Comet impact caused large-scale changes on Earth 13,000 years ago: ScienceAlert

Imagine being a hunter-gatherer about 13 thousand years ago. You’re doing your job – gathering berries and hunting wild animals – when suddenly a huge ball of fire appears in the sky.

It explodes with a deafening roar, sending tremors across the ground and causing trees and houses to collapse.

New research suggests this is how the first seeds of agriculture were sown in Syria, a necessary adaptation to improve the community’s chances of survival.

After fragments of a comet hit the Earth’s atmosphere, the climate changed dramatically, and the plants and animals they relied on disappeared.

A global team presented its case after analyzing sediment layers from the Neolithic site Abu Hureyra, which was excavated before it sank below Lake Assad due to the construction of the Euphrates Dam in the 1970s. Abu Hureyra is known as the site with the oldest evidence of the shift from hunter-gatherer to farming.

The group contributed four peer-reviewed papers to the study of the Younger Dryas impact hypothesis. This much-contested hypothesis proposes that a cosmic impact caused the Younger Dryas (YD) period—a sudden, severe, and prolonged interruption in the warming of Earth’s climate.

The authors write, “We present substantial new quantitative evidence and explanations supporting the hypothesis that comet fragments triggered a near-global change in climate about 12,800 years ago, and that an aerial explosion destroyed the village of Abu Hureyra. Had done it.”

The sediment layers revealed several factors, including the types of plants collected on hot, humid days before the YD climate change and on cool, dry days after.

The team adds, “These data include changes in building architecture, diet, early stages of continuous cultivation of domesticated types of grains and legumes, and early penning of livestock, marking the beginning of continuous agriculture and animal husbandry.”

Rochester Institute of Technology archaeologist Andrew Moore and team’s extensive analysis also identified shock-fractured quartz grains, consistent with impact, and evidence of a large-scale fire.

“Shocked quartz is the best-known and probably the strongest proxy for a cosmic impact,” says James Kennett, an Earth scientist at the University of California Santa Barbara.

“In the papers, we describe what shapes these shock fractures in these low-pressure events.”

Their initial study confirmed low-shock fractures in quartz rocks in meteor craters formed by the impact of the Barringer meteorite.

Importantly, similar fractures occur in quartz exposed to nuclear explosions in air, even if no impact crater is formed. This has major implications, suggesting that an asteroid or comet striking close enough to the Earth’s surface could cause these fractures to send a shockwave around the world.

“For the first time,” says Kennett, “we propose that shock metamorphism in quartz grains exposed to nuclear explosions occurs essentially the same way as in low-altitude, low-pressure cosmic windblasts.”

The second study is the first to identify shock-fractured quartz in the Abu Hureyra sediment layer from the limit (beginning) of the YD period. Detailed analysis revealed that some of the quartz grains in this layer are similar to quartz found in nuclear explosions and meteor craters.

“We wanted to compare it to what exists in shock-fractured quartz in the YD limit,” says Kennett, “to see if there were any comparisons or similarities between what we see at the Trinity Nuclear Test Site and other nuclear bombings.” Yes or No. ,

Most of these quartz grains have been exposed to temperatures of at least 1713 °C (3115 °F), which is the melting point of quartz, and possibly even higher than its boiling point of 2200 °C.

Extremely high temperatures and pressures cause the quartz particles to shatter and melt and molten silica enters the fractures. When this non-crystalline silica is found in fractured rocks, it is a clear indication of impact, as opposed to slow tectonic movement.

The third study found tiny diamonds, special crystals, and small spheres made of silica and iron in YD boundary layer sediments. Some of these substances could only be formed under conditions of higher temperatures or pressures than any human technology could produce at the time.

Small pieces of bone (blue) scraped from melted glass (tan)
Molten glass on small pieces of bone. (Moore et al., Science Open: Airburst and cratering effects, 2023)

One substance, tiny spheres called meltglass, made up about 1.6 percent of the sediment and were found on tools, bones and mud walls, suggesting that the impact indeed disrupted life in the village. The melted glass pieces also have detailed impressions of plants.

Notably, the team did not find similar materials in sediments deposited by humans over thousands of years above the YD boundary layer.

The final study presents new evidence of a direct link between cosmic impacts, environmental changes, and major changes in human society.

“Our investigation reveals slow changes in site use by humans over the centuries leading up to and just after the onset of the YD,” the authors write, “with a significant, sudden change occurring immediately at the onset of the YD.”

The team proposes that the changes came from a disintegrated large comet about 100 kilometers wide. This event is likely to cause major climate change in the Northern Hemisphere.

Graph showing changes in sediment over time
Dietary changes gave preference to home-grown cultivated foods. (Moore et al., Science Open: Airburst and cratering effects, 2023)

“There was a shift from more humid conditions with diverse sources of food for foragers and hunter-gatherers to drier, cooler conditions when they could no longer survive solely as hunter-gatherers,” says Kennett.

“Villagers began growing barley, wheat and beans. The evidence clearly shows this.”

There is no doubt that something has completely changed the way people live in Abu Hureyra.

The authors concluded, “This change was an important early step in the transition from exclusive hunting-gathering to sustained agriculture and animal husbandry.”

The study has been published in Science Open: Airburst and cratering effects: Here, here, here, and here.

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