UC SANTA BARBARA (US) — A nearly 13,000-year-old layer of sediment buried in the floor of Lake Cuitzeo in central Mexico contains an exotic assemblage of materials, including nanodiamonds, that are the result of a cosmic body hitting Earth.
Conducting a wide range of exhaustive tests, researchers conclusively identified a family of nanodiamonds, including the impact form of nanodiamonds called lonsdaleite.
The researchers also found spherules that had collided at high velocities with other spherules during the chaos of impact. Such features could not have formed through anthropogenic, volcanic, or other natural terrestrial processes, says James Kennett, professor of earth sciences at the University of California, Santa Barbara. “These materials form only through cosmic impact.”
Images of single and twinned nanodiamonds show the atomic lattice framework of the nanodiamonds. Each dot represents a single atom.
These new data, published in Proceedings of the National Academy of Sciences, are the latest to strongly support of a controversial hypothesis proposing that a major cosmic impact with Earth occurred 12,900 years ago at the onset of an unusual cold climatic period called the Younger Dryas.
The data suggest that a comet or asteroid—likely a large, previously fragmented body, greater than several hundred meters in diameter—entered the atmosphere at a relatively shallow angle. The heat at impact burned biomass, melted surface rocks, and caused major environmental disruption.
“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 explains.
The sediment layer identified by the researchers is of the same age as that previously reported at numerous locations throughout North America, Greenland, and Western Europe. The current discovery extends the known range of the nanodiamond-rich layer into Mexico and the tropics. In addition, it is the first reported for true lake deposits.
In the entire geologic record, there are only two known continent-wide layers with abundance peaks in nanodiamonds, impact spherules, and aciniform soot. These are in the 65-million-year-old Cretaceous-Paleogene boundary layer that coincided with major extinctions, including the dinosaurs and ammonites; and the Younger Dryas boundary event at 12,900 years ago, closely associated with the extinctions of many large North American animals, including mammoths, mastodons, saber-tooth cats, and dire wolves.
“The timing of the impact event coincided with the most extraordinary biotic and environmental changes over Mexico and Central America during the last approximately 20,000 years, as recorded by others in several regional lake deposits,” Kennett says. “These changes were large, abrupt, and unprecedented, and had been recorded and identified by earlier investigators as a ‘time of crisis’.”
Contributing scientists are from the University of Oregon, Northern Arizona University, Harvard University, DePaul University, the U.S. Geological Survey, Universidad Michoacana de San Nicólas de Hidalgo, National Taiwan University, SRI International; Lawrence Berkeley National Laboratory, GeoScience Consulting, and the National Institute for Materials Science.
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