Paper No. 9
Presentation Time: 4:20 PM
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°01'15.62N; 85°24'30.21W; 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.