A new method to extract Th, Th and Pa from seawater using a bulk-extraction technique with Nobias PA-1 chelating resin.

The long-lived radioisotopes of Th and Pa are unique tracers for quantifying rates of biogeochemical processes in the ocean. However, their generally low concentrations (sub-fg/kg for Th and Pa and pg/kg for Th) in seawater make them difficult to measure. Here, we present a new approach to determine Th and Th using Nobias PA-1 chelating resin following a bulk-extraction technique, and report for the first time the use of this resin to measure Pa concentrations. This method has high extraction efficiency (>80%) at pH of 4.4 ± 0.2 and the lowest procedural blanks reported in the literature: 1.0 ± 0.2 pg, 0.10 ± 0.03 fg, and 0.02 ± 0.01 fg for Th, Th, and Pa, respectively, representing 3%, 0.02%, and 0.01% of the total dissolved Th, Th, and Pa found in 5 L of a typical low-concentration surface seawater sample from the subtropical Pacific Ocean. The procedure yields data with high precision for all three isotopes (0.76% for Th, 0.89% for Th, and 0.96% for Pa, 2σ), allowing us to reliably measure Th and Pa in the oceans even at concentrations as low as those found in surface waters of the South Pacific Ocean. The accuracy of this method was confirmed by the analysis of well-characterized standard solutions (SW STD 2010-1 and SW STD 2015-1) and seawater samples collected aboard the FS Sonne (cruise SO245) during the UltraPac cruise in the South Pacific Ocean. Simultaneous and rapid extraction of Th, Th and Pa from seawater, as well as the high precision and accuracy of this method makes it ideal for both spatially and temporally high-resolution studies.