Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Project Information
• Funding

Field campaigns to sample groundwater were conducted in Umatilla, Oregon, USA during September 2020 and in Tucson, Arizona, USA during November 2021. Water samples for dissolved noble gas isotope measurements were collected in 6-liter (L) pre-evacuated stainless steel flasks (Restek TO-can air sampling flasks sealed with a Swagelok SS-4H valve), leaving 1 L of headspace. The sample gases were equilibrated between the dissolved phase in the sample water and the gas phase in the headspace on an orbital shaker for at least 3 days at a constant known temperature. After the sample water was drained, the headspace gas was transferred and gettered to remove non-inert gases. The remaining gas sample was measured on a dynamic isotope ratio mass spectrometer (IRMS), and the measured isotope ratios were corrected using known solubilities (Seltzer et al., 2019) and the recorded equilibration temperature to obtain the original isotope ratios of the water sample (Ng et al., 2023). Umatilla samples were stored for a year at room temperature prior to sampling. Tucson samples were stored for 2 weeks at room temperature prior to sampling.

Data Processing:
Corrections were made to account for 1) partitioning of the sample gas between the dissolved phase and the headspace and 2) matrix effects (ME) which are known to affect the apparent isotopic ratio of a trace gas in a mixture of gases. We empirically determine ME by measuring air and air-equilibrated water (AEW) standards and employing an optimization routine, such that squared deviations between measured AEW isotope ratios and known solubility equilibrium values are minimized.

BCO-DMO Processing:
- removed 'NaN' as a missing data identifier (missing data are blank/empty in the final csv file);
- renamed fields to comply with BCO-DMO naming conventions;
- converted the Sampling_date column to YYYY-MM-DD format.

NSF Award Abstract:
The proposed work brings together the fields of chemical oceanography, ocean modeling, and solid Earth geochemistry to develop the stable isotope composition of heavy noble gases dissolved in seawater as novel physical tracers of air-sea gas exchange. Noble gases represent ideal tools for quantifying physical processes due to the fact that they are chemically inert. Because argon (Ar), krypton (Kr), and xenon (Xe) isotope ratios have distinct solubility and diffusivity ratios, as recently quantified in laboratory experiments, they complement existing bulk noble gas measurements in seawater by adding new constraints with unique sensitivities. Precise constraints on air-sea exchange of inert gases are paramount to properly quantifying production, consumption, and physical transport of biogeochemically important gases (such as carbon dioxide and oxygen) as well as ventilation age tracers (such as sulfur hexafluoride and CFCs). Additionally, global circulation models (GCMs) routinely underestimate deep-ocean ventilation compared to noble gas observations. Introducing these new isotopic constraints into model simulations will help identify physical processes related to deep-water formation that require improvement in future GCM development. Because the overturning circulation is closely tied to projections of future climate, by both the transports of radiative gases and heat into the deep ocean, there is broad international interest in improving future model projections. Therefore, adding high-precision noble gas isotope measurements to the existing body of research on inert gases in seawater will provide valuable new constraints for both the marine biogeochemistry and physical oceanography communities. Education and training of a graduate student and postdoctoral scholar will contribute to the human resource base of the United States.

The proposed work will develop high-precision Ar, Kr, and Xe stable isotope ratios in seawater as new oceanographic tracers. Along with a 2018 pilot study, the proposed measurements represent the first high- precision Kr and Xe isotope ratio analyses in seawater. A key goal of this project is to test two specific hypotheses for the observed undersaturation of Ar, Kr, and Xe throughout the deep ocean: (1) rapid cooling-induced gas uptake by the surface ocean during deep-water formation with insufficient time for equilibration before sinking, or (2) subsurface cooling caused by melting of glacial ice, leading to the dissolution of air bubbles trapped in ice. Whereas both of these non-mutually exclusive processes produce similar patterns of heavy noble gas undersaturation, the isotope ratios of these gases are well suited to distinguish the relative importance of each process. Specifically, theoretical predictions suggest a decrease in heavy-to-light isotope ratios from the kinetic fractionation associated with rapid surface ocean gas uptake, but an increase in these ratios from the input of gravitationally enriched glacial meltwater. Other goals include: (a) comparing observations to model simulations to identify successes and shortcomings of GCM representations of deep-water formation processes, and (b) a year-long time series of surface-ocean observations from the SIO pier to test models of isotopic fractionation associated with bubble injection and upwelling, with implications for the saturation of biogeochemically important gases. This work will improve upon a recent method for dissolved noble gas isotopic analysis by increasing sample sizes and refining purification techniques to achieve a >60% improvement in precision.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.