Dataset: 234Th

See complete methodology in Haskell et al. (2016). In summary:
This study is part of an effort aimed at characterizing the biological response to upwelling at SPOT on 21 cruises between January 2013 and June 2014; the Upwelling Regime In-Situ Ecosystem Efficiency (Up.R.I.S.E.E.) study.

Thorium-234: Vertical profiles from the surface to 200 m were collected for thorium via Niskin/CTD on every cruise. Ten liters were collected at eight depths, chosen based on the fluorescence profile observed during the CTD's descent. A 229Th spike of known activity was added to the samples as they were being transferred from Niskins into 10 L or 20 L polycarbonate carboys (to an activity ~0.9 dpm/L) and allowed to equilibrate for at least 24 h. The recovery yield of 229Th in each sample was used in all calculations of 234Th to correct for methodological efficiency. The samples were coprecipitated with MnO2 using the technique originally developed by Rutgers van der Loeff and Moore (1999) and detailed in Haskell et al. (2013). Samples were filtered onto a 0.45 um Pall Supor Membrane filter (142 mm). The filters were dried at room temperature, placed in a plastic test tube, and placed in an Ortec low background gamma detector (intrinsic germanium, well-type, 150cc active volume). 234Th has readily identifiable gamma peaks at 63.2 keV (branching ratio ~4%) and 92.4+92.8 keV (branching ratio ~5.5%). All reported activities of 234Th have been calculated using the sum of these two peaks. Each sample was counted until counting uncertainty was below 8%. Counts were corrected for 234Th decay, ingrowth from 238U between collection and filtration, and production from co-precipitated 238U, which was measured by re-counting multiple samples from each profile six months after collection (45 half-lives). Standardization was done using a solution of known 238U activity.