Ancient Glacial Sediments Drag Down Louisiana's Sinking Coast

The weight of these sediments is causing a large section of Earth's crust to sag at a rate of 0.04 to 0.3 inches a year, the study determined.

The sediments pose a particular challenge for New Orleans, causing it to sink irreversibly at a rate of about 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.

"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," warned co-author Ron Blom of NASA's Jet Propulsion Laboratory.

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 in Pasadena, California, hypothesized that Earth's very slow gravitational flow response to the weight of the sediments, combined with a 427 foot rise in sea level that took place thousands of years ago, are contributing to the sinking of a 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.

Looking north up the Atchafalaya River Delta where the Louisiana coast is slowly sinking (Photo courtesy U.S. Army Corps of Engineers)

"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, director of the Louisiana State Center for Geoinformatics.

"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," he said.

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 is not 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."

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."

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.

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