(234U)/(238U) DISEQUILIBRIUM AND TRACE ELEMENTS IN GROUNDWATER THROUGH TIME: A SPELEOTHEM RECORD FROM SOUTHERN ILLINOIS

There are few methods for examining water-rock interaction over geologic time scales. One paleohydrologic record comes from speleothems. Here we present combined U-series chronology and geochemistry of a 65 cm long stalagmite from Fogelpole Cave in Southern Illinois. The host cave lies within the St. Louis limestone and is overlain by Illinoisan glacial sediments. We characterized growth fabric using petrography and CL and determined ²³⁸U-²³⁴U-²³⁰Th and ²³⁵U-²³¹Pa ages by Thermal Ionization Mass Spectrometry. Ages for the two independent systems are concordant for 7 samples, range from ~80,000 years to ~30,000 years and progressively decrease up the sample. The growth rate of the stalagmite decreases significantly through time from ~230 mm/yr to ~3 mm/yr. Despite changing growth rates, U concentrations are continuously high (40-60 ppm) while Th concentrations are low (~ppb). d ¹⁸O (– 4.5~ – 3 per mil) and d ¹³C (– 4.5 ~ – 2.5 per mil) do not appreciably vary, suggesting fairly stable vegetation over this time. This contrasts with data from nearby Crevice Cave (Dorale et al., 1998) that indicate significant shifts between forest and prairie vegetation at this time.

The initial activity ratio for ²³⁴U/²³⁸U (d ²³⁴U₀) varies from -210 to -70 and correlates well with the growth rate of the stalagmite and Mg concentrations. Both d ²³⁴U₀ and Mg likely reflect changes in water-rock interaction in the overlying sediments. However, ⁸⁷Sr/⁸⁶Sr, another indicator of water-rock interaction, shows no variation over the length of the stalagmite.

An examination of the global database shows that speleothem growth rates often correlate with d ²³⁴U₀. Most studies find d ²³⁴U₀ > 0 that increases as growth rate decreases. In contrast, our stalagmite shows d ²³⁴U₀ < 0 that decreases with decreasing growth rate. A self-consistent explanation for these observations is that changes in d ²³⁴U₀ reflect mixing of U from two endmember components: 1) mineral interiors having d ²³⁴U₀ close to zero and 2) mineral surfaces having d ²³⁴U₀ far from equilibrium. During wetter conditions greater amounts of mineral dissolution occurs so that d ²³⁴U₀ is closer to zero compared to drier conditions where surfaces dominate. If growth rate reflects paleo-precipitation, variations in d ²³⁴U₀ through time may provide another important record of paleoclimate change.