SPELEOTHEM ISOTOPE RECORDS OF PALEOCLIMATE VARIABILITY, RACCOON MOUNTAIN CAVE, EAST TENNESSEE, USA

A preliminary petrographic and stable isotopic study of a speleothem (flowstone) from the Raccoon Mountain Cave in East Tennessee (35°01'15.62”N; 85°24'30.21”W; 258 m elev.) provides evidence for changing paleoclimate and vegetations in this region during the Holocene epoch. The Raccoon Mountain Cave formed in limestone deposits of Monteagle Formation (Upper Mississippian, Chesterian) and contains a variety of active speleothem formations. Micromorphological study reveals two carbonate microfacies in this flowstone. One is a very dense, gray to grayish brown, microcrystalline to microsparitic calcite that is very finely laminated at scales ranging from 0.1 mm to 1 mm thick, although some laminae are extremely thin (<0.1mm). Another is a white to grayish white, coarsely crystalline sparry calcite with a very porous structure and layer thicknesses from 1-3 mm. The δ¹⁸O values of calcite in these microfacies exhibit a large range of values from -11.07 ‰ to -6.53 ‰ PDB, with the averaged value of -8.19 ± 1.15 ‰ PDB (n=67). Fluctuations in the δ¹⁸O values of the speleothem suggest the paleoclimate or the source of precipitation in East Tennessee has varied significantly during speleothem formation. The δ¹⁸O variations are comparable to the fluctionations of atmospheric methane concentration (CH4) during the Holocene period. Lower δ¹⁸O values (more negative) during the middle Holocene indicate a cooling in this region and are consistent with low CH4 concentrations in atmosphere, suggesting lower amounts of precipitation associated with low ocean surface temperature. The δ¹³C values are averaged at -9.79± 0.67 ‰ PDB (n=67) and show less variation than oxygen isotope ratios. Although the δ¹³C results may have been overprinted by the contamination from the limestone, high frequency variations in δ¹³C appear to co-vary with δ¹⁸O values. Significant fluctuations and covariation of δ¹⁸O and δ¹³C in the speleothem with atmospheric CH4 concentration suggest that stable isotope techniques may be useful for speleothem studies in this region.