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Thorium-232, Thorium-230 and Protactinium-231 measurements from the GEOTRACES compliant cruise GPc01 (SO202)

Responsible investigator

Prof Robert Anderson
email: boba@ldeo.columbia.edu
Lamont-Doherty Earth Observatory
PO Box 1000, 61 Route 9W, Palisades, 10964-1000
USA

Dr Martin Q. Fleisher
email: martyq@ldeo.columbia.edu
Lamont-Doherty Earth Observatory
PO Box 1000, 61 Route 9W, Palisades, 10964-1000
USA

Data contributor

Dr Christopher Hayes
email: christopher.t.hayes@usm.edu
Lamont-Doherty Earth Observatory
PO Box 1000, 61 Route 9W, Palisades, 10964-1000
USA

Laboratory of analysis

Lamont-Doherty Earth Observatory

Dataset description

Thorium-232, Thorium-230 and Protactinium-231 measurements from the GEOTRACES compliant cruise GPc01 (SO202)

Acquisition description

Sampling methodology

Water samples were collected with a Sea-Bird Electronics CTD carousel fitted with 24 10-liter PVC Niskin bottles. The carousel was lowered from the ship with steel wire. Niskin bottles caps were held together with rubber tubing. After collection seawater was drained with Teflon-lined TygonTM tubing and filtered through Pall AcropakTM 500 filters on deck (gravity filtration, 0.8/0.45 µm pore size) into Fisher I-Chem series 300 LDPE cubitainers. Approximately 9-10 L was collected per desired depth. Prior to the cruise, the tubing, filters and cubitainers were cleaned by immersion in 1.2 M HCl (Fisher Scientific Trace Metal Grade) for 4-5 days. Once filtered, samples were adjusted to a pH ~2 with ultra-clean 6 M HCl (Tama Chemicals, TAMAPURE-AA-100 grade), double-bagged and stored at room temperature as packaged until analysis.

Analytical methodology

Analysis involved Fe hydroxide co-precipitation, acid digestion, anion exchange chromatography and isotope dilution inductively-coupled plasma mass spectrometry. Laboratory methods were slightly modified from Anderson et al. (2012) as described by Hayes et al (2015).

In the on-shore laboratory, samples were weighed to determine sample size, taking into account the weight of the cubitainer and of the acid added at sea. Four to five liters of the original sample was used for Th/Pa analysis, the remaining sample kept as archive. Then weighed aliquots of the artificial isotope yield monitors ²²⁹ Th (20 pg) and ²³³ Pa (0.5 pg) and 15 mg dissolved Fe were added to each sample. After allowing 1 day for spike equilibration, the pH of each sample was raised to 8-8.5 by adding ~10 mL of concentrated NH ₄ OH (Fisher Scientific OPTIMA grade) 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 Milli-Q H ₂ O to remove the major seawater ions. The precipitate was then dissolved in 16 M HNO ₃ (Fisher Scientific OPTIMA grade) and transferred to a Teflon beaker for a high-temperature (180-200°C) digestion with HClO ₄ 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 Mill-Q H ₂ O, centrifuged, and dissolved in 12 M HCl 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. The final column elutions were dried down at 180°C in the presence of 2 drops of HClO ₄ and taken up in approximately 1 mL of 0.16 M HNO ₃ /0.026 M HF for mass spectrometric analysis.

Concentrations of ²³² Th, ²³⁰ Th and ²³¹ Pa were calculated by isotope dilution using nuclide ratios determined on a VG Elemental AXIOM Single Collector Magnetic Sector ICP-MS with a Resolving Power of ~400 to ensure the highest sensitivity. All measurements were done using a peak jumping routine in ion counting mode. A solution of SRM129, a natural U standard, 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, used to correct for the instrument background count rates on the masses measured. To correct for potential tailing of ²³² Th into the minor Th and Pa isotopes, beam intensities were measured at the half masses above and below each mass for ²³⁰ Th, ²³¹ Pa, and ²³³ Pa. Tailing under each minor isotope was estimated as the log mean intensity of the half masses on either side of each minor isotope.

Water samples were analyzed in batches of 10-12. Procedural blanks were determined by processing 4-5 L of Milli-Q water in an acid-cleaned cubitainer acidified to pH ~2 with 6 M HCl as a sample in each batch. An aliquot of an intercalibrated working standard solution of ²³² Th, ²³⁰ Th and ²³¹ Pa, SW STD 2010-1 referred to by Anderson et al. (2012), was added to a separate cubitainer with 5 L of Milli-Q water (acidified to pH 2) and also processed like a sample in each batch. Total procedural blanks for ²³² Th, ²³⁰ Th, and ²³¹ Pa ranged from 7.1-24.3 pg, 0.8-1.6 fg, and 0.02-0.2 fg respectively. One batch had an anomalously high ²³² Th blank of 140 pg (with ²³⁰ Th and ²³¹ Pa in the reported range). Application of this blank correction to the analyzed SW STD caused an anomalously low estimate of its ²³² Th concentration (approximately 990 ± 15 pg/g). From this we concluded the blank was due to random contamination of the procedural blank and it should not be used to blank-correct samples. Instead, we determined what magnitude of blank correction would be necessary for the analyzed SW STD to achieve the intercalibrated concentration. This was 12 pg ²³² Th, which is close to the average blank value measured throughout the course of this project.

Further details on sampling and analysis are given by Anderson et al. (2012).

References Cited

Anderson, R. F., M. Q. Fleisher, L. F. Robinson, R. L. Edwards, J. Hoff, S. B. Moran, M. M. Rutgers van der Loeff, A. L. Thomas, M. Roy-Barman, and R. Francois, 2012. GEOTRACES intercalibration of ²³⁰ Th, ²³² Th, ²³¹ Pa, and prospects for ¹⁰ Be, Limnol. Oceanogr. Methods, 10, 179-213.

Hayes, C. T., R. F. Anderson, Fleisher M. Q., Huang K-F., Robinson L.F., Lu Y., Cheng H., Lawrence Edwards R., Bradley Moran S., 2015. 230Th and 231Pa on GEOTRACES GA03, the U.S. GEOTRACES North Atlantic transect, and implications for modern and paleoceanographic chemical fluxes, Deep Sea Research Part II: Topical Studies in Oceanography, Volume 116, Pages 29-41, ISSN 0967-0645, http://dx.doi.org/10.1016/j.dsr2.2014.07.007.

BODC Data Processing Procedures

Data were submitted to BODC in Excel spreadsheet format. The mapping between the originator's channels and BODC parameter codes is detailed in the table below. The originators quality control flags were changed to BODC quality control flags.

Data Quality Report

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