More than 12,800 years ago
Quartz samples support the idea that a fragmented comet crashed into Earth’s atmosphere 12,800 years ago, triggering widespread climate change.
Among other things, that event led to an abrupt reversal of the Earth’s warming trend and an anomalous near-glacial period called the Younger Dryas.
UC Santa Barbara Professor Emeritus James Kennett and colleagues report the presence of tracers associated with the cosmic airburst distributed across several separate sites in the eastern United States (New Jersey, Maryland, and South Carolina), materials indicative of the force and temperature involved in such an event, including platinum, microspheres, molten glass, and shock-fractured quartz. The study appears in the ScienceOpen journal Airbursts and Cratering.
“What we found is that the pressures and temperatures were not characteristic of large, crater-forming impacts, but were consistent with so-called ‘landing’ airbursts that don’t form many craters,” Kennett said.
The Earth is bombarded every day by tons of celestial debris, in the form of tiny dust particles. At the other end of the scale are extremely rare and cataclysmic impacts like the Chicxulub event that 65 million years ago caused the extinction of dinosaurs and other species. Its 150 kilometer wide impact crater can be found on the Yucatan Peninsula in Mexico.
Somewhere in between are impacts that leave no craters on Earth’s surface but are destructive. The blast wave from the 1908 Tunguska event leveled 2,150 square kilometers of forest when the roughly 40-meter-diameter asteroid struck the atmosphere nearly 10 kilometers above the Siberian taiga.
The comet believed to be responsible for the Younger Dryas cooling episode is estimated to have been 100 kilometers wide, much larger than the Tunguska object, and fragmented into thousands of pieces. The layer of sediment associated with the airburst extends across much of the Northern Hemisphere, but can also be found in places south of the equator. This layer contains unusually high levels of rare materials associated with cosmic impacts, such as iridium and platinum, and materials formed under high pressures and temperatures, such as magnetic microspheres (cooled metal droplets), molten glass, and nanodiamonds.
Researchers are particularly interested in the presence of shocked quartz, indicated by a pattern of lines, called laminae, that show a strain great enough to deform the crystal structure of quartz, a very hard material. This “crème de la crème” of cosmic impact evidence is present in impact craters, yet linking shocked quartz to cosmic airbursts proved a greater challenge.
“In the extreme form, like when an asteroid hits the Earth’s surface, all the fractures are very parallel,” Kennett explained. In the realm of cosmic airbursts, there are different variables present in the realm of cosmic airbursts. “If you think about it, the pressures and temperatures that produce these fractures will vary depending on the density, the angle of entry, the altitude of impact and the size of the impactor.”
These fractures are seen in an irregular, web-like pattern of intersecting and meandering lines and surface and subsurface fissures, in contrast to the parallel, planar deformations of impacted, impact-associated quartz found in craters. (Europa Press)
