August 2009 Geology and GSA Today media ...(ors of dynamic late Paleozoic climate ch...)

U-Pb zircon ages from the southwestern Karoo basin, South Africa -- Implications for the Permian-Triassic boundary
Andrea Fildani et al., Chevron Energy Technology Company, 6001 Bollinger Canyon Rd, San Ramon, California 94583, USA. Pages 719-722.

The most extreme biological crisis on Earth, the end-Permian mass extinction, is recorded in terrestrial vertebrate fossil assemblages and paleosol horizons of the spectacularly preserved Karoo Basin, South Africa. By analyzing and dating 205 zircon grains extracted from deep-marine ash beds of the western Karoo (using an high-resolution microprobe [SHRIMP-RG] at Stanford University), Fildani et al. have discovered that the important Permian-Triassic boundary is marine in the southwestern sector of the basin, making the Karoo the only basin known to have a Permian-Triassic boundary identified in both marine and terrestrial sections. This discovery offers the unique opportunity to study, in close vicinity, impacts of the end-Permian crisis across an environmental spectrum.

Geological and archaeological implications of strontium isotope analysis of exposed bedrock in the Chicxulub crater basin, northwestern Yucatan, Mexico
Adrian Gilli et al., Geological Institute, ETH Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland. Pages 723-726.

The existence of a large buried impact crater beneath the Yucatan Peninsula of Mexico has been known for almost two decades. This crater, named after the town Chicxulub, was formed by a giant meteorite impact about 65 million years ago. The impact may have been responsible for the great extinction event that occurred at the Cretaceous-Paleogene boundary, including the extinction of dinosaurs. Although a several-kilometer-thick cover of sedimentary rocks buries the crater, the outline of the crater is still visible at the surface by a semicircular rin
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