Refining the date of dinosaur extinction (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 25 Apr 2008 15:33:24 GMT
Media Relations
University of California-Berkeley
Contact: Robert Sander
Phone: (510) 643-6998
24 April 2008
Refining the date of dinosaur extinction
BERKELEY -- Scientists at the University of California, Berkeley, and the
Berkeley Geochronology Center have pinpointed the date of the dinosaurs'
extinction more precisely than ever thanks to refinements to a common
technique for dating rocks and fossils.
The argon-argon dating method has been widely used to determine the age of
rocks, whether they're thousands or billions of years old. Nevertheless,
the technique had systematic errors that produced dates with uncertainties
of about 2.5 percent, according to Paul Renne, director of the Berkeley
Geochronology Center and an adjunct professor of earth and planetary
science at UC Berkeley.
Renne and his colleagues in Berkeley and in the Netherlands now have
lowered this uncertainty to 0.25 percent and brought it into agreement
with other isotopic methods of dating rocks, such as uranium/lead dating.
As a result, argon-argon dating today can provide more precise absolute
dates for many geologic events, ranging from volcanic eruptions and
earthquakes to the extinction of the dinosaurs and many other creatures at
the end of the Cretaceous period and the beginning of the Tertiary period.
That boundary had previously been dated at 65.5 million years ago, give or
take 300,000 years.
According to a paper by Renne's team in the April 25 issue of Science, the
best date for the Cretaceous-Tertiary, or K/T, boundary is now 65.95
million years, give or take 40,000 years.
"The importance of the argon-argon technique is that it is the only
technique that has the dynamic range to cover nearly all of Earth's
history," Renne said. "What this refinement means is that you can use
different chronometers now and get the same answer, whereas, that wasn't
true before."
Renne noted that the greater precision matters little for recent events,
such as the emergence of human ancestors in Africa 6 million years ago,
because the uncertainty is only a few tens of thousands of years.
"Where it really adds up is in dating events in the early solar system,"
Renne said. "A 1 percent difference at 4.5 billion years is almost 50
million years."
One major implication of the revision involves the formation of
meteorites, planetessimals and planets in the early solar system, he said.
Argon-argon dating was giving a lower date than other methods for the
formation of meteorites, suggesting that they cooled slowly during the
solar system's infancy.
"The new result implies that many of these meteorites cooled very, very
quickly, which is consistent with what is known or suggested from other
studies using other isotopic systems," he said. "The evolution of the
early solar system -- the accretion of planetessimals, the differentiation
of bodies by gravity while still hot -- happened very fast. Argon-argon
dating is now no longer at odds with that evidence, but is very consistent
with it."
Renne has warned geologists for a decade of uncertainty in the argon-argon
method and has been correcting his own data since 2000, but it took a
collaboration that he initiated in 1998 with Jan R. Wijbrans of the Free
University in the Netherlands to obtain convincing evidence. Wijbrans and
his Dutch colleagues were studying a unique series of sediments from the
Messinian Melilla-Nador Basin on the coast of Morocco that contain records
of cycles in Earth's climate that reflect changes in Earth's orbit that
can be precisely calculated.
Wijbrans' colleague Frits Hilgen at the University of Utrecht, a co-author
of the study, has been one of the world's leaders in translating the
record of orbital cycles into a time scale for geologists, according to
Renne. Renne's group had proposed using the astronomical tuning approach
to calibrate the argon-argon method as early as 1994, but lacked ideal
sedimentary sequences to realize the full power of this approach. The
collaboration brought together all the appropriate expertise to bring this
approach to fruition, he said.
"The problem with astronomical dating of much older sediments, even when
they contain clear records of astronomical cycles, is that you're talking
about a pattern that is not anchored anywhere," Renne said. "You see a
bunch of repetitions of features in sediments, but you don't know where to
start counting."
Argon-argon dating of volcanic ash, or tephra, in these sediments provided
that anchor, he said, synchronizing the methods and making each one more
precise. The argon-argon analyses were conducted both in Berkeley and
Amsterdam to eliminate interlaboratory bias.
Argon-argon dating, developed at UC Berkeley in the 1960s, is based on the
fact that the naturally-occurring isotope potassium-40 decays to argon-40
with a 1.25-billion-year half-life. Single-grain rock samples are
irradiated with neutrons to convert potassium-39 to argon-39, which is
normally not present in nature. The ratio of argon-40 to argon-39 then
provides a measurement of the age of the sample.
"This should be the last big revision of argon-argon dating," Renne said.
"We've finally narrowed it down to where we are talking about
fractions-of-a-percent revisions in the future, at most."
Klaudia Kuiper, the lead author of the Science paper, was a Ph.D. student
in Amsterdam working with study coauthors Wijbrans, Hilgen and Wout
Krijgsman when the study was initiated. She also conducted lab work with
Renne and Alan Deino, a geochronologist with Renne at the Berkeley
Geochronology Center who was also one of the study's coauthors.
The work was funded by the U.S. and Dutch National Science Foundations and
the Ann and Gordon Getty Foundation.
IMAGE CAPTION:
[http://www.berkeley.edu/news/media/releases/2008/04/images/kt.jpg (25KB)]
At Zumaia in the Basque country of northern Spain, sediments laid down
around the end of the Cretaceous period show layers of light limestone and
dark marl reflecting warm and cool periods, respectively, in Earth's
climate. These alternating climatic periods are caused by 100,000-year and
405,000-year cycles in Earth's orbital eccentricity. Because Earth's
orbit, and thus the relative ages of the sediment layers, can be precisely
calculated, dating of the sediments by the argon-argon method provided a
much-needed calibration of the method and made it possible to pinpoint the
Cretaceous/Tertiary boundary at 65.95 million years ago. (Image courtesy
of PNAS)
.
- Prev by Date: Re: Convection inside a star
- Next by Date: Mining for Dark Matter (Forwarded)
- Previous by thread: How Michel Nostradamus stopped the Randi-Dawkins Corp.....
- Next by thread: Mining for Dark Matter (Forwarded)
- Index(es):
Relevant Pages
|
|
