(Â鶹´«Ã½ release Web site: http://www.unl.edu/pr/releases.html)
(Science news release Web site: http://www.unl.edu/pr/science.html)
Lincoln (Neb.) - May 11, 2000 - A funny thing happened on the way to Tammy Rittenour's master's thesis.
Rittenour, a first-year doctoral candidate in geosciences at the University of Nebraska-Lincoln, was working on her master's degree at the University of Massachusetts at Amherst in the fall of 1997 when she and her colleagues, UMass geosciences professor Julie Brigham-Grette and postdoctoral researcher Michael E. Mann (now an assistant professor of environmental sciences at the University of Virginia) made an unanticipated discovery.
In a project funded by the National Science Foundation, National Geographic Society and the University of Massachusetts, Rittenour and her colleagues were looking for evidence of the way New England's glacial Lake Hitchcock drained at the end of the last Ice Age. They found that evidence, but they also found something that surprised them - evidence of El Nino effects in New England's climate 17,500 to 13,500 years ago during the late Pleistocene era.
"We did not expect that," she said. "There's the idea, based on recent archaeological evidence from South America, that El Ninos are warm-weather conditions and there was no evidence prior to this research that they occurred during glacial time periods. For them to be seen in North America, let alone right at the toe of the last ice sheet, is unexpected."
The discovery was announced in the May 12 issue of Science, the global weekly journal of research, in an article titled "El Nino-like Climate Teleconnections in New England During the Late Pleistocene."
Scientists define El Nino as a disruption of the ocean-atmosphere system in the tropical Pacific, having important consequences for weather around the globe. A weakening of the trade winds allows unusually warm currents in the western Pacific to flow eastward across the equatorial Pacific to the western coast of South America. This exceptionally large area of warm ocean surface waters occurs cyclically causing serious changes in global weather patterns.
Lake Hitchcock covered what is now the Connecticut River valley from southern Connecticut to northern Vermont. Geologists knew it had been formed when the farthest advance of the Laurentide Ice Sheet built a terminal moraine that acted as a natural dam to the runoff when the ice sheet melted. But no one was sure how the lake had drained - gradually, in stages or catastrophically.
The UMass campus in Amherst sits on the shoreline of the ancient lake and Rittenour's team used well-drilling equipment at a site on a campus athletic field to pull two cores from the ancient lake bed, one of 105 feet and one of 25 feet. The longer core was missing the uppermost portion of the geologic record, so the second core was taken and the two cores were matched up to form one 110-foot core.
Each year of sediment consisted of two distinct layers, one light and one dark. Like modern glacial lakes and streams, Lake Hitchcock and its tributaries were filled with silt and clay and their waters probably had a turquoise-blue color. During the summer, when the lake's waters were stirred by wind and glacial runoff, only the heavier, light-colored silt settled to the bottom. When the lake was ice-covered in winter and its waters were calm, the lighter-weight, dark-colored clay settled. The next year, the process repeated itself, and so on for thousands of years.
"I counted 1,389 varves, or years' worth, of sedimentation," said Rittenour, who was the lead author in the Science article. "Also, by obtaining a radio-carbon date from the sediments I could determine the time that the ice retreated from Amherst and how long glacial Lake Hitchcock existed in Massachusetts."
As she counted the varves, Rittenour noticed immediately that there was a variance in the thickness of the annual layers. She began to suspect that there might be a pattern to them and she knew if there was, it had to be climate-related.
She was also familiar with the work of Ernst Antevs. A Swedish geologist who came to North America in the early 20th century, Antevs developed what is known as the New England varve chronology by examining and measuring outcrops left by Lake Hitchcock and other now-dry glacial lakes in New York and New England. He measured layers throughout Lake Hitchcock and the other lakes and correlated them, year-for-year and lake-for-lake, developing a chronology that covered 4,000 years of sedimentation.
"The reason he could correlate between lakes was because the amount of glacial melting that was coming into all of these lakes was controlled by climate," Rittenour said. "I correlated my year-for-year thickness with his and there was a series of five varves that had a really distinctive 'M' shape, and I used those to link the whole chronology together. It was almost as if I took his data and plotted it again.
"This is pretty remarkable for a lake. Lakes usually have different sedimentation in different regions, so there was something special about these glacial lakes - something that allowed sediment to be deposited throughout glacial Lake Hitchcock and the other glacial lakes in the region with the same variability each year."
Rittenour and her colleagues eliminated the earliest, thickest varves from their study because they were formed when the ice sheet was almost on top of the core site and their thickness only reflected the local effect of the melting ice sheet, not climate. They then performed a statistical analysis to look for climate signals and a spectral analysis to look for periodicities.
What they found was a grouping of three peaks of varve thickness - one between 2.5 and 2.8 years, one between 3.3 and 3.5 years and another from four to five years.
"This is right in the modern El Nino climate frequency," Rittenour said. "The fact that there are three peaks is related to the way El Nino operates and this gives us a better idea that this is El Nino. We expected some climate signals were recorded in the sediments because of how the thickness of the lake sediments change from year to year. But we expected the climate to be influenced by something called the North Atlantic Oscillation, which is closer to New England, and not a climate signal from the tropical Pacific."
Rittenour said evidence of an Ice Age El Nino provides an important new clue in learning how El Ninos operate and what to expect from them in the future.
"We know so little about the El Nino system and how it operates under different climate conditions," she said. "We need to study it over a long period of geologic history, through a lot of different climate changes. If we know how it responded in the past, we can forecast how it will respond in the future.
"Prior to this, people really didn't think El Ninos existed during glacial periods. By publishing this paper, we're showing that they do occur during glacial time periods and that may suggest that El Ninos are a perpetual, continual aspect of the climate system. They've been around for at least 17,000 years, and there's new evidence published this year that they existed 120,000 years ago during the last interglacial period. We shouldn't just expect them to go away."
Lake Hitchcock, by the way, drained in stages and that finding was the main subject of Rittenour's thesis, "The Drainage of Glacial Lake Hitchcock, Northeastern United States." El Nino provided only a chapter in that document, but earned publication in the most prestigious American scientific journal.
For her new research in the Mississippi River valley, Rittenour recently won the 2000 J. Hoover Mackin Award given by the Geological Society of America's Quaternary Division. This is the nation's highest award given to a geology Ph.D. candidate. Brigham-Grette won the same award 19 years ago.
A native of Appleton, Minn., Rittenour earned her bachelor's degree in geology and biology at the University of Minnesota at Morris (1996) and completed her master's in geosciences last year at Massachusetts.
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