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At the Cretaceous-Tertiary (K/T) boundary 65 million years ago, a projectile
10 to 15 kilometers wide smashed into the Yucatan peninsula in Mexico, leaving
a gaping hole at least 180 kilometers in diameter (Geotimes
January 2004) and killing 75 percent of all species and 50 percent of genera,
including the dinosaurs. When the projectile hit, it ballistically slung tons
of melted material high into the atmosphere, which then rained down over Earth.
Researchers have found a thin K/T boundary layer in spots all over the world,
containing soot and the tell-tale signs of the impact — iridium, nano-diamonds
and shocked quartz grains.
Since the discovery of this extraterrestrial impact, scientists have hypothesized
about what chain of events led to the demise of the dinosaurs. High soot content
found in K/T boundary rocks has led some researchers to hypothesize that the
impact ignited a global wildfire, forcing the extinctions. But this popular
hypothesis has recently come under fire.
In December, Claire Belcher (Royal Holloway University of London), Alan Hildebrand
(University of Calgary) and colleagues published a paper in Geology suggesting
that a global wildfire was not possible, based on new charcoal evidence from
six outcrops in North America. The same month, David Kring (University of Arizona)
and Daniel Durda (Southwest Research Institute) published an article in the
newsmagazine Scientific American suggesting that fast-moving debris heated
the atmosphere as particles from the huge explosion rained down and created
spontaneous wildfires that spread across the globe in a matter of hours and
days following the impact.
Belcher and colleagues collected samples from six sites in a south-to-north
transect across North America from Colorado to Saskatchewan to test K/T boundary
rocks for charcoal. The stratigraphy of rocks around the K/T boundary in North
America includes two distinctive claystone layers: a layer that is considered
the ejecta layer and a layer sometimes called the impact layer, which includes
an iridium anomaly, shocked minerals and soot. Belcher hypothesized that if
there was indeed a fire of global proportions, charcoal should be abundant in
these layers close to the impact site. Charcoal is chemically different from
soot and can only be found where biomass has burned, whereas the smaller soot
particles can be carried airborne for thousands of kilometers.
Instead of finding charcoal, Belcher's team found that the K/T boundary and
lowest Tertiary rocks contain "no charcoal or below-background levels of
charcoal and a significant quantity of noncharred organic materials," the
authors write. The presence of noncharred plant matter indicates that there
was "no distinctive wildfire across the North American continent related
to the K/T event." Furthermore, the lack of charcoal indicates that the
amount of thermal power delivered from the impact was significantly less than
previously thought. If the thermal power was insufficient to ignite vegetation
just a few thousand kilometers from the crater, the authors write, there couldn't
have been a global inferno.
Wendy Wolbach, a geochemist with DePaul University in Chicago who has long
been involved in fire research at the K/T boundary, cautions that while Belcher's
work is intriguing, it is premature to throw out the whole global fire hypothesis.
Furthermore, large-scale biomass burning could have been patchy, due to better
or poorer burning conditions at different places, Wolbach says, and this would
not necessarily mean that biomass did not burn or that fires were not global.
In the model Durda and Kring developed (published in the Journal of Geophysical
Research in 2002), they suggest that in order to determine the extent of
the fires, geoscientists must first determine the thermal input and the trajectory
path of the initial impactor. And one way to do that is to look for charcoal
presence at terrestrial sites, "so I'm happy to see Belcher and colleagues
looking into the rock record," Kring says.
In past studies, researchers have found charcoal in K/T boundary rocks in outcrops
in northern New Mexico and Mexico, "evidence that there was biomass burning
somewhere in the vicinity at that time," Wolbach says. And the pesky global
soot layer, contemporaneous with the impact, further supports biomass burning.
So at the least, Kring says, the impact generated local fires.
More research is needed, Wolbach says. "I look forward to seeing [Belcher's]
group apply their techniques to more geographically distant sites." Hildebrand
agrees, saying "six sites are good to look at, but so are another six."
Belcher and colleagues plan to look for charcoal at other sites across North
America in the near future.
Hildebrand says that he now believes that the thermal pulse could not have been great enough to cause global conflagrations. Still, something burned somewhere, he says, "so now we need to figure out where all the soot came from," a puzzle that is not easy to solve.
Megan Sever
Links:
"Unraveling
the Chicxulub Case," Geotimes January 2004
Kring
and Durda's Wildfire Image
K/T
Boundary Information from Royal Holloway University of London
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