One big question was, did an extraterrestrial impact occur near the end of the last ice age, about 13,000 years ago as the ice sheets covering Canada were melting, and cause an abrupt cooling that thrust the northern hemisphere back into the ice age for an extra 1,200 years?
That event is known as the Younger Dryas (YD) event that occurred in the Northern Hemisphere with abrupt cooling over a time interval of decades, with temperatures possibly reaching 15°C colder than the present.
Some scientists believed the event – which cooled the Earth by about 3 degrees Centigrade, a huge amount – was caused by an extraterrestrial impact with the Earth, such as a meteor collision.
A new study on the ancient sediment appears to solve the mystery of why the Earth cooled suddenly about 13,000 years ago.
The Hall’s Cave has a sediment record extending over 20,000 years, and he first began researching the cave in 2017. Scientists found the evidence left in layers of sediment in Hall’s Cave was almost certainly the result of volcanic eruptions.
Alan Brandon, professor of geosciences at the University of Houston and head of the research team, said, “This work shows that the geochemical signature associated with the cooling event is not unique but occurred four times between 9,000 and 15,000 years ago. Thus, the trigger for this cooling event didn’t come from space. Prior geochemical evidence for a large meteor exploding in the atmosphere instead reflects a period of major volcanic eruptions.”
“I was skeptical. We took every avenue we could to come up with an alternative explanation, or even avoid, this conclusion. A volcanic eruption had been considered one possible explanation but was generally dismissed because there was no associated geochemical fingerprint.”
Scientists noted, “After a volcano erupts, the global spread of aerosols reflects incoming solar radiation away from Earth and may lead to global cooling post-eruption for one to five years, depending on the size and timescales of the eruption.”
Steven Forman, professor of geosciences at Baylor said, “The Earth’s climate may have been at a tipping point at the end of Younger Dryas, possibly from the ice sheet discharge into the North Atlantic Ocean, enhanced snow cover and powerful volcanic eruptions that may have in combination led to intense Northern Hemisphere cooling.”
Michael Waters, director of The Center for The Study of the First Americans and Distinguished Professor at Texas A&M University, said, “This period of rapid cooling coincides with the extinction of several species, including camels and horses, and the appearance of the Clovis archaeological tradition.”
By completing the isotopic analysis of sediments collected from Hall’s Cave, scientists found that elements such as iridium, ruthenium, platinum, palladium, and rhenium were not present in the correct proportions. It means a meteor or asteroid could not have caused the event.
The chemical anomalies found in sediments dating to the beginning of the Younger Dryas are the result of volcanism and not an extraterrestrial impact.
University of Houston scientist Nan Sun said, “The isotope analysis and the relative proportion of the elements matched those that were found in previous volcanic gases.”
Forman said, “The Younger Dryas cooling lasted about 1,200 years, so a sole volcanic eruptive cause is an important initiating factor, but other Earth system changes, such as cooling of the oceans and more snow cover were needed to sustain this colder period.”
- N. Sun et al. Volcanic origin for Younger Dryas geochemical anomalies ca. 12,900 cal B.P. Science Advances, 2020 DOI: 10.1126/sciadv.aax8587