Dissolved radium isotope activity around Guaymas Basin from samples collected by CTD and HOV Alvin during R/V Atlantis cruise AT42-05 in November 2018 (NCEI Accession 0291428)

Acquisition Description:
Samples for this dataset were separated into different worksheets depending on the sample collection method (worksheets have been combined into one dataset; "Collection_Method" column indicates the source worksheet).

"CTD" samples were collected from 20L Niskin bottles associated with CTD casts by slowly (<1 L/min) draining the sample via a sampling tube from the spigot on the Niskin bottle through a dry, 25 g aliquot of acrylic fiber impregnated with MnO 2 (Moore, 1976). These "Mn fibers" quantitatively sorb Ra isotopes from the aqueous phase.

"Core-top waters" were collected by gravity siphoning water overlying sediment cores that were collected as push cores by HOV Alvin, filtering the samples through 0.45 mm cellulose filters, then slowly (< 1 L/min) passing the effluent over 25 g Mn fiber.

"Alvin Niskins" were recovered from one or several of the 1.25 L Niskin bottles on Alvin. On the ship, these water samples were drained from the Niskin bottles and slowly (<1 L/min) passed over 25 g Mn fiber.

"Porewater" samples were collected by sectioning sediments cores under an inert (Ar) atmosphere at 4-5 cm intervals into 50 mL centrifuge tubes, centrifuging the samples at 5,000 RPM for 15 minutes, then decanting the supernatant fluids and filtering them through 0.45 mm syringe filters. Effluent samples were measured for volume, then diluted with 1 L Ra-free seawater before being passed slowly (< 1L/min) twice through Mn fibers.

After passing the water samples over the Mn fibers, the fibers were rinsed with Ra-free fresh water, then dried using a compressed air stream to a suitable humidity (Sun and Torgersen, 1998). The fibers were then counted immediately on a Radium Delayed Coincidence Counter (Moore and Arnold, 1996) for total Ra-224 and Ra-223 activity. The fibers were counted again after 3 weeks to measure the supported Ra-224 activity from any sorbed Th-228 on the fibers. This activity is subtracted from the total Ra-224 activity derived from the initial measurement to compute the excess Ra-224 activity (the activity reported in the dataset). Ra-226 activities were measured by sealing the Mn fibers in air-tight cartridges for ~1 week and measuring the accumulated Rn-222 (daughter product of Ra-226) on a radon emanation line (Peterson et al., 2009). Fibers were then counted an additional time on the Radium Delayed Coincidence Counter after ~1 year from collection for Ra-228 (as the change in Th-228 activity from the 3-week measurement; Moore, 2008). Analytical uncertainties are based on counting statistics (as 1-s standard deviation of the total counts logged, propagated through the activity computations).