How accurate are 234 Th measurements in seawater based on the MnO 2 ‐impregnated cartridge technique?

In the past decade, a MnO 2 ‐impregnated cartridge technique has been widely used to extract 234 Th 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 MnO 2 cartridges. In this study, an intercalibration between the cartridge technique and a small‐volume MnO 2 coprecipitation technique was carried out to test this assumption. We demonstrated that the collection efficiency for 234 Th could be substantially overestimated by the MnO 2 cartridge technique. This may be the result of organic complexation of a significant portion of 234 Th in seawater, causing this fraction of Th to pass through the MnO 2 cartridges. The overestimate in collection efficiency may explain the deep‐water 234 Th 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 234 Th‐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.