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Total 234Th from GEOTRACES GA02 cruises PE319 and PE321

Responsible investigator

Pere Masqué ¹ , Michiel M Rutgers van der Loeff ² , Viena Puigcorbe ¹

¹ Independent University of Barcelona Institute of Science and Environmental Technology
² Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research

Originator's Protocol for Data Acquisition and Analysis

Sample collection

A total of 33 stations, were sampled to analyze total 234Th concentrations during the cruises 64PE319 (28th of April to 25th of May) and 64PE321 (11th of June 8th of July), on board of the R/V Pelagia (northern section of the GA02 transect). Total 234Th was measured from 4 L of seawater collected at 12 discrete depths over the upper 1000 m and processed following the MnO ₂ co-precipitation technique (Pike et al., 2005). Basically, immediately upon recovery, samples were acidified to a pH < 2 with concentrated nitric acid and shaken vigorously. Th-230 (Eckert and Ziegler; 178 ± 2 mBq g-1) was added as internal tracer and the samples were shaken again and allowed to equilibrate for at least 12h. Passed this time, pH was raised to ~8 using concentrated ammonia. Next, to form manganese precipitate, KMnO4 and MnCl2 were added in separate doses and samples were shaken in between. Samples were left for 8h and then filtered through quartz microfiber filters (QMA, Whatman; nominal pore size = 1 µm Ø 25 mm). The filters were dried in the oven (~50 °C) overnight and prepared for beta counting by placing them on plastic supports (beta-holders) and covering them with a layer of Mylar and a double layer of aluminum foil.

Samples were counted onboard using a gas flow proportional low-level beta counter (RISØ, Denmark). Beta counting was continued till counting uncertainty was below 3% RSD (relative standard deviation). At the beginning of the cruise 238U standards and background counts were measured to confirm correct operation of the RISØ detectors. The gas used during both cruises was a mixture of argon with 1% isobutane. Samples were recounted >6 months later to determine background activities before processing for chemical recoveries using ICPMS. Deep samples replicates (3000 m) were collected at selected stations for calibration purposes. After each use, 234Th sample bottles and filtration heads were rinsed with a solution of 1M HNO ₃ with ~1% H ₂ O ₂ and then with milli-Q.

The analysis of thorium recovery was done by dissolving the MnO ₂ precipitate in 8M HNO ₃ /10% H ₂ O ₂ solution and adding a known amount of 229Th (Eckert and Ziegler; 3.63 ± 0.13 Bq g-1) as a second yield tracer. Samples were sonicated and allowed to stand covered for 6 h. Once the filter was dissolved the remaining solution was evaporated to dryness and reconstructed to 5% HNO ₃ /0.08% HF and then filtered using Acrodisc 0.2 µm HT Tuffryn membrane syringe filters (Whatman Laboratory) into HDPE (high-density polyethylene) vials for archive, after taking and aliquot of 120 µl that was collected in a 4 ml ICP-MS polypropylene scintillation vial (Fisher) were 880 µl of 2.2% HNO ₃ were added before measuring the 230Th/229Th using ICPMS (adaptation of the method described in Pike et al., 2005). The average recovery obtained was 87 ± 13% (n = 335).

Uranium-238 activities were deduced from salinity data using the relationship from Owens et al. (2011), even though 3 L samples for 238U were also collected at some stations and analyzed to confirm the U-Salinity relationship.

References Cited

Owens, S.A., Buesseler, K.O., Sims, K.W.W., 2011. Re-evaluating the 238U-salinity relationship in seawater: Implications for the 238U-234Th disequilibrium method. Mar. Chem. 127, 31-39.

Pike, S.M., Buesseler, K.O., Andrews, J., Savoye, N., 2005. Quantification of Th-234 recovery in small volume seawater samples by inductively coupled plasma-mass spectrometry. J. Radioanal. Nucl. Chem. 263, 355-360.

BODC Data Processing Procedures

Data were received and were loaded using established BODC data banking procedures. This document covers data submitted using the following GEOTRACES parameters: