TWO DIFFERENT CARBON SOURCES FOR SPELEOTHEM FORMATION IN RACCOON MOUNTAIN CAVE, EAST TENNESSEE

Speleothem d¹³C values have been a controversial proxy for paleovegetation because the dissolved inorganic carbon (DIC) pool in cave systems can be complex. Here we provide one such example of DIC pool complexity within the Raccoon Mountain Cave (RMC) located in east Tennessee. RMC has formed in Monteagle Fm. (Upper Miss., Chesterian) limestone deposits and contains a variety of active speleothem formations. We have conducted stable C and O isotopic analyses of fossil and modern speleothems, cave seepage water, pond water, surface soil, and limestone collected throughout this cave.  Host limestone has a fairly homogenous d¹³C composition ranging from ~2.75 ‰ to ~2.94 ‰ PDB. The average d¹³C values of a fossil stalagmite collected from our northern site (ns) and a fossil flowstone collected from our southern site (ns), along with modern stalactites and stalagmites collected from both sites, exhibit significant spatial variation over relatively short distances (~260m).  The fossil stalagmite has d¹³C values averaging -2.35±0.81 ‰ PDB (n=33), while the average d¹³C value of the fossil flowstone is -9.79± 0.67 ‰ PDB (n=67). Modern speleothems collected from these sites show a similar shift in d¹³C values, suggesting that the speleothem formation on these two sites tapped distinct sources of CO₂. Currently, Raccoon Mountain Cave is covered by mesophytic forest, which is a C₃ dominated ecosystem.  This C₃ dominance appears to be recorded by the carbon isotope composition of the speleothems at the ss, but not the ns.  Seepage water collected from the ns has d¹⁸O values very similar to that of mean annual meteoric water for the region, but d¹⁸O values of seepage water from the ss vary closely with surface precipitation events. The difference of d¹⁸O values suggests that both groundwater and surface precipitation are playing a role in speleothem formation within the cave. This potential spatial variation in carbon isotopes caused by different carbon sources dominating in different parts of the cave should be taken into consideration by researchers trying to reconstruct temporal paleovegetation changes using speleothems.  Further isotopic investigations of modern cave system hydrology and DIC pools will help improve the accuracy of such paleoenvironmental reconstructions in the future.