Evaluation of Direct-Precipitation and Gas-Evolution Methods for Radiocarbon Dating of Ground Water

Abstract

The extraction of dissolved carbonate species for age dating from a 100L water sample by the direct-precipitation method (DPM) and by the gas-evolution method (GEM) has been investigated. Stable carbon-isotope fractionation between initial and final carbon dioxide evolved was ca 11 per mil by GEM and 1 per mil by DPM. GEM will produce isotopically lighter carbon dioxide compared with DPM if carbonate recovery is low. Extraction efficiency of > 95% can be achieved by GEM in 3 hours using nitrogen gas at a sweeping rate of 2000cc per minute. DPM requires precipitates to settle overnight to assure > 95% recovery. GEM is little affected by a high concentration of sulfate ions, whereas DPM is greatly affected by sulfate resulting in less yield.