Mass extinctions manifest in Earth's geologic record were turning points in biotic evolution. We present (40)Ar/(39)Ar data that establish synchrony between the Cretaceous-Paleogene boundary and associated mass extinctions with the Chicxulub bolide impact to within 32,000 years. Perturbation of the atmospheric carbon cycle at the boundary likely lasted less than 5000 years, exhibiting a recovery time scale two to three orders of magnitude shorter than that of the major ocean basins. Low-diversity mammalian fauna in the western Williston Basin persisted for as little as 20,000 years after the impact. The Chicxulub impact likely triggered a state shift of ecosystems already under near-critical stress.

Potassium isotopic compositions measured by MC-ICP-MS in cold-plasma, high-resolution mode indicate terrestrial variability in 41K/39K ratios.

Microbial communities within deep subsurface environments were analyzed by 16S rRNA gene cloning. Clone libraries from 27 borehole fluid, 7 mining-contaminated, and 5 rock samples were compared. Borehole fluids derived from deep fractures were populated by microbial communities with low diversity with an average of 11 and 5 bacterial and archaeal OTUs respectively. Low taxa richness was likely driven by limited biogeochemical reactions available for growth and not extreme parameters such as pH and tempera-

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