Seawater sampling: Procedures used during GEOTRACES GP17-ANT were identical to those described for previous U.S. GEOTRACES cruises (Kadko, 2017; Kadko et al., 2020; He et al., 2025). Briefly, a centrifugal pump on deck was used to draw 500-750 liters (L) of seawater from predetermined depths via a 1.5-inch PVC hose to fill large plastic tanks on deck. Be-7 was then extracted by pumping this seawater through 200 grams of iron-impregnated acrylic fibers at a flowrate of ~10 L per minute (Lai et al., 1988; Krishnaswami et al., 1972; Lee et al., 1991). A temperature/depth recorder was attached close to the end of the hose to confirm an accurate sampling depth.

Aerosol sampling: Bulk aerosol samples were collected following procedures described for previous U.S. GEOTRACES cruises (Shelley et al., 2015; Buck et al., 2019; Marsay et al., 2018, 2022). Briefly, for each deployment, one of five Tisch Environmental high-volume (~1 cubic meter air per minute) aerosol samplers was loaded with 12 replicate 47-millimeter (mm) diameter Whatman 41 filters on open-face filter holders (Advantec MFS) installed on a PVC adaptor plate (Shelley et al., 2015). Filters were acid-washed before use to lower trace element blanks (Morton et al., 2013). Samplers were installed above the bridge of RVIB Nathaniel B. Palmer and operation was controlled by wind speed and direction through a Campbell Scientific CR800 data-logger interfaced with a sonic anemometer set up alongside the samplers. In this way, sampling was restricted to periods when relative wind speed and direction were >0.5 meters per second and from within ±60 degrees of the bow of the ship for at least five continuous minutes.

Analytical procedures: Samples were counted for Be-7 using low-background germanium gamma detectors at Florida International University (FIU). For aerosols, three replicate filters from each deployment were stacked in a petri dish for counting. For seawater, the fibers were dried and then ashed. The iron hydroxide powder remaining after ashing was pressed into a 5.8-centimeter (cm) diameter pellet and the pellet thickness measured, then placed in a petri dish for counting. Be-7 has a readily identifiable 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 W41 filters (for aerosol samples) and pellets of various thickness (for seawater samples) to derive calibration curves using peaks associated with the following isotopes: Sn-113 at 392 keV, Sr-85 at 514 keV, Cs-137 at 662 keV. Counting efficiencies of the four detectors at FIU ranged from 0.035304 to 0.05686 for Be-7 on aerosol filters, and from 0.07873 to 0.083779 for pellets derived from seawater samples.