Water-column total Th-234 from R/V Thomas G. Thompson cruise TN303 in the Eastern Tropical Pacific in 2013 (U.S. GEOTRACES EPZT project) (NCEI Accession 0278080)

Acquisition Description:
Sampling details:
Super stations: approximately 28 discrete total Th-234 samples were collected throughout the water column, a minimum of 12 depths were chosen in the upper 1000 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.
Hemi station: 16 discrete total Th-234 samples were collected from the upper 3000 m.
Demi and Shelf stations: 12 discrete total Th-234 samples were collected, at shelf stations depths spanned the entire water column, at demis the upper 1000 m was sampled.

The shipboard procedures follow Pike et al. (2005).

Thorium-234:
Thorium-234 was determined by the widely-adopted 4 L method. Generally, shallow (< 1000 m) samples were collected from the ODF Rosette and deep samples (>1000 m) were collected from niskin bottles hung above in situ pumps. Data are decay corrected to the mid-point time between when bottles 1 and 12 were fired for shallow casts, when the messenger was dropped for deep casts, and the time of collection for underway samples. All underway samples were collected directly from the ship’s underway system in the aft ODF wet lab (denoted as bottle 99, cast 0, depth 3 m). All data have been recovery-corrected using the Th-230/Th-229 recovery method (see references below) to account for any loss of sample material during processing. All samples were analyzed using RISO Laboratory Anti-coincidence Beta Counters, using a helium/1% butane mixture. Overall method efficiency was determined by minimizing the percent difference between mean U-238 and Th-234 values for samples from >1000 m to > 500 m from the bottom (45.72 %). Only stations 6 through 16 and 23 through 36 were used to determine method efficiency so as not to include samples with potential coastal or hydrothermal influences.

Uranium-238:
Value and uncertainty from equation derived 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).

For more information on methods and intercalibration procedures, refer to the GEOTRACES Intercalibration Report (PDF) Supplemental File.