干旱区生态环境知识资源中心

Natural waters universally show fractionation of uranium series (U-series) parent-daughter pairs, with the disequilibrium between U-234 and U-238 (U-234)/(U-238) commonly used as a tracer of groundwater flow. Because speleothems provide a temporal record of geochemical variations in groundwater precipitating calcite, (U-234)/(U-238) variations in speleotherns 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 speleotherns and drip waters from Fogelpole Cave in southern Illinois. Data from all speleothems from the cave show an inverse correlation between (U-234)/(U-238) and U concentration, following the pattern observed in groundwaters globally. Within a 65-cm-long stalagmite, concordant U-234-U-238-Th-230 and U-235- Pa-231 ages for 5 samples indicate accurate chronology from 78.5 ka to 30 ka. Notably, (U-234)/(U-238)(o) which differs from most speleotherns by having (U-234)/(U-238)(o)< 1 positively correlates with speleothem growth rate. We generalize this to the observation that speleothents globally show (U-234)/(U-238)(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 (U-234)/(U-238) 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 (U-234)/(U-238)(o). Based on this result, we suggest that (U-234)/(U-238)(o) in speleothems may record changes in hydrologic flux with time and thus could provide a useful proxy for long term records of paleoprecipitation. (c) 2005 Elsevier B.V. All rights reserved.