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

Activities of Pa-231 and Th-230 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 Pa-231(xs)/Th-230(xs) ratios throughout the Canadian and Eurasian Basins and the high particle flux Chukchi slope region are essentially invariant (average = 0.063 +/- 0.014, N = 39) and significantly below the water column production ratio (0.093). Pa-231(xs)/Th-230(xs) ratios are also below the production ratio in the seasonally high productivity Greenland-Norwegian Sea basins (average=0.082 +/- 0.024, N = 13), though they are similar to 30% higher than the interior Arctic. The low sediment Pa-231(xs)/Th-230(xs) ratios are attributed to the net export of similar to 39% of dissolved Pa-231 produced in intermediate/deep waters, as opposed to similar to 10% of Th-230, through Fram Strait. The negligible Th-230 export, combined with the reported low Th-230(xs) inventory of basin sediments and high Th-230(xs) inventory in Chukchi slope sediments, points to significant boundary scavenging of Th-230 in the Arctic. The invariant Pa-231(xs)/Th-230(xs) ratios further indicate that scavenging of Th-230 is comparable to Pa-231 (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-231 and Th-230 occurs in the Arctic, there is a distinct lack of basin-margin fractionation and a sizable export of Pa-231. There is also likely to be export of other similarly particle-reactive radionuclides (e.g., Be-10, Pu isotopes) out of the Arctic. (c) 2005 Elsevier B.V. All rights reserved.