Water column Th-234 activity determined using a 4-L technique from samples collected on R/V Melville cruise MV1008 in the Costa Rica Dome in 2010 (CRD FLUZiE project) (NCEI Accession 0278122)

Acquisition Description:
4-L samples for analysis of water column 234 Th concentrations were typically taken at 12 depths spanning the upper 150 m on two casts per experimental cycle. On two cycles in the core of the CRD, an additional set of 8 samples was taken to assess 234 Th deficiency down to 500-m depth. Samples were analyzed by standard small volume procedures (Benitez-Nelson et al. 2001, Buesseler et al. 2001, Pike et al. 2005). Immediately after collection, samples were acidified to a pH < 2 with concentrated HNO 3 , spiked with 1 mL 230 Th tracer (0.17 Bq mL -1 ), shaken vigorously, and allowed to equilibrate for 4-9 hours. Samples were then basified to a pH of 8-9 with NH 4 OH. 100 uL each of KMnO 4 (7.5 g L -1 ) and MnCl 2 (33 g L -1 ) were added and samples were shaken then allowed to sit for >8 hours as Th co-precipitated with manganese oxide. Samples were then filtered at high pressure onto a quartz microfiber (QMA) filter, dried in a drying oven, and mounted into RISO sample holders for beta counting.

Samples were beta counted at the University of South Carolina on a RISO low-level beta multi-counter within two months of the collection date. After an additional 8 months, samples were recounted to determine background beta emissions. Detector efficiency was calibrated using four samples taken from 2000-m depth (in water columns exceeding 3000-m depth), which were assumed to be at equilibrium with respect to 234 Th and 238 U. After background counts yield analyses were conducted as follows. Manganese oxide precipitate was dissolved from filters in 10 mL 8M HNO 3 /10% H 2 O 2 . 1 mL of 229 Th (1.66 Bq g -1 ) tracer was then added gravimetrically, and the sample was sonicated for 20 minutes and allowed to sit for >6 hours. An aliquot of this solution was then filtered through a 0.1-mm syringe filter and diluted with 2.2M HNO 3 /1%HF for mass spec analysis of the 229:230 Th tracer ratio at the Woods Hole Oceanographic Institution Analytical Facility.