Dataset: Neodymium (Nd) isotopes measured from dissolved seawater on 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

Sampling methods at sea followed the GEOTRACES cookbook (Cutter et al., 2017). Water samples were collected in 10-liter Niskin bottles with nylon-coated closure springs and Viton O-rings. The seawater collected was filtered with Pall Acropak 500 filters (0.8- and 0.45-micrometer (μm) pore size) into LDPE cubitainers. Shallow casts of the ODF rosette collected single 10-liter samples for Th isotopes, 231Pa, and Nd isotopes. Otherwise, 4 to 5 liters were collected for each sample. After filtering, samples were adjusted to a pH of ~2 with 20 milliliters (mL) 6 M HCl (redistilled Fisher Scientific Trace Metal grade HCl). Samples were stored and shipped to labs double-bagged at ambient temperatures.

All Neodymium (Nd) isotope samples were preconcentrated using C18cartridges (Waters Corp., Sep-Pak classic, 360 mg, 55-105 μm) loaded with complexing agent of 2-ethylhexyl hydrogen phosphate (HDEHP) and 2-ethylhexyl dihydrogen phosphate (H2MEHP) mixture (Sigma Aldrich), following Pahnke etal., 2012. Specifically, C18 cartridges were first cleaned in a 0.5 N HCl bath overnight, passed through 10 mL of 6 N HCl and then flushed with >500 mL of Milli-Q water. Prior to sample introduction, 300 μL of complexing agent HDEHP/H2MEHP was loaded on a clean cartridge. Seawater samples were adjusted to pH ≈ 3.5 by adding Optima® ammonium hydroxide, then were pumped through the cartridges at 20 mL per minute by a peristaltic pump. Cartridges were first eluted with 10 mL of 0.01 N HCl to remove barium. The cartridges were then eluted with 35 mL of 6 N HCl at 10 mL per minute by a peristaltic pump to collect rare earth elements (REEs). REEs were dried and further purified using column chemistry (Biorad® AG-50 followed by Eichrom® Ln-Spec) to separate Nd from the other REEs (Pahnke etal., 2012). All Nd isotopes were measured at Oregon State University (OSU) with a Nu3 multicollector-inductively coupled plasma-mass spectrometer (MC-ICP-MS). The instrument was coupled with an Apex-2 desolvating nebulizer. All measured Nd isotopic compositions were corrected for mass fractionation using an exponential law with 146Nd/144Nd = 0.7219. The standard JNdi-1 was measured between every sample as sample-standard bracketing. JNdi-1 standards of 50 parts per billion (ppb), yielded internal reproducibility (2σ) of ±0.30 εNd units. External reproducibility was determined by multiple (n= 55) runs of an in-house Specpure Nd standard (2σ error = 0.5 εNd units; See Table 1). The total procedural blank was measured by quadrupole ICP-MS at ~3.5 picomoles (pM) Nd. Our Limit Of Detection (LOD) for Nd isotopic analyses on the Nu3 was set to <2 V total Nd; generally equivalent to a solution of ~3 ppb Nd, or 3 nanograms (ng) Nd. This LOD is somewhat arbitrary, but was established in light of unacceptable error on the 143Nd/144Nd ratios produced below this level.