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Pinpointing Louisiana's natural sinking

Louisiana’s Gulf Coast is sinking — that is not in question. How rapidly it is sinking, however, is highly controversial. Although scientists have yet to agree on a rate, new research is shedding light on exactly what could be causing the sinking. For residents of the hurricane-prone state, such information cannot come soon enough.

Sinking, or “subsidence,” along the Gulf Coast is a danger to the region’s wetlands in particular, which provide protection during hurricanes and other storms. The net subsidence in the Gulf region has two components — one that comprises the surface itself sinking, primarily as a result of sediment compaction that is exacerbated by human activities, and one that comprises changes deeper in Earth’s crust. The latter, called natural tectonic subsidence, is sinking due to faulting and the sinking of sediments that are more than 10,000 years old.

To study these ancient sediments’ contribution to modern-day subsidence, Torbjörn Törnqvist, a geologist at Tulane University in New Orleans, and colleagues, turned to natural peat. Peat levels deep belowground can show relative sea-level changes over time — and thus sinking rates — because peat forms in coastal marshes when the land drops below sea level, Törnqvist and colleagues reported in the August Geology.

Because deposits older than 10,000 years are difficult to date and do not contain sensitive indicators of former sea-level positions, Törnqvist says, he and colleagues used more modern peat levels from three different study areas along the delta — to reconstruct the sea levels from present day to about 8,000 years ago. The younger overlying sediments have recorded the sinking of the older “basement” deposits, he says, thus indicating how rapidly the older sediments underlying the Mississippi Delta are sinking.

Surprisingly, the team found that these ancient sediments are “much more stable than we thought,” Törnqvist says. To be sure, they compared the peat-measured sea-level changes to rates recorded in more tectonically stable areas such as Florida and the Bahamas, and took GPS and optical survey measurements of the land, as well as radiocarbon dated the peat. All evidence points to tectonic subsidence being approximately a fraction of a millimeter per year — or less than a few meters in 8,000 years.

Parts of the Gulf Coast, including New Orleans and especially the “birdsfoot” area of the Mississippi Delta, are known to have higher rates of surface subsidence, however, Törnqvist says. For example, a June 1 Nature paper by Tim Dixon and Shimon Wdowinski at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science in Florida, and colleagues described current surface subsidence rates in New Orleans of a few millimeters a year to upward of 20 millimeters per year, using synthetic aperture radar measurements.

Törnqvist’s team’s research suggests that tectonic subsidence is an unlikely cause of those higher sinking rates, leaving recent sediment compaction as the likely primary cause, Wdowinski says. The pumping out of water, oil and other fluids from the region’s ground may also be causing these high subsidence rates seen in New Orleans, says Jeff Williams, of the U.S. Geological Survey in Woods Hole, Mass.

The debate is not over, however, Williams says. Other recent studies of the delta region have found that tectonic movement is responsible for up to 17 millimeters per year of subsidence in the region — as much as 100 times Törnqvist’s numbers. Williams says the geologic record just doesn’t show that, but the exact rates are still unknown.

While none of these recent studies is “the final answer,” Wdowinski says, they all emphasize the need for more cooperative research and further observations. “We need to compare the geologic record, what happened in the last 10,000 years, to what has happened in the last 10 years, and what will happen in the next 10 years,” he says. Only by tying these studies together, Williams adds, can policy-makers take steps forward to promote sustainable development on the Gulf Coast.

Megan Sever

"When Levees Fail," Geotimes, August 2006
"Confusion over sinking coasts in Gulf," Geotimes, August 2005

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