Evidence that has been at the center of the hypothesis to support the Younger Dryas impact event has been called into question by a new study led by U.S.L Geological Survey scientists, who believe that the evidence is actually the result of natural processes.
Elevated levels of iridium, magnetic spherules, and titanomagnetite grains, collectively called “impact markers,” have made up the bulk of evidence to support the Younger Dryas impact event hypothesis. According to the USGS, the “Younger Dryas impact hypothesis contends that an extraterrestrial object, possibly a comet, exploded over North America about 12,900 years ago, resulting in dramatic climate change, massive wildfires, and the extinction of many large herbivores and their predators.”
However, researchers found high levels of the reported “impact markers” in deposits called black mats, the remains of old marshes and swamps. These discoveries were located at several sites in the southwestern U.S. and the Atacama Desert of northern Chile, and ranging in age from 6,000 to more than 40,000 years in areas far removed from the purported impact location.
The report concludes that these markers accumulated naturally in wetlands and are not the result of any catastrophic impact event.
“Luis and Walter Alvarez’s proposal that an extraterrestrial impact was responsible for extinctions at the Cretaceous-Tertiary boundary eventually moved from unlikely hypothesis to accepted theory, and with its acceptance came the temptation to apply this explanation to any rapid change in Earth’s conditions,” said USGS Director Marcia McNutt. “The results of this study demonstrate the importance of maintaining a healthy skepticism and multiple working hypotheses.”
“When the idea was first promoted in 2007, those of us familiar with black mats suspected that normal depositional processes in wetlands might be responsible,” said Dr. Jeff Pigati, a USGS geologist and lead investigator of the new study. Indeed, this is what Pigati and coauthors now report in this week’s issue ofProceedings of the National Academy of Sciences, USA.
“This is a great object lesson for how scientific hypotheses are done and undone,” said Paul Baker, Professor of Earth and Ocean Sciences at Duke University and a member of National Geographic Society’s Committee for Research and Exploration.