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Miami geologist, colleagues: New evidence argues against prehistoric extraterrestrial impact


Jason Rech, associate professor of geology, in the Atacama Desert, Chile
Miami University geologist Jason Rech is part of a new collaborative study led by U.S. Geological Survey scientists that casts doubt on a contested possible extraterrestrial impact 12,900 years ago. Instead, the team suggests the environmental markers tied to climate change and extinctions from that time period are the result of natural processes.

The controversial 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.

Elevated levels of iridium, magnetic spherules, and titanomagnetite grains, collectively called “impact markers,” form the bulk of the evidence for the Younger Dryas impact hypothesis, according to the study authors.

In this study, led by Jeff Pigati of the U.S Geological Survey (USGS), scientists found high levels of the reported markers in deposits called black mats, the organic-rich remains of old marshes and swamps, at several sites in the southwestern U.S. and the Atacama Desert of northern Chile.

Markers were found in black mats ranging in age from 6,000 to more than 40,000 years in areas far removed from the purported impact location. These findings indicate the markers accumulated naturally in wetlands and are not the result of a catastrophic impact event.

The full report, “Accumulation of impact markers in desert wetlands and implications for the Younger Dryas impact hypothesis” is published in the online, Early Edition of the Proceedings of the National Academy of Sciences (PNAS) USA the week of April 23.

The Younger Dryas impact hypothesis, if true, might imply a greater risk to humanity than previously imagined, due to the recency of such a large impact, according to the USGS.

“When the idea was first promoted in 2007 (by Firestone,, those of us familiar with black mats suspected that normal depositional processes in wetlands might be responsible,” said Pigati, a USGS geologist and lead investigator of the new study.

Rech said “my co-authors and I were immediately suspicious as these wetland environments are not high-resolution depositional environments, like many lakes and oceans. In general, black mats are thin (a few centimeters) and highly mixed due to vegetation and small organisms that live in these environments. Black mats are not the type of deposits that I would use to look for evidence of a very recent extraterrestrial impact.”

“Therefore, we designed a study to examine black mats of various ages in North America and South America (away from the proposed impact) to see if we would find similar concentrations of iridium and other markers that Firestone and his colleagues used as supporting evidence for an impact. We found that there were similar concentrations of these markers in deposits of various ages and in both the Northern and Southern Hemispheres, indicating that they cannot be the result of an extraterrestrial impact event,” Rech said.

The research was funded by grants from the National Geographic Society’s Committee for Research Exploration; the U.S. Geological Survey’s Global Change Research Program; the Millennium Science Initiative and Chile’s National Commission on Scientific Research and Technology.

Co-authors are Jeff Pigati, USGS, Denver; Claudio Latorre, Pontifica Universidad Catolica de Chile, Santiago, Chile and Institute of Ecology and Biodiversity, Santiago, Chile; Rech, associate professor of geology and environmental earth science, Miami University; Julio Betancourt, USGS, Tucson, Ariz.; Katherine Martinez, Pontifica Universidad Catolica de Chile, Santiago, Chile; and James Budahn, USGS, Denver.

Material also contributed by USGS

The scientific process

"One of my areas of expertise is on the use of wetland deposits to reconstruct climate and environmental change in arid lands," Rech said. "Wetland deposits are laid down around springs and marshes and include organic-rich sediments known as black mats, as well as other deposits indicative of standing or flowing water and an abundance of vegetation.
In general, they form during wetter periods and can be used to identify the timing and magnitude of past wet periods."

"The 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."

Jason Rech (and future scientists), Atacama Desert


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