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Total 234Th measured during ANT-XXIV/3

Responsible investigator

Dr Michiel Rutgers van der Loeff
email: mloeff@awi.de
Marine Geochemistry
Alfred Wegener Institute for Polar and Marine Research
Am Handelshafen 12
D-27570 Bremerhaven
Germany

Data contributor

Dr Pinghe Cai
email: caiph@xmu.edu.cn
Xiamen University
State Key Laboratory of Marine Environmental Science
Xiamen
CHINA

Laboratory of analysis

Alfred Wegener Institut für Polar- und Meeresforschung (AWI), Germany

Originator's Protocol for Data Acquisition and Analysis

Objectives

The objectives of the project are to acquire accurate estimates of upper ocean POC export fluxes in the Southern Ocean using 234Th/238U ratios

A total of 20 depth profiles of total 234Th in 4-L water samples were collected using the AWI CTD system. Sampling, analysis, calibration and data reduction followed Cai et al. (2006) and Pike et al. (2005). For the analysis of total 234Th, the samples were coprecipitated with MnO2 ppt.

Procedure total 234Th

For the analysis of total 234Th, the samples were acidified to a pH of < 2.0, spiked with a known amount of 230Th. After 12-24 hours, the pH was then brought up to 8.15-8.30, thorium was coprecipitated on MnO2 by adding 0.25 ml KMnO4 (3.0 g/L) and 0.25 ml of MnCl2 (8.0 g MnCl24H2O/L). The samples were heated in a water bath at > 90 °C for 3 hours, cooled down to room temperature, the MnO2 ppt was then filtered onto a 25 mm 1.0 mm QMA filter. The QMA filter with MnO2 ppt was dried overnight in an oven, mounted under a layer of Mylar film and a layer of Al foil (8.00 mg/cm2), and counted on a RISO beta counter onboard. After 6 months, the background of the MnO2 ppt was counted on a RISO counter.

Procedure Thorium recovery

The QMA filter with MnO2 ppt was dismounted, and a known amount of 229Th was added as a second spike. The MnO2 ppt was dissolved in 8 M HNO3 +H2O2 solution and sonicated for 20 minutes. Thorium isotopes were isolated and purified using classic column exchange chemistry. The 230Th/229Th ratio was measured on an ICP-MS. For samples where 'no spike recovery was determined; the average recovery of 0.957±0.022 was applied.'

Procedure calculation parent 238U

238U (dpm/L) = 0.0713 * Salinity, the associated error is about 3% (Pates and Muir, 2007)

The 238U data errors that should be used is 0.07 dpm/L

Standards

Total 234Th: calibration with aged deep waters assumed to be in radioactive equilibrium with 238U.

Figures of merit
estimated precision: 3% at 2.50 dpm/L
overall accuracy: 0.10 dpm/L

References

Cai, P., M. Dai, D. Lv, and W. Chen (2006), An improvement in the small-volume technique for determining thorium-234 in seawater, Marine Chemistry, 100, 282-288, doi:10.1016/j.marchem.2005.10.016

Pike, S. M., K. O. Buesseler, J. A. Andrews, and N. Savoye (2005), Quantification of 234Th recovery in small volume sea water samples by inductively coupled plasma mass spectrometry, Journal of Radioanalytical and Nuclear Chemistry, 263, 355-360, doi:10.1007/s10967-005-0062-9

Pates, J.M., Muir, G. K. P., 2007. U-salinity relationships in the Mediterranean: Implications for 234Th:238U particle flux studies. Marine Chemistry, 106, 530-545, doi:10.1016/j.marchem.2007.05.006

BODC Data Processing Procedures

Data received from the originator were loaded into the BODC database using established BODC data banking procedures. 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. The 234Th/238U total ratio was not transferred since this data can easily be calculated.

Data Quality Report

None reported