Stalagmites yield

Eric Cherland

The climate and vegetation of the central United States changed often between 75,000 and 25,000 years ago, a group of University researchers has determined by studying stalagmites.
The efforts of a group of University geologists studying climate history in the central United States recently culminated in new climate and vegetation information.
Stalagmites are limestone structures made up of calcium carbonate that rise up from the floors of caves. The calcium carbonate is found in water that drips from the cave ceiling. The stalagmites form rings as they grow.
The technique for this research was developed in part by Larry Edwards, a professor of geology and geophysics at the University. By dating the radioactive decay of uranium in the stalagmites, it is possible to determine the structures’ ages.
Jeff Dorale, a geology graduate student, is the lead author of the study. The other researchers are Emi Ito, a professor of geology and geophysics at the University and Luis Gonzalez, an associate professor of geology at the University of Iowa.
The researchers studied the rings of four stalagmites from Crevice Cave, about 75 miles south of St. Louis, Mo.
After determining the age of the various rings in the stalagmites, the researchers examined the content of carbon and oxygen to figure out the temperature at the time the substance was deposited.
From this research, the geologists were able to conclude the type of vegetation predominant in the area.
“Shifts of a few degrees can really push us over the threshold to forest or grassland,” Dorale said.
The evidence showed a change from forest 75,000 years ago to savannah 71,000 years ago. The climate changed back to prairie 59,000 years ago and then back to forest 55,000 years ago, which remained until 25,000 years ago.
“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,” Dorale said.
The cave studies have been ongoing for two years. Dorale believes that further research should be done to expand the climatological studies into the past and future.