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It forms under very high temperatures and pressures consistent with a cosmic impact," Kennett said.

"The type of diamond we have found - Lonsdaleite - is a shock-synthesized mineral defined by its hexagonal crystalline structure. The thin layer of iridium-and-quartz-rich sediment dates to the transition of the Cretaceous and Tertiary periods, which mark the end of the Mesozoic Era and the beginning of the Cenozoic Era. They were found in sediment dating to massive asteroid impacts 65 million years ago in a layer widely known as the K-T Boundary. Such soot and diamonds are rare in the geological record. The diamonds were found in association with soot, which forms in extremely hot fires, and they suggest associated regional wildfires, based on nearby environmental records. The nano-sized diamonds were pulled from Arlington Canyon on the island of Santa Rosa that had once been joined with three other Northern Channel Islands in a landmass known as Santarosae. They date to the end of Clovis - a Paleoindian culture long thought to be North America's first human inhabitants.

These tiny diamonds and diamond clusters were buried deeply below four meters of sediment. Kennett and colleagues from nine institutions and three private research companies report the presence of shock-synthesized hexagonal diamonds in 12,900-year-old sediments on the Northern Channel Islands off the southern California coast. In a paper appearing online ahead of regular publication in the Proceedings of the National Academy of Sciences, University of Oregon archaeologist Douglas J. ScienceDaily (July 21, 2009) - A 17-member team has found what may be the smoking gun of a much-debated proposal that a cosmic impact about 12,900 years ago ripped through North America and drove multiple species into extinction. Science Daily: California's Channel Islands Hold Evidence Of Clovis-age Comets. Scientists have found what they call "the smoking gun" in Holocene impact research: evidence of Clovis-age comets confirming Holocene catastrophe, Democritus's worlds in collision, and Plato's Timaeus.