Comment on “How accurate are ²³⁴Th measurements in seawater based on the MnO₂‐impregnated cartridge technique?” by Pinghe Cai et al.

Reply to comment by Chin‐Chang Hung et al. on “How accurate are 234Th measurements in seawater based on the MnO2‐impregnated cartridge technique?”

“…We disagree that the comparison of the small‐volume MnO 2 coprecipitation method and the MnO 2 cartridge method of Hung et al [2008] is adequate. This is because at least some of the small‐volume 234 Th data used for their comparison relied on an early protocol of the small‐volume method that did not include a recovery correction.…”

“… Hung et al [2008] argued that the best approach for calibrating the cartridge method is to add a known amount of 234 Th in equilibrium with 238 U to a blank MnO 2 cartridge and count it in a gamma spectrometer. It should be noted that an ideal standard for gamma measurements should have identical matrix and geometry as real samples to eliminate uncertainties in counting efficiency and self absorption [ Rutgers van der Loeff et al , 2006].…”

“…As overestimation of the extraction efficiency of 234 Th could occur to the cartridge method [ Cai et al , 2006a] and the extent of this overestimation may vary with depth, temperature and dissolved organic carbon (DOC) concentrations [ Santschi et al , 2006]; hence those using deep‐water MnO 2 cartridge samples as standards should be cautious. Hung et al [2008] cited studies in the equatorial Pacific as verification that deep water 234 Th: 238 U equilibration was achieved [ Bacon et al , 1996], but ignored the fact that a 234 Th deficiency throughout the whole water column was observed in some of the studies with the cartridge method [e.g., Baskaran et al , 1996; Guo et al , 2002; Hung et al , 2004; Lepore and Moran , 2007; Lepore et al , 2007; Moran et al , 2005; Santschi et al , 1999; Shimmield et al , 1995].…”

“…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 MnO 2 coprecipitation method with the MnO 2 cartridge method of Hung et al [2008] is inadequate. This is because at least some of the small‐volume 234 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).…”

“…This is because at least some of the small‐volume 234 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 arguments put forth by Hung et al [2008] are flawed.…”