Shallow vs. deep-water scavenging of Pa-231 and Th-230 in radionuclide enriched waters of the Atlantic sector of the Southern Ocean

The scavenging of Pa-231 and Th-230 was investigated in the Atlantic Sector of the Southern Ocean by combining results from sediment trap and in situ filtration studies. We present the first high-resolution profile of dissolv ed Th-230 and Pa-231 in surface waters across the ACC, showing a dramatic s outhward increase of both radionuclides around the southern ACC Front at 53 degreesS. High dissolved Pa-231/Th-230 ratios combined with low Th-230/Pa- 231 fractionation factors (F) in these surface waters result in extremely h igh Pa-231(xs)/Th-230(xs) ratios of material collected in the shallow traps . Particulate Pa-231(xs)/Th-230(xs) ratios in a shallow trap near Bouvet Is land increase continuously during the productive period in austral summer, and drop back in the low flux period. This behavior, following the Rayleigh fractionation principle, is interpreted to be due to an increase in the di ssolved Pa-231/Th-230 ratio in the euphotic zone resulting from preferentia l scavenging of Th-230 relative to Pa-231, even in opal-dominated regions. In the post-bloom stage, the depleted radionuclide concentrations are reple nished by upwelling of Circumpolar Deep Water. The high particulate Pa-231( xs)/Th-230(xs) Signal is weakened during downward transport of the bloom pa rticles in the water column by incorporation of deep suspended particles, w hich have a lower Pa-231(xs)/Th-230(xs) ratio. It is shown that under the s pecial hydrographic conditions in the Southern Ocean scavenging from the up per water column significantly influences the budgets of Th-230 and Pa-231 in the sediment. Nevertheless, the budgets are still made up primarily by s cavenging from the large standing stock of deep suspended particles. (C) 20 00 Elsevier Science Ltd. All rights reserved.