Radium isotopes and 228Th in water column samples collected on POLARSTERN expedition PS100

Radium isotopes are produced by decay of an isotope of thorium, a highly particle reactive element. Radium is relatively soluble in seawater such that once a thorium isotope in suspended or bottom sediments decays, a fraction of the produced radium isotope may be released to solution. 228Ra is strongly enriched in shelf waters and in the Transpolar Drift (TPD) and its half-life (5.8 years) is well suited to study the fate of this shelf-derived water in the central Arctic. The 228Th/228Ra ratio in the subsurface water column can be used to trace particle fluxes. In previous studies up to PS94 (2015) we had identified the strong and increasing 228Ra signal in the Transpolar Drift. During PS100 we studied the distribution of 228Ra in Fram Strait to investigate how this 228Ra signal in the TPD is changed by dilution and decay on its way to the East Greenland Current. Water from in situ pumps was filtered and passed over MnO2-coated cartridges to absorb the Ra and Th. On board, short-lived Radium isotopes and 228Th were analysed using RaDeCC counting. In the lab, the cartridges were leached in refluxing HCl and long-lived Radium isotopes were measured by gamma spectroscopy. At depths where in situ pumps were deployed with two cartridges in series, the Ra adsorption efficiency was determined from the ratio of the Ra activities in the two cartridges. At depths where only one cartridge was mounted, we used the average collection efficiency of all other cartridge pairs, corrected after a mass spectrometric analysis of 226Ra activities in discrete samples (Vieira et al., 2021, doi:10.1002/lom3.10428) (82 ± 8%).