DURHAM, N.C. -- Analyzing ancient sediments laid down in a North Dakota lake thousands of years ago, ecologists and earth scientists have found evidence of century-scale cycles of drought and moisture. The finding not only sheds light on ancient drought cycles, but also offers hints that global warming may tip a precarious balance, with the increased aridity in continental interiors sending the Central Plains and other such areas into such cycles.
The researchers prepared their findings for presentation Wednesday at the annual meeting of the Ecological Society of America in Snowbird, Utah. The research team is led by Duke University Professor of Botany James Clark. Co-authors are Eric Grimm, of the Illinois State Museum; Joseph Donovan, University of West Virginia; Sherry Fritz, University of Nebraska; and Daniel Engstrom, University of Minnesota. The work is sponsored by the National Science Foundation.
"Grassland ecosystems all over the globe are predicted to be particularly sensitive to increased aridity with global warming," Clark said. "In areas such as Africa, relatively modest climatic shifts that might not have much impact in more temperate areas can have large effects. We have even seen such effects in the 20th-century."
He said the Dust Bowl of in the Central Plains in the 1930s is a good example. The sensitivity arises, Clark said, because the Central Plains draws most of its moisture from the Gulf of Mexico, and only slight variations in atmospheric circulation can influence the critical flow of Gulf moisture long distances northward.
Clark explained that the analysis of sediments from Kettle Lake in North Dakota was designed to reveal in greater detail earlier hints of ancient climate variability in the region.
"For some time, we've had paleo evidence suggesting that larger climate changes than we see today occurred in prehistoric times in some of these grasslands," he said. "So, to understand that variability on a finer scale, we decided to perform sampling in a different way than had been done before."
Thus, the researchers measured the minerals in the sediment layers, which reflect the atmospheric dust. Higher levels of such atmospheric dust indicated periods of ancient drought.
In contrast, measurements of the mineral aragonite in the sediments indicated periods of higher humidity, since more of that mineral is produced in lake waters when humidity is high, Clark said.
The scientists also measured charcoal levels in the lake sediments as an indication of the extent of fires, and since the fires required fuel, the changing abundance of grass. Pollen analysis gave the scientists an indication of the extent of vegetation during the period.
The scientists' sediment measurements revealed that during a 600-year period about 7,000 years ago, the Kettle Lake area showed well-defined 80- to 100-year cycles of alternately arid and humid conditions. During humid intervals, grasses expanded, providing fuel for fires. During intervening arid decades, grass cover declined, fires were limited by low amounts of fuel, and erosion of exposed soils increased dust in the atmosphere. Grasses were replaced by weedy plants.
"These findings imply that if the climate in these grassland regions become more arid, they could flip into a somewhat different mode, in which drought cycles become prevalent," said Clark. Productivity of grasses, the role of fire and erosion rates would all be affected.
The scientists' next efforts will be to extend their data to include lake studies not only in western North Dakota, but across a swath that runs from the forests of Wisconsin and Minnesota through the tallgrass prairies of western Minnesota and the eastern Dakotas.
"We'd like to find out how the variability in droughts has changed over the past 10,000 years, across the entire Northern Plains, to determine whether these cyclic drought patterns vary regionally and over time," Clark said.