Dataset: GN01 Particulate Thorium and Protactinium

Sampling Methods at Sea:
Sampling methods at sea followed the GEOTRACES cookbook (Cutter et al., 2017). Large-volume size-fractionated particulate samples were collected using McLane Research in-situ pumps (WTS-LV) that had been modified to accommodate two flowpaths (Lam and Morris, 2013). The wire-out was used to target depths during deployment and both a self-recording Seabird 19plus CTD deployed at the end of the line and an RBR pressure logger attached to the pump at the middle of the line were used to correct for actual depths during pumping. The RBR was used for all casts in the entire cruise, whereas the CTD was only deployed in the northbound leg due to electronic problems.

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 filter holder/flowpath was loaded with paired 0.8 µm Pall Supor800 polyethersulfone filters behind a 51 µm Sefar polyester mesh prefilter ("Supor-side") and the other filter holder/flowpath was loaded with paired 1 µm Whatman QMA quartz fiber filters behind a 51 µm Sefar polyester mesh prefilter ("QMA-side") (see Xiang and Lam, 2020 for particulate sampling methodology). Each cast also had “dipped” filter blanks deployed. These were the full filter sets (prefilter followed by paired Supor or paired QMA filters) sandwiched within a 1 µm polyester mesh filter, loaded into perforated polypropylene containers, attached with plastic cable ties to a pump frame, and deployed. "Dipped" filter blanks were exposed to seawater for the length of the deployment, processed and analyzed as regular samples, and thus functioned as full seawater particulate procedural blanks. For the "small particulate" dataset, we analyzed either quarter top (or bottom) Supor filter cuts (LDEO) or eighth paired Supor filter cuts (UMN) from the "dipped" Supor filter blanks from 1 or more depths for nearly every station (18 of 20). For the "large particulate" dataset, UMN analyzed eighth filter cuts from the "dipped" Sefar prefilter blanks from 1 or more depths for every station where samples were selected for analysis (10 of 10).

All filters and filter holders were acid leached prior to use according to methods recommended in the GEOTRACES cookbook (Cutter et al., 2017).

Analytical Methods at LDEO:
Quarter filter cuts of the Supor filters ("small particulate", 0.8-51 µm particle size class, "suspended" size fraction) were folded into 60 mL Teflon jars and weighed aliquots of artificial isotope yield monitors Th-229 (1 pg) and Pa-233 (0.05-0.12 pg) and 5 mg dissolved Fe were added to each sample, which then sat overnight in 5 mL concentrated (16 M) HNO3 (Fisher Scientific OPTIMA grade). The next day, the filters were heated for ~1 hour at 180°C, at which point 4-5 mL of concentrated HClO4 (Fisher Scientific OPTIMA grade) were added and the hot plate temperature was increased to 220°C. Samples were heated until dense white perchloric fumes appeared. After 10-20 minutes, the samples were covered with a Teflon watch glass. 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 jar walls were rinsed down with 2 mL concentrated (16 M) HNO3 (Fisher Scientific OPTIMA grade) and placed back on the hot plate at 220°C. Once the samples were back to fuming perchloric, 0.5 mL of concentrated HF (Fisher Scientific OPTIMA grade) were added to the samples and heated at 220°C back to perchloric fumes before another 0.25-0.50 mL of concentrated HF (Fisher Scientific OPTIMA grade) were added. Following the second addition of concentrated HF, the samples were dried down at 220°C to a viscous residue.

The sample residue was taken up in dilute (0.6 M) HCl and transferred to 50 mL centrifuge tubes with Milli-Q H2O rinses. Then, ~1.5 mL of concentrated NH4OH (Fisher Scientific OPTIMA grade) were added to raise the pH to 8.0-8.5 when iron (oxy)hydroxide precipitated. This precipitate was then centrifuged and the supernatant was decanted. The precipitate was then dissolved in concentrated (16 M) HNO3 (Fisher Scientific OPTIMA grade), 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 180-200°C in the presence of 2 drops of concentrated HClO4 (Fisher Scientific OPTIMA grade) and taken up in 0.5 mL of 0.16 M HNO3/0.026 M HF for mass spectrometric analysis.

Concentrations of Th-232, Th-230 and Pa-231 were calculated by isotope dilution, relative to the calibrated tracers Th-229 and Pa-233 added at the beginning of sample processing. Analyses were carried out on a Thermo-Finnigan ELEMENT XR Single Collector Magnetic Sector ICP-MS, equipped with a high-performance Interface pump (Jet Pump Aridus I™), and specially designed sample (Jet) and skimmer (X) 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; Th-229, Th-230, Pa-231, and Pa-233 were measured in Counting mode, while the Th-232 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 U-238 and U-235 were both measured in Counting mode, allowing us to determine the mass bias/amu (typical values varied from -0.6%/amu to 0.3%/amu). In the other, more concentrated solution, U-238 was measured in Analog mode and U-235 was measured in Counting mode, yielding a measurement of the Analog/Counting Correction Factor (typical values varied from 0.9 to 1.3). 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 Th-232 into the minor Th and Pa isotopes, a series of Th-232 standards were run at concentrations bracketing the expected Th-232 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 Th-232 standards. The Th-232 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 Th-232 signal intensity in the Th fraction. The regressions of Th-229, Th-230, Pa-231, and Pa-233 signals as a function of the Th-232 signal in the standards was used to correct for tailing of Th-232 in samples. Only in rare cases was a tail correction of Th-232 on Pa-231 and Pa-233 necessary, while it was always the case that tail corrections of Th-232 on Th-229 and Th-230 were performed.

"Small particulate" samples (0.8-51 µm particle size class, "suspended" size fraction) were analyzed in batches of 15. Analysis of the paired Supor filters represents a particle size class approximating 0.45-51 µm (Bishop et al., 2012), while the top filter alone represents 0.8-51 µm and it is this size class referred to here as "small particulate". A selection of 14 top and bottom Supor filters were measured separately for radionuclides and it was found that the bottom filters had radionuclide levels that were indistinguishable from procedural blanks ("dipped" filter blanks). Therefore, whether or not samples were analyzed as paired Supor filters, or the top Supor filter alone, the "small particulate" data was inferred to represent the 0.8-51 µm particle size class or "suspended" size fraction. "Small particulate" samples were analyzed as top Supor filter alone and procedural blanks ("dipped" filter blanks) were analyzed as top or bottom Supor filter alone (top and bottom "dipped" filter blanks had radionuclide levels that were indistinguishable from one another). Procedural blanks were determined by analyzing "dipped" filter blanks, mentioned above, which represent the total blank associated with sample collection and handling in addition to the laboratory procedure. An aliquot of two intercalibrated working standard solutions of Th-232, Th-230, and Pa-231, SW STD 2010-1 referred to by Anderson et al. (2012) and SW STD 2015-1 which has ~6 times lower Th-232 activity, were added to separate acid-cleaned Teflon beakers along with weighed aliquots of Th-229 and Pa-233 spike. Spikes and SW STD were equilibrated for at least 1 day. They were then dried down and dissolved in concentrated (12 M) HCl (Fisher Scientific OPTIMA grade) for a series of anion-exchange chromatography and processed like a sample with each batch. Samples were corrected using the pooled average of all procedural blanks ("dipped" filter blanks) analyzed during processing of HLY1502 particulate samples. The average procedural blanks ("dipped" filter blanks) for the "small particulate" data from HLY1502 for a quarter filter cut of top (or bottom) Supor filters for Th-232, Th-230, and Pa-231 were 18.78 ± 7.42 pg, 0.89 ± 0.49 fg, and 0.04 ± 0.03 fg, 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 2 standard deviations above the average of the procedural blanks. Our LOD for Th-232, Th-230, and Pa-231 were 33.61 pg, 1.86 fg, and 0.09 fg, respectively, or about 1.8x, 2.1x, and 2.5x greater than the blank amount, respectively.

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

Analytical Methods at UMN:
Eighth filter cuts of the paired (top and bottom) Supor filters ("small particulate", 0.8-51 µm particle size class, "suspended" size fraction) and the Sefar prefilters ("large particulate", >51 µm particle size class, "sinking" size fraction) were folded into 30 mL Teflon beaker and weighed aliquots of the artificial isotope yield monitors Th-229 (1 pg) and Pa-233 (0.2-0.6 pg) were added to the filters. Filters were then completely submerged in 20-25 mL of 7 M HNO3 combined with 10 drops of concentrated HF, tightly covered with a Teflon threaded cap and heated for 10 hours at ~90°C so that the particulate sample was dissolved/leached under pressure. The leachate and the rinse solution (rinsed twice with ICP-MS solution, 0.16 M HNO3/0.026 M HF) were then transferred to a second acid-cleaned Teflon beaker separate from the residual filter. Five drops of concentrated 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 taken up in dilute (2 M) HCl and transferred to 15 mL centrifuge tubes along with a dilute (2 M) HCl rinse. About 0.5 mg of dissolved Fe (one drop of FeCl3 solution with Fe3+ concentration at ~1%) and six to nine drops of concentrated NH4OH were added to raise the pH to 8.0-8.5 at which time iron (oxy)hydroxide precipitated. This precipitate was then centrifuged, decanted, washed with deionized H2O (>18 MΩ), centrifuged, 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 (internal diameter ~0.35 cm) with a 0.55 mL column volume (CV). The sample was loaded in 0.55 mL (1 CV) of 7 M HNO3, followed by 0.825 mL (1.5 CV) of 7 M HNO3 (to wash Fe and other undesired elements off the resin), 1.65 mL (3 CV) of 8 M HCl (to collect Th fraction), and 1.65 mL (3 CV) of 8 M HCl combined with 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 0.55 mL column volumes) using similar elution schemes. The final Pa and Th fractions were then dried down in the presence of 2 drops of concentrated HClO4 and dissolved in weak nitric acid for analysis on the mass spectrometer.

Concentrations of Th-232, Th-230, and Pa-231 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 U-233-U-236 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) used to correct for the instrument background count rates on the masses measured.

"Small particulate" (0.8-51 µm particle size class, "suspended" size fraction) and "large particulate" (>51 µm particle size class, "sinking" size fraction) samples were analyzed in batches of 37-39. Analysis of the paired Supor filters represents a particle size class approximating 0.45-51 µm (Bishop et al., 2012), while the top filter alone represents 0.8-51 µm and it is this size class referred to here as "small particulate". A selection of 14 top and bottom Supor filters were measured separately for radionuclides at LDEO and it was found that the bottom filters had radionuclide levels that were indistinguishable from procedural blanks ("dipped" filter blanks). Therefore, whether or not samples were analyzed as paired Supor filters, or the top Supor filter alone, the "small particulate" data was inferred to represent the 0.8-51 µm particle size class or "suspended” size fraction. "Small particulate" samples and procedural blanks ("dipped" filter blanks) were analyzed as paired Supor filters. Procedural blanks were determined by analyzing "dipped" filter blanks, mentioned above, which represent the total blank associated with the sample collection and handling in addition to the laboratory procedure. An aliquot of one of two intercalibrated working standard solutions of Th-232, Th-230, and Pa-231, SW STD 2010-1 referred to by Anderson et al. (2012) and SW STD 2015-1 which has ~6 times lower Th-232 activity, was added to a separate acid-cleaned Teflon beaker along with weighed aliquots of Th-229 and Pa-233 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. Samples were corrected using the pooled average of all procedural blanks ("dipped" filter blanks) analyzed during processing of HLY1502 particulate samples. The average procedural blanks ("dipped" filter blanks) for the HLY1502 "small particulate" dataset for an eighth filter cut of paired Supor filters for Th-232, Th-230, and Pa-231 were 11.36 ± 3.20 pg, 0.64 ± 0.30 fg, and 0.01 ± 0.01 fg, respectively. The average procedural blanks ("dipped" filter blanks) for the HLY1502 "large particulate" dataset for an eighth filter cut of Sefar prefilters for Th-232, Th-230, and Pa-231 were 12.55 ± 8.26 pg, 0.35 ± 0.15 fg, and 0.01 ± 0.01 fg, 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 2 standard deviations above the average of the procedural blanks. For the HLY1502 "small particulate" dataset, our LOD for Th-232, Th-230, and Pa-231 were 17.76 pg, 1.23 fg, and 0.02 fg, respectively, or about 1.6x, 1.9x, and 1.9x greater than the blank amount, respectively. For the HLY1502 "large particulate" dataset, our LOD for Th-232, Th-230, and Pa-231 were 29.07 pg, 0.65 fg, and 0.04 fg, respectively, or about 2.3x, 1.8x, and 2.9x greater than the blank amount, respectively.

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

Notes on Derived Parameters:
Th_230_SPT_XS_CONC_PUMP:
The small particulate excess Th-230 refers to the measured small 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 small particulate Th-232 and a lithogenic Th-230/Th-232 ratio of 4.0e-6 (atom ratio) as determined by Roy-Barman et al. (2002), 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 factor converts picomoles to micro-Becquerels.

Th_230_SPT_XS_CONC_PUMP = Th_230_SPT_CONC_PUMP – 4.0e-6 * 1.7473e5 * Th_232_SPT_CONC_PUMP

Pa_231_SPT_XS_CONC_PUMP:
The small particulate excess Pa-231 refers to the measured small 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 small particulate Th-232 and a lithogenic Pa-231/Th-232 ratio of 8.8e-8 (atom ratio) which is derived from assuming an average upper continental crustal U/Th ratio (Taylor and McClennan, 1995) and secular equilibrium between Pa-231 and U-235 in the lithogenic material, 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_SPT_XS_CONC_PUMP = Pa_231_SPT_CONC_PUMP – 8.8e-8 * 4.0370e5 * Th_232_SPT_CONC_PUMP

Th_230_LPT_XS_CONC_PUMP:
The large particulate excess Th-230 refers to the measured large 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 large particulate Th-232 and a lithogenic Th-230/Th-232 ratio of 4.0e-6 (atom ratio) as determined by Roy-Barman et al. (2002), 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 factor converts picomoles to micro-Becquerels.

Th_230_LPT_XS_CONC_PUMP = Th_230_LPT_CONC_PUMP – 4.0e-6 * 1.7473e5 * Th_232_LPT_CONC_PUMP

Pa_231_LPT_XS_CONC_PUMP:
The large particulate excess Pa-231 refers to the measured large 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 large particulate Th-232 and a lithogenic Pa-231/Th-232 ratio of 8.8e-8 (atom ratio) which is derived from assuming an average upper continental crustal U/Th ratio (Taylor and McClennan, 1995) and secular equilibrium between Pa-231 and U-235 in the lithogenic material, 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_LPT_XS_CONC_PUMP = Pa_231_LPT_CONC_PUMP – 8.8e-8 * 4.0370e5 * Th_232_LPT_CONC_PUMP

Th_230_SPT_ADS_CONC_PUMP:
The small particulate adsorbed Th-230 concentration refers to the measured small particulate Th-230 corrected for a contribution of Th-230 locked within mineral lattices. To estimate the particulate Th-230 supported by decay of U within mineral lattices, we use measured small particulate Th-232 and a lithogenic Th-230/Th-232 ratio of 4.0e-6 (atom ratio) as determined by Roy-Barman et al. (2002). However, because some fraction of the Th-232 is also adsorbed (and not merely found within mineral lattices), we use the dissolved Th-232/Th-230 ratio to estimate the fraction of the small particulate Th-232 that is adsorbed and furthermore to calculate the correction for adsorbed Th-230. Conversion factors are also necessary to convert picomoles to micro-Becquerels. When dissolved Th data do not exist at the same depth as the particulate samples, we linearly interpolated the dissolved data onto the depths of the particulate samples. See Hayes et al. (2015) for more details.

Th_230_SPT_ADS_CONC_PUMP = [ Th_230_SPT_CONC_PUMP – 4.0e-6 * 1.7473e5 * Th_232_SPT_CONC_PUMP ] / [ 1 – 4.0e-6 * 1.7473e5 * Th_232_D_CONC_BOTTLE / Th_230_D_CONC_BOTTLE ]

Pa_231_SPT_ADS_CONC_PUMP:
The small particulate adsorbed Pa-231 concentration refers to the measured small particulate Pa-231 corrected for a contribution of Pa-231 locked within mineral lattices (as opposed to adsorbed onto particle surfaces). To estimate the particulate Pa-231 supported by decay of U within mineral lattices, we use measured small particulate Th-232 and a lithogenic Pa-231/Th-232 ratio of 8.8e-8 (atom ratio) which is derived from assuming an average upper continental crustal U/Th ratio (Taylor and McClennan, 1995) and secular equilibrium between Pa-231 and U-235 in the lithogenic material. However, because some fraction of the Th-232 is also adsorbed (and not merely found within mineral lattices), we use the dissolved and particulate Th-232/Th-230 ratio to estimate what fraction of the small particulate Th-232 is adsorbed and furthermore to calculate the correction for adsorbed Pa-231. Conversion factors are also necessary to convert picomoles to micro-Becquerels. When dissolved Pa data do not exist at the same depth as the particulate samples, we linearly interpolate the dissolved data onto the depths of the particulate samples. See Hayes et al. (2015) for more details.

Pa_231_SPT_ADS_CONC_PUMP = Pa_231_SPT_CONC_PUMP – 8.8e-8 * 4.0370e5 * ( Th_232_SPT_CONC_PUMP – Th_232_D_CONC_BOTTLE * [ ( Th_230_SPT_CONC_PUMP – 4.0e-6 * 1.7473e5 * Th_232_SPT_CONC_PUMP ) / ( Th_230_D_CONC_BOTTLE – 4.0e-6 * 1.7473e5 * Th_232_D_CONC_BOTTLE ) ] )

Th_230_LPT_ADS_CONC_PUMP:
The large particulate adsorbed Th-230 concentration refers to the measured large particulate Th-230 corrected for a contribution of Th-230 locked within mineral lattices. To estimate the particulate Th-230 supported by decay of U within mineral lattices, we use measured large particulate Th-232 and a lithogenic Th-230/Th-232 ratio of 4.0e-6 (atom ratio) as determined by Roy-Barman et al. (2002). However, because some fraction of the Th-232 is also adsorbed (and not merely found within mineral lattices), we use the dissolved Th-232/Th-230 ratio to estimate what fraction of the large particulate Th-232 is adsorbed and furthermore to calculate the correction for adsorbed Th-230. Conversion factors are also necessary to convert picomoles to micro-Becquerels. When dissolved data do not exist at the same depth as the particulate samples, we linearly interpolated the dissolved data onto the depths of the particulate samples. See Hayes et al. (2015) for more details.

Th_230_LPT_ADS_CONC_PUMP = [ Th_230_LPT_CONC_PUMP – 4.0e-6 * 1.7473e5 * Th_232_LPT_CONC_PUMP ] / [ 1 – 4.0e-6 * 1.7473e5 * Th_232_D_CONC_BOTTLE / Th_230_D_CONC_BOTTLE ]

Pa_231_LPT_ADS_CONC_PUMP:
The large particulate adsorbed Pa-231 concentration refers to the measured large particulate Pa-231 corrected for a contribution of Pa-231 locked within mineral lattices (as opposed to adsorbed on to particle surfaces). To estimate the particulate Pa-231 supported by decay of U within mineral lattices, we use measured large particulate Th-232 and a lithogenic Pa-231/Th-232 ratio of 8.8e-8 (atom ratio) which is derived from assuming an average upper continental crustal U/Th ratio (Taylor and McClennan, 1995) and secular equilibrium between Pa-231 and U-235 in the lithogenic material. However, because some fraction of the Th-232 is also adsorbed (and not merely found within mineral lattices), we use the dissolved and particulate Th-232/Th-230 ratio to estimate what fraction of the large particulate Th-232 is adsorbed and furthermore to calculate the correction for adsorbed Pa-231. Conversion factors are also necessary to convert picomoles to micro-Becquerels. When dissolved Pa data do not exist at the same depth as the particulate samples, we linearly interpolate the dissolved data onto the depths of the particulate samples. See Hayes et al. (2015) for more details.

Pa_231_LPT_ADS_CONC_PUMP = Pa_231_LPT_CONC_PUMP – 8.8e-8 * 4.0370e5 * ( Th_232_LPT_CONC_PUMP – Th_232_D_CONC_BOTTLE * [ ( Th_230_LPT_CONC_PUMP – 4.0e-6 * 1.7473e5 * Th_232_LPT_CONC_PUMP ) / ( Th_230_D_CONC_BOTTLE – 4.0e-6 * 1.7473e5 * Th_232_D_CONC_BOTTLE ) ] )