The Younger Dryas refers to a period of abrupt cooling about 13,000 years ago that coincided with the extinction of many large mammals, including the woolly mammoth, saber-toothed jaguar and many sloths.
The climate shift, evident in Greenland ice cores, was thought to be the result of the complex global climate system, possibly spurred by a slowdown of the thermohaline ocean circulation in North America.
Two years ago, a group of researchers reported finding high iridium concentrations in terrestrial sediments dated during this time period. A dense element in the platinum family, iridium is rare in Earth’s crust but abundant in meteorites. So the researchers theorized that the impact of a large meteorite, asteroid or comet instigated the Younger Dryas climate change.
François Paquay was intrigued. Although no crater was evident, advocates of the impact theory speculated that wildfires triggered by an airburst could account for the black mat layer across North America that correlates to the Younger Dryas climate shift. A University of Hawaiʻi at Mānoa doctoral candidate in geology and geophysics, Paquay led a team of American, Belgian and Canadian researchers to investigate further.
The researchers set out to replicate the original iridium findings, look for osmium isotopes that also indicate extraterrestrial matter and compare concentrations in other settings. They looked at marine sediments from the Gulf of California and the Cariaco Basin off the east coast of Venezuela and terrestrial sediments from across North America and a section in Belgium. Independent analysis at Mānoa and in Belgium produced similar results: no evidence of extraterrestrial enrichment of the deposits.
The results, reported at the American Geophysical Union fall meeting and published in the Dec. 22, 2009 Proceedings of the National Academy of Sciences, do not support the impact theory.
UH Mānoa Associate Professor Gregory Ravizza and researchers from five other universities in Europe and the United States were part of the team. The project was supported by the Geological Society of America and the National Science Foundation. Sediment samples were provided by the Integrated Ocean Drilling Program.