Clean seawater samples were collected using a GEOTRACES CTD referred to as GT-C/12L GoFlo, and also from the Super-GeoFISH towed surface vehicle. 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; Hatta et al., 2015). Filtered water was collected in 125-milliliter (mL) HDPE bottles (Nalgene) that had been precleaned by soaking in hot 1.2 M HCl (reagent grade) for at least 8 hours with subsequent thorough rinsing with ultrapure distilled deionized water (Barnstead E-pure).

Dissolved Rare Earth Elements (REEs including Y), were determined using 14 mL of sample, which was spiked with a mixture of isotopically-enriched Nd-145, Sm-149, Eu-153, Gd-155, Dy-161, Er-167, and Yb-171 (Oak Ridge National Labs). Each spike was >90% enriched in the listed isotopes. The sample/spike ratio was chosen so as to have the analytical isotope ratios approximately the geometric mean of the natural and enriched spike isotope ratios. Samples were then extracted/pre-concentrated using a SeaFAST system (Elemental Scientific, Inc.) operated in offline mode. A similar online SeaFAST extraction procedure is described by Hathorne et al., 2012. The extracted samples were subsequently analyzed using a Thermo-Fisher high-resolution ICP-MS with an Apex-FAST high-efficiency sample introduction system with Spiro desolvator (Elemental Scientific, Inc.). The instrument was operated in low resolution. The enriched isotope spikes also served to provide counts/sec calibration factors for elements that were not spiked with enriched isotopes. This calibration was also examined with a standard made in dilute nitric acid. Precision and recovery were checked by analysis of a large-volume composite seawater sample. Spiked (with a natural isotopic abundance elemental spike) and unspiked aliquots of this sample were analyzed twice in each analytical run. A Ba standard was also run to check for BaO+ interference on several isotopes and Ba in the extracted samples was also monitored. Because the extraction resin in the SeaFAST system (Nobias PA-1) discriminates against Ba, plus the reduction of the BaO+ interference by the desolvation system, BaO+ was less than 0.1% of the counts in Eu-151, Eu-153, Gd-155, and Gd-157. Tests also revealed no significant low REE oxide interference on mid-/high-REEs.

Dissolved Nd was determined in a separate seaFAST extraction, but with essentially the same methodology as the transition metals as described in related dataset "GP15 Dissolved Ba Cd Cu Ga Mn Ni and Pb Leg 2" (https://www.bco-dmo.org/dataset/836121). The samples were spiked with isotopically-enriched Nd-145. Nd was determined in low resolution.

932559_v1_gp15_dissolved_rees_leg2.csv