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

Abstract

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 small234U 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.

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