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CNN: Magnetism May Reveal Age of Dinosaur Eggs (long)

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Traces of Magnetism May Reveal Age of Dinosaur Eggs
AP    10-APR-99      AUCA MAHUIDA, Argentina (AP) -- How old are the

 When scientists announced the discovery last November, they could only
estimate that the eggs were some 70 million to 90 million years old.
That was based on the kinds of animal fossils found in the same
geological formation. 

 The job of narrowing that uncertainty has fallen to geologist Lowell
Dingus, 47, an earnest, bearded man of few words who takes the field
under a cowboy hat.  He's president of the InfoQuest Foundation, based
in San Diego and New York, which supports fieldwork in paleontology and

 On the expedition, he prowled buttes with Julia Clarke, a 25-year-old
Yale graduate student, and Alberto Carlos Garrido, 27, an Argentine
working on a geology thesis. They were looking for rock with a long

 They were hoping to exploit a curious habit of Earth. Every few hundred
thousand or million years, it has flipped its magnetic field around so
that sometimes a compass needle would point south instead of north.
After a while, it would flip back again 

 As layers of mud build up underwater, tiny magnetic particles line up
with the prevailing magnetic field. Once the mud hardens into rock, the
particles are locked into position. So a lab test can reveal whether a
particular rock formed while the Earth's field was normal or reversed. 

 Scientists have been able to put dates on these field reversals. So
once you can place a sample in one of the normal or reversed periods,
you can date it. The  problem is, if you just have one sample, or a
series of samples showing the same  magnetic field orientation, you
can't tell which normal or reversed it's from. 

 You need to take a lot of samples from rocks of different ages from a
site, so you can discern a pattern that can be used to find the
corresponding pattern in the dated geological record. 

  All the samples Dingus had taken before this field trip showed the
reversed orientation. Now, working in younger rocks, he wanted to find
some with  normal orientation, a step toward finding the pattern that
would let him know when all these rocks formed. 

 He also came here with another, greater goal: finding a layer of ash
blown away from an ancient volcano. A lab test of such ash can date it
to within about  200,000 years. That would give Dingus an anchor of time
in these sediments and let him date the eggs within a million years or

  One day on the expedition, while Dingus explored on his own, Clarke
and Garrido carved out chunks of stone for the magnetic field analysis.
Dressed in long-sleeved shirts, baseball caps, long pants and hiking
boots, they climbed from spot to spot on the buttes. They looked for
steep slopes; gentle slopes are covered with lots of eroded dirt that
gets in the way of finding rock. 

 Once they found a promising place, they hacked out a small area with
their picks or heels, the crumbling surface dirt rattling down the slope
in tiny avalanches. If  they found rock, they drew an arrow on it
pointing north. Then they hacked free a chunk at least the size of a
fist, containing the arrow. They wrapped the chunk in aluminum foil to
keep it from crumbling, bound the foil with masking tape, and marked a
sample number on the tape. 

 It was time-consuming work. This morning, they collected three samples
before lunch. Now it's up to a lab to determine the secrets locked

 Weeks later, Dingus finally got his deposit of volcanic ash. It's not
clear whether it's well-preserved enough to provide a good date, but at
least it gives hope for finding better stuff when the scientists return
next year. 
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