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Sea-Level Rise Gives Clue to Big Chill

John Roach
for National Geographic News
December 6, 2004
 
Many scientists believe that about 8,200 years ago a glacial lake more than twice the size of the Caspian Sea poured into the North Atlantic and triggered a precipitous drop in temperature just as Earth was exiting the last ice age.

Climate modelers are keenly interested in accurately re-creating the conditions that drove this big chill, known as the 8.2 ka event, so they can predict if and when a similar scenario might occur in the future.

"One thing they would like to get a better constrain on is exactly how much freshwater drained into the North Atlantic," said Torbjörn Törnqvist, an assistant professor of earth and environmental sciences at the University of Illinois at Chicago.



Reporting in the December 11 online issue of the science journal Geophysical Research Letters, Törnqvist and his colleagues present evidence that the influx of water rose sea level no more than 4 feet (1.2 meters).

By converting the sea-level rise to a measurable amount of freshwater, the researchers will get an idea by how much water was involved in the 8.2 ka event. This finding indicates the amount of water was less than several previous studies suggested.

"What meltwater does is it totally messes up ocean circulation," Törnqvist said. "Freshwater suddenly released into the North Atlantic basically reduces the salinity of the ocean water [and] salinity plays an important role in driving ocean circulation."

The North Atlantic climate is driven by a giant ocean conveyor belt known as the thermohaline circulation. It carries warm, salty surface water from southern latitudes to the north where it cools, which makes it denser. The dense, cool water sinks to the deep ocean and is exported south.

The addition of freshwater to the system in the North Atlantic lowers the density of the water, counteracting the cooling process that increases density. A weakening of the circulation can lead to a drastic cooling of northern regions because less warm water is brought north, according to Törnqvist.

Scientists believe a sudden influx of freshwater from North America's ancient Lake Agassiz to the North Atlantic weakened the thermohaline circulation 8,200 years ago and triggered a cooling of the region by up to 10 degrees Fahrenheit (about 6 degrees Celsius).

Hans Renssen, a climate modeler at the Free University in Amsterdam, The Netherlands, said "a crucial thing that we don't know very well is how sensitive the thermohaline circulation is to freshwater perturbations, as different climate models give different responses."

He and his colleagues use the 8.2 ka event as a test case of the circulation's sensitivity. "For such a test case, it is important to constrain the amount of freshwater as precisely as possible."

Törnqvist's research was supported in part by a grant from the National Geographic Society's Committee for Research and Exploration.

Sea-Level Rise

Previous studies have calculated the amount of freshwater dumped into the North Atlantic based on the pre-drain size of the ancient Lake Agassiz, which was located south of the Laurentide Ice Sheet in North America. The lake, several times the size of today's lake Superior, once covered what are now parts of Manitoba, Ontario, and Saskatchewan in Canada, and North Dakota and Minnesota in the United States.

These calculations, however, do not take into account the melted ice which scientists believe also flowed into the North Atlantic 8,200 years ago, Törnqvist said.

"Our study is different than all the other ones in that it looks at a different part of the story—it looks strictly at the sea-level signal," he said. "From that point of view it's the first record that can provide an estimate of the maximum amount of meltwater that ended up in the North Atlantic Ocean during that event."

Törnqvist and his colleagues were able to get a read on the sea-level rise associated with the 8.2 ka event by studying cores of sediment taken from the Mississippi River delta in Louisiana.

The cores contain layers of peat accumulated from grasses that grow in saltwater marshes that form in the region between mean sea level and mean high tide.

With the aid of global positioning satellites, the researchers are able to measure the height of the peat layers, which correspond to past sea levels, and they are also able to determine the age of the layers with radiocarbon dating.

The team retrieved two cores from the Mississippi delta, one with a peat layer dated to just before and one with a peat layer dated to just after the 8.2 ka event. "So that means the sea-level rise that was a result of this event must have happened between those two samples," Törnqvist said.

The elevation difference between those two layers is about 4 feet (1.2 meters), indicating a maximum sea-level rise from the 8.2 ka event of about 4 feet.

Additionally, the peat layer in the deeper of the two cores is covered by a muddy deposit that contains seashells characteristic of open water, indicating the salt marsh was rapidly drowned and turned into a lagoon as a result of an abrupt sea-level rise, Törnqvist said.

Refining Models

To further constrain the amount of sea-level rise associated with the 8.2 ka event, Törnqvist hopes more information on the rise will be collected from other locations around the world.

The task may prove difficult, he said. The abundant peat deposits in the Mississippi River delta may not always be present in other deltas. Furthermore, the Gulf of Mexico, where the Mississippi delta is located, has minimal tides, which make it easier than areas with big tides to discern past sea levels.

"If you take those together, we have a fairly unique type of setting," Törnqvist said. "There will be more than this one, but probably not that many of them."

Renssen, the climate modeler, said the study results will help scientists in the climate research field to better determine the thermohaline circulation's sensitivity to influxes of freshwater.

"This is highly relevant, as most climate models predict a weakening of the thermohaline circulation within the 21st century under [the] influence of human-induced global warming," he said.

Today, the threat of a freshwater influx comes from Greenland, where ice sheets are expected to experience rapid melting in response to warming temperatures.

Torbjörn Törnqvist's research was funded in part by the National Geographic Society's Committee for Research and Exploration.

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