Dataset: GPpr09 Particulate Th and Pa
Deployment: SO245

Large-volume particulate samples were collected using McLane Research in-situ pumps (WTS-LV Standard Model). The wire-out was used to target depths during deployment, and a self-recording Seabird 19plus CTD deployed at the end of the line was used to correct for actual depths during pumping.

Filter holders used were 142 mm-diameter filter holders with a titanium baffle. Filter holders were loaded with paired 0.8 µm Pall Supor800 polyethersulfone filters. Each cast also had "dipped blank" filters deployed. These were single filters, either 0.2 µm or 0.8 µm, deployed inside Ziploc bags with holes cut in them to expose the filters to seawater, 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 1/4 portions of the dipped blank from each pump cast.

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:
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°C, at which point 4-5 ml HClO4 was added and the hot plate temperature was increased to 220°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°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°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™ 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 2015-1, we 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.

We pooled all procedural blank-corrected "dipped" blanks to determine filter blank corrections. That is, the data were corrected by the average dipped blank values. Averages for “dipped” filter blanks from SO245 for a 1/4 filter fraction for 232Th, 230Th, and 231Pa at LDEO were 8.67 ± 4.26 pg, 1.56±0.7 fg, and 0.09 ± 0.04 fg, respectively.

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

Derived Parameters:
Th_230_TP_XS_CONC_PUMP - The total particulate excess Th-230 refers to the measured total particulate Th-230 corrected for a contribution of Th-230 originating from U-bearing minerals or lithogenic Th-230. Some of this lithogenic Th-230 will be still intact within minerals and some after partial dissolution will have adsorbed to particle surface to contribute in part to the total adsorbed Th-230. Using the measured total particulate Th-232 and a lithogenic Th-230/232 atomic ratio of 4e-6, and not taking into account the fact that some of the measured particulate Th-232 is adsorbed, corrects for all of the lithogenic Th-230 whether it be adsorbed or within intact minerals. This excess Th-230 is what should be used in scavenging or particle flux studies where it is desired to compare particulate Th-230 concentrations to Th-230 production by decay of uranium dissolved in seawater. An additional conversion factors converts picomoles to micro-Becquerels.

Th_230_TP_XS_CONC_PUMP = Th_230_TP_CONC_PUMP – 4.0e-6 * 1.7473e5 * Th_232_TP_CONC_PUMP

Pa_231_TP_XS_CONC_PUMP - The total particulate excess Pa-231 refers to the measured total particulate Pa-231 corrected for a contribution of Pa-231 originating from U-bearing minerals or lithogenic Pa-231. Some of this lithogenic Pa-231 will be still intact within minerals and some after partial dissolution will have adsorbed to particle surface to contribute in part to the total adsorbed Pa-231. Using the measured total particulate Th-232 and a lithogenic Pa-231/Th-232 atomic ratio of 8.8e-8, and not taking into account the fact that some of the measured particulate Th-232 is adsorbed, corrects for all of the lithogenic Pa-231 whether it be adsorbed or within intact minerals. This excess Pa-231 is what should be used in scavenging or particle flux studies where it is desired to compare particulate Pa-231 concentrations to Pa-231 production by decay of uranium dissolved in seawater. An additional conversion factor converts picomoles to micro-Becquerels.

Pa_231_TP_XS_CONC_PUMP = Pa_231_TP_CONC_PUMP – 8.8e-8 * 4.0370e5 * Th_232_TP_CONC_PUMP