Dataset: Water column total and particulate Thorium-234 from Leg 1 (Seattle, WA to Hilo, HI) of the US GEOTRACES Pacific Meridional Transect (PMT) cruise (GP15, RR1814) on R/V Roger Revelle from September to October 2018

Total water-column Thorium-234
234Th was determined by the widely-adopted 4 L method (now reduced to a 2 L method, see Pike et al. 2005). (Buesseler et al., 2001), which has been utilized previously for other GEOTRACES efforts (e.g. Owens et al., 2015). An exact 1 mL aliquot of 230Th (50.03 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 mm discs, covered with a mylar layer and 2 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 (mBq kg⁻¹). Activities for 234Th are generally reported in dpm L-1, but have been converted here using a standard density of 1.025 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 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.

In order to calculate flux, Th-234 disequilibrium from its parent U-238 must be calculated. 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 (16-24 depths per station, see also PMT data from Lam group). All samples were analyzed using Risø Laboratory Anti-coincidence Beta Counters, using a helium/1% butane mixture. All Th-234 are measured 3 times for at least 12 hours or to <5% error. All data were decay corrected back to mid-pumping times.

>51 µm Th-234 (LPT, large particulate Thorium)
Mesh screens were provided by the Lam 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 406 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 Lam 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 41 L (1042 L for entire QMA).