A new large volume equilibration method for high precision measurements of dissolved noble gas stable isotopes

We build on prior methods for measuring noble gas stable isotopes at high precision with a new large volume equilibration (LVE) method wherein sample gases are equilibrated in the sample flask between the dissolved phase and the headspace. The original dissolved gas composition is determined by measuring the headspace gases and correcting for the equilibrium dissolved gas content of the discarded water using known solubilities and fractionation factors. We evaluate the accuracy and precision of this method with air-equilibrated water standards of known noble gas composition RESULTS: Replicate air-equilibrated water standards and field measurements demonstrate that the LVE method achieves comparable precision to prior methods, with major advantages of measuring the Ne content as a constraint on excess air and allowing for long term sample storage. Isotope ratios measured with the LVE method in replicate samples are consistent between two laboratories, and LVE elemental noble gas abundances agree closely with replicate samples measured using established copper tube methods and static noble gas mass spectrometry CONCLUSIONS: The new LVE method enables reconstruction of past water table depths at ±1 m precision along with excess air, recharge temperature, and age and hydrogeochemical indicators. It has wide application to investigating climate signals and physical gas exchange processes in groundwater and seawater.