Sea-level clue to climate change

It sounds like the plot for a disaster film: rising temperatures melt
polar ice, causing a flood of freshwater to rapidly enter the salty
North Atlantic. As the fresh and salty water mixes, density changes,
altering the Gulf Stream ocean currents that moderate the North
Atlantic climate. In just a few years, average temperatures plummet,
ushering in a deep freeze that lasts a century or more before fresh and
salty water is back in balance, ocean currents adjust and temperatures
return to normal.

It sounds like the plot for a disaster filmScience fiction? Not to a growing number of geologists and
climatologists who've studied facts showing a precipitous 6-degree
Centigrade drop in Greenland's average temperature some 8,200 years ago
as the Earth was exiting the last ice age and polar ice sheets were
melting in retreat.

Many scientists believe a catastrophic flood of
freshwater entering the North Atlantic clipped the flow of the Gulf
Stream current. The suspected source of the floodwater was a glacial
reservoir called Lake Agassiz. A popular theory suggests that Lake
Agassiz's huge volume of freshwater -- more than twice that of today's
Caspian Sea -- may have breached an ice dam or tunneled under Hudson
Bay's ice sheets, then gushed into the North Atlantic, perhaps in a
period lasting only months. Scientists call it the largest mega-flood
of the last 100,000 years.

Torbj?rn T?rnqvist, an assistant professor of earth and
environmental sciences at the University of Illinois at Chicago,
reports in the Dec. 11 online issue of Geophysical Research Letters
about a new set of 8,200-year-old core samples that indicate an abrupt
sea-level rise. The finding adds credence to the theory that a
catastrophic freshwater flood into the North Atlantic triggered the
great chill around that date.

"Few would argue it's the most dramatic climate change in the
last 10,000 years," said Tornqvist. "We're now able to show the first
sea-level record that corresponds to that event."

The discovery came by coincidence. Tornqvist and his graduate
students are conducting ongoing studies into sea-level changes along
the Gulf of Mexico, using core samples of peat retrieved from the
swamps and marshes in the Mississippi River delta in Louisiana. Samples
gathered in 2003 from a saltwater marsh in an area known as Bayou Sale
held the clue.

As sea levels rise, peat deposits are formed. These deposits
can be accurately radiocarbon dated. They also contain organic debris
that can suggest whether water was salty or fresh at the time of
deposition, based on plant salt tolerance. Analyzing his samples,
Tornqvist discovered them to be around 8,200 years old and found
evidence that a saltwater marsh was abruptly flooded and turned into a
lagoon, indicating a sudden sea-level rise at the time.

Tornqvist said that if comparable sea-level readings can be
taken from other coastal areas on Earth, it could add evidence that a
catastrophic freshwater flood into the North Atlantic 8,200 years ago
did cause ocean current disruption and the consequent abrupt climate
change.

"We happened to sample along the Gulf of Mexico, but there's no
reason you can't study this in, say, China or New Zealand as well,"
said T?rnqvist.

"The oceans are all connected. If we can measure the
amount of sea level rise that occurred 8,200 years ago, we will be able
to convert that back into a measurable amount of freshwater. With our
first data, we now know the amount of sea-level rise was probably less
than 1.2 meters - which is less than several previously published
estimates. In the future, we hope to come up with a more accurate
number. Climatologists urgently need this type of information to run
their climate models in order to understand the conditions that can
produce such an abrupt climate change."

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