HR: 0830h
AN: V51C-0307    [PDF]
TI: Do ($^{234}$U)/($^{238}$U)$_{o}$ Variations in Speleothems Record Paleohydrologic Flux?
AU: * Zhou, J
EM: zhou1@uiuc.edu
AF: University of Illinois, 1301 W.Green, 245 NHB, Urbana, IL 61801
AU: Lundstrom, C
EM: lundstro@uiuc.edu
AF: University of Illinois, 1301 W.Green, 245 NHB, Urbana, IL 61801
AB: Analysis of U-series disequilibria in speleothems not only provides a means of accurate chronology but also a record of ($^{234}$U)/($^{238}$U) variations in the source groundwaters through time. We present high precision TIMS data from speleothems and drip waters from Fogelpole Cave in Southern Illinois and suggest that ($^{234}$U)/($^{238}$U)$_{o}$ in drip waters is controlled by two parameters, paleohydrologic flux and the oxidation state of the waters. For a 65 cm long stalagmite, concordant $^{238}$U-$^{234}$U-$^{230}$Th and $^{235}$U-$^{231}$Pa ages for 5 samples indicate accurate chronology, and U concentrations remain high and constant around 50 ppm. As growth rate decreases from ~230 to ~3 mm/yr, ($^{234}$U)/($^{238}$U)$_{o}$ decreases from 0.94 to 0.78. This differs from most speleothems globally that have ($^{234}$U)/($^{238}$U)$_{o}$$>$1 which increases with decreasing growth rate. A generalized observation is that ($^{234}$U)/($^{238}$U)$_{o}$ deviates farther from secular equilibrium at slower growth rates and approaches 1 at faster grow rates. A explanation for this is that ($^{234}$U)/($^{238}$U)$_{o}$ is dependent on fluid flow rates, possibly due to the role of 24 day half-life $^{234}$Th during reactive transport. If so, ($^{234}$U)/($^{238}$U)$_{o}$ could be an indicator of paleohydrologic flux. At the same time, we observe an inverse relationship between ($^{234}$U)/($^{238}$U)$_{o}$ and U concentation for all of the speleothems (5) from Fogelpole cave following the systematics of groundwaters globally. Four U rich samples have $^{234}$U deficits and 1 sample poorer in U has slight $^{234}$U excess As interpreted previously, this probably indicates that changes in U redox chemistry also affect ($^{234}$U)/($^{238}$U)$_{o}$.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting