Sampling Methods at Sea

BOTTLE Samples: Sampling methods at sea followed the GEOTRACES cookbook (Cutter et al., 2017). Water samples were collected with a Sea-Bird Electronics CTD carousel fitted with 36 10-liter PVC Niskin bottles, managed and operated by the Ship-based Science Technical Support and the Ocean Data Facility (ODF) of Scripps Institution of Oceanography. The rosette was lowered from the ship on a standard conducting hydrowire. Niskin bottles were equipped with nylon-coated closure springs and Viton O-rings. After collection, seawater was drained through Teflon-lined Tygon tubing and filtered through Pall Acropak 500 filters on deck (gravity filtration, 0.8/0.45 micrometer (μm) pore size) into LDPE cubitainers. In a departure from previous US GEOTRACES cruises, and at the request of the group measuring neodymium (Nd) isotopes, shallow casts of the ODF rosette collected single 10-liter samples for thorium (Th) isotopes, Protactinium-231 (231Pa), and Nd isotopes, rather than each group getting a 5-liter sample for themselves. In the rest of this description, volumes should be doubled for the single 10-liter samples. Approximately 4-5 liters were collected per desired depth for each dissolved sample. Once filtered, samples were adjusted to a pH of ~2 with 20 milliliters (mL) 6 M HCl (redistilled Fisher Scientific Trace Metal grade HCl diluted 1:1 with 18.2 MΩ H2O ), double-bagged, stored in pallet boxes on-deck until the end of the cruise, and then at room temperature once shipped to the participating laboratories for analysis.

FISH Samples: Parameter names include Th_232_D_CONC_ FISH, Th_230_D_CONC_ FISH, and Pa_231_D_CONC_ FISH. Selected samples were collected using a towed pumping system designed to collect uncontaminated water at 2-3 meters depth, indicated by FISH in the parameter name. FISH samples were filtered by a 0.2 µm Osmonics filter capsule. FISH samples from GP15 that were analyzed for Th and Pa were processed at UMN.

Analytical Methods at LDEO

In this section, it should be noted that the following reagents were Fisher Scientific OPTIMA grade: Ammonium Hydroxide (NH4OH), Perchloric Acid (HClO4), and Hydrofluoric Acid (HF). Hydrochloric Acid (HCl) and Nitric Acid (HNO3) were Fisher Trace Metal Grade acids that had been redistilled in Savillex Teflon Sub-Boiling Stills. In the on-shore laboratory, seawater samples were weighed and then aliquots of the artificial isotope yield monitors (spikes) 229Th (~1 picogram (pg)) and 233Pa (0.05-0.17 pg), and 25 milligrams (mg) dissolved iron (Fe), were added to each sample. The 10-liter samples got twice the amount of dissolved Fe as the 5-liter samples, but the same amounts of the yield monitors. After allowing 1 day for spike equilibration, the pH of each sample was raised to ~8.5 by the addition of concentrated NH4OH, which caused iron (oxy)hydroxide precipitates to form. Each sample cubitainer was fitted with a spigot cap, inverted, and the Fe precipitate was allowed to settle for 2 days. After 2 days, the spigots were opened and the overlying water was slowly drained, leaving only the iron oxyhydroxide precipitate and 250-500 mL of water. The Fe precipitate was transferred to centrifuge tubes for centrifugation and rinsing with Milli-Q H2O. The precipitate was then dissolved in 16M HNO3 and transferred to a Teflon vial for a high-temperature (180-200°C) digestion with concentrated HClO4 and HF on a hotplate in a HEPA-filtered laminar flow hood. After total dissolution of the sample, another precipitation of iron (oxy)hydroxide followed and the precipitate was washed with Milli-Q H2O, centrifuged, and dissolved 16 M HNO3. After conversion to concentrated HCl, Th isotopes and Pa were purified by anion-exchange chromatography using 6 mL polypropylene columns each containing 1 mL of Bio-Rad AG1-X8, 100-200 mesh size resin. For 10-liter samples, 2 mL of the same resin was used for the primary column. Details can be found in Anderson et al., 2012. Separate, purified Th and Pa fractions were dried down at 180-200°C in the presence of 2 drops of concentrated 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-Finnigan ELEMENT XR Single Collector Magnetic Sector ICP-MS. To ensure the highest possible sensitivity, the instrument was equipped with a high-performance interface pump (Pfeiffer OnTool Booster 150 "Jet Pump"), high-performance sample (Jet) and skimmer (X) cones, and a desolvating nebulizer, CETAC Aridus I. For increased signal stability, an Elemental Scientific (ESI) Continuum syringe pump system was employed for sample introduction to the Aridus I. Sample uptake rate was ~100 microliters per minute (µL/min) and sample analysis time was on the order of 3 minutes.

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 discrete dynode Secondary Electron Multiplier (SEM). 229Th, 230Th, 231Pa, and 233Pa were measured in Counting mode, while the 232Th signal was large enough that it had to be measured in Analog mode. Two solutions of SRM129, a natural uranium (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 (values varied from -0.5%/amu to +0.2%/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 (typical values varied from 0.9 to 1.1). These corrections assume that the mass bias and Analog/Counting 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 (0.16 M HNO3/0.026 M HF), which was used to correct for the instrumental background count rates. To correct for tailing of 232Th into the minor Th and Pa isotopes, a set of external 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 were quite low after separation of Pa from Th during anion-exchange chromatography, reflecting mainly reagent blanks, compared to the 232Th signal intensity in the Th fraction. The regressions of 229Th, 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. Only in rare cases was a tail correction of 232Th on 231Pa and 233Pa necessary, while it was almost always the case that tail corrections of 232Th on 229Th and 230Th were performed.

Water samples were analyzed in batches of 15. Procedural blanks were determined by processing 4-5 liters of Milli-Q H2O in an acid-cleaned cubitainer acidified to pH ~2 with 6 M HCl (Fisher Scientific OPTIMA grade) as a sample in each batch. Two procedural blanks were processed with each batch, with about half of the procedural blanks acidified at sea during RR1814-15 and the other half acidified in the on-shore laboratory before sample processing. The difference in the procedural blank values for 232Th, 230Th, and 231Pa between acidifying procedural blanks at sea or in the on-shore laboratory was statistically insignificant. An aliquot of intercalibrated in-house standard solutions of 232Th, 230Th, and 231Pa; SW STD 2010-1, referred to by Anderson et al. (2012) was added to an acid-cleaned Teflon beaker along with weighed aliquots of 229Th and 233Pa spike. Spiked SW STDs were equilibrated for at least 1 day. They were then dried down and dissolved in concentrated (12 M) HCl (Fisher Scientific OPTIMA grade) and processed with samples for each batch.

The same amount of the SW STD 2010-1, together with 229Th and 233Pa solutions, were also added to an acid-cleaned cubitainer with ~4-5 liters of Milli-Q H2O. Spikes and SW STD were equilibrated for at least 1 day. The cubitainer with the SW STD was processed equivalently to all sample cubitainers.

Samples were corrected using the pooled average of all procedural blanks analyzed during the processing of RR1814-15 dissolved samples. The average procedural blanks for 232Th, 230Th, and 231Pa are shown in the "Table 1" Supplemental File. The limit of detection (LOD) is the smallest quantity of each isotope in samples that can reliably be detected or that can be statistically distinguished from a procedural blank. The LOD was considered to be two standard deviations above the average of the procedural blanks.

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

Analytical Methods at USM

Processing of samples at USM was very similar to that at LDEO with the main differences being spiking ratios (due to a slightly lower sensitivity ICP-MS) and acid chemistry (no perchloric acid is allowed at the USM location). In the on-shore laboratory, seawater samples were weighed to determine sample size, taking into account the weight of the cubitainer and of the acid added at sea. Then, weighed aliquots of the artificial isotope yield monitors 229Th (10 pg) and 233Pa (~0.8 pg) and 10 mg dissolved Fe were added to each sample. After allowing 1 day for spike equilibration, the pH of each sample was raised to 8-9 by adding ~10-14 mL of concentrated NH4OH (Fisher Scientific OPTIMA grade) which caused iron (oxy)hydroxide precipitates to form. Each sample cubitainer was fitted with a nozzle cap, inverted, and the Fe precipitate was allowed to settle for 2 days. After 2 days, the nozzle caps were opened and the pH~8-9 water was slowly drained, leaving only the iron oxyhydroxide precipitate and 250-500 mL of water. The Fe precipitate was transferred to centrifuge tubes for centrifugation and rinsing with Milli-Q H2O (>18 MΩ) to remove the major seawater ions. The precipitate was then dissolved in 8M HNO3 (Fisher Scientific OPTIMA grade) and transferred to a Teflon beaker for acid digestions. First, the nitric sample solution was dried to near dry at 180-200°C. The sample was then taken up in 1-2 mL 8 M HNO3, the beakers capped and the samples refluxed at 180°C for at least 3 hours. The sample was then cooled, uncapped, retaining all sample drops in the beaker, heated again to 180°C for an HF (Optima) addition of 1 mL. This solution was dried at 180°C to a white precipitate that is dissolvable in optima HCl. After total dissolution of the sample, another precipitation of iron (oxy)hydroxide followed and the precipitate was washed with Milli-Q H2O, centrifuged, and dissolved in 8M HCl (Fisher Scientific OPTIMA grade) for a series of anion-exchange chromatography using 6 mL polypropylene columns each containing a 1 mL bed of Bio-rad resin (AG1-X8, 100-200 mesh size) and a 45 μm porous polyethylene frit (Anderson et al., 2012). The final column elutions were dried down at 180-200°C in the presence of 2 drops of concentrated HNO3 (Fisher Scientific OPTIMA grade) and taken up in 1.0 mL of 0.32 M HNO3 (Fisher Scientific OPTIMA grade) for mass spectrometric analysis. Digestions and columns were done in a standard fume hood, but whenever samples were sealed (i.e., no acid fumes) they were handled in a benchtop HEPA-filtered laminar flow hood.

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-Finnigan ELEMENT XR Single Collector Magnetic Sector ICP-MS. This model lacks the high-performance Interface pump (Jet Pump Aridus I™) mentioned above, but we did utilize the specially designed sample (Jet) and skimmer (X) cones which increased 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™ 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 -0.5%/amu to +0.2%/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 (typical values varied from 0.9 to 1.1). These corrections assume that the mass bias and Analog/Counting 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 (0.32 M HNO3), which was used to correct for the instrumental background count rates. Tailing of 232Th into the minor Th and Pa isotopes was monitored by counting at the half-masses surrounding 230Th and 231Pa. Tailing corrections were typically small (<0.5% and often negligible).

Water samples were analyzed in batches of 14 to 22 (12 batches total). Procedural blanks were determined by processing 4-5 L of Milli-Q H2O in an acid-cleaned cubitainer acidified to pH ~2 with 6 M HCl (Fisher Scientific OPTIMA grade) as a sample in each batch (n = 12 total procedural blanks). A smaller number (n = 3) of "at-sea" blanks were analyzed which were cubitainers filled with MQ-H2O and acidified at sea. "At-sea" blanks fell into the range of blanks reported below. In addition to the procedural blanks, with every batch an aliquot of one of two intercalibrated working standard solutions of 232Th, 230Th, and 231Pa, SW STD 2010-1 referred to by Anderson et al. (2012) and SW STD 2015-1 which has ~6 times lower 232Th activity, were added to acidified MQ-H2O and treated like a sample. Sample concentrations were corrected using the procedural blank analyzed within each batch of samples. Procedural blank, limit of detection and the results of the reference material solutions are reported in the "Table 2" Supplemental File. The limit of detection (LOD) is the smallest quantity of each isotope in samples that can reliably be detected or that can be statistically distinguished from a procedural blank. The LOD was considered to be two standard deviations above the average of the procedural blanks and we have scaled the limit of detection into the equivalent concentration in a 5 liter sample. In some cases our sample analyses approach or go below these limits of detection and in these cases we have flagged those data as below detection. Our results for SWS 2010-1 are within the consensus range from the intercalibration exercise (Anderson et al., 2012). Consensus values for SWS2015-1 have not been yet been coordinated but they agree with the reports of the LDEO lab. As an additional measure of our internal consistency, we analyzed a set of 4 replicate samples that were Niskin bottles fired at the same depth at a station but from a different cast (casts were designed to overlap for at least one depth). Our %error agreement with these replicates were similar to the %RSD reported for the standard reference material solutions.

Analytical Methods at UMN

The procedures described below apply to FISH samples as well as to BOTTLE samples. All of the FISH samples from GP15 analyzed for 232Th, 230Th, and 231Pa were processed at UMN.

In the on-shore laboratory, 1-liter aliquots of the seawater samples were weighed to determine sample size, taking into account the weight of the subsample container and of the acid added at sea. Then, weighed aliquots of the artificial isotope yield monitors 229Th (1 pg) and 233Pa (0.2-0.6 pg) and 3 mg dissolved Fe were added to each sample. After allowing 3 days for spike equilibration (at a temperature of about 40°C), the pH of each sample was raised to 8.0-8.5 by adding concentrated NH4OH which caused iron (oxy)hydroxide precipitates to form. This precipitate was allowed to settle for 1-2 days before the overlaying seawater was siphoned off. The Fe precipitate was transferred to centrifuge tubes for centrifugation and rinsing with deionized H2O (>18 MΩ) to remove the major seawater ions. The precipitate was then dissolved in 14 M HNO3 and transferred to a Teflon beaker. It was then dried down and taken up in 7 M HNO3 for anion-exchange chromatography using Bio-rad resin (AG1-X8, 100-200 mesh size) and a polyethylene frit. Initial separation was done on Teflon columns with a 0.75 mL column volume (CV). The sample was loaded in 0.75 mL (1 CV) of 7 M HNO3, followed by 1.125 mL (1.5 CV) of 7 M HNO3 (to wash Fe and other undesired elements off the resin), 2.25 mL (3 CV) of 8 M HCl (to collect Th fraction), and 2.25 mL (3 CV) of 8 M HCl/0.015 M HF (to collect Pa fraction). The Pa and Th fractions were then dried down in the presence of 2 drops of concentrated HClO4 and taken up in 7 M HNO3. They were each passed through second and third columns (each with 0.5 mL column volumes) using similar elution schemes. The final Pa and Th fractions were then dried down in the presence of 2 drops of concentrated 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 Multicollector ICP-MS. 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 the run solution (weak nitric acid), which was used to correct for the instrument background count rates on the masses measured.

Water samples were analyzed in batches of 28-56. Procedural blanks were determined by performing a complete chemical procedure on 1 L of Milli-Q water with each batch of samples. An aliquot of one of two intercalibrated working standard solutions of 232Th, 230Th and 231Pa, SW STD 2010-1 referred to by Anderson et al. (2012) and SW STD 2015-1 which has ~6 times lower 232Th activity, was added to a separate acid-cleaned Teflon beaker along with weighed aliquots of 229Th and 233Pa spike. Spikes and SW STD were equilibrated for 3 days. They were then dried down and taken up in 7 M HNO3 for anion-exchange chromatography and processed like a sample with each batch. RR1814-15 dissolved samples were corrected using the procedural blank analyzed during the same sample batch. The average procedural blanks for 232Th, 230Th and 231Pa were 0.6021 ± 0.0045 pg/kg, 0.33 ± 0.19 fg/kg, and 0.037 ± 0.010 fg/kg, respectively. The limit of detection (LOD) is the smallest quantity of each isotope in samples that can reliably be detected or that can be statistically distinguished from a procedural blank. The LOD was considered to be two standard deviations above the average of the procedural blanks. Our LOD for 232Th, 230Th and 231Pa were 0.009 pg/kg, 0.38 fg/kg, and 0.02 fg/kg, respectively. Procedural blank, limit of detection, and the results of the reference material solutions are reported in the "Table 3" Supplemental File.