A. Scott et al., Geophysical Research Letters (2010)

A match? Ancient, supposedly impact-generated carbonaceous spherules (left, top, and bottom) versus lightly charred fungal debris (right, top, and bottom).

'Mammoth-Killer' Nothing More Than Fungus and Bug Poop

Dick writes about Earth and planetary science for Science magazine.

Proponents of the idea that an exploding comet wiped out mammoths, giant sloths, and other megafauna 12,900 years ago have pointed to unusual organic debris in the soil from this period—debris, they say, that could have formed only in extreme wildfires raging across North America. But in a new study, a team argues that this debris is just fungal remains and bug poop.

In a paper just published in Geophysical Research Letters, Andrew Scott of Royal Holloway, University of London in Egham, United Kingdom, and colleagues used four kinds of microscopy to take a closer look at the odd debris, known as carbonaceous spherules. The spherules have a honeycombed appearance and are a few hundred micrometers across. Scott spent 30 years studying charcoal from modern and ancient fires, but the spherules were too cryptic to bother with, at least until the mammoth-killer impact started showing up in TV documentaries.

Scott's team says it found a good match between carbonaceous spherules from 12,900 years ago and so-called fungal sclerotia (see figure). These are balls that fungi form during times of environmental stress and that can germinate if more favorable conditions return. When charred at relatively low temperatures, the resemblance increases. Some of the more elongate particles are "certainly fecal pellets, probably from termites," says Scott. And using a reflected-light technique to gauge the temperature at which charring occurred, the researchers conclude that the 12,900-year-old spherules were heated in low-intensity natural wildfires, if that. "There's certainly no evidence they're 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."

Scott says the work should shift interest back to the idea that early Americans, climate change, or disease wiped out mammoths and their ilk.

But comet proponents aren't swayed. "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." Kennett, a professor emeritus at the University of California, Santa Barbara, is one of several leaders of a loosely confederated group of 25 or so researchers who proposed and largely continues to support the impact. And, he says, Scott and others have yet to explain an unusual feature in some carbonaceous spherules: nanodiamonds, nanometer-size bits of diamond that comet proponents say could have formed only in the extreme conditions of an impact.

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