Lake Cuitzeo is located in central Mexico, and it’s the second-largest freshwater lake in the country. At the bottom of the lake is a sediment layer, roughly 12,900 years old, containing rare microscopic materials that can potentially greatly further our understanding of Earth’s history. “These materials form only through cosmic impact,” said James Kennett, professor of Earth Science at UCSB. These microscopic findings have names like nanodiamonds, aciniform soot, and impact spherules, and Kennett and other researchers say they help explain a strange geologic time in the planet’s past.
Nanodiamonds are miniature crystals formed by explosions, but the particular kinds of nanodiamond at the bottom of Lake Cuitzeo are only formed by something from outer space hitting Earth. A team of researchers jointly led by Kennett discovered them under Lake Cuitzeo and recently published their findings in the Proceedings of the National Academy of Sciences.
These nanodiamonds, they claim, support a controversial hypothesis that a comet or asteroid hit Earth 12,900 years ago. This would be right at the onset of a period called the Younger Dryas, known colloquially as the Big Freeze, and it’s when species like mastodons, woolly mammoths, saber-tooth cats, and dire wolves went extinct. “The youngest fossils of these animals are about 12,900 years old. There used to be a large diversity of animals in North America, but once the Younger Dryas came about, that changed,” said Kennett. According to researchers, these sediment findings in Lake Cuitzeo provide further evidence that a cosmic impact altered the Earth’s climate and caused these extinctions.
Outside of Mexico, this sediment layer has already been identified throughout North America, Greenland, and Western Europe, and this finding shows that the layer goes all the way down to the tropics. In the geologic record, there are only two known continent-wide layers with an abundance of the same things Kennett and his team found under Lake Cuitzeo. One is the 65-million-year-old layer that coincided with the extinction of the dinosaurs. The other is 12,900 years old, just when the Younger Dryas began. “The idea that an extraterrestrial body caused the extinction of the dinosaurs is a hypothesis, too,” Kennet said. “You never prove anything in science. You just find evidence that supports a hypothesis.”
“The timing of the impact event coincided with the extraordinary biotic and environmental changes over Mexico and Central America,” said Kennett. “These changes were large, abrupt, and unprecedented.” Because the Younger Dryas came about so abruptly, and caused so many extinctions, it has been of particular interest to geologists. “The driving forces of climate say that the planet should have been warming,” Kennett explained. “But over a number of decades the planet cooled tremendously. In all of Earth’s other periods like this, the planet was primed for cooling.”
Kennett and his team say that the data suggest that a comet or asteroid greater than several hundred meters in diameter entered the atmosphere at a relatively shallow angle, burning plant life, melting surface rocks, and generally causing 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 explained.