October 25, 2006
APPALACHIAN
MOUNTAINS,
CARBON DIOXIDE CAUSED LONG-AGO GLOBAL COOLING
The
rise of the Appalachian Mountains may have caused a major ice age
approximately 450 million years
ago, an Ohio
State University
study has found.
The
weathering of the mountains
pulled carbon dioxide (CO2) from the atmosphere, causing the opposite
of a
greenhouse effect -- an "icehouse" effect.
Scientists have suspected
that our current ice age, which began 40 million
years ago, was caused by the rise of the Himalayas. This
new study
links a much earlier major ice age --one that occurred during the Ordovician period
-- to the
uplift of the early Appalachians.
It also reinforces the
notion that CO2 levels in the atmosphere are a major
driver of Earth's climate.
Seth Young, a doctoral
student in earth science at Ohio State, reported
the new study October 25 at the Geological Society of America Meeting
in Philadelphia.
Because we are currently
living in an ice age -- or, more precisely, in a
slightly warmer interglacial period within an ice age -- CO2 levels
worldwide
would ordinarily be low; but scientists believe that humans have raised
CO2
levels by burning fossil fuels.
Matthew Saltzman,
professor of geological sciences and Young's advisor,
looks for evidence of ancient climate change to help scientists gain
perspective on the climate change of today. He believes the geologic
record can
help solve current debates.
One such debate is whether
atmospheric carbon dioxide truly drives Earth's
climate. The planet has shifted between greenhouse conditions and
icehouse
conditions throughout its history, and research from Saltzman's team
strongly
suggests that carbon dioxide levels are a key cause.
"In this study, we're
seeing remarkable evidence that suggests
atmospheric CO2 levels were in fact dropping at the same time that the
planet
was getting colder. So this significantly reinforces the idea that CO2
is a
major driver of climate," Saltzman said.
This study builds on work
the same team published in 2005, when they used
quartz sandstone deposits in Nevada
and two
sites in Europe to
determine when the
Ordovician ice age began -- approximately 450 million years ago.
They've now analyzed the
same set of rock samples in a different way,
comparing the ratio of two isotopes of the element strontium,
strontium-87 and
strontium-86.
They found that,
immediately prior to the time that the Ordovician ice age
began, the strontium ratio dropped dramatically. The likely cause: a
vast
amount of volcanic rock was being eroded away, and the resulting
sediment was
being deposited in the world oceans.
"We observed a major shift
in the geochemical record, which tells us
something must have changed in the oceans," Young said.
The timing of the
strontium ratio decline matches the rise of the Appalachian
Mountains. The crustal plate underneath what
is now the Atlantic Ocean pushed against the eastern side of North America, lifting ancient
volcanic rock up from the seafloor and
onto the continent.
This kind of silicate rock
weathers quickly, Young explained. It reacts with
CO2 and water, and the rock disintegrates. Carbon from the CO2 is
trapped in
the resulting sediment.
The chemical reaction that
weathered away part of the Appalachians
would have consumed large amounts of CO2 from the atmosphere
–- right around
the time that the Ordovician ice age began.
The Ordovician period
started out warm, with high sea levels worldwide. It ended
cold, with low sea levels as glaciers covered the poles and portions of
the
continents. According to the Ohio State
study, most of the
Appalachian weathering took place over 7 or 8 million years -- a very
short
time, by geological standards -- as the climate moved from one extreme
to the
next.
The crossover between
greenhouse and icehouse conditions set the stage for
mass extinctions around the planet at the end of the Ordovician.
"We are seeing a mechanism
that changed a greenhouse state to an icehouse
state, and it's linked to the weathering of these unique volcanic
rocks,"
Young said.
This kind of rock is often
called "island arc" rock, because it
forms curved chains of volcanic islands such as Indonesia
and Japan.
"Those rocks are around
today, where you have ocean crust being
subducted under a crustal plate," Young explained. "What's unusual
about the Ordovician period is that those island arcs were being
uplifted onto
a continent. The ones in the Pacific
Ocean now
are mostly underwater, so they're not weathering away like the
Appalachian rock
did."
The rise and subsequent
weathering of the Himalayas
may have caused our current ice age, the one that began 40 million
years ago.
"In the Himalayas, the process would
have been the same -- silicate rocks are exposed to the atmosphere,
weathering
sucks CO2 out of the atmosphere and chills the planet," Saltzman said.
"This may be the only
effective way to bring CO2 levels down to a
threshold that's cool enough for ice to start building up."
Coauthors on the study
included Kenneth Foland, a professor, and Jeff
Linder, a research associate, both in earth sciences at Ohio State. The
National Science Foundation funded this research.
##
Contact:
Matthew
Saltzman
Ohio State University
614-292-0481
Saltzman.11@osu.edu
This text derived from:
http://researchnews.osu.edu/archive/wethring.htm
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