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Geochemical and Biological Evidence for a Catastrophic end to the Triassic

Olsen, P E, polsen@ldeo.columbia.edu, LDEO, Rt. 9W, Palisades, NY 10964 United States; Koeberl, C, Institute of Geochemistry, University of Vienna, Vienna, Austria; Huber, H, Institute of Geochemistry, University of Vienna, Vienna, Austria, Montanari, A, Osservatorio Geologico do Coldigioco, I-62020, Frontale di Apiro, Italy, Fowell, S J, Dept. Geology & Geophysics, Univ. Alaska Fairbanks, Fairbanks, AK 99775 United States

The Triassic-Jurassic (Tr-J) boundary is one of the "big five" mass extinctions of the last half billion years. In many of the exposed rift basins of the Atlantic passive margin of North America and Morocco, the boundary is identified as an interval of stratigraphically abrupt floral and faunal change within cyclical lacustrine strata (1). A comparatively thin interval of Jurassic age strata separates the boundary from extensive overlying basalt flows, for which the most reliable dates (201ñ2) are virtually indistinguishable from recent dates on tuffs from marine boundary sequences (2). The pattern and magnitude of biotic changes at the Triassic boundary is remarkably similar to that at the K-T boundary. The similarity has fueled debates on the cause of the Tr-J extinctions, with hypotheses focusing on bollide impacts (3,4) and CO2 warming due to flood basalt volcanism (4,5). Here we present the results of detailed geochemical, mineralogical, and paleontological sampling of four sections that cross the biologically identified Tr-J boundary in the Newark rift basin (NY, NJ, PA: USA), where it has been most intensely studied. Major and trace elements were assayed by XRF, and Ir by gamma-gamma coincidence spectrometry after neutron activation (detection limit of about 0.02 ppb Ir). The samples show variations in Ir from 19 to 285 ppt, and results obtained thus far do show a small Ir anomaly at the boundary in four sections. The fact that the anomaly is intimately associated with the previously identified spore spike (1) and floral and faunal last appearances in these sections invites comparison to the K-T boundary in the western US (6), which has similar features. Based on Milankovitch calibration of accumulation rates (7), the pollen and spore transition occurred over an interval of less than 2000 years. Abundant, well-preserved reptile footprints around the boundary attest to a very abrupt extinction of Triassic tetrapods (< 20 ky). It is also possible that the relatively weak Ir anomaly seen thus far is a consequence of dilution by the coarse sampling level (ca. 3 cm / per sample) required by the very high accumulation rates (ca. 1 m / 2000 yr) in this part of the Newark basin. Our search for shocked quartz proved unsuccessful. Only a few grains with subplanar deformation features were found, but none had features diagnostic of shock metamorphism. This result is in agreement with previously published attempts (8). Taken together, however, the geochemical and biological data are consistent with a catastrophic end to the Triassic. Present data are insufficient to discriminate between a terrestrial (e.g. volcanic) or extraterrestrial (e.g. bollide) cause of the apparent catastrophic extinctions, although there are contending "smoking guns" for both (3-5).

(1) Fowell, S.J., B. Cornet, B., P.E. Olsen. 1994. Geol. Soc. Am. Spec. Pap. 288L:197-206; Fowell, S.J. and P.E. Olsen. 1993. Tectonophysics 222:361-369. (2) P lfy, J., J.K. Mortensen, E.S. Carter, P.L. Smith, R.M. Friedman, W.W. Tipper. 2000. Geology 28:39-42. (3) Olsen, P.E., N.H. Shubin, M. Anders. 1987. Science 237:1025-1029; Mory. A.J., R.P. Iasky, A.Y. Glikson, F. Pirajno. 2000. Earth Planet. Sci. Lett., 177:119-128. (4) Olsen, P.E. 1999. Science 284:604-605. (5) McHone, J.G. 1996. Geology 24:319-322. (6) Tschudy, R.H., C.L. Pillmore, C.J. Orth, J.S. Gimore, J.D. Knight. 1984. Science 225:1030-1032. (7) Olsen P.E, R.W. Schlische, M.S. Fedosh. 1996. Mus. North. Ariz. Bull. 60:11-22. (8) Mossman, D.J., R.C. Grantham, F. Lagenhors. 1997. Can. Jour. Earth Sci., 35:101-109.

Olsen, P. E., Koeberl, C., Huber, H., Montanari, A., and Fowell, S J, 2000, Geochemical and Biological Evidence for a Catastrophic end to the Triassic. Eos, Transactions, American Geophysical Union, Supplement, v. 81, no. 48,  p. F1325.