Changing Continental Runoff Patterns Could Change Ocean Circulation

October 29, 1996

Denver, Colo. -- Human alteration of major waterways may cause more problems than drought downstream, according to a Penn State geologist who is studying circulation models of the ancient oceans.

"We have already seen some of the consequences of changing surface waterways," says Karen L. Bice, graduate student in geosciences. "The Colorado River barely flows to the ocean and the Aral Sea is drying up. We do have the capacity to alter the amounts of freshwater that run off into the oceans and that could alter ocean circulation patterns."

Bice, Dr. Eric J. Barron, professor of geosciences and director of Penn State's Earth System Science Center, and William H. Peterson, Earth System Science Center, are using a supercomputer-based General Circulation Model to look at the effects of continental runoff on ocean circulation patterns in the early Eocene about 55 million years ago when the Earth was substantially warmer than it is now.

The geography of the Eocene was different. The Tibetan plateau had not formed, India was the fastest moving continent and there was a large seaway in the subtropical middle latitudes called the Tethys Sea.

"What we found is that during the early Eocene, the oceans' deep water formation patterns may have been sensitive to variations in continental runoff," Bice told attendees today (Oct. 29) at the annual meeting of the Geological Society of America in Denver.

Researchers using the general circulation models have not previously considered continental runoff.

The reason that continental runoff can change ocean circulation patterns is that the fresh water running out of continental rivers decreases the salinity of the oceans' surface. Natural evaporation in the Tethys sea tended to increase the salinity of the surface water. Circulation in the oceans can be caused by colder water sinking below warmer water or saltier, denser water sinking below less salty water. If runoff stops, new salty areas could form to produce new deep water formation sites.

In today's oceans, deep water forms near the poles with cold water sinking and then traveling toward the equator, while warmer water travels poleward near the surface. Temperature, not salinity, seems to control ocean circulation today.

One reason why the researchers are looking at the early Eocene is that it was the most recent warm period in the Earth's history. Researchers believe that we are heading toward a warmer period, possibly much like the early Eocene.

"From various indicators in the geological record, we know that during the Eocene, the bottom water in the ocean was about 18 degrees Fahrenheit warmer than it is today," says Bice. "And with this warmer water globally, we believe that there were deep water formation sites in the subtropics."

While the general circulation model used by Bice does not yet mirror history, it does show that changes in surface salinity will affect circulation patterns. According to Bice, it is not just the changes in warm surface currents that could affect the environment, but changes in deep water formation may also alter carbon dioxide concentrations.

"Cold water near the poles that is the source of current deep water formation dissolves large amounts of carbon dioxide from the atmosphere," says Bice. "It takes one to two thousand years for this carbon dioxide to be returned to the atmosphere through upwelling elsewhere."

As in carbonated soft drinks, carbon dioxide dissolves better in cold liquids than in warm liquids. If deep water is being formed in the warm subtropics, it will carry less carbon dioxide from the atmosphere and sequester less of this greenhouse gas in the oceans.

"It appears that salinity in the Tethys Sea was the mechanism that switched deep water formation from the cold poles to the warmer subtropics," says Bice. "In a future, warmer Earth, human alterations to fresh water runoff and surface salinity might have the same effect."

Ms. Bice may be reached at 814-865-1073 or
Dr. Barron may be reached at 814-865-1619.

Penn State

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