Mar. 26, 1999 ATHENS, Ga. -- The Middle Holocene Period -- from 5,000 to 7,000 years ago -- was a crucial time in what is now the Southeastern United States. The human population was increasing and pine trees began to spread north in vast numbers, displacing deciduous forests.
Until recently, some scientists suggested that the Middle Holocene period in the Southeast was marked by a warm, dry climate somewhat like that of the Upper Midwest. Recent studies by a University of Georgia geographer and his graduates students, however, have found that the climate was, in fact, warm and wet.
"There's really no good physical evidence for the warm-and-dry theory, and all the lines of evidence we have developed indicate quite the contrary," said Dr. David Leigh.
Leigh presented his information today at the Southeast Section meeting of the Geological Society of America, which was held here. Also meeting with the geographers were the southeastern sections of the National Association of Geoscience Teachers, the Paleontological Society and the Society for Sedimentary Geology. Between 600 and 700 people are attending the meetings here this week.
The idea that the Southeast was warm and dry in the Middle Holocene has been generally accepted for decades because of what Leigh calls "very good evidence" of such climate from Illinois to Texas and the Dakotas. The idea of a warm, dry Southeast was extrapolated from that data by some, though little research has been done in the Southeast on the issue.
Beginning in 1991, Leigh started to gather evidence about the climate in the Middle Holocene, using several measures to understand the weather and climate of that fertile period in the South's prehistory. There are at least three reasons for supposing a warm, wet climate here, Leigh reported today: large meandering paleochannels from the period that indicate intensified flooding, possibly from pronounced monsoonal circulation; fossil pollen data from peat bogs that show an increase in wetland plants; and a relative lack of windblown sand that would be present in certain areas if the period had been very dry.
"We're talking about a climate that was only a few degrees centigrade warmer than now," said Leigh, "but even modest changes in climate can make substantial changes in river flooding and soil moisture."
In an earlier study of Ogeechee River Basin in southeast Georgia, Leigh found large dry channels that were once flowing river "meanders" during the Middle Holocene. These meanders were probably caused by a climate much wetter than what we now know, perhaps because of a change in position of the prevailing pressure gradient called the "Bermuda high," which rules summer weather over the Southeast. It's even possible that the area received heavy, near-monsoonal rains in the period, though this remains unclear. The research on the Ogeechee River Basin was published several years ago in the journal Geology.
Fossil pollen also provides considerable evidence for a wet Middle Holocene. Leigh and a former master's degree student, Fong Brook, took sections from a peat bog in the Ogeechee Basin and studied the fossil pollen record using powerful microscopic and statistical techniques.
"We saw an increasing abundance of pollen from wetland aquatic plants, which you would not have seen if the climate was warm and dry," said Leigh. "The bogs just wouldn't be present in a very dry climate such as that of the Upper Midwest."
Other researchers have argued, using the fossil pollen record, that the period in the South was wetter than previously thought, but their work has remained a minority opinion until the research of Leigh and Brook added weight to their findings.
It is known that the South's ubiquitous pine trees began arriving during the Middle Holocene, impinging on deciduous forests that once ruled the area. Just why this occurred is unclear, though fire and humans may have played a role, since pine stands depend on fire for long-term health. Leigh said that it is possible that an increase in thunderstorm activity during the period also increased lightning-set fires, opening the way for the takeover of pine forests. Though this is speculative, it fits the overall idea of a wetter climate.
Finally, drought conditions would have likely given rise to large tracts of sand dunes from the period.
"In fact, there is very little evidence at all of widespread Middle Holocene windblown sand in the Southeast," said Leigh. He based his opinion on his own research and that of his graduate student Andrew Ivester.
Paleoclimate modeling has become an academic discipline unto itself in the past decade, and Leigh said the physical evidence collected in the South is consistent with computer modeling done by other scientists. Modeling for paleoclimate, however, remains difficult, especially in understanding rainfall, though models have been improved considerably in recent years.
Just how the new information might help in developing models to predict future climate remains unclear. Problems abound. For one thing, a glacial ice sheet still covered much of what we now call Canada until the Middle Holocene, affecting climate worldwide in ways that are no longer applicable. Still, with global warming a serious concern as the new century nears, understanding how climate changes is crucial to scientists and governments everywhere.
"One point that's good to make is that in terms of climate, what's happening in Illinois might be completely different from what's happening in the Southeast," said Leigh.
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