Overhunting or Impact?
Researchers believe that they have found physical evidence of a cosmic impact that might have caused the extinction of an estimated 35 mammal and 19 bird genera about 12,900 years ago. Their findings are inconsistent with the popular theory that overhunting by Clovis people led to these extinctions.
Lonsdaleite Diamonds
The researchers have found hexagonal nanodiamonds within a dark layer of sediment on Santa Rosa Island off the coast of Southern California. These are not ordinary diamonds. Instead, they are lonsdaleite, a shock-synthesized mineral with a hexagonal crystalline structure. It forms under extreme temperatures and pressures, similar to those produced by a cosmic impact.
Lonsdaleite diamonds, are typically found in meteorites, impact craters and impact ejecta. Their presence in the dark sediment layer suggests that materials in the layer might be ejecta produced by a large impact event.
One of the researchers involved with the study has found similar diamonds in sediment layers at six different North American locations. These sediment layers date to about 12,900 years ago, consistent with the bird and mammal extinctions and the disappearance of the Clovis culture (a culture associated with the first human presence in North America).
In addition to the diamonds, the dark layer of sediment is rich in soot, which forms during extremely hot fires. The scientists believe that heat produced by cosmic impact produced massive wildfires that contributed soot to the dark sediment layers.
A Series of Comet Strikes
The researchers believe that this distribution of nanodiamonds and soot at multiple locations might have been produced by a series of comet strikes across North America at the end of Clovis.
These comet strikes might have been similar to the Comet Shoemaker-Levy 9 impacts that collided with Jupiter in July of 1994. The impacts would have thrown large amounts of dust into the atmosphere and ignited wildfires which produced smoke and soot. These together might have been sufficient to modify climate and cause the extinctions.
A Material Assemblage With Few Explanations
One of the researchers, James P. Kennett, a paleoceanography and professor emeritus at the University of California, Santa Barbara said: "It is hard to explain this assemblage of materials without a cosmic impact event and associated extensive wildfires. This hypothesis fits with the abrupt cooling of the atmosphere as shown in the record of ocean drilling of the Santa Barbara Channel. The cooling resulted when dust from the high-pressure, high-temperature, multiple impacts was lofted into the atmosphere, causing a dramatic drop in solar radiation."
A team of seventeen researchers will publish a more detailed account of their study in the Proceedings of the National Academy of Sciences. The researchers are: Douglas J. Kennett, Jon M. Erlandson and Brendan J. Culleton, all of the University of Oregon; James P. Kennett of UC Santa Barbara; Allen West of GeoScience Consulting in Arizona; G. James West of the University of California, Davis; Ted E. Bunch and James H. Wittke, both of Northern Arizona University; Shane S. Que Hee of the University of California, Los Angeles; John R. Johnson of the Santa Barbara Museum of Natural History; Chris Mercer of UC Santa Barbara and National Institute of Materials Science in Japan; Feng Shen of the FEI Co.; Thomas W. Stafford of Stafford Research Inc. of Colorado; Adrienne Stich and Wendy S. Wolbach, both of DePaul University in Chicago; and James C. Weaver of the University of California, Riverside.
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| Dark sediment layer exposed in Arlington Canyon on Santa Rosa Island. Image by James Kennett.Enlarge
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 | Hexagonal nanodiamond discovered on Santa Rosa Island. Image by James C. Weaver. Enlarge Image
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