TRACE ELEMENT CONCENTRATIONS IN SPELEOTHEMS FROM RIVER BLUFF CAVE, GREENE COUNTY, MISSOURI: PRELIMINARY RESULTS

River Bluff Cave in Greene County, Missouri contains numerous speleothems that have not been touched by humans. Speleothem ages range from 1.8 million years ago to the present, making it an ideal location in which to investigate recent climate variation. Speleothem samples were collected from two locations in the cave. Powdered carbonate samples were collected by dental drill from growth zoning visible in the cross section of the speleothems. Sample masses ranging from 11 to 27 mg were dissolved in aqua-regia and analyzed for Ba, Co, Cu, Fe, Mg, Mn, Pb, Sr, and Zn, using ICP-OES. Total metal concentrations were adjusted for dilution during dissolution and are reported here as micrograms metal per gram of sample (ppm). Trace metal concentrations varied widely. Barium, copper, and zinc were not found to occur at concentrations above the detection limit. Lead occurred only intermittently, ranging from 0 to 44.1 ppm. Other metals present include (in ppm): cobalt (0.3-113); iron (6.56-80.7); magnesium (93.5-151.5); manganese (0.17-1.99); and strontium (9.3-24.0). Studies indicate that variations in magnesium, iron, and strontium concentrations may be used to infer relative changes in the volume of water moving through the cave system. Since most cave water is meteoric in origin, these variations may indicate wetting or drying climates. The relationship to concentration is inverse for magnesium and strontium and direct for iron. Samples from River Bluff cave show an increase in Sr and corresponding decrease in Fe, indicating drying over the time period during which speleothem growth occurred. Mg concentrations do not exhibit a similar trend, likely due to thermal variation which changes the mg-calcite partitioning coefficient. In addition, samples exhibited spikes in Pb and Co concentrations. These spikes may be the result of high flows which tend to enrich metals that commonly are associated with colloids. Both Pb and Co may be adsorbed to the surfaces of clays or as amorphous metal oxides that can be mobilized during high water flow events. These data hint at an overall climate drying event punctuated by periods of increased moisture. The next step will be to link these data to geologic ages and to stable isotope compositions in an effort to account for the impact of temperature change on speloethem chemistry over time.