Radon samples were collected and analyzed in two ways: as discrete grab samples and continuously from the ship’s underway system. This dataset contains the discrete grab samples (including February and June/July collections). The underway (continuous) measurements from July 2023 are provided in the related dataset.

For discrete radon-222 samples, six-liter seawater samples were collected into an 8 L Nalgene high density polyethylene jerrican (hereafter referred to as jerrican) with a three-port cap fitted with tubing, as described by Stringer and Burnett (2004). This approach is similar to the commercially available Big Bottle System (Big Bottle System Manual, 2018) but allows for a sufficient headspace volume of gas, to increase the total sample volume and thus a lower detection limit. Samples were immediately sealed with screw-compressor clamps on the tubing in two locations to prevent radon-222 loss from the headspace. All discrete radon-222 samples were analyzed within 48 hours of collection. Jerricans were connected either to a RAD8 or RAD7, both commercially available radon-in-air detectors (Durridge, Inc.). Detectors were initially purged with Drierite to reduce internal humidity to below 10% and background counts were recorded. Following Stringer and Burnett (2004), jerrican samples were degassed with the built-in air pump for 60 minutes in a closed-air loop. Samples were counted for a minimum of three hours (“Sniff” mode) to reach statistically significant counts. Following, sample volume was precisely measured using a graduated cylinder.

For underway radon-222 analyses, water was pumped into a showerhead gas equilibrator (RAD-AQUA, Durridge, Inc.) at a flow rate > 2 liters min^-1 with a fifteen-minute counting interval, following Burnett et al. (2001).

For both discrete and underway measurements, data files were downloaded using CAPTURE software (Durridge, Inc.). Radon-in-air was calculated by dividing the total counts-per-minute (cpm; background corrected) in window-A by the RAD7/8 detector efficiency. For discrete samples, additional corrections were made as the ratio of total air volume (volume of tubing/drying column connector, jerrican, and chamber) to chamber air volume. For both discrete and underway measurements, in-situ seawater salinity was taken from the ship underway system or CTD (bottom waters), and temperature of the radon-222 sample from the internal air temperature of the RAD7/8 system. Radon-in-air to radon-in-water solubility corrections were made following Schubert et al. (2012), and samples were decay-corrected to the time of collection. Errors are propagated from counting statistics and detector efficiencies. Specific activities are reported as total radon-222 (supported from the in-situ decay of its parent Ra-226 plus excess radon-222).