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

• Autorzy: Pálfy J. , Mundil R.
• Konferencja: International Congress on the Jurassic System (7 ; 06-18.09.2006 ; Kraków, Poland)
• Języki publikacji: EN

Abstrakty

EN

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.

Słowa kluczowe

EN

extinction; Triassic/Jurassic boundary; U-Pb dating; time scale; recovery

• Wydawca: Państwowy Instytut Geologiczny - Państwowy Instytut Badawczy
• Czasopismo: Volumina Jurassica
• Rocznik: 2006
• Tom: Vol. 4, no. 1
• Strony: 294--294
• Opis fizyczny: Bibliogr. 5 poz.

Twórcy

autor

Pálfy J.

autor

Mundil R.

• Research Group for Paleontology, Hungarian Academy of Sciences-Hungarian Natural History Museum, P.O. Box 137, Budapest, H-1431, Hungary,

Bibliografia

• 1. Knight K. B., Nomade S., Renne P. R., Marzoli A., Bertrand H. and Youbi N. 2004. The Central Atlantic Magmatic Province at the Triassic-Jurassic boundary: paleomagnetic and ⁴⁰Ar/³⁹Ar evidence from Morocco for brief, episodic volcanism. Earth and Planetary Science Letters, 228, 1-2: 143-160.
• 2. Mattinson J. M. 2005. Zircon U-Pb chemical abrasion („CA-TIMS”) method: Combined annealing and multistep partial dissolution analysis for improved precision and accuracy of zircon ages. Chemical Geology, 220: 47-66.
• 3. Min K., Mundil R., Renne P. and Ludwig K. R. 2000. A test for systematic errors in 40Ar/39Ar geochronology through comparison with U-Pb analysis of a 1.1 Ga rhyolite. Geochimica et Cosmochimica Acta, 64, 1: 73-98.
• 4. Pálfy J., Mortensen J. K., Carter E. S., Smith P. L., Friedman R. M. and Tipper H. W. 2000. Timing the end-Triassic mass extinction: First on land, then in the sea? Geology, 28, 1: 39-42.
• 5. Schoene B., Crowley J. L., Condon D. J., Schmitz M. D. and Bowring S. A. 2006. Reassessing the uranium decay constants for geochronology using ID-TIMS U-Pb data. Geochimica et Cosmochimica Acta, 70: 426-445.