Sea Level History Locked In Mississippi Mud

Previous studies of delta geological records from the past 8,000 years give a conflicting history. Some researchers find a pattern of a smooth, gradual rise in relative sea levels. Others indicate a step-like pattern suggesting long periods of stability, followed by dramatic elevations in sea level. But a new study led by Torbjörn Törnqvist, an assistant professor of earth and environmental sciences at the University of Illinois at Chicago, lends credence to the gradual rise hypothesis.

The evidence for Törnqvist's findings lies deep beneath the Mississippi Delta mud, or more precisely, in the peat that's packed beneath the muddy swamps.

"There's a lot of confusion about how sea level has changed in the delta over the last 7,000-8,000 years. Land loss is a potential problem in many coastal areas, but even more so in the Mississippi Delta because it is subsiding very rapidly," said Törnqvist.

For two summers, Törnqvist and his researchers took peat samples from 30 sites stretching between Baton Rouge and New Orleans, digging as deep as 12 meters to get the evidence they were looking for.

"Peat is organic matter. Plant material accumulates and is preserved in this wet environment. Peat contains remnants of plants that we can sieve out, like seeds or little pieces of wood. Some of these species are indicators of brackish water. If we find that, we know we must have been very close to sea level," he said.

Törnqvist's samples from different elevations in the eastern delta covered an area of about 20 square kilometers. The material collected was then carbon 14 dated using state-of-the-art accelerator mass spectrometry. Findings were plotted for sample age against depth below mean sea level.

"Our evidence shows that the natural pattern of sea level rise has been very smooth," said Törnqvist. The new data contradicts studies suggesting alternating long periods of stable sea levels, punctuated by short periods of extreme rises. The findings were reported in the Nov. 12 issue of Eos, the weekly newsletter of the American Geophysical Union.

Törnqvist's use of peat samples to read for geological clues is a first for the Mississippi delta region. He's excited about the technique.

"This is a perfect technique to see if there are variations in subsidence rates within the Mississippi Delta, or the entire coastal Louisiana," he said. "If that's the case, this will help enormously in making predictions of which areas are the most sensitive to wetland loss and coastal erosion. It may not be the same throughout all of coastal Louisiana. There's probably regional variation."

Törnqvist's Eos article was co-authored by UIC doctoral student Juan González, Lee Newsom of Pennsylvania State University, and Klaas van der Borg and Arie de Jong of Utrecht University in the Netherlands. The research was supported by a grant from the National Science Foundation.

Törnqvist's research team is presently collecting samples from the western Mississippi delta with the aim of finding peat that may yield relative sea level information about the past 3,000 years. Samples collected from the eastern delta region show the period from 3,000 to 8,000 years before the present.

"It's not that such samples don't exist in the eastern delta. There's probably loads of sites," said Törnqvist. "It's a logistical problem. Most of the areas we work in are surrounded by swamps," he explains. "They're just impossible to get into." The western delta study sites are more accessible.

Not that Törnqvist minds a bit of mud on his work boots. "Swamps are beautiful. I love to get into them," he said, "… when I'm in a boat."