Dataset: Thorium-234 along the Western Antarctic Peninsula (2016-2020)
Deployment: LMG1601

Total 234Th was measured using standard small volume techniques (Benitez-Nelson et al., 2001; Pike et al., 2005). 4-liter (L) samples for total 234Th were sampled using a CTD Niskin rosette (typically from eight depths spanning the epipelagic). Samples were acidified to a pH of <2 with HNO3. A tracer addition of 230Th was added and samples were mixed vigorously. Samples were allowed to equilibrate for 4-9 hours and then adjusted to a pH of 8-9 with NH4OH. KMnO4 and MnCl2 were added and samples were mixed and allowed to sit for ~12 hours as Th co-precipitated with manganese oxide. Samples were then vacuum-filtered at high pressure onto QMA filters, dried, and mounted in RISO sample holders.

Samples were beta counted on a RISO low-level background beta counter at Palmer Station and re-counted >6 half-lives later. Samples for water-column 234Th were then dissolved in HNO3/H2O2 solution and 229Th tracer was added. Samples were evaporated and reconstituted in dilute nitric acid / hydrofluoric acid. They were then analyzed by inductively-coupled plasma mass spectrometry at the Woods Hole Oceanographic Institution Analytical Lab to determine the ratio of 229:230Th to determine the initial yield of the 234Th filtration. 238U activity (for estimating 238U-234Th deficiency) was estimated from a linear relationship with salinity (Owens et al., 2011).