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

Danskin

et al. 2019

Nat Commun

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Constraining the magnitude of past hydrological change may improve understanding and predictions of future shifts in water availability. Here we demonstrate that water-table depth, a sensitive indicator of hydroclimate, can be quantitatively reconstructed using Kr and Xe isotopes in groundwater. We present the first-ever measurements of these dissolved noble gas isotopes in groundwater at high precision (≤0.005‰ amu−1; 1σ), which reveal depth-proportional signals set by gravitational settling in soil air at the time of recharge. Analyses of California groundwater successfully reproduce modern groundwater levels and indicate a 17.9 ± 1.3 m (±1 SE) decline in water-table depth in Southern California during the last deglaciation. This hydroclimatic transition from the wetter glacial period to more arid Holocene accompanies a surface warming of 6.2 ± 0.6 °C (±1 SE). This new hydroclimate proxy builds upon an existing paleo-temperature application of noble gases and may identify regions prone to future hydrological change.

Section: Resultsmentioning

confidence: 96%

Section: Introductionmentioning

confidence: 99%

Deglacial water-table decline in Southern California recorded by noble gas isotopes

Danskin

et al. 2019

Nat Commun

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“…Ar, Kr, and Xe isotopic solubility fractionation in seawater (markers and solid lines: this study) and freshwater (dashed lines: Seltzer et al, ) versus temperature. The standard errors of seawater ε sol values for the isotope ratios considered in this study range from ±4 per meg for 40 Ar/ 36 Ar to ±7 per meg for 86 Kr/ 82 Kr.…”

Section: Methodsmentioning

confidence: 66%

“…In total, 12 experiments were carried out: five each for Kr and Ar isotopes (at 34.6 ± 0.1 psu and 35.4 ± 0.1 psu, respectively), and two for Xe isotopes (at 36.8 ± 0.1 psu). Figure 1 shows the results of these seawater solubility fractionation experiments as ε values (where ε sol ≡ α sol − 1, in per mil) along with freshwater ε values (Seltzer et al, 2019). For Ar, Kr, and Xe at a given temperature, we find greater relative enrichment of heavy-to-light isotopes dissolved in seawater than in freshwater.…”

Section: Laboratory Determination Of Isotopic Solubility Fractionatiomentioning

confidence: 97%

See 1 more Smart Citation

Heavy Noble Gas Isotopes as New Constraints on the Ventilation of the Deep Ocean

Geophysical Research Letters

Pavia

et al. 2019

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Past studies of noble gas concentrations in the deep ocean have revealed widespread, several percent undersaturation of Ar, Kr, and Xe. However, the physical explanation for these disequilibria remains unclear. To gain insight into undersaturation set by deep‐water formation, we measured heavy noble gas isotope and elemental ratios from the deep North Pacific using a new analytical technique. To our knowledge, these are the first high‐precision seawater profiles of 38Ar/36Ar and Kr and Xe isotope ratios. To interpret isotopic disequilibria, we carried out a suite of laboratory experiments to measure solubility fractionation factors in seawater. In the deep North Pacific, we find undersaturation of heavy‐to‐light Ar and Kr isotope ratios, suggesting an important role for rapid cooling‐driven, diffusive air‐to‐sea gas transport in setting the deep‐ocean undersaturation of heavy noble gases. These isotope ratios represent promising new constraints for quantifying physical air‐sea gas exchange processes, complementing noble gas concentration measurements.

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

Tyne

Rapid Comm Mass Spectrometry

et al. 2023

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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