Researchers say a nearly 13,000-year-old layer of thin, dark sediment buried in the floor of a lake in central Mexico suggests a cosmic body crashed into Earth.

Writing in the Proceedings of the National Academy of Sciences, researchers say exotic materials in the sediment layer strongly support a controversial hypothesis that a major cosmic impact with Earth 12,900 years ago coincided with the onset of an unusual cold climatic period called the Younger Dryas.

Analyzing the sediment, researchers said they identified an impact form of nanodiamonds called lonsdaleite that could not be formed through volcanic or other natural terrestrial processes.

"These materials form only through cosmic impact," said James Kennett, professor of earth science at the University of California, Santa Barbara.

The data suggest a comet or asteroid at least several hundred yards in diameter entered the atmosphere at a relatively shallow angle, with the resulting impact with Earth causing burned biomass, melted surface rocks, and major environmental disruption, researchers said.

"These results are consistent with earlier reported discoveries throughout North America of abrupt ecosystem change, megafaunal extinction, and human cultural change and population reduction," Kennett said.

"These changes were large, abrupt, and unprecedented, and had been recorded and identified by earlier investigators as a 'time of crisis.'"