Seawater sampling: The procedures used on the GEOTRACES GP17-OCE cruise were identical to those described for the GEOTRACES GP15 and GP16 cruises by Kadko et al. (2017, 2020). Briefly, a 1 horsepower (hp) centrifugal pump on deck pulled seawater from selected depths via a 1.5-inch PVC hose. 400 to 700 liters (L) of seawater were delivered to large plastic tanks in this manner. Beryllium-7 (Be-7) was then extracted by pumping the seawater through 200 grams of iron impregnated acrylic fibers at a rate of ~10 liters per minute (Lai et al., 1988; Krishnaswami et al., 1972; Lee et al., 1991). A portable CTD (model: YSI EXO1) was attached to the end of the hose so that temperature, depth, and salinity could be recorded.
Aerosol sampling: Aerosol samples were collected according to protocols set forth in the GEOTRACES cookbook using a Tisch TE-5170V-BL high-volume aerosol sampler modified to collect 12 replicate samples on 47-millimeter (mm) diameter Whatman-41 (W-41) filters (Wallace et al, 1977; Baker et al., 2006). In order to minimize filter blanks, the W-41 filters were pre-cleaned inside a HEPA-filtered laminar flow hood using three cycles of leaching with 0.5M HCl (Optima) and then rinsing with ultra-high purity water (UHP water) according to trace element protocols (Morton et al., 2013; similar to Baker et al., 2006). Sector control, which was accomplished via a Campbell Scientific data logger, was set such that the sampler would only operate during winds of >0.5 meters per second (m/s) and from ± 60° of the ship's bow to avoid sampling air influenced by the ship's exhaust. Three replicate filters from each deployment, placed in PetriSlides, were sent to FIU for Be-7 analysis.
Analytical procedures: Samples were counted for Be-7 onshore using low background germanium gamma detectors at Florida International University (FIU). FIU's facilities include four HPGe detectors: three Ortec Coaxial GEM series detectors and one Canberra Broad Energy gamma detector. For aerosols, the three replicate 47 mm filters were stacked in a Petri dish for counting. For seawater, the fibers were dried and then ashed. The powder remaining after ashing was pressed into a 5.8-centimeter (cm) diameter pellet, and the pellet thickness was measured. The iron hydroxide pellet was placed in a Petri dish for gamma counting. Be-7 has a readily identifiable gamma peak at 478 kiloelectron volts (keV), and the spectra were evaluated using Maestro (Ortec) or Genie 2000 (Canberra) software.
The detectors were calibrated for each geometry by adding a commercially prepared mixed isotope solution of known gamma activities (Eckert & Ziegler Mixed Nuclide Solution 7500) to three W-41 filters (for aerosol samples) and pellets of various thicknesses (for seawater) to derive a calibration curve using peaks associated with following isotopes: Sn-113 @ 392 keV, Sr-85 @ 514 keV, Cs-137 @ 662 keV. The counting efficiencies of the four detectors ranges from 0.05686 to 0.11415 for Be-7 on aerosol filters. And for the seawater pellets from this cruise, the counting efficiencies ranged from 0.03480 to 0.08590.
