Louisiana Is Sinking, But Why?

July 26, 2006 — While there is no doubt southern Louisiana is sinking, experts disagree on the extent, speed and primary cause of its slow drop into the sea.

Two recent scientific studies point to two very different causes and rates of subsidence. Scientists fear the apparent contradiction might send a confusing message to policy makers, who are under pressure to make decisions now about the future of New Orleans and other coastal areas.

A study in the August issue of the journal Geology blames the gradual compaction of river sediments, using radiocarbon-dated peat sediments deposited as far back as 8,000 years ago as evidence.

Because the peat was left behind by sea-level marshes, the sediments show past changes in sea level over time.

"We sample (the peat) and measure the elevation using GPS," said Torbjörn Törnqvist of Tulane University.

Törnqvist and his colleagues determined past sea levels and dated the peat layers to better understand coastal areas in Florida and other parts of the Caribbean.

"Our conclusion is, of course, that the land surface is subsiding," said Törnqvist.

Törnqvist detected, on average, a loss of a tenth of a millimeter per year from the compaction of sediments.

That comes to just a tenth of a meter drop per 1,000 years. That rate of subsidence is in stark contrast to the rate — up to 170 times greater — reported in another recent study limited to the past 50 years.

"The problem with using the peat layers is that they are time-averaged," said geologist Roy Dokka of Louisiana State University. In other words, the peat can’t show changes over shorter periods, like 50 years.

Yet it’s the last 50 years and the next 50 that are the most important for policy makers, Dokka said.

Dokka led a team that recently completed a rigorous survey of data from same region and found evidence of almost 17 millimeters per year of subsidence in some places.

These are changes affecting the region now, he stressed. Dokka and his team published their results in the April issue of Geology.

"(Törnqvist’s) data doesn’t even relate to what’s happening right now," said Dokka.

Dokka has identified what looks like a gigantic landslide feature giving way in metropolitan New Orleans and slumping far out in the Gulf of Mexico. That’s something entirely beyond human control — unlike soil compaction, which can be exacerbated by oil, gas and water extraction, and levee building.

For his part, Törnqvist agreed that Dokka could be seeing something happening over a much shorter time scale. That’s one reason he and his team are now working hard in the New Orleans area to see what the peat might say about subsidence there.

"Maybe we’ll find something surprising," said Törnqvist. "You have to have an open mind. It’s an incredibly complicated issue and there is so much at stake."

Both Törnqvist and Dokka agree that the controversy is unfortunate and not particularly helpful to policy makers.

On the other hand, it’s hardly unusual in science for different groups using very different methods to arrive at conflicting results, said Törnqvist. What it usually means is they have found different parts of a highly-nuanced predicament.

"This is something we have to resolve," Törnqvist said.