The recently developed 2‐5 L small‐volume MnO2 coprecipitation method for
determining 234Th in seawater has provided a new way to substantially increase, both
temporally and spatially, the sampling resolution of 234Th and 234Th‐based particulate
organic carbon export estimates in the upper ocean. In this study, we further optimize
the technique by reducing reagent quantities and the use of an additional water bath
heating step. This optimization allows the filtration of the MnO2 precipitate onto a
25‐mm diameter, 1.0 μm pore size QMA filter to be completed within < 30 minutes for
unfiltered waters from the South China Sea. In addition, we have modified the MnO2
purification procedure to allow for alpha spetrometric measurements of 234Th recoveries.
Results from recovery experiments suggest that reagent amounts can be reduced to
0.0375 mg KMnO4 and 0.1 mg MnCl2∙4H2O per liter of sample, while still maintaining
high 234Th recovery. This study further confirms that the addition of a yield monitor is
necessary for the application of this small‐volume method.
