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

Abstract

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– 238U– 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.