234Th distributions in coastal and open ocean waters by non-destructive β-counting

Abstract

Non-destructive β-counting analyses of particulate and dissolved 234Th activities in seawater are simpler but no less precise than traditional radioanalytical methods. The inherent accuracy limitations of the non-destructive β-counting method, particularly in samples likely to be contaminated with anthropogenic nuclides, are alleviated by recounting the samples over several half-lives and fitting the counting data to the 234Th decay curve. Precision (including accuracy, estimated at an average of 3%) is better than 10% for particulate or 5% for dissolved samples. Thorium-234 distributions in the Skagerrak indicated a vigorous, presumably biological, particle export from the surface waters, and while bottom sediment resuspension was not an effective export mechanism, it did strip thorium from the dissolved phase. In the Greenland and Norwegian Seas, we saw clear evidence of particulate export from the surface waters, but at 75 m, total 234Th activities were generally in equilibrium with 238U.