More Evidence Chicxulub Was Too Early

http://www.geosociety.org/news/pr/06-14.htm

News Release

29 March 2006
GSA Release No. 06-14

Contact: Ann Cairns,
Director-Communications and Marketing
(303) 357-1056, fax 303-357-1074

More Evidence Chicxulub Was Too Early

Boulder, Colo. - A new study of melted rock ejected far from the
Yucatan's Chicxulub impact crater bolsters the idea that the famed
impact was too early to have caused the mass extinction that killed the
dinosaurs 65 million years ago.

A careful geochemical fingerprinting of glass spherules found in
multiple layers of sediments from northeast Mexico, Texas, Guatemala,
Belize and Haiti all point back to Chicxulub as their source. But the
analysis places the impact at about 300,000 years before the infamous
extinctions that mark the boundary between the Cretaceous and Tertiary
periods, a.k.a. the K-T boundary.

Using an array of electron microscopy techniques, Markus Harting of the
University of Utrecht in the Netherlands has found that chemical
compositions of the spherules all match what would be expected of rocks
melted at the Chicxulub impact. The spherules are now found in several
layers because after they originally hit the ground, they were
"reworked" by erosion to create later layers of sediments, he said.
It's
this reworking long after the impact that has misplaced some of the
spherules into sediments that, based on the fossils in the same
sediments, are misleadingly close to the K-T boundary.

Harting is scheduled to present his latest findings on Monday, 3 April
Backbone of the Americas-Patagonia to Alaska. The meeting is
co-convened
by the Geological Society of America and the Asociaci?n Geol?gica
Argentina, with collaboration of the Sociedad Geol?gica de Chile. The
meeting takes place 3-7 April in Mendoza, Argentina.

"The whole story is that it's a single impact event," said Harting of
his analysis of the multiple spherule layers. In fact, the original
spherule layer is not particularly hard to make out, since its
spherules
are not as abraded and damaged as those which were moved around and
re-deposited in later, higher sediments. Above these, and younger
still,
Harting has also identified the famous layer of extraterrestrial
iridium
in sediments worldwide which was originally touted as the smoking gun
for an impact somewhere on Earth at the K-T boundary.

"In most of the sections we found spherules we also found the iridium
layer at or near the K-T boundary," said Harting. "That makes the
mismatch with Chicxulub even more obvious."

The sediments from the region are also providing clues to what
transpired during those 300,000 years between the impact and the K-T
boundary die-offs. "Nothing happened between them," said Harting. "The
K-T iridium layer is a totally different event."

Disconnecting the Chicxulub impact from the K-T boundary also helps
make
sense of some other oddities in the iridium layer. In the Gulf of
Mexico, close to the impact site, iridium is found at a weak
concentration, just one part per billion, says Harting. Yet farther
away
in Denmark, higher concentrations of iridium are found. "This doesn't
really make sense," he said, unless, of course, the impact and iridium
layer are not related.

All this begs the question: What, then, created the worldwide iridium
layer, if not a humongous impact? One possibility is that Earth and
perhaps the entire solar system was passing through a thick cloud of
cosmic dust 65 million years ago.

"You probably have a time when lots of meteorites are coming down and
never touching the ground," said Harting. Instead they burned up as
"shooting stars," depositing their iridium in the atmosphere. There it
was quickly rained out, washed into lakes and oceans and buried in
contemporary sediments.

Another burning question is whether the massive impact - which
undoubtedly occurred and was certainly catastrophic - is responsible
for
any extinction at all. Maybe, answers Harting. There is the case of the
ammonites, the once ubiquitous nautilus-like sea creatures that died
out
at about the same time as the Chicxulub impact and before the K-T
boundary, he said.

But whether the impact was the ammonite killer is not at all clear,
according to Harting. Early models of the Chicxulub impact called on a
"nuclear winter" scenario, in which a dust-shrouded world went cold and
plant life died away for years, to cause mass extinctions. Yet
sun-loving animals like crocodiles and turtles appear to have glided
right through without any ill effects. And that is, perhaps the silver
lining to Chicxulub's fall from the status of
most-massive-of-all-murderers: Even giant impacts aren't necessarily
global catastrophes.


WHEN & WHERE

Backbone of the Americas - Patagonia to Alaska
Centro de Congresos
Mendoza, Argentina
Monday, 3 April

View abstract at
http://gsa.confex.com/gsa/06boa/fina...act_101180.htm