NEW CONSTRAINTS ON THE TIMING OF THE TRIASSIC-JURASSIC EXTINCTION AND RECOVERY

The understanding of the extinction and recovery/origination of life, and the underlying causes, during the Triassic-Jurassic (Tr-J) transition is critically dependant on the availability of a reliable time scale. Ages and uncertainties from time-scale compilations are typically taken at face value although their data base is often unacceptably weak. With this presentation we hope to draw the attention of end users to the Triassic/Jurassic transition, the calibration of which is particularly fragile.

Most constraints for the timing of the Tr-J transition are from terrestrial sequences which have been studied in detail in eastern N America as well as NW Africa. ⁴⁰Ar/³⁹Ar ages from CAMP (Central Atlantic Magmatic Province) volcanics cluster around 199.9 Ma and have been documented to coincide with an abrupt change of both fauna and flora on land (Knight et al., 2004).

In contrast, the preserved marine geologic record is scarce. The only recent radio-isotopic age, a U/Pb zircon date of 199.6 ± 0.3 Ma (Palfy et al., 2000), was obtained from a volcanic ash predating the Tr-J boundary. Although in apparent agreement with the ⁴⁰Ar/³⁹Ar age for the CAMP volcanism, recent results from several studies applying U/Pb and ⁴⁰Ar/³⁹Ar analyses to minerals from the same rocks show that there is a systematic bias between the two isotopic systems, with ⁴⁰Ar/³⁹Ar ages being ca 1% younger than U/Pb ages. Accounting for this discrepancy consequently shifts the expected U/Pb age for the main pulse of the CAMP volcanism to 202 Ma and therefore predates the U/Pb age for the marine extinction by more than 2 Ma. Additional constraints come from a new preliminary age of 198.0 ± 0.6 Ma (single-crystal IDTIMS U/Pb analyses) applied to a volcanic layer within early Sinemurian marine sediments in S Hungary. The latter age suggests that the Tr-J boundary might be older than 199.6 Ma, unless the lowermost stage of the Jurassic (Hettangian) is extremely short which has severe implications for both the cause of the extinction as well as the rate of origination and recovery following the Tr-J biotic crisis.

A number of potential scenarios arise, none of which can be unambiguously accepted from the currently available database, and therefore await a rigorous test.

Knight et al., Earth and Planetary Science Letters 228, 143– 160 (2004)

Palfy et al., Geology 28, 39-42 (2000)