NASA Finds Glacial Sediments Adding to Louisiana Coast's Sinking
01.31.08
PASADENA, Calif. - A study by NASA and Louisiana State University
scientists finds that sediments deposited into the Mississippi River
Delta thousands of years ago when North America's glaciers retreated are
contributing to the ongoing sinking of Louisiana's coastline. The weight
of these sediments is causing a large section of Earth's crust to sag
at a rate of 0.1 to 0.8 centimeters (0.04 to 0.3 inches) a year.
The sediments pose a particular challenge for New Orleans, causing it to
sink irreversibly at a rate of about 0.4 centimeters (0.17 inches) a year,
according to data from a network of global positioning system stations and
a model of sediment data collected from the northern Gulf of Mexico and
the Mississippi Delta.
Hurricanes Katrina and Rita in 2005 focused national attention on the Gulf
coast's vulnerability to hurricanes due to loss of wetlands and sea level rise.
These new findings add another factor for scientists, government agencies and
the public to consider when assessing the vulnerability of the region to
hurricanes and large storms.
A science team led by Erik Ivins of NASA's Jet Propulsion Laboratory, Pasadena,
Calif., hypothesized that Earth's very slow gravitational flow response to the
weight of the sediments, combined with a 130-meter (427-foot) rise in sea level
that took place thousands of years ago, are contributing to the sinking of a
199,000-square kilometer (77,000-square-mile) section of coastal Louisiana.
To test their theory, the team developed a physical model of sinking caused by
both the weight of the sea level rise and the flow of glacial sediments into the
Gulf of Mexico following the retreat of the great ice sheet that covered much of
North America some 22,000 years ago. The model spanned the past 750,000 years.
Results were compared with actual global positioning system measurements and
other geophysical data for southern Louisiana and the Gulf, collected from
multiple sources over the past 60 years.
The scientists found the model results were in good agreement with the actual
geophysical data, predicting sinking of between 0.1 centimeters (0.04 inches)
and 0.8 centimeters (0.3 inches) a year. The highest sinking rates were observed
where coastal land loss is greatest, near the center of the Mississippi and
Atchafalaya River Delta complexes.
"Our study shows that the weight of these sediments on Earth's crust can explain
between 0.1 and 0.8 centimeters (0.04 and 0.3 inches) of sinking per year,"
said study co-author Roy Dokka of Louisiana State University, Baton Rouge.
"These sediments contribute a part of the region's sinking that's inevitable
and must be considered when predicting rates of sinking and future sea level
change in coastal Louisiana."
The scientists say when these results are combined with sinking totaling about
0.3 centimeters (0.12 inches) per year caused by other factors such as compaction
and oxidation of sediments, pumping of oil and water by humans, faulting and sea
level rise, the overall outlook isn't bright. "Louisiana is slowly losing its
battle with the Gulf of Mexico," said Ivins. "Our model predicts this rate of
sinking will continue for hundreds of years. Continued sinking, along with the
sediment starvation of the coast caused by construction of flood control levees
along the Mississippi River, will ultimately lead to the drowning of the coast."
Co-author Ron Blom of JPL adds that New Orleans is particularly vulnerable.
"When the effect of this sinking near New Orleans is combined with a potential
0.9 centimeter (0.35 inch) annual sea level rise that could result should ice
sheet melting accelerate as projected by many climate models, it is possible New
Orleans could see a relative sea level rise of roughly one meter (3.3 feet) in
the next 90 years," Blom said.
The good news, the authors say, is that, with refinement, their model may help
the region prepare better for future large storms and the gradual inundation of
the coast. "Our model gives civil engineers and disaster preparedness managers
very precise predictions of how the landscape is changing so that they can better
mitigate the effects of this sinking," said Ivins. "Understanding all of the
processes affecting the coast is essential for engineering effective solutions."
Louisiana's coast is far from the centers of North America's former ice sheets,
which were once as large as Antarctica, but successive periods of glacier
formation and retreat have affected the region through sea level rises and changes
in the flow of glacial sediments into the Gulf of Mexico. This process has been
ongoing in cycles caused by regular variations in Earth's orbit. When the glaciers
retreated, the resulting flooding transported large volumes of sediments down today's
major river systems -- three to four times more than the Mississippi River currently transports.
The authors say the relative fluidity of Earth's upper mantle beneath the Gulf
coast is the primary factor that determines how Earth's crust responds to deposited
sediments. Earth's crust and uppermost mantle is weaker beneath the Gulf of Mexico
than it is beneath eastern Canada.
Results of the study are published in Geophysical Research Letters. The research was
funded by NASA, the National Science Foundation and the Louisiana Board of Regents.
For more information, visit:
http://www.nasa.gov .
Additional media contact for this story: Ernie Ballard, Louisiana State University,
225-578-5685, eballa1@lsu.edu .
JPL is managed for NASA by the California Institute of Technology in Pasadena.
Media contact: Alan Buis 818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif.
Alan.buis@jpl.nasa.gov
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