Glacial climate regime may have enhanced Southwest Monsoon
By Summit Voice
SUMMIT COUNTY — Geologists and paleoclimatologists have long known that the great basins of the intermountain West were once filled with water, forming vast inland seas. At the peak of the last glacial cooling period, about 14,000 to 20,000 years ago as much as a quarter of Nevada and Utah were covered with water.
What’s not exactly clear is where and when the water came from, but a new study led by a Texas A&M researcher offers additional clues, suggesting that the additional moisture came from a powerful, enhanced summer monsoon.
First, the scientists set out to test the prevailing hypothesis that the water resulted from a shift in the winter storm track that now generally carries storm to the north of the Great Basin, into northern California, Washington and Oregon.
“Large ice caps profoundly altered where storms went during glacial periods. Before this study, it was assumed that Pacific winter storms that now track into Washington and Canada were pushed south into central and southern California,” said Texas A&M oceanographer Mitch Lyle. “”However, by comparing timing between wet intervals on the coast, where these storms would first strike, with growth of the inland lakes, we found that they didn’t match,” Lyle said.
The researchers were able to time wet periods along the California coast from pollen buried in marine sediments from cores archived by scientists at the Integrated Ocean Drilling Program at Texas A&M. They evaluated lake level studies from southeast Oregon, Nevada, Utah, eastern California, New Mexico, and west Texas to find when lakes filled in different parts of the west, identifying a new water-cycle connection between the southwest U.S. and the tropics.
Only southern California coastal wet intervals matched with the progression of high lakes inland, pointing to the development of a tropical connection, where storms cycled into the region from the tropical Pacific, west of southern Mexico.
“We think that the extra precipitation may have come in summer, enhancing the now weak summer monsoon in the desert southwest. But we need more information about what season the storms arrived to strengthen this speculation,” Lyle said.
“Many teams of scientists have been working on this problem since the 1950s, when radiocarbon dating first allowed ages to be put on old shorelines,” Lyle added. “The data we synthesized covers a wide latitude so that we could determine how the glacial wet intervals operated.”
Not only is the development of the glacial lakes important from a paleoclimate perspective, but it is likely that the lakes were important to the migration of people into North America, Lyle believes. Many of the archaeological sites where early Indians settled when they first came into the U.S. are rock shelters at the edges of these ancient lakes. The lakes were a major source of fish, and a gathering place for deer and wildfowl at that time.
“What we need to do now is look at all of this on a finer scale,” Lyle points out. “We need to understand better the processes that directed the storms thousands of years ago, and to predict better what changes might occur in the future.”
Lyle was joined in the study by colleagues from Columbia University, University of California-Santa Cruz, Stanford University, Hokkaido University of Japan, Brown University and the U.S. Geological Survey. Their work, funded by the National Science Foundation, is published in the current issue of Science magazine.