Cause of Upper Triassic climate crisis revealed by Re-Os geochemistry of Boreal black shales

The Triassic Period is bracketed by two of the 'big five' Phanerozoic mass extinctions. Though long viewed as a period of climatic stability, emerging data suggest multiple climatic swings and at least one severe ecological crisis. Linking these climatic instabilities with probable causes is hampered by poor age control within the Triassic time scale. Here we present new Re-Os ages for shale sections straddling Middle-Upper Triassic stage boundaries. Nominal Re-Os isochron ages of 236.6 and 239.3 Ma for the top and base of the Ladinian (upper Middle Triassic) bring absolute time into the contentious Triassic time scale, and place the beginning of the Late Triassic about 12 m.y. earlier than previously assigned. A marked decrease in initial 187Os/188Os in Upper Ladinian shale records input from Wrangellian flood basalts - an instigator in the Carnian (Late Triassic) Pluvial Event and accompanying radiation of key fossil groups (e.g., dinosaurs and calcareous nanoplankton). An absolute time scale is proposed for the Anisian-Ladinian-Carnian boundaries based on Re-Os geochronology.