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

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Publication Date

3-30-1998

Abstract

Precise230Th234U ages were obtained on thirty-one growth laminae in speleothem samples which are self-consistent with the detailed layer stratigraphy. Samples with low232Th238U ratios give ages with analytical uncertainties of 40 years at 2 ky and 1000 years at 90 ky. Some growth zones with high but variable232Th238U were dated by intermal isochrons. This permits the determination of the initial230Th232Th assuming equilibrium of232Th and238U series in the source of the high232Th component. This shows initial (230Th232Th) (in activity units) of from 1.3 to 2.9. The calculated atomic ratios of232Th238U for the high232Th component range from 1.08 to 2.4 which is well below the average crustal value. Speleothem materials with high232Th238U are found to exhibit clear correlations of232Th with Si, Al and Fe, while238U correlates with Sr and Ba. Analyses of Soreq cave drip waters show that the particulates in the waters have high232Th concentrations and a232Th238U ratio much lower than that found in the high232Th component in speleothems but with230Th232Th) = 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 initial234U238U)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 of234U238U)0 with the δ18O values. There is a drop of δ18O in the time interval 20 to 15 ky which then remains relatively constant to recent times. As the high δ18O values have been related to rainfall and associated climatic conditions, we suggest that the234U238U in the speleothem reflects the effects of rainfall and soil weathering conditions on drip-water composition and may provide a proxy for climate change.

Keywords

Uranium-thorium dating, Climate change, Asia, Israel

Geographic Subject

Asia; Israel

Document Type

Article

Notes

Volume 156, Issues 3-4

Identifier

K26-05453

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