Lake Sediments Spark Controversy: Did a Comet Cause the Ice Age Catastrophe?

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Ice ages, prehistoric civilizations, extinctions, and celestial impacts—these are being discussed more and more. What is the truth? Or could these things happen in our future?

Lake Sediments Bring Controversy: Did a Comet Cause the Ice Age Catastrophe?

“Maybe it was the overhunting by the Clovis people over centuries, rather than a devastating impact, that led to the extinction of North American mammoths and other megafauna,” researchers say.

Jacquelyn Gill, a paleoecologist at the University of Wisconsin–Madison, says that after clearing through layers of ancient lake sediments, her team found no evidence to support the theory that a comet caused the Ice Age biological extinction.

“There is no natural sign to suggest this was an impact event,” Gill stated at the Ecological Society of America meeting held here on Tuesday. “If this was an impact event... it did not have the ecological impact that people previously claimed.”

In 2007, Richard Firestone and his colleagues at the Lawrence Berkeley National Laboratory published evidence suggesting that a comet exploded in the atmosphere near the Great Lakes 12,900 years ago, causing massive fires across North America. These fires supposedly led to the rapid extinction of several organisms, including the Clovis civilization on the continent and megafauna such as mammoths, ground sloths, and 33 other genera of large mammals.

However, skeptics point to several facts: no associated impact craters have been found, and the evidence for continental forest fires and rapid population decline is insufficient. Furthermore, if such an event truly occurred, how did small mammals and birds survive? (Additionally, a recent study has questioned the likelihood of comet impacts being the cause of multiple extinction events in Earth’s history.)

Gill and her team decided to look for clues of a comet impact, not on land, but in three lakes in Indiana and Ohio, where the conditions of ancient plant pollen and minerals have remained stable, allowing for the reconstruction of ecological environments dating back thousands of years. They searched for all mineral samples that could be linked to a comet impact as evidence, including ash, charcoal, magnetic grains, silicate particles, and elements such as titanium and chromium. She did not look for rare earth elements like iridium, which other researchers consider signatures of an impact.

The team did not find any information indicating that a single catastrophic event occurred 12,900 years ago. Moreover, in one of the lakes, they found that while titanium content decreased, carbon content actually increased. “This is clearly not an impact event,” Gill said.

Gill also questioned whether the timing of the animals’ deaths coincided with the timing of the comet impact.

A type of fungal spore called Sporormiella, which is closely associated with the dung of large animals, began to decline in number 14,600 years ago, followed by the onset of the ice age. One lake showed that the spore record was interrupted 13,600 years ago and only recovered in the past few centuries, rising in areas used for pasture. At the same time, the pollen of plants eaten by large animals—such as ash, ironwood, and hop hornbeam—began to increase, indicating that plant growth was no longer being suppressed by large herbivores.

Gill says that more recently discovered fossils of megafauna from 13,600 to 12,900 years ago were likely the last surviving species decimated by the Clovis people with their characteristic spears.

However, Firestone was not swayed by this. As he said, “The argument adds nothing and plays no role.” He stated that he did not expect to find a large amount of evidence in lakes because magnetic minerals would rust away and remain suspended in the water rather than concentrating at the bottom of the lake. To him, the data still supports the theory of destruction caused by extraterrestrial factors. “I think the debate will not make much progress,” he said.

Original text from Scientific American:

Lake Sediments Cast More Doubt That a Comet Caused Ice Age Extinctions

Overhunting by Clovis people over centuries, not a catastrophic impact, may have wiped out North American mammoths and other megafauna, researchers say

ALBUQUERQUE—After combing through layers of ancient lake sediments, paleoecologist Jacquelyn Gill of the University of Wisconsin–Madison says her team has found no evidence to support a controversial comet theory for an ice age extinction event.

"There’s no physical trend to suggest that there was an impact event," Gill said Tuesday at the Ecological Society of America meeting held here this week. "If there was an impact event...it’s not having the ecological effects [previously] suggested."

In 2007 Richard Firestone at the Lawrence Berkeley National Laboratory and colleagues published evidence suggesting that a comet exploding in the atmosphere 12,900 years ago near the Great Lakes set off massive fires in North America. These fires supposedly led to the rapid disappearance of the continent’s Clovis culture as well as megafauna including mammoths, ground sloths and 33 other large mammal genera.

But skeptics point to the fact that no associated impact craters have been found, and evidence for continental forest fires and a rapid decline in human populations is sketchy. Besides, if such an episode had occurred, small mammals and birds somehow survived. (Another recent study has called into question the likelihood of comet impacts being responsible for more than one extinction event during Earth’s history.)

Gill and her team decided to look for hints of the comet’s impact not on land but in three lakes in Indiana and Ohio, where pollen and minerals have settled daily, creating an ecological record dating back millennia. She scoured core samples for evidence of ash, charcoal, magnetic grains, tiny silicate spheres, and elements such as titanium and chromium that could be associated with impact events. She did not look for rare earth minerals like iridium, which other researchers rely on as a signature of impacts.

The team failed to find a consistent signal that would indicate that a single catastrophe occurred around 12,900 years ago. At one lake, titanium actually decreased at the same time charcoal was increasing. "It’s clearly not an impact event," Gill said.

Gill also questioned the view that animals died off right at the time of the proposed impact event.

Fungal spores called Sporormiella, associated with the dung of large mammals, actually begin to decline 14,600 years ago, soon after the end of the last ice age. The spores wink out of the record at one lake 13,600 years ago and only recover in the past few centuries with the rise in cattle grazing. At the same time, pollen from the plants that megafauna munched on—ash, ironwood and hop hornbeam—starts piling up, suggesting the plants’ growth was no longer being kept in check by leaf-loving large mammals.

More recent megafauna fossils, dated between 13,600 and 12,900 years ago, were likely the last hangers-on as the Clovis people decimated species with their characteristic spears, Gill says.

But Firestone was not swayed by the new study, which he says "adds nothing to the argument." He says that he would not expect to find much evidence in lake sites because magnetic material would rust away, while the tiny spheres float and would not collect on the lake bottom. For him, the data still points toward an extraterrestrial impact. "As far as I’m concerned the debate is settled," he said.

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