Fossil Evidence Casts Doubt on Younger Dryas Impact Theory
American Geophysical Union (PR 10–13) | 16 June 2010
Fungus, not comet or catastrophe, accounts for carbonaceous spherules in the Younger Dryas "impact layer"
New findings challenge a theory that a meteor explosion or impact thousands of years ago caused catastrophic fires over much of North America and Europe and triggered an abrupt global cooling period, called the Younger Dryas. Whereas proponents of the theory have offered "carbonaceous spherules" and nanodiamonds -- both of which they claimed were formed by intense heat -- as evidence of the impact, a new study concludes that those supposed clues are nothing more than fossilized balls of fungus, charcoal, and fecal pellets. Moreover, these naturally-occurring organic materials, some of which had likely been subjected to normal cycles of wildfires, date from a period of thousands of years both before and after the time that the Younger Dryas period began -- further suggesting that there was no sudden impact event.
The Younger Dryas impact event theory holds that a very large meteor struck Earth or exploded in the atmosphere about 12,900 years ago, causing a vast fire over most of North America, which contributed to extinctions of most of large animals on the continent and triggered a thousand-year-long cold period. While there is much previous evidence for the abrupt onset of a cooling period at that time, other researchers have theorized that the climatic change resulted from increased freshwater in the ocean, changes in ocean and atmospheric circulation patterns, or other causes unrelated to impacts.
The impact-theory proponents point to a charred layer of sediment filled with organic material that they say is unique to that period as evidence of such an event. These researchers described carbon spheres, carbon cylinders, and charcoal pieces that they conclude are melted and charred organic matter created in the intense heat of a widespread fire.
Scott and his fellow researchers analyzed sediment samples to determine the origins of the carbonaceous particles. After comparing the fossil particles with modern fungal ones exposed to low to moderate heat (less than 500 degrees Celsius, or 932 degrees Fahrenheit), Scott's group concludes that the particles are actually balls of fungal material and other ordinary organic particles, such as fecal pellets from insects, plant or fungal galls, and wood, some of which may have been exposed to regularly-occurring low-intensity wildfires.
Comet cause for climate change theory dealt blow by fungus
- Geophysical Research Letters, doi: 10.1029/2010GL043345, in press (accepted 01 June 2010)
PhysOrg | Earth Sciences | 17 June 2010
'Mammoth-Killer' Nothing More Than Fungus and Bug Poop
A team of scientists - led by Professor Andrew C Scott of the Department of Earth Sciences at Royal Holloway, University of London - have revealed that neither comet nor catastrophe were the cause for abrupt climate change some 12,900 years ago.
Theories of impacts and their influence on animal extinctions and climate change are receiving increasing attention both in the scientific and popular literature. Despite increasing evidence to dispute the theory, the idea that onset of the Younger Dryas (‘Big Freeze’) climate interval, mega-faunal extinctions, including mammoths, the demise of the North American Clovis culture, and a range of other effects, is due to a comet airburst and/or impact event has remained alive both through written and television media despite growing negative scientific evidence.
One key aspect of this claim centers on the origin of ‘carbonaceous spherules’ that purportedly formed during intense, impact-ignited wildfires. Theorists have used these ‘carbonaceous spherules’ as evidence for their comet impact-theories, but this new study concludes that those supposed clues are nothing more than fossilized balls of fungus, charcoal, and fecal pellets. These naturally-occurring organic materials also date from a period thousands of years both before and after the Younger Dryas period began, further suggesting that there was no sudden impact event.
NOW | 17 June 2010
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 to be 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.