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Re: Deccan produced global warming that was over before the K-T boundary



"David Marjanovic" <david.marjanovic@gmx.at> writes:


> Interesting is the date issue -- the authors** put the K-T boundary 
> at 65.5
> Ma ago instead of at 65.0. Is this just the usual difference between
> argon-whatever and uranium-lead?


For Upper Cretaceous ages, a 500,000-year discrepency between the U-Pb
and K-Ar methods is not unusual.

Gradstein et al. (1995) give 65.00+/- 0.04 for the K-T boundary.
(Gradstein, F.M., F.P. Agterberg, J.G. Ogg, J. Hardenbol, P. Van Veen, J.
Thierry, and Z. Huang. 1995. Mesozoic timescale, pp. 95-126, IN Berggren,
W. A., D.V. Kent, C.C., Swisher, M. Aubry, and J. Hardenbol (eds),
Geochronology, Time Scales and Global Stratigraphic Correlation. SEPM
Special Publication No. 54).

Is Gradstein et al.'s K-T date still authoritative?

The boundary occurs in the upper portion of geomagnetic chron C29r
(approximately at C29r.7).

OTOH, the most accurate estimate for the age of the melt rock at the
bottom of the Chixculub crater is 65.07 +/- 0.1 ma (K-Ar method).  See: 
Swisher, C.C., N.J.M. Grajales, A. Montanari, S.V. Margolis, P. Claeys,
W. Alvarez, P. Renne, P.E. Cedillo, F. J-M. Maurrasse, R. Curtis, J.
Smit, and M. McWilliams. 1992. Coeval Ar-Ar ages of 65 million years ago
from Chixculub crater melt-rock and Cretaceous-Tertiary boundary
tektites. Science 257: 954-958.

I believe that the melt rock in the crater also has a reversed
geomagnetic polarity.

<pb>
--








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