New research reveals evidence of cometary outbursts from 12,800 years ago

Researchers continue to build the case for the Younger Dryas Impact hypothesis. The idea holds that a fragmented comet slammed into Earth’s atmosphere 12,800 years ago, triggering widespread climate change that led to, among other things, an abrupt reversal of Earth’s warming trend and an anomalous near-glacial period called the Younger Dryas.

Now, UC Santa Barbara professor emeritus James Kennett and colleagues report the presence of proxies associated with the cosmic outburst, spread across several separate locations 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, melt glass and shock-fractured quartz. The research appears in the journal Air explosions and craters.

“What we found is that the pressures and temperatures are not characteristic of large crater-forming impacts, but are consistent with so-called ‘touchdown’ air explosions that produce very few craters,” Kennett said.

The Earth is bombarded by tons of celestial debris, in the form of tiny dust particles, every day. At the other end of the scale are the extremely rare and cataclysmic impacts, such as the Chicxulub event that caused the extinction of dinosaurs and other species 65 million years ago. The 150-kilometer-wide impact crater is located on the Yucatán Peninsula in Mexico.

Somewhere in between are the impacts that leave no craters on the Earth’s surface but are still destructive. The 1908 Tunguska event devastated 2,150 square kilometers (830 square miles) of forest when the 40-meter (130-foot) diameter asteroid slammed into the atmosphere nearly 10 kilometers (6 miles) above the Siberian taiga.

The comet thought to be responsible for the Younger Dryas cooling was estimated to be 100 kilometers (62 miles) wide, much larger than the Tunguska object, and fragmented into thousands of pieces. The sediment layer associated with the airburst extends across much of the Northern Hemisphere, but can also be found in locations 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.

Shocked quartz and amorphous silica

The researchers are particularly interested in the presence of shocked quartz, indicated by a pattern of lines called lamellae that exhibit strains large 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, but linking shocked quartz to cosmic airbursts has proven more challenging.

“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 air explosions, there are several variables present in the realm of cosmic air explosions. “If you think about it, the pressures and temperatures that cause these fractures are going to vary depending on the density, the angle of impact, the height of the impact, and the size of the impactor.

“What we found – and this is characteristic of the impact layer, called the Younger Dryas Boundary – is that while we do occasionally see examples of the ‘traditional’ shocked quartz with parallel fractures in the quartz grains, we mostly see grains that are not parallel,” he said. These fractures appear in an irregular, web-like pattern of intersecting, meandering lines and fissures at the surface and subsurface, unlike the parallel and planar deformations of shock-associated quartz found near craters. These subparallel and subplanar deformations are largely due to the relatively lower pressures produced by explosions that occur above ground, the researchers say, as opposed to impacts that make contact with Earth.

What these sediments have in common with the shocked quartz at crater sites is the presence of amorphous silica (molten glass) in these fractures. And that, the researchers say, is evidence of the combination of pressure and high temperatures (greater than 2,000 degrees Celsius) that could result from a low-altitude bolide eruption. Similar fractured quartz grains and molten glass have been found in more modern samples from above-ground explosions, such as at the Trinity nuclear bomb test site in New Mexico. The roughly 20-kiloton bomb was detonated atop a 30.5-meter (100-foot) tower.

These lower-pressure shocked quartz grains join a growing number of impact proxies that together make a case for a fragmented comet that not only caused widespread fires, but also caused abrupt climate change that resulted in the extinction of 35 species of megafauna in North America, such as such as mammoths and giant ground sloths, and led to the collapse of a thriving human culture called Clovis, according to the researchers.

“There’s a whole range of different types of shocked quartz, so we need to have a well-documented demonstration that they are indeed important for interpreting the cosmic impact, even though they don’t reflect a traditional large crater-forming event,” Kennett said. “These are from very low-altitude ‘touchdown’ explosions that are almost certainly related to a comet impact.”