Sampling Methods at Sea:
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 either 12 30-liter or 36 10-liter PVC Niskin bottles, managed and operated by Ship-based Science Technical Support in the Arctic and the Ocean Data Facility of Scripps Institution of Oceanography, or with a Sea-Bird Electronics CTD carousel fitted with 24 12-liter GO-FLO bottles (the GEOTRACES Clean carousel). The 12-place 30 L Niskin bottle rosette was used for stations 1-10 and 26, the 36-place 10 L Niskin bottle rosette was used for stations 12-19, 30-38, and 43-66, and the 24-place 12 L GO-FLO bottle rosette was used for station 41. Carousels were lowered from the ship with steel wire. Niskin bottles were equipped with nylon-coated closure springs and Viton O-rings. After collection, seawater was drained with Teflon-lined Tygon™ tubing and filtered through Pall Acropak™ 500 filters on deck (gravity filtration, 0.8/0.45 μm pore size) into Fisher I-Chem series 300 LDPE cubitainers. Ice hole seawater samples were collected from under the ice (at approximately 1, 5, and 20 m) using a battery-powered pump and Teflon-lined PVC tubing, then filtered and stored in the same manner as seawater samples collected from a rosette using a Niskin bottle. Surface (1 m) seawater samples were also collected from a small boat upstream of the ship using the same pumping system used to collect ice hole seawater samples, except they were passed through a 0.2 µm Acropak™ 200 filter capsule before being transferred to cubitainers. Melt pond samples were collected using a battery-powered peristaltic pump and silicone tubing, bulk sea ice samples were collected using a trace metal clean ice corer and melted overnight in LDPE melting chambers, and both were also passed through a 0.2 µm Acropak™ 200 filter capsule before cubitainer storage. Approximately 4-5 liters were collected per desired depth for each dissolved sample. Prior to the cruise, the tubing, filters, and cubitainers were cleaned by immersion in dilute (1.2 M) HCl (Fisher Scientific Trace Metal Grade) for 4-5 days. Once filtered, samples were adjusted to a pH of ~2 with ultra-clean 6 M HCl (Fisher Scientific OPTIMA grade), 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.

Analytical Methods at LDEO:
In the on-shore laboratory, seawater, sea ice, and melt pond 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 Th-229 (1 pg) and Pa-233 (0.05-0.17 pg) and 25 mg dissolved Fe were added to each sample. After allowing 1 day for spike equilibration, the pH of each sample was raised to 8.3-8.7 by adding ~12 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~8.3-8.7 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 concentrated (16 M) HNO3 (Fisher Scientific OPTIMA grade) and transferred to a Teflon beaker for a high-temperature (180-200°C) digestion with concentrated HClO4 and HF (Fisher Scientific OPTIMA grade) 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 in concentrated (16 M) HNO3 (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 HClO4 (Fisher Scientific OPTIMA grade) and taken up in 0.5 mL of 0.16 M HNO3/0.026 M HF (Fisher Scientific OPTIMA grade) 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 -0.5%/amu to 0.2%/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 1.1). 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 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.

Water samples were analyzed in batches of 15. 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. Two procedural blanks were processed with each batch, with about half of the procedural blanks acidified at sea during HLY1502 and the other half acidified in the on-shore laboratory before sample processing. The difference in the procedural blank values for Th-232, Th-230, and Pa-231 between acidifying procedural blanks at sea or in the on-shore laboratory was statistically insignificant. 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 samples with each batch. Samples were corrected using the pooled average of all procedural blanks analyzed during processing of HLY1502 dissolved samples. The average procedural blanks for Th-232, Th-230, and Pa-231 were 5.72 ± 3.22 pg, 0.14 ± 0.08 fg, and 0.07 ± 0.08 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 12.15 pg, 0.29 fg, and 0.23 fg, respectively, or about 2.1x, 2.1x, and 3.1x greater than the blank amount, respectively.

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

Analytical Methods at UMN:
In the on-shore laboratory, 1-liter aliquots of the seawater, sea ice, and melt pond 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 Th-229 (1 pg) and Pa-233 (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 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), 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 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. HLY1502 dissolved samples were corrected using the procedural blank analyzed during the same sample batch. The average procedural blanks for Th-232, Th-230, and Pa-231 were 0.83 ± 0.80 pg, 0.03 ± 0.03 fg, and 0.02 ± 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 2.44 pg, 0.09 fg, and 0.08 fg, respectively, or about 2.9x, 2.5x, and 3.1x 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_D_XS_CONC_BOTTLE:
The dissolved excess Th-230 concentration refers to the measured dissolved Th-230 corrected for a contribution of Th-230 due to the partial dissolution of uranium-bearing minerals, or lithogenics. Thereby the dissolved excess represents solely the fraction of Th-230 produced in the water by decay of dissolved uranium-234. We estimate the lithogenic Th-230 using measured dissolved 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 a conversion factor to convert picomoles to micro-Becquerels.

Th_230_D_XS_CONC_BOTTLE = Th_230_D_CONC_BOTTLE – 4.0e-6 * 1.7473e5 * Th_232_D_CONC_BOTTLE

Pa_231_D_XS_CONC_BOTTLE:
The dissolved excess Pa-231 concentration refers to the measured dissolved Pa-231 corrected for a contribution of Pa-231 due to the partial dissolution of uranium-bearing minerals, or lithogenics. Thereby the dissolved excess represents solely the fraction of Pa-231 produced in the water by decay of dissolved uranium-235. We estimate the lithogenic Pa-231 using measured dissolved 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. An additional conversion factor is needed to convert picomoles to micro-Becquerels.

Pa_231_D_XS_CONC_BOTTLE = Pa_231_D_CONC_BOTTLE – 8.8e-8 * 4.0370e5 * Th_232_D_CONC_BOTTLE

Th_230_D_XS_CONC_BOAT_PUMP:
The dissolved excess Th-230 concentration refers to the measured dissolved Th-230 corrected for a contribution of Th-230 due to the partial dissolution of uranium-bearing minerals, or lithogenics. Thereby the dissolved excess represents solely the fraction of Th-230 produced in the water by decay of dissolved uranium-234. We estimate the lithogenic Th-230 using measured dissolved 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 a conversion factor to convert picomoles to micro-Becquerels.

Th_230_D_XS_CONC_BOAT_PUMP = Th_230_D_CONC_BOAT_PUMP – 4.0e-6 * 1.7473e5 * Th_232_D_CONC_BOAT_PUMP

Pa_231_D_XS_CONC_BOAT_PUMP:
The dissolved excess Pa-231 concentration refers to the measured dissolved Pa-231 corrected for a contribution of Pa-231 due to the partial dissolution of uranium-bearing minerals, or lithogenics. Thereby the dissolved excess represents solely the fraction of Pa-231 produced in the water by decay of dissolved uranium-235. We estimate the lithogenic Pa-231 using measured dissolved 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. An additional conversion factor is needed to convert picomoles to micro-Becquerels.

Pa_231_D_XS_CONC_BOAT_PUMP = Pa_231_D_CONC_BOAT_PUMP – 8.8e-8 * 4.0370e5 * Th_232_D_CONC_BOAT_PUMP

Th_230_D_XS_CONC_SUBICE_PUMP:
The dissolved excess Th-230 concentration refers to the measured dissolved Th-230 corrected for a contribution of Th-230 due to the partial dissolution of uranium-bearing minerals, or lithogenics. Thereby the dissolved excess represents solely the fraction of Th-230 produced in the water by decay of dissolved uranium-234. We estimate the lithogenic Th-230 using measured dissolved 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 a conversion factor to convert picomoles to micro-Becquerels.

Th_230_D_XS_CONC_SUBICE_PUMP = Th_230_D_CONC_SUBICE_PUMP – 4.0e-6 * 1.7473e5 * Th_232_D_CONC_SUBICE_PUMP

Pa_231_D_XS_CONC_SUBICE_PUMP:
The dissolved excess Pa-231 concentration refers to the measured dissolved Pa-231 corrected for a contribution of Pa-231 due to the partial dissolution of uranium-bearing minerals, or lithogenics. Thereby the dissolved excess represents solely the fraction of Pa-231 produced in the water by decay of dissolved uranium-235. We estimate the lithogenic Pa-231 using measured dissolved 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. An additional conversion factor is needed to convert picomoles to micro-Becquerels.

Pa_231_D_XS_CONC_SUBICE_PUMP = Pa_231_D_CONC_SUBICE_PUMP – 8.8e-8 * 4.0370e5 * Th_232_D_CONC_SUBICE_PUMP

Th_230_ICE_D_XS_CONC_CORER:
The dissolved excess Th-230 concentration refers to the measured dissolved Th-230 corrected for a contribution of Th-230 due to the partial dissolution of uranium-bearing minerals, or lithogenics. Thereby the dissolved excess represents solely the fraction of Th-230 produced in the water by decay of dissolved uranium-234. We estimate the lithogenic Th-230 using measured dissolved 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 a conversion factor to convert picomoles to micro-Becquerels.

Th_230_ICE_D_XS_CONC_CORER = Th_230_ICE_D_CONC_CORER – 4.0e-6 * 1.7473e5 * Th_232_ICE_D_CONC_CORER

Pa_231_ICE_D_XS_CONC_CORER:
The dissolved excess Pa-231 concentration refers to the measured dissolved Pa-231 corrected for a contribution of Pa-231 due to the partial dissolution of uranium-bearing minerals, or lithogenics. Thereby the dissolved excess represents solely the fraction of Pa-231 produced in the water by decay of dissolved uranium-235. We estimate the lithogenic Pa-231 using measured dissolved 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. An additional conversion factor is needed to convert picomoles to micro-Becquerels.

Pa_231_ICE_D_XS_CONC_CORER = Pa_231_ICE_D_CONC_CORER – 8.8e-8 * 4.0370e5 * Th_232_ICE_D_CONC_CORER

Th_230_D_XS_CONC_MELTPOND_PUMP:
The dissolved excess Th-230 concentration refers to the measured dissolved Th-230 corrected for a contribution of Th-230 due to the partial dissolution of uranium-bearing minerals, or lithogenics. Thereby the dissolved excess represents solely the fraction of Th-230 produced in the water by decay of dissolved uranium-234. We estimate the lithogenic Th-230 using measured dissolved 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 a conversion factor to convert picomoles to micro-Becquerels.

Th_230_D_XS_CONC_MELTPOND_PUMP = Th_230_D_CONC_MELTPOND_PUMP – 4.0e-6 * 1.7473e5 * Th_232_D_CONC_MELTPOND_PUMP

Pa_231_D_XS_CONC_MELTPOND_PUMP:
The dissolved excess Pa-231 concentration refers to the measured dissolved Pa-231 corrected for a contribution of Pa-231 due to the partial dissolution of uranium-bearing minerals, or lithogenics. Thereby the dissolved excess represents solely the fraction of Pa-231 produced in the water by decay of dissolved uranium-235. We estimate the lithogenic Pa-231 using measured dissolved 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. An additional conversion factor is needed to convert picomoles to micro-Becquerels.

Pa_231_D_XS_CONC_MELTPOND_PUMP = Pa_231_D_CONC_MELTPOND_PUMP – 8.8e-8 * 4.0370e5 * Th_232_D_CONC_MELTPOND_PUMP