The age of the Triassic/Jurassic boundary: new data and their implications for the extinction and recovery

Providing accurate and precise radio-isotopic ages for the Triassic/Jurassic (T/J) boundary is a goal for time scale calibration but it also bears on the timing and causality of the end-Triassic mass extinction and subsequent recovery. The currently used estimate of the T/J boundary age hinges on a multi-grain zircon U-Pb age from a marine T/J boundary section in the Queen Charlotte Islands, Canada (199.6±0.4 Ma, Pálfy et al. 2000), and ⁴⁰Ar/³⁹Ar ages of volcanism of the Central Atlantic Magmatic Province (CAMP) cluster around 199.9 Ma (Knight et al. 2004), supporting a hypothesis that implicates this Large Igneous Province in triggering severe environmental changes and the biotic extinction. However, the apparent synchrony of the end-Triassic extinction and CAMP volcanism requires scrutiny for at least two reasons. Multi-grain zircon U-Pb analyses are prone to leave slight Pb loss undetected, hence producing marginally younger ages. Comparison of ⁴⁰Ar/³⁹Ar and U-Pb dates is affected by a current revision of the decay constant of ⁴⁰K and the age of fluence monitors that may require recalculation of all Phanerozoic ⁴⁰Ar/³⁹Ar ages and their upward adjustment by ˜1% (e.g. Min et al. 2000). Here we present new single-crystal zircon U-Pb ages that provide new constraints on the T/J boundary. Applying the pretreatment of thermal annealing and chemical abrasion (Mattinson 2005) resulted in a coherent cluster of zircon ages with a preliminary ²⁰⁶Pb/²³⁸U age of 198.0±0.6 Ma from a volcanic tuff layer within Early Sinemurian sediments in the Mecsek Mts., southern Hungary. The same method yielded an age of 200.6±0.3 Ma for a volcanic ash layer in ammonite-bearing Middle Hettangian marine sediments at Puale Bay, Alaska. The Early Sinemurian age provides an upper limit for the earliest Jurassic recovery interval. The two new ages suggest that the T/J boundary might be older than previously thought, likely >201 Ma. Considering the systematic bias in existing ⁴⁰Ar/³⁹Ar ages for the CAMP, synchrony of volcanism and extinction remains a strong possibility but requires further tests. Significantly, a single-crystal 206Pb/238U age of 201.3±0.3 Ma has been reported from the North Mountain basalt, a CAMP flow in Nova Scotia, Canada (Schoene et al. 2006). Further dating effort is needed to re-analyze critically important volcanic ash layers from the T/J boundary sections of the Queen Charlotte Islands.