The latest study to discredit the controversial theory that a cosmic impact 12,900 years ago triggered the Younger Dryas cold period comes from the University of California, Davis.
     The Younger Dryas lasted a thousand years between two major glaciations, and coincided with the extinction of mammoths and the disappearance of the Clovis culture. In the 1980s, some researchers put forward the idea that the period began when a comet or meteor struck North America.
     In the new study, scientists analysed soil samples from four sites in northern Syria dating back 10,000 to 13,000 years, comparing them to similar droplets previously suggested to be the result of a cosmic impact at the onset of the Younger Dryas.
     "For the Syria side, the impact theory is out," says lead author Peter Thy, a project scientist in the UC Davis Department of Earth and Planetary Sciences.
     Several findings support that conclusion. The composition of the droplets was related to the local soil, not to soil from other continents as one would expect from an intercontinental impact. The texture, thermodynamic modelling, and other analyses showed the droplets were formed by short-lived heating events of modest temperatures, and not by the intense, high temperatures expected from a large impact event. Finally, the samples were collected from archaeological sites spanning 3,000 years. "If there was one cosmic impact," Thy said, "they should be connected by one date and not a period of 3,000 years."
     Where did the droplets come from? House fires. The study area in modern Syria was associated with early agricultural settlements along the Euphrates River. Most of the locations include mud-brick structures, some of which show signs of intense fire and melting. The study concludes that the droplets formed when fires destroyed buildings made of a mixture of soil and straw.

Edited from PhysOrg (6 January 2015)