Holocene Forcing of the Indian Monsoon Recorded in a Stalagmite from Southern Oman
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S. J. Burns et al., J. Geophys. Res.107, 4434 (2002).
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Analytical procedures for the separation and purification of Th and U were performed as described in ( 32 ). Th-U measurements were in part performed on a MC-TIMS (Finnigan MAT 262 RPQ) and in part on a MC-ICPMS (Nu Instruments). TIMS measurements are described in detail in ( 6 ). The Nu Instruments MC-ICPMS is equipped with three ion-counting electron multipliers one of them placed behind a retardation (WARP) filter and a Cetac Aridus desolvating nebulizer system. A mixed 236 U- 229 Th spike was added before sample dissolution. U was measured in static mode with ion counters for masses 236 and 234 and correcting for instrumental fractionation with the natural 238 U/ 235 U ratio. Multiplier gains were calibrated with the National Institute of Standards and Technology U050 standard. Th was measured in a two-cycle sequence measuring masses 230 and 229 alternately in the WARP-equipped multiplier which was calibrated with an in-house Th standard. Doping with a Th-free solution of natural U enabled instrumental fractionation correction. The reproducibility of 234 U/ 238 U ratios and 232 Th concentrations is as given in ( 6 ) for the TIMS measurements. Activity ratios for MC-ICPMS data were calculated with the decay constants described in ( 33 ). This results in 234 U activities 4‰ lower than previously used values. The resulting shift in ages is well within the experimental error.
For oxygen stable-isotope ratio determinations ∼5 mg of powder was drilled from the sample and analyzed with an on-line automated carbonate preparation system linked to a VG Prism ratio mass spectrometer. Results are shown as the per mil difference between sample and the Vienna Pee Dee belemnite standard in delta notation. Reproducibility of standard materials is 0.08‰.
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We thank D. Sanz for caving assistance and H. Al-Azry (Directorate of Minerals Ministry of Commerce and Industry Sultanate of Oman) for support during fieldwork. This work was supported by the Swiss National Science Foundation (grants 2021-52472.97 and 2000-059174.99) and the National Science Foundation of the United States (ATM-0135542).