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I am the Chicago Center for
Cosmochemistry Postdoctoral Research
Fellow. The Center includes scientists from the University of Chicago, the Field Museum, and Argonne
National Laboratory.
Cosmochemists study what the Solar System is
made of, how it formed, and what physical and chemical conditions
prevailed in the early Solar System.
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We are using the CHARISMA
instrument, a unique resonant
ionization mass spectrometer at Argonne,
to study presolar
grains for what they tell us about the synthesis of the chemical
elements. Resonance ionization is an especially sensitive and
elementally selective technique, and we have greatly improved the
isotopic precision attainable by this method. The poster
we presented at the 39th Lunar and Planetary Science Conference in
March, 2008, summarizes how far we've come in making precise and
reproducible measurements of chromium and iron isotopes.
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With colleagues at
the TANDAR
Laboratory (part of the Argentine Comisión
Nacional de Energía Atómica), Technical University
of Munich, and VERA
Laboratory in Vienna, I studied a meteorite that fell in
Córdoba Province, Argentina approximately 410,000 years
ago. We determined this age by accelerator mass spectrometry of
long-lived cosmogenic isotopes. The exceptionally long survival
of this meteorite on Earth is likely due to its chemical composition
and a history of burial. Our paper is accepted for publication in
Meteoritics and Planetary Science, but until publication, one must be
content with our preliminary results, presented as a poster
(3 MB PDF file) at the 38th
Lunar and Planetary Science Conference in March, 2007.
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I received my Ph.D. in physics
from the University of California,
Berkeley
in 2004, working with Rich Muller.
My dissertation investigated the meteoroid bombardment history of the
Moon (and, by extension, of the Earth) by measuring the ages of lunar
glass spherules, part of the soil samples collected by the Apollo
astronauts. With Paul Renne, we dated
individual spherules at
the Berkeley Geochronology Center.
A paper summarizing our findings was published in 2005 in
Geophysical Research Letters.
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We measured the ages of each
spherule by measuring the amount of argon in each one that was due to
the radioactive decay of potassium. However, spherules contain
argon from other sources, and measuring this argon allows us to
constrain how the Sun has behaved over billions of years, and how the
soil on the Moon is mixed and stirred by repeated meteoroid
impacts. Our results were published in a 2007 paper in
Geochimica et Cosmochimica Acta.
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Robert Rohde and I extensively
modeled the diffusion of argon in lunar spherules to complement our
dating experiment above by trying to learn the spatial distribution of
argon within the spherules. This would be important for
understanding how energetic is the solar wind, for example, which is
the source of most of the non-radiogenic argon in the spherules.
However, as our 2006
paper in the Journal of Geophysical Research shows, this modeling
instead showed that spherules retain material and chemical complexities
from their parent materials, which constrain how spherules form.
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While at Berkeley, I had the
privilege of working on a number of other
interesting projects. These included a measurement of cosmic
iridium in dust trapped in Greenland ice samples. Our 2003
paper in Geochimica et Cosmochimica Acta describes this work, and
gives our estimate of the rate of extraterrestrial dust accretion to
the Earth. This project represented a collaboration with
scientists at Lawrence Berkeley National Laboratory and the University
at Buffalo (SUNY).
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Another of my projects
investigated the history of the ice ages of the last 850,000
years. Climate is recorded in sediments, and benthic foraminifera
are particularly useful recorders of the extent of glacial ice during
their lifetime. Our benthic stack, published in 2002 in
Paleoceanography, can be used to test proposals of astronomical and
physical mechanisms for natural climate change.
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I have a long-standing interest
in the dynamics of planetary orbits. I have written a computer
program (in Matlab) that calculates the amount of solar radiation
reaching any point on the Earth on any date during the last 3,000,000
years. My insolation program is not available on-line, but send me an e-mail if you
would like a copy.
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I have also constructed movies
of the dynamics of the solar system. One
is available on-line, showing the evolution of the inner planets'
orbits for the last 3 million years.
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