Speleothem carbonates are precipitated continuously from inorganic carbon dissolved in seepage water infiltrating from the land surface, that reflects a mixture of atmospheric CO2, respired soil carbon, and epikarst sources, each with distinct δ13C values. To aid in deconvolving these signatures, soil samples were collected above the Blue Spring cave system in Sparta, Tennessee, USA and subjected to a series of incubation experiments, in order to constrain the correlation between CO2 respiration rates and soil moisture. This relationship is used to parameterize a simple mixing model which predicts the relationship between δ13C and soil moisture in fluids infiltrating into the underlying cave system.