Dataset: GP15 Ba Isotope Ratios Leg 2

The sampling procedure is similar to what was used in the previous U.S. GEOTRACES cruises (GA03 and GP16). Particles were sampled using in situ battery-powered pumps (McLane WTS-LV) that were set up with two "mini-MULVFS" filter holders (QMA and PES; Bishop et al., 2012). Each filter holder contained a 142 mm diameter, 51 μm pore-size polyester pre-filter, and a pair of quartz fiber Whatman QMA filters ('QMA side') or a pair of 0.8 μm pore-size Supor brand polyethersulfone filters ('PES side') downstream of the pre-filter. (Note that only the top PES filter was processed for Ba isotope analysis.)

All filters were acid-leached in a HEPA-filtered clean environment prior to the cruise, and assembled onto the filter holders in a HEPA-filtered trace-metal clean bubble on board. Pumps were deployed on a metal-free hydrowire (Hytrel-jacketed Vectran) and operated for 4 h, pumping seawater in situ through both filter holders in parallel. The pumping speed was initially programmed as 8 L/min. Typically 1,000–1,200 L (median: 1091 L) of seawater passed through the QMA side, and 350–450 L (median: 403 L) through the Supor side flowpaths. Two complete sets of blank filters ("QMA" set and "Supor" set) were deployed with pumps in each cast. The dipped blank filters were processed identically to samples.

Immediately upon recovery of the pumps, the sample holders were brought into the clean bubble, and the filters were cut using a ceramic rotary blade (Cadence Inc.). For the small size fraction (SSF, 0.8–51 μm), Supor filters from the Supor side filter holder were cut into 1/16 or 1/8 wedges. SSF filter wedges were then dried at room temperature on acid-leached polystyrene "eggcrate" grids in a laminar flow bench and then stored in particle-free cleanroom polyethylene bags (KNF Flexpak).

Filter wedges were prepared for analysis by leaching in 40 mL of 0.6 M HCl (hydrochloric acid) at 80 °C for 16 h. (Note that particulate samples from St. 29 and 39 of GP15 were dissolved using the protocols developed in Phoebe Lam’s lab, and thus differ slightly from those used at the other stations. Given that both methods achieve near-quantitative yields for particulate Ba (e.g., Bishop et al., 2012; Planquette & Sherrell, 2012), we do not expect any systematic offsets arising from these different pre-treatments.) In both cases, the leachate was dried and reconstituted in 1 M HCl, from which an aliquot containing ~10 % of the sample was taken for multi-element analysis. The aliquot was reconstituted in 2 % HNO3, spiked with indium to achieve a final [In] of ~1 ng/mL and analyzed using a ThermoFisher iCAP reverse quadrupole ICP-MS (inductively coupled plasma mass spectrometer) at the WHOI Plasma Facility. Quantification was achieved by comparing indium- and blank-corrected ion beam intensities measured in samples to those of a reference curve constructed from serial dilutions of a reference standard containing known element concentrations. These values were used to calculate particulate trace metal concentrations in seawater, and guide spiking for Ba isotope analysis.

Samples containing sufficient Ba for isotope analysis were spiked with an appropriate quantity of ¹³⁵Ba-¹³⁶Ba double spike to achieve a spike:sample ratio of between 1-2. Following spiking, samples were dried, fluxed overnight in a 1 mL of mixture containing equal parts concentrated nitric acid and hydrogen peroxide, dried, and reconstituted in 2 M HCl for ion-exchange chromatography. Chromatography protocols are detailed in Horner et al. (2015). Following purification, samples were reconstituted in 2 % nitric acid and analyzed for δ¹³⁸Ba (Ba isotope compositions) using a ThermoFinnigan Neptune multicollector ICP-MS, also situated at the WHOI Plasma Facility. Samples were aspirated at 140 μL/min, desolvated using an Aridus II, and introduced into the instrument using 1 L/min Ar carrier gas containing 2-5 mL/min admixed nitrogen. Samples are measured in low-resolution mode relative to concentration- and spike:sample-matched aliquots of NIST SRM 3108 (≡0 ‰). Samples are analyzed between 2-4 times, and Ba-isotopic compositions calculated using an iterative, geometric-based deconvolution of spike-sample mixtures. Resultant δ¹³⁸Ba are normalized to a moving average of the nearest four analyses of NIST SRM 3108. The accuracy of the NIST normalization is monitored using a secondary standard, analyzed in place of every 11th sample. Typical 2σ precision for the complete protocol is ±0.03 ‰ for samples containing >40 ng of Ba, which is an appropriate level of uncertainty given replicate analyses of filters conducted during the course of this study.