Stalagmites from a Missouri cave have yielded a clear picture of climate and vegetation change in the mid-continental region during the millennia leading up to the last ice age (75,000 to 25,000 years ago), a time period for which such data have been sketchy. The records show that average temperature fluctuations of four degrees C were associated with profound changes in vegetation, including a sharp shift from prairie to forest 55,000 years ago, when temperatures fell and ice sheets began to grow. The study, to be published Friday (Dec. 4) in the journal Science, reveals the value of stalagmites in reconstructing past climate, said lead author Jeffrey Dorale, a graduate student in geology at the University of Minnesota.
"It hasn't been clear how climate and vegetation changed between 120,000 years ago, when conditions were similar to today, and 20,000 years ago, when the last Ice Age was at its peak," said Dorale. "Much of the data comes from the oceans, and while that's good in determining global patterns it's less helpful in figuring out local and regional climate histories."
Climatic change in continental regions is most frequently studied by examining pollen in lake sediments, Dorale said. Unfortunately, most lakes in the middle of the United States are younger than 15,000 years, which limits their value for studying earlier times. But stalactites (growing down like icicles) and stalagmites (growing up from cave floors) can be much older. These limestone structures, built from calcium carbonate deposited by running water, are layered like rings in a tree, with stalagmites usually having sharper, more easily studied rings.
"Techniques developed in the last decade by Larry Edwards and others at the University of Minnesota have made possible this kind of resolution in dating calcium carbonate materials such as stalagmites and corals," said Dorale. The dating techniques are based on the radioactive decay of uranium and allow scientists to determine the ages of material much older than can be determined using carbon dating. Edwards, a professor of geology and geophysics at the university, is a co-author of the paper. Other co-authors are Emi Ito, also a professor of geology and geophysics at the university, and Luis Gonzalez, associate professor of geology at the University of Iowa.
The researchers examined four stalagmites from Crevice Cave, about 75 miles south of St. Louis. The stalagmites appeared to have been broken by natural forces such as floods or earthquakes and were found about 80 feet below the ground surface, Dorale said. The team determined how long ago the stalagmite layers were deposited, then deduced the general types of vegetation growing in the vicinity during that era by examining the carbon and oxygen within the calcium carbonate. Carbon and oxygen exist in different forms, called isotopes. The mixture of oxygen isotopes in a layer of stalagmite reveals the temperature when that layer was deposited. Similarly, the mixture of carbon isotopes indicates whether the plants were warm-season grasses or cold-season grasses and trees. From this information the researchers constructed a temperature/vegetation profile of the area.
The profile showed forest in the area 75,000 years ago. The forest changed to savannah 71,000 years ago, followed by prairie at 59,000 years and forest again at 55,000 years. The forest persisted until 25,000 years ago. This pattern is consistent with climatological records from the ocean, which indicate global cooling between 55,000 and 25,000 years ago, said Dorale.
The study also revealed that in the mid-continental region, small temperature shifts could affect the form of vegetation, which is sensitive to the balance between temperature and moisture.
"Shifts of a few degrees can really push us over the threshold to forest or grassland," said Dorale. The researchers cautioned that further study is needed to determine how wide a geographic area is represented by the climatic history uncovered in Crevice Cave.
"We think this research should continue," said Dorale. "Caves should not be overlooked as a resource in climatological studies."
Source: ScienceDaily
