http://www.psu.edu/ur/2003/cretaciousclimatechange.html
Dinosaurs Experienced Climate Changes Before K-T Collision
Pennsylvnia State University
January 14, 2003
University Park, Pa. - Climate change had little to do with the demise of
the dinosaurs, but the last million years before their extinction had a
complex pattern of warming and cooling events that are important to our
understanding of the end of their reign, according to geologists.
"The terrestrial paleoclimate record near the K-T is historically
contradictory and poorly resolved," says Dr. Peter Wilf, assistant professor
of geosciences at Penn State. "In contrast, the resolution of K-T marine
climates that has emerged over the last 10 years is excellent. Our work
brings the terrestrial record up to speed so that we can look for global
climate events that occurred for both land and sea."
Wilf worked with Kirk R. Johnson, curator of paleontology, Denver Museum of
Nature & Science, who provided the data on land plant fossils and Brian T.
Huber, curator of Foraminifera, National Museum of Natural History,
Smithsonian Institution, who provided the marine data.
An extraterrestrial object that impacted the Earth near the Yucatan in
Mexico 65.51 million years ago doomed the dinosaurs and 70 percent of the
Earth's other species, vaporizing itself and the surrounding rocks and
throwing enough ash, soot and debris into the atmosphere to effectively stop
photosynthesis worldwide. This impact radically altered the natural
progression of evolution. The time of the impact is called the K-T boundary
and marked the end of Cretaceous Period and the beginning of the Tertiary
Period.
"It could be argued that we are still recovering from that impact and the
mass extinctions of dinosaurs, mammals, insects, plants and sea life that it
caused," says Wilf, who worked on this project at the University of Michigan
before coming to Penn State. "For example, not only the dinosaurs, but also
80 to 90 percent of the Cretaceous plant species, including all the dominant
species, disappeared."
According to Wilf, there is a lingering minority argument that the K-T
extinction was caused by climate change, but the research team's results,
published in a recent issue of the Proceedings of the National Academy of
Sciences, both document the climate changes and show that they were not the
principal cause.
Wilf, Johnson and Huber first worked to create a finely resolved terrestrial
temperature record, based on plant fossils, and then correlated that record
with the existing marine records.
Plant fossils from the one million-year period before the extinction that
are abundant and well preserved in a fine time sequence are found only in
New Mexico and North Dakota. Of the two, the North Dakota sites are
comparably much more intensively collected and studied and enabled Johnson
to collect 22,000 plant fossils of more than 300 fossil plant species.
"Only in the last year, with the publication of an entire volume filled with
new research results on the Hell Creek Formation in North Dakota, can we do
this work and tie the plant fossil record there to actual dates in millions
of years rather than relative dates," says Wilf. Johnson is a co-editor and
contributor for the Hell Creek volume.
Fossils can be dated relatively by their position in the stratigraphy or
layers of sediment using a simple rule. In undisturbed layers, the oldest
fossils are in the lowest layers and the most recent fossils are in higher
levels. Tying relative dates to real dates is not easy, especially keeping
within the 100,000 year sensitivity available in the marine record, which
comes from the scientific results of the ocean drilling program.
Luckily, the K-T extinction occurred during a short interval in the Earth's
magnetic pole reversals. Periodically, the Earth's poles switch polarity
making North negative and South positive. Eventually, another switch occurs
making North positive and South negative. A record of the Earth's
paleomagnetism is recorded in the rocks as they are laid down.
"Three hundred and thirty-three thousand years before the extinction, a pole
reversal occurred," says Wilf. "Two hundred and seventy thousand years after
the extinction, another reversal occurred."
Because the researchers have three datable points --the two reversals and
the K-T impact - they could attach ages to the layers and the fossils within
and correlate the terrestrial and marine data at much finer resolution than
ever before.
Simply equating the layers, however, was not enough. The researchers needed
to estimate the temperature of the environment in which each fossil grew.
For the plants, this turned out to be simple, using a method first developed
in 1915 that is still widely used today.
Modern forests have two types of trees, those with toothed leaves and those
with smooth leaves. The cooler the climate, the higher the percentage of
species with toothed leaves.
"The presence of palm species also suggests a warm climate as these plants
cannot survive the ground freezing," says Wilf.
The researchers found from the plants that the long, slow cooling that
occurred for millions of years of the Late Cretaceous was broken by a
warming event that began about 66 million years ago and peaked 300,000 to
100,000 years before the K-T collision. The temperatures then returned to
baseline just before the collision and stayed nearly constant before and
after the collision. The plant record agreed strongly with the marine data,
which comes from ocean coring projects in the South Atlantic, Antarctica and
off the shores of New Jersey and Florida, and is based on the oxygen isotope
ratios in the skeletons of marine-shelled micropredators called
Foraminifera. The colder the water, the more of the heavier oxygen isotope
is incorporated in the calcium carbonate of the shells. The sediments that
entomb the forams also record the paleomagnetic reversals around the K-T.
Because the marine data come from four different locations and the
terrestrial data from a fifth, the warming and cooling trends seem global,
according to Wilf. The marine data also show that warm water forams migrated
from the tropics as far as New Jersey and Antarctica.
While the mean annual temperature in North Dakota today is 43 to 45 degrees
Fahrenheit, during the warmest part of the warming episode, the mean annual
temperature was from 65 to 68 degrees Fahrenheit. The North Dakota site was
then at the same latitude as Quebec City, Canada, and not only palm trees,
but alligators and turtles thrived too.
"The K-T impact affected the Earth's living things severely and
dramatically, but the climate changes right before the impact, by
comparison, did not," says Wilf. "Understanding the climate and vegetation
before the impact gives us insight into what kind of world the meteorite
struck, and shows us that it was warming, cooling, lushly forested and
otherwise functioning the way it always has done. The dinosaurs were well
adapted to global warming and cooling, but not to giant speeding rocks from
space."
The American Chemical Society, National Science Foundation and Smithsonian
Institution funded this research.
**aem**
Contacts:
A'ndrea Elyse Messer (814) 865-9481 [EMAIL PROTECTED]
Vicki Fong (814) 865-9481 [EMAIL PROTECTED]
EDITORS: Dr. Wilf is at 814-865-6721 or at [EMAIL PROTECTED] by e-mail.
Dr. Johnson is at 303-370-6448 or at [EMAIL PROTECTED] by e-mail. Dr. Huber
is at 202-786-2658 or at [EMAIL PROTECTED] by e-mail.
______________________________________________
Meteorite-list mailing list
[EMAIL PROTECTED]
http://www.pairlist.net/mailman/listinfo/meteorite-list
