230Th and 231Pa in the Arctic Ocean: implications for particle fluxes and basin-scale Th/Pa fractionation

We report concentrations of 231Pa, 230Th, and 232Th measured in unfiltered seawater samples from three stations occupied during the 1994 Arctic Ocean Section; at the western edge of the Canada Basin, in the Makarov Basin, and in the Amundsen Basin at the North Pole. The North Pole data agree well with previously published profiles from the Amundsen Basin. 230Th and 231Pa concentrations in the Canada and Makarov Basins are intermediate between those reported previously for the Beaufort Sea and the Alpha Ridge, reinforcing the observation that scavenging rates are variable in the western and central Arctic and can be as rapid as in the Eurasian Basin and other oceans. Thorium-232 data indicate that relatively high rates of scavenging under the permanent ice cover are driven at least in part by offshore transport of detrital particulate material. A comparison of water column 230Th 231Pa ratios throughout the Arctic suggests that there may be little basin-scale fractionation of these two isotopes in this ocean, contrary to expectations based on the relatively large area of the continental shelves and their contrasts with the deep basins. Models of shelf–basin exchange and 230Th/231Pa scavenging, incorporating available constraints, support this suggestion and point to the need for more data from the Arctic shelves, where the imprint of Th/Pa fractionation, if any, is expected be most pronounced.