Geochemistry of speleothem records from southern Illinois: Development of (²³⁴U)/(²³⁸U) as a proxy for paleoprecipitation

Natural waters universally show fractionation of uranium series (U-series) parent-daughter pairs, with the disequilibrium between ²³⁴U and ²³⁸U (²³⁴U)/(²³⁸U) commonly used as a tracer of groundwater flow. Because speleothems provide a temporal record of geochemical variations in groundwater precipitating calcite, (²³⁴U)/(²³⁸U) variations in speleothems provide a unique method of investigating water-rock interaction processes over millennium time scales. We present high precision Thermal Ionization Mass Spectrometric (TIMS) U-series analyses of speleothems and drip waters from Fogelpole Cave in southern Illinois. Data from all speleothems from the cave show an inverse correlation between (²³⁴U)/(²³⁸U) and U concentration, following the pattern observed in groundwaters globally. Within a 65-cm-long stalagmite, concordant ²³⁴U-²³⁸ U-²³⁰Th and ²³⁵U-²³¹Pa ages for 5 samples indicate accurate chronology from 78.5 ka to 30 ka. Notably, (²³⁴U)/(²³⁸U)_o which differs from most speleothems by having (²³⁴U)/(²³⁸U)_o <1, positively correlates with speleothem growth rate. We generalize this to the observation that speleothems globally show (²³⁴U)/ (²³⁸U)_o deviating farther from secular equilibrium at lower growth rates and approaching secular equilibrium at higher grow rates. Based on the Fogelpole observations, we suggest that groundwater (²³⁴U)/(²³⁸U) is controlled by the U oxidation state, the U concentration of the water and the fluid velocity. A transport model whereby U-series nuclides react and exchange with mineral surfaces can reproduce the observed trend between growth rate and (²³⁴U)/(²³⁸U)_o. Based on this result, we suggest that (²³⁴U)/(²³⁸U)_o in speleothems may record changes in hydrologic flux with time and thus could provide a useful proxy for long term records of paleoprecipitation.