Synchrony between Early Jurassic extinction, oceanic anoxic event, and theKaroo-Ferrar flood basalt volcanism

A well-known second-order mass extinction took place during the Pliensbachi an and Toarcian Stages of the Early Jurassic. First recognized as a minor P liensbachian peak in the global extinction rate, it has alternatively been interpreted as a regional response to the early Toarcian oceanic anoxic eve nt. Detailed studies established it as a global long-term event spanning fi ve successive ammonoid zones. Here we present a revised time scale based on high-precision U-Pb ages resolved to the zone level, which suggests that e levated extinction rates were sustained for about 4 m.y, and peak extinctio n occurred at 183 Ma. Recent isotopic dating of flood basalts from the sout hern Gondwanan Karoo and Ferrar provinces documents a culmination in volcan ic activity ca. 183 Ma. The onset of volcanism is recorded as an inflection and start of a rapid rise of the seawater Sr-87/Sr-86 curve. The synchrony of voluminous flood basalt eruptions and biotic crises, as already noted f or three of the major mass extinctions, permits a causal relationship, whic h in this case may be mediated by widespread oceanic anoxia.