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

Abstract

Rationale: Noble gases are widely used as physically based climate proxies, notably in dissolved water samples as tracers of past recharge temperature in groundwater and air-sea gas exchange processes in seawater. Recent advances in measuring largevolume samples of dissolved noble gas isotopic ratios at high precision have expanded the range of climate parameters that can be interpreted. Methods: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 were consistent between two laboratories, and LVE elemental noble gas abundances agreed 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. | INTRODUCTIONNoble gases are valuable physical tracers of past climate and gas exchange processes due to their chemical and biological inertness, and their presence in various terrestrial, marine, and polar climate archives. In groundwater and ice cores, noble gases are a wellestablished tool to reconstruct paleotemperatures at the time of recharge or air-bubble closure in ice based on their temperaturedependent solubilities. [1][2][3] In seawater, noble gas measurements help distinguish physical processes of air-sea gas exchange from biogeochemical ones. 4,5 Dissolved noble gases in groundwater or seawater are typically sampled in 10-50-ml copper tubes for measurement of elemental abundances via static mass spectrometry (herein, the CT method). 6 Recently, high-precision measurements of dissolved argon (Ar), krypton (Kr), and xenon (Xe) stable isotope ratios-alongside their elemental concentrations-have allowed for reconstructions of additional paleoclimatic parameters beyond traditional

Solubility Equilibrium Isotope Effects of Noble Gases in Water: Theory and Observations

“…The headspace gas was purified by exposure to a chemical getter (Ti sponge) at 900 °C and cryogenically transferred to a Thermo MAT 253 Plus mass spectrometer using a silica gel-containing dual-valve dip tube as an intermediary, following the method of ref . Small corrections were made for the noble gases contained in drained and residual water, following ref . Although isotopes of Ar, Kr, and Xe, in addition to Kr/Ar and Xe/Ar elemental ratios, were measured, here we report only Ar isotope ratios ( 40 Ar/ 36 Ar and 38 Ar/ 36 Ar) due to the fact that Kr and Xe isotope and elemental ratios in water are used to constrain instrumental matrix effects in this method, and thus our measured values are implicitly tied to published values for Kr and Xe SEIEs. , Our new Ar isotope measurements are made entirely independently of prior studies and thus provide an opportunity to comprehensively redetermine SEIEs.…”

“…In this study, experiments were carried out at temperatures between ∼275 and 296 K and salinities between 0 and ∼33 g kg –1 . A new technique for high-precision dynamic mass spectrometry measurements of dissolved noble gas isotopes in water was used , in which evacuated 6-L stainless steel flasks were filled part way (∼2 to 4 L of water), equilibrated on a shaker table at constant temperature (∼295 K) for a minimum of 3 days, and drained to remove equilibrated water, leaving behind the headspace gas and a small (quantified) residual amount of water. The headspace gas was purified by exposure to a chemical getter (Ti sponge) at 900 °C and cryogenically transferred to a Thermo MAT 253 Plus mass spectrometer using a silica gel-containing dual-valve dip tube as an intermediary, following the method of ref .…”

“…38 In this study, experiments were carried out at temperatures between ∼275 and 296 K and salinities between 0 and ∼33 g kg −1 . A new technique for high-precision dynamic mass spectrometry measurements of dissolved noble gas isotopes in water was used 9,10 in which evacuated 6-L stainless steel flasks were filled part way (∼2 to 4 L of water), equilibrated on a shaker table at constant temperature (∼295 K) for a minimum of 3 days, and drained to remove where T is water temperature (K), S is salinity (g kg −1 ), and ε 293 is the ε sol value at 293 K in freshwater. We note that the published helium isotope SEIE function is quadratic in form, 24 but that our linear approximation is accurate to within ∼0.1‰ (between 273 and 298 K and 0−100 g kg −1 ), which is well below the precision of noble gas mass spectrometry for dissolved helium isotopes in water.…”

“…38 In this study, experiments were carried out at temperatures between ∼275 and 296 K and salinities between 0 and ∼33 g kg −1 . A new technique for high-precision dynamic mass spectrometry measurements of dissolved noble gas isotopes in water was used 9,10 in which evacuated 6-L stainless steel flasks were filled part way (∼2 to 4 L of water), equilibrated on a shaker table at constant temperature (∼295 K) for a minimum of 3 days, and drained to remove 36 Ar and 38 Ar/ 36 Ar) due to the fact that Kr and Xe isotope and elemental ratios in water are used to constrain instrumental matrix effects in this method, and thus our measured values are implicitly tied to published values for Kr and Xe SEIEs. 11,14 Our new Ar isotope measurements are made entirely independently of prior studies and thus provide an opportunity to comprehensively redetermine SEIEs.…”

“…For example, dissolved helium isotope ratios in groundwater − and seawater − have been used to determine water-mass residence times, quantify volcanic volatile inputs, and constrain physical and biogeochemical fluxes in the ocean. Recent analytical developments − for precise measurement of dissolved argon, krypton, and xenon isotope ratios in groundwater and seawater have enabled new applications for paleoclimate, groundwater hydrogeology, and atmosphere–ocean gas exchange. , In addition to their broad applications in the geosciences, noble gas isotopes are also of fundamental interest to physical chemistry because of their inherent simplicity as inert, monatomic gases. For example, vapor–pressure isotope effects (VPIEs) of noble gasesi.e., the difference in vapor pressure between isotopes of the same noble gasprovide a useful means of evaluating physical models rooted in interatomic potential functions via comparison to measurements of noble gas isotope fractionation. − …”