U-Th isotope systematics from the Soreq cave, Israel and climatic correlations

Precise Th-230-U-234 ages were obtained on thirty-one growth laminae in speleothem samples which are self-consistent with the detailed layer stratigraphy. Samples with low Th-232/U-238 ratios give ages with analytical uncertainties of 40 years at 2 ky and 1000 years at 90 ky. Some growth zones with high but variable Th-232/U-238 were dated by internal isochrons. This permits the determination of the initial Th-230/Th-232 assuming equilibrium of Th-232 and U-238 series in the source of the high Th-232 component. This shows initial (Th-230/Th-232) (in activity units) of from 1.3 to 2.9. The calculated atomic ratios of Th-232/U-238 for the high Th-232 component range from 1.08 to 2.4 which is well below the average crustal value. Speleothem materials with high Th-232/U-238 are found to exhibit clear correlations of Th-232 with Si, Al and Fe, while U-238 correlates with Sr and Ba. Analyses of Soreq cave drip waters show that the particulates in the waters have high Th-232 concentrations and a Th-232/U-238 ratio much lower than that found in the high Th-232 component in speleothems but with (Th-230/Th-232) = 1.0 to 2.4. We infer that the trapped high-Th component in speleothems is from particulate matter in water with a large concentration of adsorbed U and not simply from detrital material, The speleothems have only small U-234 excess. The initial (U-234/U-238)(0) show a range of 1.02 to 1.14 that was found to correlate with age over the past 25 ky. The youngest samples have values in the same range as the modem drip waters. There appears to be a correlation of (U-234/U-238)(0) with the delta(18)O values. There is a drop of delta(18)O in the time interval 20 to 15 ky which then remains relatively constant to recent times. As the high delta(18)O values have been related to rainfall and associated climatic conditions, we suggest that the U-234/U-238 in the speleothem reflects the effects of rainfall and soil weathering conditions on drip-water composition and may provide a proxy for climate change. (C) 1998 Elsevier Science B.V. All rights reserved.