Clean seawater samples were collected using a GEOTRACES CTD referred to as GT-C/12-liter GoFlo, and also from the Super-GeoFISH towed surface vehicle and from the ship's underway system for 1 sample where bad weather did not allow for the GeoFISH deployment. For more information, see the cruise report. Water samples were filtered through pre-cleaned, 0.2-micrometer (µm) Pall Acropak Supor filter capsules as described elsewhere (e.g., Cutter et al., 2014). Filtered water was collected in 125-milliliter (mL) HDPE bottles (Nalgene) that had been precleaned following the GEOTRACES protocol for trace-metal clean sampling.

Seawater samples were analyzed using a standard methodology for determining dissolved barium (Ba) concentrations in seawater samples. This process involves the use of barium-135 (135Ba) isotope dilution coupled with analysis via inductively coupled plasma mass spectrometry (ICP-MS) using a ThermoScientific iCAP Q operated in Kinetic Energy Dispersion (KED) mode. Solutions containing 100 microliters (μL) of seawater sample, 100 μL of a tellurium (Te) solution, which serves as a monitor for instrumental mass bias, and 100 μL of a 135Ba spike solution were analyzed, eluted to 2 mL with 2% HNO3. The enriched 135Ba isotope spike is BaCO3 obtained from Oak Ridge National Laboratories (batch no.142890) The ratios of barium isotopes, specifically the ratio of 135Ba/137Ba, are measured in the samples. The spike-to-sample ratio is then calculated using standard isotope dilution equations, and the resultant [Ba] in the sample (in nanomoles) was determined based on the quantity of spike added. Optimal spiking is achieved when spiked samples exhibit 135Ba/137Ba between 1.6 and 32.7 (Stracke et al., 2014), noting that our samples exhibited 135Ba/137Ba between 5.65 and 23.40. [Ba] was converted to nanomoles per kilogram (nmol/kg) based on the ocean's average density at the surface, 1.027 grams per cubic centimeter (g/cm3).