Sampling details:
Approximately 28 discrete total Th-234 samples were collected throughout the water column, a minimum of 12 depths were chosen in the upper 1000 meters (m).
Full stations: approximately 20 discrete total Th-234 samples were collected throughout the water column, a minimum of 12 depths were chosen in the upper 1000 m.
Demi and Shelf Stations: 13 discrete total Th-234 samples were collected. At shelf stations, depths spanned the entire water column, and at demi stations, the upper 1000 m was sampled.
The shipboard procedures follow Pike et al. (2005) and Clevenger et al., 2021.
Total water-column Thorium-234:
234Th was determined by following the methods of Pike, et al. (2005) and Clevenger, et al. (2021) on 2-liter (L) water column samples, which have been utilized previously for other GEOTRACES efforts (e.g. Owens et al., 2015). An exact 1-milliliter (mL) aliquot of 230Th (50.03 disintegrations per minute per gram (dpm g⁻¹)) was used as the yield monitor and added during initial acidification of the samples. QMAs were used to collect the precipitate from the 2 L process and immediately dried. Once dried, they were mounted onto plastic 25-millimeter (mm) discs, covered with a mylar layer and two layers of aluminum foil, and immediately beta counted at sea. The filters were counted again, 5 to 6 months later to quantify the background radioactivity due to the beta decay of long-lived natural radionuclides that are also precipitated. The mean value of the at-sea counts (decay-corrected to the time of collection) minus the background value for each filter is reported as the 234Th activity (milliBecquerels per kilogram (mBq kg⁻¹)). Activities for 234Th are generally reported in disintegrations per minute per liter (dpm L⁻¹) but have been converted here using a standard density of 1.028 kilogram per liter (kg L⁻¹) and 1 dpm = 16.667 mBq. Data are decay-corrected to the mid-point time between when the first and last bottles were fired for shallow casts, when the messenger was dropped for deep casts, and the time of collection for fish tow samples. Generally, shallow water column (< 1000 meters (m)) samples were collected from the ODF Rosette, and deep samples (>1000 m) were collected from Niskin bottles hung above in-situ pumps. All fish samples (namely intermediate surface and surface samples) were collected directly from the clean fish tow (denoted as either arriving or intermediate fish, depth 3 m). All data have been recovery-corrected using the 230Th /229Th recovery method (see References) to account for any loss of sample material during processing. All samples were analyzed using Risø Laboratory Anti-Coincidence Beta Counters, using a helium/1% butane mixture.
Uranium-238:
U-238 can be calculated via the equation described in Owens et al. (2011):
U-238 (dpm/L) = (Salinity * 0.0786) – 0.315
Salinity was measured on board the ship by the Scripps Ocean Data Facility (ODF).
Particulate Thorium-234:
Particulate material was collected using in situ McLane pumps (5-24 depths per station, see also GP17-OCE data from Ohnemus group). Approximately 24 discrete particulate Th-234 samples were collected throughout the water column for super stations before losing the pumps on December 25, 2022. The discrete particulate samples per station decreased to an average of 6 per station after December 25. All samples were analyzed using Risø Laboratory Anti-Coincidence Beta Counters, using a helium/1% butane mixture. All Th-234 are measured three times for at least 12 hours or to <5% error. All data were decay-corrected back to mid-pumping times.
>51-micrometer (µm) Th-234 (LPT, large particulate Thorium):
Mesh screens were provided by the Ohnemus group. A whole or partial (3/4) mesh screen was rinsed onto a 25 mm silver filter for beta counting. The mean volume pumped through the whole Supor mesh screens was 800 L.
1-51 µm Th-234 (SPT, small particulate Thorium):
Whole QMAs, located below a mesh screen in the filter head housing, were provided by the Ohnemus group and oven-dried upon recovery. A 25 mm subsample was taken from this whole filter for beta counting for Th-234. The mean effective volume for the 25 mm QMA subsample was 32 L (804 L for entire QMA).
