Dataset: Water column dissolved radium-226 and radium-228 from Leg 1 (Seattle, WA to Hilo, HI) of the US GEOTRACES Pacific Meridional Transect (PMT) cruise (GP15, RR1814) on R/V Roger Revelle from September to October 2018

At sea, Radium-226 samples were collected following the GEOTRACES cookbook. Samples were collected from a 30-liter (L) Niskin mounted directly above a McLane large volume in situ pump WTS-LV (>1000 meters) or on a CTD Rosette (<1000 meters). These samples (10-25 L) were gravity-filtered through 15 grams (g) of manganese oxide impregnated acrylic fiber at approximately 0.5 liters per minute (L/min). The filtrate was collected inside of a graduated rigid sided container and the volume was recorded. A mass was also obtained by using a hand-held digital balance.

The Mn fibers from the Niskin bottles were rinsed with deionized water back at Woods Hole Oceanographic Institution (WHOI), partially dried, and sealed within a fiber holder after being flushed with helium equivalent to 30 column volumes. Samples were stored for a minimum of 12 days and analyzed for 226Ra via 222Rn ingrowth using alpha scintillation counting (Key et al., 1979). Samples were counted for 1-20 hours depending on the activity on the fiber, which resulted in counting uncertainties of ~2% dependent upon 226Ra content on the fiber and sample volume.

Radium-228 was collected on manganese-coated cartridges filtered at depth using an adapted McLane in situ pump (WTS-LV). The pumps were programmed to filter 1500-2000L of seawater per deployment. The cartridges were rinsed with radium-free fresh water and excess moisture was removed using compressed air. Samples were analyzed multiple times using a delayed coincidence counter (RaDeCC) for determination of radium-228 via thorium-228 ingrowth. At selected stations, samples were ashed in a muffle furnace at 820 degrees Celsius for 24 hours and the fiber ash was transferred to polystyrene vials, sealed with epoxy (to prevent 222Rn loss), and counted on high purity, well-type germanium detectors to measure 228Ra, using the lines of 228Ac (338 keV, 911 keV, and 969 keV).

In addition to the water column samples, large volume surface samples were collected by pumping from a depth of 2 meters through a single Mn-cartridge accompanied by a 15-25-liter sample that was passed through a 15 g Mn-fiber column to retain radium-226.