University of Illinois
Rising sea levels
and subsiding land are combining to gobble up more than two acres of
the Mississippi Delta every hour. There is little dispute that this
ecologically and economically important region -- and more significantly,
the future of metropolitan New Orleans -- is under threat. What isn't
clear, however, is how fast different parts of the delta are subsiding.
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 Torbjorn Tornqvist, 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 Tornqvist'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 Tornqvist.
For two summers, Tornqvist
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.
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.
shows that the natural pattern of sea level rise has been very smooth,"
said Tornqvist. 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.
Tornqvist'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."
Tornqvist's Eos article
was co-authored by UIC doctoral student Juan Gonzalez, 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.
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 Tornqvist. "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 Tornqvist
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."