Article

Geophysical surveys

, Volume 2, Issue 2, pp 217-239

Validity of radiocarbon dates on ground water

  • M. A. TamersAffiliated withLife Sciences Center, Nova University

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Abstract

Carbonate species, dissolved in appreciable quantities in all ground waters, can be extracted and used for radiocarbon dating. The material is mainly of biogenic origin, but contains some bicarbonate produced from limestone. This causes a dilution of the radiocarbon activity of the total carbonate species in the water and a correction is necessary in order to determine absolute radiocarbon ages, i.e., the time since the waters went underground.

Three methods of evaluating the limestone contributions are examined critically: carbonate species ratios, carbon-13 variations, and concurrent tritium measurements. All possess uncertainties. The largest errors occur in the carbon-13 procedure, due to the wide natural fluctuation of the stable isotope content in plants and the occurrence of isotope effects in the soil air-water carbon dioxide exchange. Comparisons of the limestone dilution correction techniques show that they produce analogous results in actual aquifer studies, but with small differences in the calculated radiocarbon ages. The reasons for these inconsistencies have not yet been unequivocally established. Two problems are: the possibility of irreversible exchange of the ground water dissolved carbonate species with limestone in the aquifer matrix, and the significance of exchange of carbon dioxide in the water with that of the soil air in the surface layer of the catchment zone. Experiments are suggested that could resolve these questions.