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A New Look at Subsidence Issues

July 24, 2006

Mike Strecker

While erosion and wetland loss are huge problems along Louisiana's coast, the basement 30 to 50 feet beneath much of the Mississippi Delta has been highly stable for the past 8,000 years with negligible subsidence rates, according to an article in the August issue of Geology, a journal published by the Geological Society of America.


Near the Mississippi River levee in the Lutcher-Gramercy area, Tulane scientist Torbjörn Törnqvist uses Global Positioning System technology in his study of coastal Louisiana subsidence. (Photo courtesy of Juan Gonzalez)

Studying sediment core samples, Torbjörn Törnqvist, an associate professor of earth and environmental sciences at Tulane University and lead author of the article, made detailed reconstructions of what sea levels were in different sections of the delta over the past 8,000 years.

Törnqvist's study covered areas near the Louisiana cities of New Iberia, Franklin, and Lutcher-Gramercy. Törnqvist and his students are currently investigating sediments in New Orleans, including an area close to one of the levee breaches along the London Avenue Canal.

"Our research could have major implications for rebuilding plans that are currently being debated," Törnqvist said. "Over the long term, comprehensive understanding of subsidence will better support rational coastal management and successful urban and land-use planning for all low-lying areas along the Gulf Coast."

Recent research has suggested that the basement of the Mississippi Delta is subsiding by about half an inch a year, but Törnqvist's study shows subsidence rates that are at least 10 times lower. A stable basement could allow rebuilding in low-lying areas thought to be sinking too rapidly to justify reconstruction.

With funding mainly from the National Science Foundation, Törnqvist and his team reconstructed the rate of sea-level rise over the past 8,000 years from three separate areas in the Mississippi Delta. Peat samples were used as sea-level indicators. According to Törnqvist, peat formation begins as soon as water levels rise above the land surface. Carbon isotope analyses verified that accumulation of the samples was directly controlled by sea-level rise.

Determining precise elevation of sampling sites was accomplished with a combination of Global Positioning System measurements and optical surveying. The team then compared its results with sea-level data spanning the same time period from areas in the Caribbean regarded as tectonically stable, including Florida and the Bahamas.

Törnqvist and his colleagues were surprised to find no difference, suggesting that large portions of the delta basement are stable. They also inferred from the data that long-term subsidence rates are more on the order of a fraction of a millimeter per year rather than 10 millimeters. Törnqvist points out that these numbers do not necessarily apply to the entire delta. He also notes that well-documented high-subsidence rates in and near the birdfoot of the delta indicate that different conditions prevail there.

"It remains to be demonstrated how rapidly the basement under metropolitan New Orleans subsides," he said. The study's imaginative approach is timely and a step in the right direction, said H. Richard Lane, program director of the National Science Foundation's sedimentary geology and paleobiology program.

"If we are to reverse the loss of Louisiana wetlands and the protection they afford New Orleans, we must think outside the box. These scientists demonstrate that innovative research can open many new possibilities for addressing environmental issues," he said.

Tulane University, New Orleans, LA 70118 504-865-5000