231 Pa and 230 Th in surface sediments of the Arctic Ocean : Implications for 231 Pa / 230 Th fractionation , boundary scavenging , and advective export

Activities of Pa and Th in surface sediments from the Arctic Ocean and Greenland–Norwegian Seas are used to examine the redistribution of these water-column produced tracers between the low productivity interior basins and high particle flux marginal areas. Sediment 231Paxs/ 230Thxs ratios throughout the Canadian and Eurasian Basins and the high particle flux Chukchi slope region are essentially invariant (average=0.063F0.014, N=39) and significantly below the water column production ratio (0.093). Paxs/ Thxs ratios are also below the production ratio in the seasonally high productivity Greenland–Norwegian Sea basins (average=0.082F0.024, N=13), though they are ~ 30% higher than the interior Arctic. The low sediment 231Paxs/ 230Thxs ratios are attributed to the net export of ~ 39% of dissolved Pa produced in intermediate/deep waters, as opposed to ~ 10% of Th, through Fram Strait. The negligible Th export, combined with the reported low 230Thxs inventory of basin sediments and high 230Thxs inventory in Chukchi slope sediments, points to significant boundary scavenging of Th in the Arctic. The invariant 231Paxs/ 230Thxs ratios further indicate that scavenging of Th is comparable to Pa (i.e., fractionation to a similar degree between the two nuclides) in both the interior basins and margins. Thus, while significant boundary scavenging of both Pa and Th occurs in the Arctic, there is a distinct lack of basin-margin