Dataset: Seawater radioisotope (234Th) and carbon from sampling conducted at the Compass Station in Bedford Basin, Nova Scotia, Canada from 2021 to 2024

Samples were collected at Compass Station in the Bedford Basin, Halifax, Nova Scotia, Canada (44.7 N, -63.6 W) on day-trips aboard Connor's Diving Ltd, vessel EASTCOM in April 2021, August 2021, January 2022, May 2023, August 2023, and January 2024.

Total 234Th in the water column:
Total (dissolved + particulate) 234Th was measured six times at the Compass Station between 2021and 2024. Water column samples were collected using 10 liter (L) Niskin bottles at 9 discrete depths. From each bottle, a 2 L subsample of unfiltered seawater was taken, spiked with a 230Th yield tracer, and processed according to the MnO2 co-precipitation technique (Pike et al., 2005). Following co-precipitation, samples were filtered onto quartz fiber filters (QMA, 25 millimeters (mm) diameter, 1 micrometer (μm) nominal pore) following Cai et al. (2006). After filtration, filters were dried (55 degrees Celsius (°C)), mounted on sample holders, covered with Mylar film (1 layer) and aluminium foil (2 layers), and their beta emissions were measured on low-level beta GM multi-counters (Risø National Laboratory, Denmark). Samples were counted until the error was typically < 3 %. After >120 days from collection (i.e., after 5-6 half-lives of 234Th had passed), samples were recounted to determine the non-234Th beta activity stemming from other radionuclides included in the precipitate, which was subtracted from the first count. After final counting was completed, the chemical recovery of 230Th was determined by a Thermo Scientific iCAP quadrupole inductively coupled plasma mass spectrometry (ICP-MS) as outlined in Clevenger et al. (2021), yielding a mean recovery of 85 ± 12 % (n = 63). The net counting rate was corrected for 234Th decay and ingrowth, counting efficiency, and chemical recovery. The uncertainties in the total 234Th activities are determined from counting statistics and the propagation of errors in relation to sample processing, including mass and volume measurements, and ICP-MS recovery analysis. The parent 238U activity is derived from salinity following the relationship given by Owens et al. (2011).

Size fractionated particulate 234Th and POC activities in water column:
Size fractionated particulate 234Th samples (234Thp) were collected by in-situ pumping of ~ 30 L at 3 depths (~20, 40, 60 m) using WTS-LV large volume pumps (McLane Laboratories). Filter heads were equipped with a 51 µm pore size Nitex filter above a 1 µm pore size QMA filter (pre-combusted). The large particle material (> 51 µm) captured on the Nitex screens was carefully rinsed onto QMA filters (1 µm pore size, 25 mm diameter, pre-combusted) using 0.2 µm filtered and ultraviolet (UV) treated seawater. The small particulate material (1–51) captured on the QMA filter was subsampled by cutting out a 25 mm diameter punch. After drying (55 °C) all particulate samples were mounted onto sample holders and analyzed on beta counters (see above). Following the second counting, particulate samples were prepared for POC measurement by fumigation under HCl vapor for 6 hours inside a glass desiccator to remove the carbonate phase. Samples were dried at 50 °C and packed into tin cups and then measured on an Elemental Analyzer (Elementar MicroCube & Isoprime 100).