Surprising discovery: a cosmic explosion in the atmosphere may have caused global cooling 12,800 years ago

New evidence supports the theory that an explosion in the atmosphere of a comet caused the Younger Dryas climate change and mass extinction 12,800 years ago. The materials found indicate low-pressure impacts rather than impacts that created craters.

New evidence bolsters the hypothesis that a disintegrating comet caused dramatic climate change 12,800 years ago, with findings from across the US indicating powerful explosions in the atmosphere ("touchdowns") without cratering.

Researchers hold the Younger Dryas impact hypothesis, which suggests that a fragmented comet collided with Earth's atmosphere about 12,800 years ago. This event is considered to be the cause of a significant climate change, which abruptly stopped the Earth's warming trend and almost put it into an ice age called the Younger Dryas.

Now, Professor Emeritus James Kent of the University of California Santa Barbara and his colleagues report the presence of cosmic explosion-related markers in the atmosphere found at several separate sites in the eastern US (New Jersey, Maryland and South Carolina). The materials testify to the forces and temperatures involved in such an event, and include platinum, microspheres, molten glass, and shock-broken quartz. The study was published in the ScienceOpen journal "Airbursts and Cratering".

"What we found was that the pressures and temperatures did not have the characteristics of impacts that create large craters, but were consistent with atmospheric 'touchdown' explosions, which do not create many craters," Kent said.

Cracked quartz

Cracked quartz grains with cracks filled with molten glass. Credit: University of California Santa Barbara. 

The Earth is bombarded every day with a huge amount of cosmic debris, in the form of small dust particles. At the other end of the scale are rare and catastrophic collisions such as the Chicxulub event that happened 65 million years ago and caused the extinction of the dinosaurs and other species. Its impact crater, 150 kilometers (93 miles) wide, is on Mexico's Yucatan Peninsula.

In the middle of the scale are the impacts that do not leave craters on the Earth's surface, but are still devastating. The shock wave from the 1908 Tunguska event felled an area of ​​2,150 square kilometers (830 sq mi) of forest, when an asteroid about 40 meters (130 ft) in diameter exploded in the atmosphere about 10 kilometers above the Siberian taiga.

The comet thought to be responsible for the Yanger Dryas cooling event is estimated to have been 100 kilometers across—much larger than the Tunguska bone—and split into thousands of pieces. The sediment layer associated with the cosmic explosion is spread over most of the Northern Hemisphere, but is also found in more southern locations. This layer contains extremely high concentrations of rare materials associated with cosmic collisions, such as iridium and platinum, and materials formed under high pressures and temperatures, such as magnetic microspheres (metal droplets that have cooled), molten glass and nanodiamonds.

Cracked quartz and amorphous silica

The researchers are particularly interested in the presence of fractured quartz, which is characterized by a pattern of lines, called lamellae, showing pressure strong enough to distort the crystal structure of quartz, which is a very hard material. One of the strongest evidences for cosmic impacts is found in impact craters, but being able to link the broken or cracked quartz to cosmic explosions in the air is a bigger challenge.

"In its extreme form, when an asteroid hits the face of the Earth, all the fragments are very parallel," Kent explained. In the category of cosmic explosions in the air, there are different variables that influence. "When you think about it, the pressures and temperatures that create the fractures vary depending on the density, the angle of entry, the height of the impact and the size of the impactor."

"What we found - and this is what characterizes the impact layer, called the Yanger Dryas boundary - is that although we occasionally see in the quartz samples 'traditional' fractured quartz crystals with parallel fractures, in most cases we see grains with non-parallel fractures," he said. These fractures appear as an irregular pattern, zigzag lines with internal explosions, compared to the parallel and flat deformations of fractured quartz found in craters. The researchers claim that these deformations are mainly due to lower pressures caused by explosions above the surface of the ground, and do not directly affect the earth.

What these deposits and the quartz fragments in the craters have in common is the presence of amorphous silica - molten glass - within the fragments. The researchers note that this is evidence of a combination of high pressures and temperatures (over 2,000 degrees Celsius) that could have been caused by an explosion in the air at low altitude. Similarly, fragments of quartz and molten glass have also been found in modern samples of above-ground explosions, such as the atomic bomb test site in New Mexico, the Trinity bomb which was detonated atop a 30.5 meter (100 ft) tower.

The low-pressure fractured quartz grains join an emerging series of vulnerability markers that bolster the theory that a fragmented comet caused not only widespread wildfires, but also a sudden climate change that led to the extinction of 35 species of large North American animals, such as mammoths and giant sloths, and the collapse of the thriving Clovis culture , so the researchers speculate.

"There are a variety of different types of fractured quartz, so we need to make a strong case that they are indeed significant for understanding cosmic impacts, even if they don't reflect impact events that created large craters," Kent said. "These are explosions in the air at low altitude, which are almost certainly related to the impact of a comet."

for the scientific article