Abstract
Development of a small volume (2-4 liter) technique for measuring 234 Th in sea water has been instrumental in bringing to light small-scale structures in upper-ocean particle removal processes previously missed by standard 234 Th measurement techniques. Further development of this method to evaluate removal efficiencies of 234 Th via MnO2 precipitation quantified using ICP-MS are presented in this work. Advantages to this approach are precise knowledge of 234 Th recovery, while maintaining high sample throughput afforded by ICP-MS analyzes. The improved technique includes the acidification of 4-liter sea water samples and the addition of 230 Th as a yield monitor prior to MnO2 precipitation. Subsequent filtration and beta-counting of the high-energy daughter, 234mPa, was followed by a final background count after 6 half-lives (144 d) of decay. Filtered precipitates were dissolved with H2O2, and an internal standard of 229 Th was added. Samples were purified using anion-exchange chromatography to remove high levels of manganese, and recoveries were determined by measured ratios of 230 Th/229 Th by ICP-MS. Application of this procedure for 234 Th derived export in the recent Southern Ocean Iron Experiment showed average recoveries of 91%. Corrections for rare low recoveries (25-80%) noticeably change 234 Th profiles, thus impacting subsequent elemental flux calculations.
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