It makes for a good movie: 12,900 years ago, a comet slams into Earth, igniting forest fires across North America and sending the planet into a thousand cold years, killing off mammoths, giant sloths, and a bunch of other big mammals. But scientists have fiercely debated whether such a movie, about the cause of the planet-wide cooling period called the Younger Dryas, should be documentary or science fiction. According to a paper recently published in the Geophysical Research Letters, new evidence–or refuted, old evidence–points to science fiction.

Those that think a comet hit the planet cite “carbonaceous spherules” and nanodiamonds found in sediment from the period of the suspected impact. They argue that these particles formed from the intense heat of the collision.

Lead author of this new study, Andrew Scott of the University of London in Egham suspects those spherules are not from a comet collision, but are bug poop, fungal spores, or charcoal pellets.

From a test that measures how much light the spherules reflect, Scott’s team has determined that the spherules were slow-roasted in a low-intensity heat (perhaps from natural wildfires) instead of in intense, comet impact heat. As shown in the figure, the researchers compare the charred spherules to fungal sclerotia, emergency cell balls created by stressed fungi that can germinate after a bad growing period is over, and saw a striking similarity.

Some of the more elongate particles are “certainly fecal pellets, probably from termites,” says Scott…. “There’s certainly no evidence [that any of these particles are] related to intense fire from a comet impact,” says Scott. Part of the problem, he says, is that “there was nobody [among impact proponents] who ever worked on charcoal deposits, modern or ancient. If you’re not familiar with the material, you can make mistakes.” [Science Now]

Scott’s team also radiocarbon dated the particles, and says those spherules aren’t unique to the collision time.

“There is a long history of fire in the fossil record, and these fungal samples are common everywhere, from ancient times to the present,” Scott says. “These data support our conclusion that there wasn’t one single intense fire that triggered the onset of the cold period.” [American Geophysical Union]

Other researchers aren’t buying it–like James Kennett, a proponent of the impact theory from the University of California, Santa Barbara.

“We disagree that charred fungal sclerotia … have the same morphology” as certain carbonaceous spherules, paleoceanographer James Kennett writes in an e-mail. “Their alternate hypothesis that the carbon spherules are simply charred fungal spores is incorrect.”[ScienceNOW]

Kennett also clings to the nanodiamonds that impact-believers say formed under the extreme conditions of the collision. The new study doesn’t address these nanodiamonds, but Scott says there is more to come.

His team has studied the nanodiamond issue, but he’s not yet able to discuss the results. He did, however, hint that the particles might not be nanodiamonds at all: Fungal spores the team examined have similar microscopic features. And, Scott said, “obviously [spores] are not nanodiamonds.” [National Geographic]

Related content:
80beats: Nano-Diamond Discovery Suggests a Comet Impact Killed the Mammoths
80beats: Spores in Mastodon Dung Suggest Humans Didn’t Kill Off Ancient Mammals
80beats: Comets Not So Likely to Smash Into Earth and Kill Us All
DISCOVER: What Caused the Great American Extinction?

Image: American Geophysical Union