[meteorite-list] Yucatan Impact Crater May Have Occured Before the Dinosaurs Went Extinct

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon May 16 12:59:25 2005
Message-ID: <200505161658.j4GGwnU08319_at_zagami.jpl.nasa.gov>

http://www.sciam.com/article.cfm?chanID=sa004&articleID=0000D7BD-7720-1264-B1DB83414B7F0000

Doubts on Dinosaurs
Yucatan impact crater may have occurred before the dinosaurs went extinct
By Barry E. DiGregorio
Scientific American
May 16, 2005

According to conventional paleontological wisdom, an asteroid or comet
10 to 14 kilometers wide crashed into the present-day Yucatan Peninsula
65 million years ago and wiped out the dinosaurs. Most scientists
currently consider the Chicxulub impact crater, perhaps about 145
kilometers wide, to be the smoking gun of this Cretaceous-Tertiary (KT)
extinction.

Not so fast, says Princeton University micropaleontologist Gerta Keller.
The collision that created the Chicxulub crater, she argues, happened
before the KT extinction--300,000 years too soon, to be more precise.
She first made the controversial assertion last year, and the dust has
yet to settle.

Keller does not dispute that a meteorite could have helped trigger the
demise of the dinosaurs. But she remains confident that Chicxulub is not
the crater scientists should be looking at, based on sediments she has
analyzed from various Chicxulub sites. She has several lines of
evidence: one in particular relates to the layer of iridium, an
extremely rare element known to be abundant in many meteorites, that
exists at the KT boundary at sites around the world. In theory, only
massive impacts can distribute the element globally.

A big collision can also produce another kind of layer, too, by melting
and vaporizing silicate rocks, which then condense into sand-grain-size
glass spheres known as microtektites. Depending on the mass of the
colliding meteorite, these tiny glass spheres can be thrown hundreds to
thousands of kilometers from the point of impact.

Keller discovered that the original Chicxulub microtektite layers lie up
to 14 meters below the KT iridium layer at the northeastern Mexico site
(the crater itself extends from the northwestern tip of the Yucatan
Peninsula into the Gulf of Mexico). "To date, no one has found iridium
associated with Chicxulub," Keller says.

Jan Smit, a paleontologist at Vrije University in the Netherlands,
doubts Keller's claims, stating that her argument about KT iridium and
Chicxulub borders on tautology: "If you uncouple all the
iridium-enriched ejecta layers from the Chicxulub impact, then of course
there is no iridium associated with Chicxulub." In any case, Smit
contends, "How and where do you hide the iridium from a large impact
such as Chicxulub?"

Keller hypothesizes that the object that made Chicxulub may have been "a
dirty snowball" type that did not have any iridium. Some meteorites do
not. Another possibility could be that measurements may not yet have
been taken from the correct rock strata.

Researchers have also raised doubts about Keller's proposed 300,000-year
age difference between Chicxulub and KT, which is based on sedimentation
rates extrapolated from the distance between the microtektite layer and
the KT boundary layer. Geoffrey Garrison, a paleontologist from the
University of Washington, wonders why the material separating the two
layers could not have been just sediment that was resuspended by the
impact and that had simply settled back to the seafloor.

Keller insists that she has already ruled out resuspension. She claims
that sediment settling after a high-energy event, such as an impact,
tsunami or storm, produces identifiable layers. Heavier grains settle
out first, followed by the finest-grained muds and clays. Such a pattern
does not appear in the Chicxulub crater, Keller reports.

Keller plans to bolster her case with an upcoming paper that argues that
meteorite impacts that leave Chicxulub-size craters and smaller cannot
by themselves cause significant species extinctions. The amount of
material ejected, she finds, is insufficient to trigger long- lasting
climatic or geographic changes from fire or floods. Sudden mass
extinctions might require the coincidence of major volcanism and a large
impact event, "but so far no one has found the source crater," Keller
says in her dismissal of Chicxulub. "The history of mass extinctions
seems to indicate that a single short-term shock to the environment can
be survived by nearly all species." Whether conventional wisdom survives
Keller's own shock to paleontology remains to be seen.

------------------------------------------------------------------------
Barry E. DiGregorio is a research associate for the Cardiff Center for
Astrobiology in Wales, U.K.
Received on Mon 16 May 2005 12:58:48 PM PDT