Weifang Chen

On the importance of the decay of 234Th in determining size-fractionated C/234Th ratio on marine particles

proxy for the export of particulate organic carbon (POC) from the upper ocean. Previous studies have shown that POC/ 234 Th ratios may vary by as much as two orders of magnitude, but the mechanism for this variability remains poorly understood. In this study, POC/ 234 Th and POC/ 228 Th ratios in three particle size classes were examined for samples collected from the upper 500 m at a station in the South China Sea. Our results indicate that changes in the POC/ 234 Th ratio with particle size at a given depth are mostly driven by simple decay of 234 Th. In fact, various trends in the POC/ 234 Th ratio versus particle size are explained by a combination of 234 Th decay and the preferential remineralization of POC relative to Th during particle aggregation. We also observed a trend of decreasing 234 Th/ 228 Th ratios with particle size. This trend is consistent with results expected from a Brownian-pumping model. Citation: Cai, P., M. Dai, W. Chen, T. Tang, and K. Zhou (2006), On the importance of the decay of 234 Th in determining size-fractionated C/ 234 Th ratio on marine particles, Geophys. Res.

Isotopic Composition and Distribution of Plutonium in Northern South China Sea Sediments Revealed Continuous Release and Transport of Pu from the Marshall Islands

The (239+240)Pu activities and (240)Pu/(239)Pu atom ratios in sediments of the northern South China Sea and its adjacent Pearl River Estuary were determined to examine the spatial and temporal variations of Pu inputs. We clarified that Pu in the study area is sourced from a combination of global fallout and close-in fallout from the Pacific Proving Grounds in the Marshall Islands where above-ground nuclear weapons testing was carried out during the period of 1952-1958. The latter source dominated the Pu input in the 1950s, as evidenced by elevated (240)Pu/(239)Pu atom ratios (>0.30) in a dated sediment core. Even after the 1950s, the Pacific Proving Grounds was still a dominant Pu source due to continuous transport of remobilized Pu from the Marshall Islands, about 4500 km away, along the North Equatorial Current followed by the transport of the Kuroshio current and its extension into the South China Sea through the Luzon Strait. Using a simple two end-member mixing model, we have quantified the contributions of Pu from the Pacific Proving Grounds to the northern South China Sea shelf and the Pearl River Estuary are 68% ± 1% and 30% ± 5%, respectively. This study also confirmed that there were no clear signals of Pu from the Fukushima Daiichi Nuclear Power Plant accident impacting the South China Sea.

How accurate are 234Th measurements in seawater based on the MnO2‐impregnated cartridge technique?

In the past decade, a MnO2-impregnated cartridge technique has been widely used to extract 234Th from seawater. One of the inherent assumptions associated with this technique is that all Th species in the dissolved phase are subject to extraction by the MnO2 cartridges. In this study, an intercalibration between the cartridge technique and a small-volume MnO2 coprecipitation technique was carried out to test this assumption. We demonstrated that the collection efficiency for 234Th could be substantially overestimated by the MnO2 cartridge technique. This may be the result of organic complexation of a significant portion of 234Th in seawater, causing this fraction of Th to pass through the MnO2 cartridges. The overestimate in collection efficiency may explain the deep-water 234Th deficit observed in some oceanographic settings (e.g., the Gulf of Mexico, the Middle Atlantic Bight, and the Gulf of Maine). Sensitivity tests show that using the cartridge technique can yield 234Th-based particulate organic carbon export rates that are overestimated by factors of up to 10. Furthermore, the frequent observed disagreements between Th fluxes recorded by shallow sediment traps and estimated using the cartridge method may be ascribed, at least partially, to this methodological issue.

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 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