Th measurements in seawater based on the MnO 2 - impregnated cartridge technique ? ’ ’

[1] We disagree that the comparison of the smallvolume MnO2 coprecipitation method and the MnO2 cartridge method of Hung et al. [2008] is adequate. This is because at least some of the small-volume Th data used for their comparison relied on an early protocol of the smallvolume method that did not include a recovery correction. Thus results are potentially biased. New data from the South China Sea confirm our earlier conclusion that the extraction efficiency for Th is substantially overestimated by the cartridge method. We therefore restate that the small-volume Thmethod that includes a recovery correction is advantageous over the cartridge technique and is recommended to more accurately quantify Th activities used to constrain the export of particulate organic carbon in the upper ocean. [2] The comment by Hung et al. [2008] argued that the analytical procedures applied by Cai et al. [2006a] are not typical of other workers and that previous applications of the MnO2 cartridge method have found it to give good agreement with U in deep water and to agree with Th measured on small volume samples. They attempted to suggest that the two methods yield comparable results, within errors, when properly applied. We restate that the analytical procedures adopted by Cai et al. [2006a] is preferable by providing additional experimental data. Furthermore, we point out that the comparison of the small-volume MnO2 coprecipitation method with the MnO2 cartridge method of Hung et al. [2008] is inadequate. This is because at least some of the small-volume Th data cited by the authors were based on an early protocol of the method that did not include a recovery correction and thus could be biased (see below). Therefore the G Geochemistry Geophysics Geosystems