The Reason For Younger Dryas Cold Snap Was More Likely To Be Massive Volcanic Eruptions, Not Comets
The study of one of the Texas caves made geologists doubt the popular theory that the so-called Dryas cooling, the last episode of the onset of glaciers, was due to the fall of a comet about 12.9 thousand years ago. The research is available in the scientific journal Science Advances.
"Our results showed that from 9 to 15 thousand years ago, four similar episodes occurred at once. Their geochemical traces are preserved in the sediments at the bottom of the Hall cave. Most likely, the reason for the Dryas cooling was not the fall of a comet or meteorite, but a series of volcanic eruptions," said Professor Alan Brandon of the University of Houston (USA).
The last cold snap and the subsequent onset of glaciers on the Earth occurred approximately 12.9 thousand years ago during the period that geologists call the late Dryas. Because of the dramatic climate change, the megafauna of North America, including mastodons and saber-toothed cats, have become completely extinct. That also gave rise to the first ancient Indian cultures.
Previously, scientists associated this cooling with a temporary stop of the "pipeline of currents" in the World's oceans. However, at the beginning of the XXI century, American geologists suggested that the cause of this cooling was the fall of a small comet or a massive asteroid.
This assumption was made by scientists after studying fragments of typically "meteoric" rocks that were found at the bottom of one of the ancient dried-up lakes in Mexico. Later, in other regions of the Earth, scientists found additional possible traces of this cataclysm, including ash from massive forest fires, which made this theory a popular explanation for how the Dryas cooling occurred.
Many other researchers still doubt this hypothesis, since its proponents have not yet found traces of a crater from the fall of this meteor. However, this crater might not have been there, since a comet or its debris could have fallen on the Earth, which might not have left visible traces on the surface of the planet.
Volcanoes or meteorites
Brandon and his colleagues in the "skeptical" camp found a new possible contradiction in this theory. They studied sedimentary rock deposits of the beginning of the Dryas from the bottom of the Hall cave, which is located in southern Texas. Scientists were interested in whether they have a thin layer with an unusual chemical composition, the so-called "boundary layer," which supporters of the "cosmic" theory of the origin of Dryas believe to be the trace of the fall of an asteroid or comet.
As a rule, such deposits are characterized by a unique ratio of isotopes of osmium, platinum, iridium, ruthenium, and other rare metals, which are more common in meteorites and comets than on the Earth's surface. In the deposits from Hall's cave, there were four or even five such layers at once. They were formed from 9 to 15 thousand years ago.
After studying their composition, the researchers concluded that these layers did not arise as a result of falling asteroids, but due to massive volcanic eruptions. In favor of this, in particular, the fact that the fractions of osmium-187 and osmium-188 these layers were much closer to volcanic ash and lava than to the matter of comets and asteroids-chondrites.
"I was very skeptical when we got these results. We have checked all possible alternative explanations to avoid this conclusion. In the past, many colleagues have been extremely negative attitude to such a hypothesis, as geochemical traces of massive volcanic eruptions in Dryas rocks, nobody has found," explained Brandon.
So far, scientists can't say whether these eruptions were just one of the factors, along with changes in the flow patterns and other climatic factors that triggered the glaciers, or their main driving force. The authors hope that they will find traces of these eruptions during the next research and will be able to convince the entire scientific community that the Dryas cooling was not caused by the fall of a comet or asteroid.