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

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

Mar 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998e%26psl.156..141k&link_type=abstract

Earth and Planetary Science Letters, Volume 156, Issue 3-4, p. 141-155.

Computer Science

19

Scientific paper

Precise 230Th-234U ages were obtained on thirty-one growth laminae in speleothem samples which are self-consistent with the detailed layer stratigraphy. Samples with low 232Th/(238U) 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 232Th/(238U) were dated by internal isochrons. This permits the determination of the initial 230Th/(232Th) assuming equilibrium of 232Th and 238U series in the source of the high 232Th component. This shows initial (230Th/(232Th)) (in activity units) of from 1.3 to 2.9. The calculated atomic ratios of 232Th/(238U) for the high 232Th component range from 1.08 to 2.4 which is well below the average crustal value. Speleothem materials with high 232Th/(238U) are found to exhibit clear correlations of 232Th with Si, Al and Fe, while 238U correlates with Sr and Ba. Analyses of Soreq cave drip waters show that the particulates in the waters have high 232Th concentrations and a 232Th/(238U) ratio much lower than that found in the high 232Th component in speleothems but with (230Th/(232Th)) = 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 234U excess. The initial (234U/(238U))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. These appears to be a correlation of (234U/(238U))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 the 234U/(238U) 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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