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

[1] We disagree that the comparison of the small-volume 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 234Th data used for their comparison relied on an early protocol of the small-volume 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 234Th is substantially overestimated by the cartridge method. We therefore restate that the small-volume 234Th method that includes a recovery correction is advantageous over the cartridge technique and is recommended to more accurately quantify 234Th 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 238U in deep water and to agree with 234Th 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 234Th 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. [3] Hung et al. [2008] questioned the quality of the MnO2 cartridges and claimed that most investigators used 1 μm or 0.5 μm polypropylene cartridges to prepare the MnO2 cartridges. This statement is incorrect and was based on selective citations (in their Table 1). Additional examples of studies that applied 5 μm pore size cartridges include at the very least, Buesseler et al. [1995] in the equatorial Pacific; Buesseler et al. [1998] in the Arabian Sea; Charette et al. [1999] in the NE Pacific; Charette and Moran [1999] in the Atlantic; Moran and Smith [2000] in the Beaufort Sea; Moran et al. [2003] in the Labrador Sea; Benitez-Nelson et al. [2000] and Charette et al. [2001] in the Gulf of Maine. [4] Hung et al. [2008] then implied that the adsorption of dissolved 234Th could be limited by the contact area of MnO2 on the cartridge, and they assumed that larger pore sizes would yield lower efficiencies at equal flow rates. This is clearly wrong even on the basis of their Table 1. For example, at a comparable flow rate, Moran et al. [1997] obtained much higher efficiency (90 ± 7%) using 5 μm cartridges than Livingston and Cochran [1987] using 1 μm cartridges (83 ± 8%). The most important criteria to assess the quality of MnO2 cartridges is to examine the theoretical extraction efficiency using the equation E = 1 − B/A [e.g., Guo et al., 2002; Hartman and Buesseler, 1994]. In our study, much lower 234Th activities were detected in cartridge B than in cartridge A, and the theoretical extraction efficiency of our cartridges was as high as previous studies [Cai et al., 2006a] (also see Table 1). This is the best proof that the MnO2 coating on our cartridges is more than sufficient for 234Th adsorption. [5] Hung et al. [2008] also questioned the reliability of the leaching procedure used by Cai et al. [2006a]. The leaching procedure used in our study allowed the use of the same set of counting systems to measure 234Th activity in the cartridge samples and in the small-volume samples with >99% of leaching efficiency for 234Th. Because all the 234Th measurements were conducted on the same beta counter, it thus minimized the uncertainties introduced by the cross standardization of different counting systems, i.e., gamma spectrometry versus beta counters. This guaranteed that any systematic offset observed in the 234Th measurements would be a real reflection of the methodological issues associated with the cartridge method. Second, because the beta counter has a much higher counting efficiency and a much lower background, the cartridge 234Th measurements in our study should be more precise than those based on the gamma spectrometry. This is especially true for 234Th measurements on cartridge B, which are critical for determination of the theoretical collection efficiency for 234Th, but were often found to be below the detection limit of gamma spectrometry [Baskaran and Swarzenski, 2007]. Most importantly, the addition of a yield tracer guaranteed that any loss of 234Th during sample processing would be monitored and corrected. This greatly minimized the uncertainty associated with the measurements of 234Th recovery. In a recent cruise to the South China Sea in May 2005, the leaching efficiency was assessed and the cartridge method was reexamined. Large-volume cartridge samples as well as small-volume samples were collected from the upper 500 m at the South East Asia Time series Station (SEATS, at 116°E, 18°N, water depth ∼3800 m). Samples were collected and processed following the procedure described by Cai et al. [2006a]. Seven out of thirty-two cartridge samples were assessed for the leaching efficiency. The leached cartridges were ashed in a muffle furnace at 500°C and the residue 234Th activities were determined. Results showed that using the procedure by Cai et al. [2006a], >99% of the 234Th was leached from the intact MnO2 cartridges (Table 2). [6] Using our new data sets, we confirm our previous conclusion that the MnO2-impregnated cartridge technique may substantially overestimate the collection efficiency for 234Th. The average theoretical collection efficiencies (TCE) for 234Th on the MnO2 cartridges are 85.7 ± 11.9% (n = 16, Table 1). The derived collection efficiencies, however, were found to be much lower, 55.9 ± 11.8% (DCE-A in Table 1, n = 16). Figure 1 shows the depth profiles of total 234Th activities based on the small-volume method and on the cartridge method. The depth profile based on the small-volume method shows either 234Th:238U secular equilibrium or 234Th excess relative to 238U below 100 m. In contrast, the depth profile based on the cartridge method shows a consistent deficiency of 234Th from the surface down to 450 m. The only exception to this pattern occurs at 500 m, where 234Th activity was apparently in equilibrium with 238U. This single point of equilibrium could however be caused by the uncertainty inherent to the theoretical collection efficiency and should not be taken as a proof that the 234Th measurements based on the cartridge method are unbiased. [7] Hung et al. [2008] argued that the best approach for calibrating the cartridge method is to add a known amount of 234Th in equilibrium with 238U to a blank MnO2 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]. However, ashing the cartridge does not guarantee the geometric similarity between standards and samples. Significant differences in the weight of ashed residue between samples and standards are common, as the amount of MnO2 adhered may vary drastically between cartridges even if the way of preparing the cartridges is strictly identical. As a consequence, the counting efficiency of the gamma detectors used may differ drastically between different laboratories. A typical example has been the intercomparison of deep-water MnO2 cartridge samples (all MnO2 cartridges ashed) conducted between Stony Brook and IAEA-MEL in the MedFlux program [Stewart et al., 2007]. Systematic offset of 234Th activities in deep-water samples was observed between the two laboratories. While the precise cause of this offset was stated to be “uncertain,” these investigators used a ratio of 1.16 ± 0.04 to force their deep-water 234Th measurements to agree with the 238U activity. As a consequence of the forcing, the deep-water 234Th deficit vanishes, and very possibly, the systematic offset in 234Th measurements between the cartridge method and other methods, i.e., the small-volume MnO2 coprecipitation and the Fe(OH)3 coprecipitation methods, will vanish as well. [8] Many researchers used deep water as an independent check on the standardization of the cartridge method [Amiel et al., 2002; Bacon et al., 1996; Benitez-Nelson et al., 2000; Buesseler et al., 2001a, 1992; Charette and Moran, 1999; Moran et al., 1997]. As overestimation of the extraction efficiency of 234Th 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 MnO2 cartridge samples as standards should be cautious. Hung et al. [2008] cited studies in the equatorial Pacific as verification that deep water 234Th:238U equilibration was achieved [Bacon et al., 1996], but ignored the fact that a 234Th 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]. [9] Hung et al. [2008] also cited the paper by Santschi et al. [1999] and claimed that equilibrium values of 234Th/238U ratios were achieved at depths where aggregate concentrations were lowest. This statement is also incorrect. Taking a closer look at Figures 3a and 3b of Santschi et al. [1999], one will see that the aggregate minima occurred at 150–300 m and 1200–1800 m, respectively, where 234Th was actually in deficit with respect to 238U. One will also see that 234Th deficit was evident essentially from the surface down to more than 2500 m [Santschi et al., 1999, Figures 1 and 3 and Tables 3 and 4]. While 234Th excess or deficit at some specific depth intervals may be a real oceanographic phenomenon, a 234Th deficiency from the surface to >2500 m throughout the water column was not observed by using any other 234Th technique. This again strongly suggests that the MnO2 cartridge method drastically overestimates the extraction efficiency of 234Th, as pointed out by Cai et al. [2006a]. [10] Hung et al. [2008] argued that laboratory tests showed 99% extraction efficiency for 234Th on MnO2 cartridges, in good agreement with the field cartridge efficiencies [Santschi et al., 1999]. Meanwhile, it has long been recognized that isotopic equilibration is a time-dependent kinetic process, which also depends highly on pH and temperature. In order to reach the equilibration between the spike and the naturally occurring Th nuclides, seawater samples need to be acidified to pH < 2.0 and stand for at least 12 hours [e.g., Bhat et al., 1969]. In contrast, in the laboratory tests of Santschi et al. [1999], 234Th spike was added directly to the seawater sample under natural pH condition and the sample was stirred for about 30 minutes. Under such conditions, it is very probable that equilibration was not reached between the added spike and the naturally occurring Th isotopes. Thus their results may not represent the real cartridge 234Th extraction efficiency for natural seawater samples. Indeed, the earliest application of MnO2 cartridge to the sampling of Th isotopes in natural seawater gave much lower extraction efficiencies, which were about 10–15% [Bacon and Anderson, 1982], much closer to those reported by Cai et al. [2006a]. [11] Early application of the small-volume method involves direct addition of reagents (KMnO4 and MnCl2) to the seawater sample to form a MnO2 precipitate. Subsequently, the precipitate is allowed to form for 8–16 hours and then filtered and measured for 234Th via the direct beta counting of its high-energy beta emitting daughter, 234mPa. This protocol assumes quantitative scavenging (i.e., 100%) of 234Th in seawater by the MnO2 precipitate and thus does not use a yield monitor [Benitez-Nelson et al., 2001; Buesseler et al., 2001b]. Recent studies showed that 234Th extraction efficiency on the MnO2 precipitate could vary from <25% to >99%, possibly owing to the effect of organic complexation of 234Th as well as physical loss of the MnO2 precipitate during sample processing [Cai et al., 2006b; Pike et al., 2005; Rodriguez y Baena et al., 2006]. As such, before addition of reagents, seawater samples should be acidified to pH < 2 and spiked with a 230Th yield monitor. The samples should then be allowed to equilibrate for >12 hours and the pH readjusted to 8 ± 0.15 with concentrated NH4OH. The recovery of the 230Th yield monitor on the MnO2 precipitate is quantified either by inductively coupled plasma-mass spectroscopy or by alpha spectrometry with addition of a 229Th (or 228Th) internal standard [Cai et al., 2006b; Pike et al., 2005; Rodriguez y Baena et al., 2006]. [12] At least some of the small-volume 234Th data, such as those from Lepore et al. [2007] that Hung et al. [2008] used for their comparison are based on the early protocol [Buesseler et al., 2001b] to measure 234Th activities in the small-volume samples. As this protocol does not use a yield tracer, one can deduce that their small-volume 234Th data may have been biased, and with low yields, the only direction for this bias is to have underestimated 234Th in non yield corrected samples. For the rest of data Hung et al. [2008] adopted for their comparison, no information was available in the original literature or from Hung et al. [2008] that would allow us to verify if a recovery correction was made. Regardless, the intercomparison of the two methods conducted by these authors is limited in its conclusion owing to the possibility of low 234Th in small volume samples. [13] While we are certain that the MnO2 cartridge method may overestimate the Th extraction, we should be conservative on the extent that this overestimation could have occurred to open ocean regimes given that our intercomparison was conducted in a marginal sea, the South China Sea. We should at the same time point out that the DOC and particulate organic carbon (POC) concentrations at the sampling site of the South China Sea are very similar to open ocean settings; that is, DOC ∼ 70–80 μM at surface and <42 μM at depth ∼ 3000 m, and POC ∼ 2–3 μmol L−1 at surface and <0.5 μmol L−1 at depth >1000 m. Unfortunately, to our knowledge, there is no direct comparison between the small-volume method and the cartridge method in the open ocean yet available. [14] We reiterate that selection of Th sampling techniques should be based on the goals of the chosen research objectives, as recommended by Rutgers van der Loeff et al. [2006]. As we pointed out [Cai et al., 2006a], the MnO2 cartridge approach eliminates the need for ion-exchange chemistry, and holds the advantage of being able to simultaneously collect multiple isotopes, which have allowed us to gain substantial knowledge on particle dynamics and export flux estimates. Nonetheless, with technological advances, the small-volume 234Th method is recommended for the determination of total 234Th in seawater. It can now be done onboard a research vessel for initial 234Th counting with minimal water volume, feasible with CTD Rosette water sampling systems. It is thus capable of mapping 234Th fluxes at a higher spatial and temporal resolution. In particular, we recommend the small-volume MnO2 precipitation technique with the addition of a yield monitor, since all losses of 234Th due to colloidal complexation and physical losses of MnO2 precipitates can be corrected by Th recoveries [Cai et al., 2006b; Pike et al., 2005; Rodriguez y Baena et al., 2006]. [15] This work was supported by the Natural Science Foundation of China (NSFC) through grants 40490264, 40676045, and 40521003. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

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