Precise determination of Ar, Kr and Xe isotopic fractionation due to diffusion and dissolution in fresh water

Abstract Dissolved noble gases are ideal conservative tracers of physical processes in the Earth systemdue to their chemical and biological inertness. Although bulk concentrations of dissolved Ar, Kr, and Xe are commonly measured to constrain physical models of atmosphere, ocean, and terrestrial hydrosphereprocesses, stable isotope ratiosof these gases (e.g. 136 Xe/ 129 Xe) are seldom used because of low signal-to-noise ratios. Here we present the first results from a new method of dissolved gas sampling, extraction and analysis that permits measurement of stable Ar, Kr, and Xe isotoperatios at or below ∼5 per meg amu −1 precision (1 σ ), two orders-of-magnitude below conventional Kr and Xe isotopicmeasurements. This gain in precision was achieved by quantitative extraction and subsequent purification of dissolved noble gases from 2-L water samples via helium spargingand viscous dual-inlet isotope ratio mass spectrometry. We have determined the solubility fractionation factors ( α sol ) for stable Ar, Kr, and Xe isotoperatios between ∼2 and 20 °C via laboratory equilibration experiments. We have also conducted temperature-controlled air- water gasexchange experiments to estimate the kinetic fractionationfactors ( α kin ) of these isotoperatios. We find that both α sol and α kin , normalized by isotopicmass difference (Δm), decrease in magnitude with atomic number but are proportional to Δm for isotoperatios of the same element. With the new ability for high precision isotopicmeasurements, we suggest that dissolved Kr and Xe isotoperatios in groundwater represent a promising, novel geochemical tool with important applications for groundwater modeling, water resource management, and paleoclimate.