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

Researchers continue to expand the arguments for the Younger Dryas Impact Hypothesis. The idea posits that a fragmented comet impacted Earth’s atmosphere 12,800 years ago, causing a widespread climate shift that led, among other things, to the abrupt reversal of Earth’s warming trend and to an anomalous near-glacial period called the Younger Dryas is called.

Now, UC Santa Barbara Professor Emeritus James Kennett and colleagues report the presence of proxies associated with the cosmic eruption spread across several distinct locations in the eastern United States (New Jersey, Maryland and South Carolina), materials indicative for the force and temperature involved in such an event, including platinum, microspheres, melting glass and shock-fractured quartz. The research appears in the journal Air bursts and craters.

“What we found is that the pressures and temperatures were not characteristic of large crater-forming impacts, but were consistent with so-called ‘touchdown’ air eruptions that don’t form much in the form of 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 the extremely rare and cataclysmic consequences, 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 devastating. The shock wave of the 1908 Tunguska event knocked down 2,150 square kilometers (830 sq mi) of forest, when the asteroid with a diameter of about 40 meters (130 ft) collided with the atmosphere nearly 10 kilometers (6 mi) above the Siberian taiga.

The comet thought to be responsible for the Younger Dryas cooling is estimated to be 100 kilometers wide (62 miles) – much larger than the Tunguska object and fragmented into thousands of pieces. The sediment layer associated with the eruption 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 pressure and temperatures, such as magnetic microspheres (cooled metal droplets), melting 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, which exhibit stress large enough to deform the crystal structure of quartz, a very hard material. This “cream of the crop” of cosmic impact evidence is present in impact craters, but linking shocked quartz to cosmic eruptions has proven more challenging.

“In the extreme form, such as when an asteroid slams into the Earth’s surface, all the fractures are very parallel,” Kennett explains. In the field of cosmic eruptions, there are several variables present in the field of cosmic eruptions. “If you think about it, the pressures and temperatures that cause these fractures will vary depending on the density, the angle of impact, the height of the impact and the size of the impactor.

“What we discovered – and this is characteristic of the impact layer called the Younger Dryas Boundary – is that although we occasionally see in the quartz grains examples of the ‘traditional’ shocked quartz with parallel fractures, we mainly 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, in contrast to the parallel and planar deformations of shock-associated quartz found at craters. These subparallel and subplanar deformations are due in large part to the relatively lower pressure caused by explosions that occur above ground, the researchers argue, 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 come from a low-altitude bolide air explosion. Similarly, crushed quartz grains and molten glass have been found in more contemporary samples from above-ground explosions, such as at the Trinity atomic bomb test site in New Mexico. The approximately 20-kiloton bomb was detonated on 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 if they don’t reflect a traditional large crater-forming event,” Kennett said. “These come from very low-altitude ‘touchdown’ explosions that are almost certainly related to a comet impact.”