Cretaceous. Linking these eclectic phenomena into a coherent Earth-system model has become a popular goal in many recent studies. The development of marine anoxia is generally regarded as the proximate cause of the Toarcian losses, with the release of gas hydrates further contributing to the high-stress environmental changes. However, the crucial evidence, from the relative timing of these phenomena, is out of kilter in the European record. Thus, the mass extinction occurs within the basal Toarcian tenuicostatum Zone and is followed by radiation and recovery in the succeeding serpentinum Zone. The black shales of the OAE are diachronous in their development being of serpentinum age in northern Europe and tenuicostatum age in southern Europe. Detailed study of the northern England record in fact reveals a significantly earlier onset of anoxia in the region indicating that there was a brief interval, in the late tenuicostatum Zone, when anoxia was European-wide. It was this geological "moment" that marked the extinction, but significantly this moment predated the C isotopic evidence for gas hydrate release. Our understanding of Toarcian events have been principally based upon the European record, but a newly discovered Toarcian record from southern Tibet greatly extends the geographic coverage. In this region an extinction of large foraminifera and bivalves in carbonate platform facies coincides with a deepening event and the development of bottom-water dysoxia. However, age determination indicates that these phenomena date to the latest Toarcian. The possibility of a single-phase, ocean-wide, early Jurassic OAE is therefore beginning to look unlikely.