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

Dissolved noble gases are ideal conservative tracers of physical processes in the Earth system due 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 hydrosphere processes, stable isotope ratios of these gases (e.g. Xe-136/Xe-129) 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 isotope ratios at or below similar to 5 per meg amu(-1) precision (1 sigma), two orders-of-magnitude below conventional Kr and Xe isotopic measurements. This gain in precision was achieved by quantitative extraction and subsequent purification of dissolved noble gases from 2-L water samples via helium sparging and viscous dual-inlet isotope ratio mass spectrometry. We have determined the solubility fractionation factors (alpha(sol)) for stable Ar, Kr, and Xe isotope ratios between similar to 2 and 20 degrees C via laboratory equilibration experiments. We have also conducted temperature-controlled air-water gas exchange experiments to estimate the kinetic fractionation factors (alpha(kin)) of these isotope ratios. We find that both alpha(sol) and alpha(kin), normalized by isotopic mass difference (Delta m), decrease in magnitude with atomic number but are proportional to Delta m for isotope ratios of the same element. With the new ability for high precision isotopic measurements, we suggest that dissolved Kr and Xe isotope ratios in groundwater represent a promising, novel geochemical tool with important applications for groundwater modeling, water resource management, and paleoclimate. (C) 2019 Elsevier B.V. All rights reserved.