July 31 (UPI) -- Geochemical signatures found in sediments recovered from a Texas cave suggest the Younger Dryas, a period of global cooling that occurred 13,000 years ago, was caused by a series of Earth-based processes, not an extraterrestrial impact.
Previously, scientists in search of an explanation for the Younger Dryas have pointed to spikes in several rare earth metals as evidence that an asteroid or comet impact triggered the cooling.
Most recently, when researchers looked at the sedimentary evidence, they found these spikes in rare earth metals actually featured relatively low concentrations of iridium, ruthenium, platinum, palladium and rhenium -- levels inconsistent with an extraterrestrial impact event.
"The isotopic signatures and concentrations can be explained by processes of volcanic eruption, condensation and transport of aerosols, which enriched the concentration and shifted the isotopic composition," Steven Forman, professor of geosciences at Baylor University, told UPI.
In addition to finding relatively low levels of the rare earth metals, researchers also found the same geochemical signature occurred several times in sediments dated between 9,000 and 15,000 years ago.
"The supposed 'comet' signature did not just occur during the Young Dryas but at four other age-levels, which is physically impossible," Forman said. "This is one piece of evidence that it was not a comet, but an aerosol signature in the atmosphere from a distant volcanic eruption."
While the new research, published Friday in the journal Scientific Advances, undermines an extraterrestrial explanation for the Younger Dryas, the study doesn't offer its own definitive trigger for the 1,200-year period of global cooling.
According to Forman, co-author of the new paper, volcanic activity accounts for the geochemical signatures, but can't entirely explain the cooling.
"The cooling associated with a volcanic eruption is greatest the year of the eruption and then is less and less for the following two to three years," he said. "Younger Dryas is about 1,200 years long, so a volcanic eruption cannot be the sole cause."
Instead researchers suggest some combination of volcanic activity and related phenomena, including melting Northern Hemisphere ice sheets and subsequent ocean cooling, likely fueled the Younger Dryas.
Forman suggests the fact that the Younger Dryas wasn't caused by an extraterrestrial impact is relevant to our understanding of climate change today.
The Younger Dryas offers proof that climate mechanisms internal to Earth are capable triggering sudden and dramatic shifts in climate, he said.
"Our planet's response to internal climate perturbations -- like increase in CO2 -- is not linear, but can be rapid and to an extreme state, like the Younger Dryas," Forman said.