Precise 230 Th 234 U ages were obtained on thirty-one growth laminae in speleothem samples which are self-consistent with the detailed layer stratigraphy. Samples with low 232 Th 238 U 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 232 Th 238 U were dated by intermal isochrons. This permits the determination of the initial 230 Th 232 Th assuming equilibrium of 232 Th and 238 U series in the source of the high 232 Th component. This shows initial ( 230 Th 232 Th) (in activity units) of from 1.3 to 2.9. The calculated atomic ratios of 232 Th 238 U for the high 232 Th component range from 1.08 to 2.4 which is well below the average crustal value. Speleothem materials with high 232 Th 238 U are found to exhibit clear correlations of 232 Th with Si, Al and Fe, while 238 U correlates with Sr and Ba. Analyses of Soreq cave drip waters show that the particulates in the waters have high 232 Th concentrations and a 232 Th 238 U ratio much lower than that found in the high 232 Th component in speleothems but with 230 Th 232 Th) = 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 234 U excess The initial 234 U 238 U) 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 modern drip waters. There appears to be a correlation of 234 U 238 U) 0 with the δ 18 O values. There is a drop of δ 18 O in the time interval 20 to 15 ky which then remains relatively constant to recent times. As the high δ 18 O values have been related to rainfall and associated climatic conditions, we suggest that the 234 U 238 U in the speleothem reflects the effects of rainfall and soil weathering conditions on drip-water composition and may provide a proxy for climate change.
Earth and Planetary Science Letters – Elsevier
Published: Mar 30, 1998
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