To laypeople, the "muck" found in certain areas of Alaska and Yukon is just dirt – dark, silty, often frozen, and full of plant material. To miners, it is somewhat of a nuisance. When dug out and left to thaw, the muck lets loose a fetid stench due to its high organic content. To scientists, however, the muck is a graveyard, and a fascinating one at that. Over the years, thousands of remains of bison, mammoth, horse, musk ox, moose, lynx, lion, mastodon, bear, caribou, and even camel have been uncovered.

More interesting than the mere presence of this zoological gold mine is the actual condition of the remains. Cached inside the frozen mucks for as long as 48,000 years, the remains are remarkably well preserved, with some carcasses mostly intact and effectively mummified. Even more curious, many animals show no signs of predation, scavenging, or decomposition, and despite disarticulated bones, seemed to be in relatively good health at the time of their demise.

This made Jonathan Hagstrum, a research geophysicist at the U.S. Geological Survey, wonder... What killed all of these animals? He and colleagues Richard Firestone, Allen West, James Weaver, and Ted Bunch share an intriguing hypothesis.

They think the seemingly sudden deaths of many of these animals in the Alaskan and Yukon mucks could be explained by airbursts and impacts from comet debris that struck Earth during the Late Pleistocene, between 11,000 and 46,000 years ago. Hagstrum and his colleagues recently presented new evidence for this idea in the journal Scientific Reports.

Examining sediments attached to nine mammoth and bison skull fragments dug out from the mucks and dated to the Late Pleistocene, the researchers found large quantities of "impact-related microspherules" in seven of them (see images of the microspherules at the top of the page). These microscopic grains have compositions and characteristics similar to those of grains discovered at known impact craters (note triangle "d" below).

Hagstrum and his colleagues suggest that the abundance of these microspherules indicates that the animals died as a result of airbursts or impacts from astronomical objects. Some of these explosive events may have been on the scale of the 1908 Tunguska event, in which a foreign object between 200 and 620 feet in diameter exploded over Siberia, knocking down some 80 million trees in a blast roughly equal to the Castle Bravo thermonuclear explosion.

"Primary blast injuries are caused by the nearly instantaneous change in environmental pressure from passage of the blast wave," the researchers write. "Blast winds from the impacts would have swept across the Beringian landscape flattening trees and killing, dismembering, and burying megafaunal carcasses or body parts, along with logs, branches, other plant material, with a matrix of redeposited loess in low-lying creek valleys."

The current findings play in to the Younger Dryas impact hypothesis, which states that cosmic impacts, not humans, may have caused most of North America's large animals to go extinct during the Late Pleistocene. This hypothesis is widely contested, however, and many scientists contend there's no good evidence for it, instead favoring the idea that humans hunted megafauna to extinction. The impact hypothesis also suffers from a conspiracy problem. Oddball thinkers like Graham Hancock have co-opted it to write than impacts during the Younger Dryas doomed the lost civilization of Atlantis, which pretty much all available evidence suggests existed purely in the realm of fiction.

The present paper lends credence to the Younger Dryas impact hypothesis, however, it suffers from a few weaknesses. First, the sample size of fossils is minuscule. Additionally, the authors should search for microspherules in a control group of animal bones taken from the Alaskan mucks in the past thousand or so years. Moreover, classification of microspherules can be very subjective and open to interpretation. As the authors admit, "further studies of fossils with well-established stratigraphic contexts are needed before the actual role of impacts in driving or contributing to the megafaunal extinctions can be inferred."

Source: Jonathan T. Hagstrum, Richard B. Firestone, Allen West, James C. Weaver & Ted E. Bunch. Impact-related microspherules in Late Pleistocene Alaskan and Yukon “muck” deposits signify recurrent episodes of catastrophic emplacement. Scientific Reports 7, Article number: 16620 (2017) doi:10.1038/s41598-017-16958-2