Showing papers by "Walter Geibert published in 2001"

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

Abstract: The scavenging of 231 Pa and 230 Th 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 dissolved 230 Th and 231 Pa in surface waters across the ACC, showing a dramatic southward increase of both radionuclides around the southern ACC Front at 53°S. High dissolved 231 Pa / 230 Th ratios combined with low 230 Th / 231 Pa fractionation factors (F) in these surface waters result in extremely high 231 Pa xs / 230 Th xs ratios of material collected in the shallow traps. Particulate 231 Pa xs / 230 Th xs ratios in a shallow trap near Bouvet Island 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 dissolved 231 Pa / 230 Th ratio in the euphotic zone resulting from preferential scavenging of 230 Th relative to 231 Pa , even in opal-dominated regions. In the post-bloom stage, the depleted radionuclide concentrations are replenished by upwelling of Circumpolar Deep Water. The high particulate 231 Pa xs / 230 Th xs signal is weakened during downward transport of the bloom particles in the water column by incorporation of deep suspended particles, which have a lower 231 Pa xs / 230 Th xs ratio. It is shown that under the special hydrographic conditions in the Southern Ocean scavenging from the upper water column significantly influences the budgets of 230 Th and 231 Pa in the sediment. Nevertheless, the budgets are still made up primarily by scavenging from the large standing stock of deep suspended particles.