2005 Salt Lake City Annual Meeting (October 16–19, 2005)
Paper No. 77-7
Presentation Time: 10:00 AM-10:15 AM

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

MUNDIL, Roland1, PÁLFY, József2, and MATZEL, Jennifer1, (1) Berkeley Geochronology Ctr, 2455 Ridge Rd, Berkeley, CA 94709-1211, rmundil@bgc.org, (2) Research Group for Paleontology, Hungarian Academy of Sciences-Hungarian Natural History Museum, POB 137, Budapest, H-1431, Hungary

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)

2005 Salt Lake City Annual Meeting (October 16–19, 2005)
General Information for this Meeting
Session No. 77
Jurassic Marine Paleobiology: Tracing the Roots of the Modern Biota
Salt Palace Convention Center: Ballroom F
8:00 AM-12:00 PM, Monday, 17 October 2005

Geological Society of America Abstracts with Programs, Vol. 37, No. 7, p. 187

© Copyright 2005 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.