New measurements made on Greenland ice cores suggest North America may have experienced a large cosmic impact about 12,900 years ago - a known abrupt climate transition previously linked to the demise of the 'Clovis' people.
     Michail Petaev and colleagues from Harvard University report a 100-fold spike in platinum concentration occurs in Greenland ice around 12,890 years old, at the same moment that rapid cooling of the climate is indicated from oxygen isotope measurements. This coincides with the start of a climatic period called the 'Younger Dryas' - a colder phase associated with the extinction of at least 17 groups of large mammals such as the mammoth, and with widespread wildfires, as well as rapid changes in atmospheric and ocean circulation.
     While as yet no impact site has been identified, the observations lend credence to earlier finds of microscopic grains of diamond and a mineral called lonsdaleite in lake sediments dated to the same time. Those measurements resemble the most recent observations of remnants of the Tunguska meteorite impact in Siberia. Sphere-shaped particles have also been identified in other sites' sediments dated to this event.

Edited from BBC News (1 August 2013)