Collection

Samples were collected on STING I and STING II during cruises AE2305 (R/V Atlantic Explorer, Feb 20th - Mar 5th 2023), EN704 (R/V Endeavor Jul 1-12, 2023), and 23WFS02 (R/V Sallenger, Jun 3-7, 2023). Surface water salinity and temperature measurements were taken from the ship underway system (AE2305 and EN704) and a flow-cell (23WFS02; YSI ProDSS; Chestang et al., 2025), and from the ship CTD for intermediate and bottom waters.

Surface samples were collected at all stations via submersible pump lowered 1.5 m below the water’s surface. Intermediate depth and bottom water samples were collected at stations along three transects that crossed the entire shelf from nearshore to the shelf break via Niskin bottles on the CTD Rosette. Samples ranged from 114 to 170 L in volume on AE2305 and EN704, and ~45 – 60 L in volume on 23WFS02 (Chestang et al., 2025).

Samples were passed through a 1 µm prefilter to remove suspended particles before being filtered at < 1L/min onto a MnO2-coated acrylic fiber to adsorb Ra. Samples were washed with Ra-free deionized water to remove salts and remaining particles, then dried to a 1:1 fiber:water mass ratio (Moore, 2008).

Measurements

Short-lived 223Ra and 224Ra were measured underway using a delayed coincidence counter (RaDeCC) (Moore and Arnold 1996), following recommended best-practices (Diego-Feliu et al. 2020). Additional measurements were performed in the laboratory one month (228Th) and two months (227Ac) post-collection to assess parent activities supporting 223Ra and 224Ra. 232Th standards in equilibrium with 228Th were measured separately in the lab and underway to assess detector efficiencies for 224Ra and 223Ra (Moore & Cai, 2013); background counts were similarly measured both in the lab and at sea. Errors are propagated from counting statistics, chance-coincidence corrections, and detector efficiencies (Diego-Feliu et al. 2020). Short-lived Ra isotope specific activity from 23WFS02, with accompanying physicochemical parameters, is reported in Chestang et al. (2025). Following, the Mn-coated acrylic fibers were combusted at 850 °C for 16 hours, homogenized, sealed with epoxy resin, and incubated for a minimum of three weeks until 222Rn was in secular equilibrium with its parent 226Ra. Long-lived Ra isotopes measured via gamma spectroscopy in a Ge well-type detector for 226Ra (352 keV photopeak) and 228Ra (average of 338 and 911 keV photopeak’s), with efficiencies calibrated using standards prepared in the same fashion as the seawater samples (Charette et al., 2001).