Iowa Professor Visits Riverbluff Cave
January 2005
Jeff Dorale from the University of Iowa is using stalagmite records to reconstruct environmental conditions of the past. Based on the pilot study of a single stalagmite found broken in the cave (probably due to the discovery blast damage), the “past” thus far encompasses the past 10,500 years. This age comes from a dating technique that measures the decay of uranium, common at low concentrations in most rocks, including stalagmites, to a daughter product called thorium (U-Th dating). The half-lives of the radioactive isotopes involved are quite long, which allows this particular dating technique to reach back about a half million years. As more materials from the cave are dated, it is likely that much older timeframes will be encountered. Only time will tell….. Past environmental conditions are deciphered from stable isotopes of carbon and oxygen, both major elements of the mineral calcite (CaCO3), which composes the stalagmites and stalactites in the cave. Oxygen isotope ratios (18O/16O) have to do with rainfall and temperature, allowing some picture of how climate has warmed or cooled in the past. Carbon isotopes are related to the type of plant community above the cave. Grasses and trees differ in their carbon isotope ratios (13C/12C), and this plant signal is translated via bacterial action in the soil to the waters that drip in caves, which feed the growing stalagmites. Thus the carbon isotope composition of stalagmites can be used as a guide to whether grasslands or forests existed over the cave in the past. Preliminary data from our one broken stalagmite show strong indications that forest –grassland oscillations have occurred over the past 10,500 years. Additional stalagmites will be studied to validate this interpretation, and to expand the history of vegetation changes to older times, when , mammoths, short-faced bears, and giant peccaries roamed the landscape.

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