Sea-Level Rise Gives Clue to Big Chill

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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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