Shulou(Shulou.com)11/24 Report--

Photo: pixabay talks about the asteroid impact before the dinosaurs became extinct, and we might think it was a distant disaster. But just over 10,000 years ago, mankind may have experienced similar natural disasters.

About 6600 million years ago, a giant asteroid crashed into what is now Mexico's Yucatan Peninsula, causing widespread wildfires and tsunamis. If the impact of wildfires and tsunamis is still limited, the smoke and dust from impacts and wildfires (or volcanoes) quickly covered the earth's surface, obscured the sun, and ushered in a long winter for the whole planet.

This asteroid is the famous Chicxulub asteroid. Shortly after the impact, the lack of sunlight destroyed the entire ecosystem, and the earth ushered in the end of the Cretaceous mass extinction-or what we commonly call the "dinosaur extinction": about 76% of species disappeared, including all non-avian dinosaurs.

You may think that such an apocalyptic scene is far away from us. But humans may have experienced similar events just over 10,000 years ago.

It should be noted that there is still a lot of controversy about the cause of the mass extinction at the end of the Cretaceous. It is generally believed that the Sixulub asteroid impact and Deccan volcano eruption caused dramatic changes in the Earth's environment and climate, thus affecting the global ecosystem. However, the academic circles have not reached a consensus on what role these two events played in this ecological disaster. (photo Source: pixabay) the story of "the Flower of doom" begins at the end of the Pleistocene, when the earth bid farewell to the last ice age (known as the last glacial maximum, LGM) and began to heat up slowly. However, 12900 years ago, this trend was suddenly interrupted-temperatures plummeted in most parts of the world, and in just a few decades, the global average temperature may have dropped by 6-8 ℃, and the earth entered a cold winter of 1, 300 years. The sudden winter ended abruptly: Greenland ice core records show that local temperatures rose rapidly by 10 ℃ in 10 years about 11,700 years ago, marking the end of this long, cold winter.

The sudden cooling is known as the "New Andromeda event", which takes its name from a plant, Dryas octopetala. Andromeda is a cold-resistant and heat-resistant plant with small white flowers that now grow mainly in cold Arctic regions. However, between 12900 and 11700 years ago, as evidence of massive cooling, the small white flowers bloomed almost all over the northern hemisphere.

The small white flowers of the fairy wood (photo source: J ö rg Hempel / wikipedia) the vast prairie is flickering with white flowers, which sounds like a beautiful sight. However, for many species, including humans, Andromeda is more like a "flower of bad luck". During the same period, mammals in North America and Europe declined significantly, especially typical giant animals such as mastodons and earth laziness withdrew from North America. The Clovis civilization of North America also disappeared.

In North America, there is a clear set of dark strata that formed at the beginning of the new Andromeda incident about 12,900 years ago, known as the "black mat". In the strata below the black cushion (older than the black cushion), a large number of giant animal bones and unique tools belonging to the Clovis civilization have been unearthed, but above the black cushion (younger than the black cushion), there is almost no trace of giant mammals and Clovis.

On the far left is the unique stone tool of the Clovis people (photo source: D. G. Anderson et al., 2011) the asteroid "visit here", like almost all climate change events in geological history, there are different opinions about the cause of the new Andromeda event. In 2007, R. B. of Lawrence Berkeley National Laboratory. Fairstone (R. B. Firestone) and colleagues found a critical set of evidence. They found that there is a very thin deposit beneath the black cushion in North America, which is less than 5 centimeters thick, but is full of microspheres, nano-diamond particles, various strange forms of carbon, charcoal and smoke. High levels of iridium were also detected in the microspheres.

These discoveries seem to be a message to geologists-asteroids "come here": first of all, microspheres are "regulars" at meteorite impact sites; at the same time, iridium is rare on the earth's surface, so large amounts of iridium tend to point to extraterrestrial objects. Although microspheres with high iridium content may still be formed by volcanic eruptions, the researchers further analyzed the composition of microspheres, comparing known meteorites with those formed in volcanoes, and eventually ruled out the possibility that they came from volcanoes.

Along with these high iridium microspheres are many spherical carbon and glassy carbon, which, together with smoke particles and charcoal, point to another source-wildfire. Similar substances can be observed in modern forest fires. As a result, the researchers suspect that there was a meteorite impact 12, 900 years ago, which ignited a wildfire, and the high temperatures caused by the impact and combustion burned large amounts of organic matter into carbon balls, glassy carbon and even nano-sized diamond particles.

Carbonaceous spheres (top) and carbonaceous glass (bottom) found in the black cushion (image source: R. B. Firestone et al., 2007) fragmented asteroids if the 2007 study opened the prelude to the impact theory of the new Andromeda event, then a large study published in 2018 paints a clearer picture. A team of 24 scientists collected ice cores or sediment samples from more than 170 locations around the world in an attempt to restore the full picture of the event more than 10,000 years ago. Because of its excessive content, the study was divided into two papers and was published in the Journal of Geology.

This time, the researchers not only found more comprehensive evidence of impact and wildfire, but also calculated the area of the wildfire more than 10, 000 years ago based on the concentration of smoke particles in the sediment-the fire burned up about 7.3% ±2.3% of the biomass and burned nearly 10 million square kilometers of land, accounting for about 10% of the land in the northern hemisphere. By contrast, less than 1/3 of the biomass burned during the mass extinction event at the end of the Cretaceous.

According to calculations, the smoke and aerosols produced by burning on this scale are fully capable of shielding the sun, bringing a thousand-year-old winter to the earth. However, there is another question: how did an asteroid impact cause such a large-scale fire?

For comparison, the Tungus explosion in Siberia more than a hundred years ago was thought to have been caused by the explosion of an asteroid in the Earth's atmosphere, which was estimated to have an impact of nearly 200atomic bombs and affected an area of 2000 square kilometers, but ignited an area of less than 200 square kilometers. If you want to ignite 10% of the northern hemisphere, it takes much more energy, perhaps 1000 megatons of TNT equivalent.

Fallen trees near the Tungus Big Bang (photo: the Leonid Kulik Expedition) so did a giant asteroid hit Earth? It is also unlikely, because "it is difficult for a larger asteroid impact to form such a widely distributed deposit." Adrian Melott, of the University of Kansas, explained that he was one of the authors of the study, "so we speculate that an asteroid is more likely to break apart and the debris hits several continents."

According to calculations, the asteroid, which needs to be more than 100 kilometers in diameter, began to disintegrate after entering Earth's orbit, and the debris collided with at least four continents or exploded over the continents, releasing energy that triggered massive wildfires. When a large number of aerosols enter the stratosphere, the sun is greatly blocked, leading to more than a thousand years of "impact winter". At the same time, NH4 and NO3, as well as other by-products of biomass burning, react in the atmosphere, possibly causing acid rain. A series of climate change, wildfires and environmental degradation have led to the decline or transformation of giant animals and human civilization.

Story without ending if the indicators in these ice cores and sediments are more indirect evidence, then a study published in the 2020 Science report may provide more direct evidence.

At the archaeological site of Aby Hureyra in northern Syria, archaeologists found a large number of molten metal particles and glassy substances, including platinum with a melting point of 1768 ℃ and iridium with a melting point of 2466 ℃. In other words, the ambient atmospheric temperature may even have reached nearly 2500 ℃ at that time.

"at such a high temperature, a car can melt completely in less than a minute," said James Kennett, a geologist at the University of California, Santa Barbara, one of the authors of the study. They believe that this energy intensity can only be caused by instantaneous high-energy phenomena-probably the result of an asteroid impact. If so, this could be the first site to record the direct impact of asteroid debris on human settlements.

At the end of the article, it should be noted that the impact hypothesis of Andromeda is only one version of the story. Another more "mainstream" hypothesis is that the rapid melting of ice caps in the northern hemisphere has slowed the temperature and salt circulation of the oceans, thus changing the global climate. Similarly, there is a lot of evidence that points to this version of the story.

Perhaps just like the extinction of dinosaurs, we can only wait for more evidence to uncover the "truth" of the story. Of course, it is more likely that we can never be sure whether we have found the "right" answer.

Related papers:

Https://www.journals.uchicago.edu/doi/10.1086/695703#_i31

Https://www.journals.uchicago.edu/doi/full/10.1086/695704

Https://www.pnas.org/doi/full/10.1073/pnas.0706977104#F3

Https://www.sciencedirect.com/science/article/pii/S1040618211002333

Https://www.nature.com/articles/s41598-020-60867-w

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by: 27, revision: Clefable

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