Watercolumn Thorium-234 (Process Cruise)

Methods

General Methods

4-L Samples for 234Th were taken from the CTD Niskin rosette at discrete depths spanning the upper 100-200 m of the water column. Samples were immediately acidified (to a pH<2) and spiked with a tracer concentration of Th-230 (1 mL at 0.17 Bq/mL), shaken, and allowed to equilibrate for 4-9 hours. Samples were then re-basified (to a pH of 8-9) and 100 microliters each of 7.5 g/L KMnO4 and 33 g/L MnCl2 were added. Samples were again shaken and allowed to sit for >4 hours as the thorium co-precipitated with manganese oxide. Samples were then vacuum filtered at high pressure onto 25-mm quartz microfiber (QMA) filters (nominally pore size = 1 micron). Samples were dried and mounted on plastic holders for a RISO beta counter. For samples from cruises P0605, P0704, P0810, and P0904: After cruise, samples were beta counted on a RISO beta counter at the University of South Carolina. After >6 half lives, the samples were again counted to determine background beta emissions. Samples were then dissolved in a solution of 10% H2O2 / 8M HNO3, spiked with a tracer concentration of Th-229 (1mL at 1.66 Bq / g), and sonicated for 20 minutes. Samples were allowed to sit for >6 hours. Samples were then purified on a Poly-Prep chromatography column containing AG1-X8 resin and eluted with HCl. HCl was evaporated off and residue was dissolved in 10%HNO3/1%HF for ICP-MS at the WHOI Analytical facility. (See Stukel et al. 2011, Limnol. Oceanogr. 56(5): 1866-1878; Stukel et al. 2013, Mar. Ecol. Prog. Ser. 491:47-65; Pike et al. 2005, J. Rad. Nuc. Chem. 263(2): 355-360) U-238 was calculated from salinity using the equation of Owens et al. (2011, Mar. Chem. 127(1-4): 31-39): U-238 = 0.0786 x Salinity - 0.315