Environmental isotope measurements

Measurement of 2H, 18O and 13 C

For measurement of 82H in water, a 5-ml water sample was reduced to H2 gas using BDH zinc shots in a closed evacuated tube at 450°C (Coleman et al. 1982). The H2 gas produced was analysed using a 602E mass spectrometer (VG Isogas, UK). The 82H values reported were with respect to SMOW (standard mean ocean water) standard. The precision of measurement was 1%c (2s). For 818O measurement of the water, a 5-ml sample was taken in a flask and equilibrated with CO2 gas at 25°C for 8 hours (Epstein & Mayeda 1953). The equilibrated CO2 was analysed using a 602E Auto mass spectrometer (VG Isogas, UK). The 818O was reported with respect to SMOW. For 813C analysis, the bicarbonates/carbonates in water were precipitated as barium carbonate by using saturated barium chloride solution in alkaline medium. The carbonate sample was treated with 100% phosphoric acid (McCrea 1950) and the carbon dioxide gas released was analysed using the 602E Auto mass spectrometer. The 813C value was reported with respect to PDB (belemnite of Pee Dee formation). The precision of measurement of both 818O and 813C was 0.2%c (2s).

Measurement 3H

Measurement of 3H of the water sample was carried out by electrolytic enrichment followed by liquid scintillation counting (Taylor 1981).

A 250-g sample of distilled water was transferred into an electrolytic cell comprising mild steel perforated cathodes and stainless steel anode. A 0.5-g sample of Na2O2 was added to 250 g of distilled water (2 g/l). A total of 708 ampere hour charge was passed, which reduced the volume from 250 g to 12 g. The temperature of the cells was maintained at c. 5 + 2°C throughout the electrolysis.

Packard (now Perkin Elmer) Hisafe Scintillator was mixed with electrolysed and neutralized water samples (8-ml water sample + 14-ml scintillator) in a 24-ml high-density polythene vial and counted in a LKB 1200 Quantulus liquid scintillation counter.

The minimum detection limit obtained was 0.5 TU.

Measurement of C

Measurement of 14C was carried out by direct absorption of carbon dioxide (obtained from the field precipitated carbonate sample) into an absorber-scintillator mixture (11 ml + 11 ml car-basorb and Permaflour V or its equivalent) followed by liquid scintillation counting (Nair et al. 1994). Carbon dioxide obtained from oxalic acid (secondary standard calibrated with respect to primary NIST standard) was also absorbed in the same quantity of absorber and scintillator mixture. Samples and standards were counted in a LKB 1200 liquid scintillation counter and each sample was counted for 1000 minutes.

Knowing the standard, background and sample count rate, 14C activity could be calculated in per cent modern carbon (pMC), where 100 pMC = 13.6dpm/g of carbon. Once corrected and normalized sample activity (ASN) and normalized standard activity (AON) are known, the uncor-rected age of the sample can be calculated using the equation:

Minimum detection limit was 1 pMC, which corresponds to a maximum measurable age of 38 000 years BP (Nair et al. 1994). Model ages can be estimated by calculating AON values using different model equations.

Fig. 2. (a) Map of Rajasthan. (b) Location map of the study area.
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