Dataset: GP16 Particulate Th and Pa
Deployment: TN303

Size-fractionated particles were collected using McLane Research in-situ pumps (WTS-LV) that had been modified to accommodate two flowpaths (Lam and Morris Patent pending).   The wire-out was used to target depths during deployment, and a self-recording Seabird 19plus CTD deployed at the end of the line and RBR data loggers attached to three of the eight pumps were used to correct for actual depths during pumping.

Filter holders used were 142 mm-diameter “mini-MULVFS” style filter holders with two stages for two size fractions and multiple baffle systems designed to ensure even particle distribution and prevent particle loss (Bishop et al. 2012).  One of two filter holder/flowpaths was loaded with a 51um Sefar polyester mesh prefilter followed by paired 0.8 um Pall Supor800 polyethersulfone filters.  Each cast also had “dipped blank” filters deployed.  These were the full filters sets (prefilter followed by paired Supor filters) sandwiched within a 1 um polyester mesh filter, loaded into perforated polypropylene containers, attached with plastic cable ties to a pump frame, and deployed.  Dipped blank filters were exposed to seawater for the length of the deployment and processed and analyzed as regular samples, and thus functioned as full seawater process blanks.  We analyzed either quarter (LDEO) or eighth (Minnesota) portions of the top and bottom filters from the “dipped” blank from 1 or more depths for all stations.

All filters and filter holders were acid leached prior to use according to methods recommended in the GEOTRACES sample and sample-handing Protocols (Geotraces 2010).

Analytical methods for particulate radionuclides:

LDEO:

Filters were folded into 60 mL Teflon jars and weighed aliquots of artificial isotope yield monitors 229Th (1 pg) and 233Pa (0.3-0.4 pg) and 7-8 mg dissolved Fe were added to each sample, which then sat overnight in 5 mL 16M HNO3  [All acids and bases used were Fisher Chemical OPTIMA grade or equivalent from Trace Metal grade acids re-distilled in a Savillex™ DST-1000 sub-boiling still]. The next day, the filters were heated for ~1 hour at 180 deg C, at which point 4-5 ml HClO4 was added and the hot plate temperature was increased to 220 deg C. Samples were heated until dense white fumes appeared. After 10-20 minutes, the samples were covered with a Teflon watch cover. After 30-60 minutes, rapid oxidation of the Supor material would occur, at which point the Supor material was almost completely broken down. The watch glasses were removed and beaker walls were rinsed down with 3 ml 8N HNO3. Ten drops of HF were added, and the samples were heated at 220 deg C until the HClO4 dried down to a viscous residue.

The sample residue was taken up in dilute HCl, and transferred to 50 mL centrifuge tubes with Milli-Q water rinses. Fifteen to 25 drops of NH4OH were added to raise pH to 8-8.5 when iron (oxy)hydroxide precipitated. This precipitate was then centrifuged, supernatant was decanted, and the precipitate was washed with Milli-Q H2O. These step were repeated. The precipitate was then dissolved in 12 M HCl, ready for a series of anion-exchange chromatography steps to purify Th and Pa, as outlined in Anderson et. al., 2012. The purified Th and Pa solutions were dried down at 180-220 deg C in the presence of 2 drops of HClO4 and taken up in 0.5 mL of 0.16 M HNO3/0.026 M HF for mass spectrometric analysis.

Concentrations of 232Th, 230Th and 231Pa were calculated by isotope dilution, relative to the calibrated tracers 229Th and 233Pa added at the beginning of sample processing. Analyses were carried out on a Thermo-Finnegan ELEMENT XR Single Collector Magnetic Sector ICP-MS, equipped with a high-performance Interface pump (Jet Pump), and specially-designed sample (X) and skimmer (Jet) cones to ensure the highest possible sensitivity. All measurements were made in low-resolution mode (∆m/M≈300), peak jumping in Escan mode across the central 5% of the flat-topped peaks. Measurements were made on a MasCom^TM SEM; 229Th, 230Th, 231Pa and 233Pa were measured in Counting mode, while the 232Th signals were large enough that they were measured in Analog mode. Two solutions of SRM129, a natural U standard, were run multiple times throughout each run. One solution was in a concentration range where 238U and 235U were both measured in counting mode, allowing us to determine the mass bias/amu (typical values varied from -0.01/amu to 0.03/amu). In the other, more concentrated solution, 238U was measured in Analog mode and 235U was measured in Counting mode, yielding a measurement of the Analog/Counting Correction Factor.  These corrections assume that the mass bias and Analog Correction Factor measured on U isotopes can be applied to Th and Pa isotope measurements. Each sample measurement was bracketed by measurement of an aliquot of the run solution, used to correct for the instrumental background count rates. To correct for tailing of 232Th into the minor Th and Pa isotopes, a series of 232Th standards were run at concentrations bracketing the expected 232Th concentrations in the samples. The analysis routine for these standards was identical to the analysis routine for samples, so we could see the changing beam intensities at the minor masses as we increased the concentration of the 232Th standards. The 232Th count rates in our Pa fractions are quite small, reflecting mainly reagent blanks, compared to the 232Th signal intensity in the Th fraction. The regressions of 230Th, 231Pa, and 233Pa signals as a function of the 232Th signal in the standards was used to correct for tailing of 232Th in samples.

In addition to laboratory procedural blanks (reagents/labware blanks) and periodic measurements of an intercalibrated working standard solution of 232Th, 230Th and 231Pa, SW STD 2010-1, referred to by Anderson et al. (2012), the participating labs also analyzed “dipped blank” filters, mentioned above, to determine the total blank, associated with the sample collection and handling in addition to the laboratory procedure. LDEO measured top and bottom dipped blanks separately, while UMN measured top and bottom dipped blanks together.

We pooled all procedural blank-corrected “dipped” blanks separately by institution to determine filter blank corrections. That is, the LDEO data were corrected by the LDEO average dipped blank values (both top and bottom included), while the UMN data were corrected by the UMN average dipped blank values. Averages for “Dipped” small particle filter blanks for a 1/4 filter fraction 232Th, 230Th, and 231Pa at LDEO were 21.52 +/- 19.32 pg, 1.17 +/- 0.9 fg, and 0.07 +/- 0.07 fg, respectively.

Further details on analysis of seawater particulate radionuclides are given by Anderson et al. (2012).

UMN:

Filters were folded into 30 mL Teflon beaker and weighed aliquots of the artificial isotope yield monitors 229Th and 233Pa. Filters were first completely submerged in 7N HNO3 acid combined with 10 drops HF, tightly covered with a Teflon threaded cap and heated for 10 hours at 200 deg F so that the particulate sample was dissolved/leached under pressure. The leach solution was then transferred to a second acid-cleaned Teflon beaker separate from the residual filter. Five drops of HClO4 were then added to the leach solution in the second beaker.  The original beaker walls and caps were washed with small amounts of weak HNO3 and the resulting solution added to the second beaker. The solution was then dried down and was taken up in 2N HCl, and transferred to 15ml centrifuge tubes along with a 2N HCl rinse. One drop of dissolved Fe and six to nine drops of NH4OH were added to raise pH to 8-8.5 at which time iron (oxy)hydroxide precipitated.  This precipitate was then centrifuged, decanted, washed with deionized H2O (>18 MΩ), centrifuged, and dissolved in 14M HNO3 and transferred to a Teflon beaker. It was then dried down and taken up in 7N HNO3 for anion-exchange chromatography using AG1-X8, 100-200 mesh resin and a polyethylene frit. Initial separation was done on Teflon columns (internal diameter ~ 0.35cm) with a ~0.55 ml column volume (CV).  The sample was loaded in one CV of 7N HNO3, followed by 1.5 CV of 7N HNO3, 3 CV of 8N HCl (collect Th fraction), and 3 CV of 8N HCl combined with 0.015N HF (collect Pa fraction).  The Pa and Th fractions were then dried down in the presence of 2 drops of HClO4 and taken up in 7N HNO3.  They were each passed through second and third columns (each with ~0.55 ml column volumes) using similar elution schemes.  The final Pa and Th fractions were then dried down in the presence of 2 drops of HClO4 and dissolved in weak nitric acid for analysis on the mass spectrometer.

Concentrations of 232Th, 230Th and 231Pa were calculated by isotope dilution using nuclide ratios determined on a Thermo-Finnigan Neptune mass spectrometer. All measurements were done using a peak jumping routine in ion counting mode on the discreet dynode multiplier behind the retarding potential quadrupole.  A solution of 233U-236U tracer was run to determine the mass bias correction (assuming that the mass fractionation for Th and Pa are the same as for U). Each sample measurement was bracketed by measurement of an aliquot of a wash solution, used to correct for the instrument background count rates on the masses measured.

Particulate samples were analyzed in batches of 37 to 39.  An aliquot of an intercalibrated working standard solution of 232Th, 230Th and 231Pa, SW STD 2010-1, was added to a separate acid-cleaned Teflon beaker along with weighed aliquots of 229Th spike and 233Pa spike. Spike and Standard were equilibrated for 3 days. The solution was then dried down and taken up in 7N HNO3 for anion-exchange chromatography using AG1-X8, 100-200 mesh resin and a polyethylene frit, and processed like a sample. In addition to laboratory procedural blanks (reagents/labware blanks), a number of “dipped blank” filters were also processed like samples, to determine the total blank, associated with the sample collection and handling, in addition to the laboratory procedure.

At UMN, averages for “Dipped” small particle filter blanks for a 1/8 filter fraction were 13.06 +/- 15.43 pg 232Th, 0.61 +/- 0.55 fg 230Th, and 0.05 +/- 0.05 fg 231Pa respectively. For large particles, the blanks for a 1/8 filter fraction were 11.89 +/- 9.90 pg 232Th, 0.57 +/- 0.60 fg 230Th, and 0.03 +/- 0.02 fg 231Pa.

Further details on Pa and Th analysis at the U. Minnesota laboratory are given in Shen et al. (2002, 2003, 2012), and Cheng et al. (2000, 2013).