Research bolsters the Younger Dryas Impact hypothesis, proposing that a comet fragment triggered dramatic climatic shifts and species extinctions 12,800 years ago through high-pressure, high-temperature airbursts. This theory is supported by the discovery of materials such as platinum and shocked quartz scattered across the eastern U.S.

Researchers are uncovering additional evidence for the Younger Dryas Impact hypothesis, which posits that a fragmented comet entered Earth's atmosphere 12,800 years ago. This event is believed to have caused a sudden climatic shift, halting the planet's warming trend and plunging it into the near-glacial conditions of the Younger Dryas period.

Figure 1. New Evidence Strengthens Theory of Fragmented Comet Causing Younger Dryas Climatic Shifts and Extinctions

Signs of Cosmic Airbursts in the Eastern United States

James Kennett, an emeritus professor at the University of California, Santa Barbara, and his team have identified impact proxies—materials linked to a cosmic airburst—at several locations across the eastern United States, including New Jersey, Maryland, and South Carolina. These materials, including platinum, microspherules, meltglass, and shock-fractured quartz, suggest the extreme pressures and temperatures generated by the event. Their research was recently published in ScienceOpen's journal Airbursts and Cratering. Figure 1 shows New Evidence Strengthens Theory of Fragmented Comet Causing Younger Dryas Climatic Shifts and Extinctions.

“What we’ve found is that the pressures and temperatures were not typical of major crater-forming impacts but align with those of 'touchdown' airbursts, which don't produce significant craters,” Kennett explained.

A Comparison of Celestial Impacts: From Tunguska to the Younger Dryas

The Earth is bombarded daily by tons of celestial debris in the form of tiny dust particles. At the other extreme are the rare, catastrophic impacts, such as the Chicxulub event 65 million years ago, which led to the extinction of the dinosaurs and many other species. The impact crater, measuring 150 kilometers (93 miles) in diameter, is located on the Yucatán Peninsula in Mexico.

In between these extremes are impacts that don’t leave visible craters on Earth’s surface but are still destructive. For example, the 1908 Tunguska event, caused by an asteroid about 40 meters (130 feet) in diameter, exploded in the atmosphere nearly 10 kilometers (6 miles) above the Siberian taiga, flattening 2,150 square kilometers (830 square miles) of forest.

The comet thought to have triggered the Younger Dryas cooling event was much larger, estimated to be around 100 kilometers (62 miles) wide, and fragmented into thousands of pieces. The sediment layer from the resulting airburst spans much of the northern hemisphere and even stretches to regions south of the equator. This layer contains unusually high concentrations of rare materials linked to cosmic impacts, including iridium and platinum, as well as substances formed under extreme pressures and temperatures, such as magnetic microspherules (cooled metallic droplets), meltglass, and nanodiamonds.

Shocked Quartz and Amorphous Silica

The researchers are particularly focused on the presence of shocked quartz, which is characterized by a pattern of lines, called lamellae, that indicate stress sufficient to alter the crystal structure of quartz, a highly durable material. While this "crème de la crème" of cosmic impact evidence is typically found in impact craters, linking shocked quartz to cosmic airbursts has proven more difficult.

“In extreme cases, such as when an asteroid strikes the Earth's surface, the fractures are very parallel,” Kennett explained. “However, with cosmic airbursts, various factors come into play.” The pressures and temperatures responsible for these fractures can differ based on factors like the density, entry angle, altitude of the impact, and the size of the impactor.

“What we found — and this is a key feature of the impact layer known as the Younger Dryas Boundary — is that while we do occasionally observe the ‘traditional’ shocked quartz with parallel fractures, the majority of the grains show irregular, web-like patterns of intersecting, meandering lines and fissures both on the surface and beneath,” he said. Unlike the parallel and planar deformations seen in impact-associated shocked quartz at craters, these subparallel and subplanar fractures are likely a result of the relatively lower pressures generated by explosions occurring above the ground, rather than direct impacts with the Earth’s surface, the researchers suggest.

“There is a wide range of shocked quartz types, so we must build a well-documented case to show that they are indeed relevant for interpreting cosmic impacts, even though they don’t reflect a traditional major crater-forming event. These fractures are likely the result of very-low-altitude ‘touchdown’ airbursts, almost certainly linked to a cometary impact.”

What these sediments share with the shocked quartz found at crater sites is the presence of amorphous silica — melted glass — within the fractures. The researchers argue that this is evidence of the extreme pressure and high temperatures (greater than 2000 degrees Celsius) that could have been generated by a low-altitude bolide airburst. Similarly fractured quartz grains and meltglass have been observed in more recent samples from above-ground explosions, such as at the Trinity atomic bomb test site in New Mexico, where a 20-kiloton bomb was detonated atop a 30.5-meter (100-foot) tower.

These lower-pressure shocked quartz grains add to a growing body of impact proxies, collectively supporting the theory that a fragmented comet not only caused widespread fires but also triggered abrupt climatic changes. These changes are believed to have contributed to the extinction of 35 genera of megafauna in North America, including mammoths and giant ground sloths, and the collapse of the Clovis human culture, according to the researchers.

Geological Significance of Impact Proxies

“There is a wide variety of shocked quartz, so we must build a well-documented case to demonstrate that they are indeed relevant for interpreting cosmic impacts, even though they don’t reflect a typical major crater-forming event,” Kennett explained. “These quartz grains are most likely from very-low-altitude ‘touchdown’ airbursts, almost certainly linked to a cometary impact.”

Source: SciTechDaily

Cite this article:

Janani R (2024), The Comet Explosion that Disrupted Climates and Triggered Extinctions 12,800 Years Ago, AnaTechmaz, pp. 130