Dataset: Noble gas concentrations from water samples collected in August and September 2016 during R/V Neil Armstrong cruise AR7-02 as part of the Overturning in the Subpolar North Atlantic Project (O-SNAP)

At each station, conductivity-temperature-depth (CTD) casts were made with a SeaBird 911Plus equipped with additional sensors measuring oxygen, beam transmission, fluorescence, and turbidity. Sensors were calibrated by the manufacturer before and after the cruise. The conductivity sensors were further calibrated using water sample salinity measurements. A rosette with twelve 10-liter Niskin bottles was deployed with the CTD to collect water samples. Noble gas samples were acquired from the Niskin bottles using gravity feed-through TYGON tubing to fill lengths of 5/8" copper refrigeration tubing (trapping ~45 grams water in replicate pairs), then each was crimp sealed using a hydraulic press.

The extracted gases are purified, separated, and measured mass spectrometrically using a third-generation, WHOI-constructed, statically operated, helium isotope mass spectrometer of branch tube design for fully simultaneous collection of 3He and 4He with improved ion optics. It employs a high-emission Nier-type ion source. The 4He branch has a Faraday Cup detector with a low-noise FET-input electrometer and precision high-frequency VFC for digital signal integration. The 3He branch uses a Galileo Channeltron operating in pulse counting mode, with high-speed preamplifier and discriminator electronics. The fully automated sample processing line is optimized for processing extracted water samples, and combines a three-stage cryogenics system (Stanley 2009) with a Pd-catalyst and dual SAES-707 getters for the removal of water vapor, the purification of reactive gases, and the quantitatively reproducible separation of the 5 noble gases (He, Ne, Ar, Kr, and Xe). Helium isotopes (3He, 4He) are measured using the magnetic sector dual-collecting mass spectrometer to a reproducibility of 0.1%, and the other noble gases using a quadrupole mass spectrometer (QMS) with a triple mass filter and an electron multiplier operated in pulse counting mode. The lighter noble gases (He, Ne, and Ar) are determined using peak-height manometry while the heavier noble gases (Kr and Xe) are measured using a newly developed, modified ratiometric multi-isotope dilution method. The system achieves reproducibility of gas standards of 0.1% for He, Ne, Ar, Kr, and Xe.