Radionuclides measured on 21 water bottle profiles during POLARSTERN cruise ANT-XXIV/3

Drake Passage is a major route for many water masses from the strong Antarctic Circumpolar Current. During the ANTXXIV-3 expedition (in 2008) the vertical distributions of dissolved and size-fractionated particulate 231Pa and thorium isotopes (230Th, 232Th and 234Th) were investigated in order to better define the scavenging regimes and the effects of the oceanic circulation on the fate of particulate material and on the Pa-Th distributions in the water column. The reversible scavenging-model applied to both 230Th and 234Th, in the upper 1500 m depth, gives estimates of the particle dynamics (settling velocities S~ 500-1300 m/y, adsorption and desorption rate constants of 0.1-0.4 1/y and 1-6 1/y respectively). Particulate 234Th/230Th activity ratio shows a depth dependence, with decreasing ratio with increasing depth in agreement with previous studies, but no relationship with particle size was found. 231Pa and thorium isotope fractionation and partition coefficients were investigated with particle size vs depth and latitude and appear to vary horizontally following a North-South gradient. This suggests that both radionuclides are mostly bound to the fine suspended particles. At Drake Passage, the 230Thxs distribution is controlled by a southward upwelling of deep water (clearly visible on the vertical section of total 230Thxs, defined as dissolved + particulate concentrations) and reversible-scavenging processes (linear increase of 230Thxs with increasing depth) with North of the Southern ACC Front, higher settling velocities and less adsorption/desorption cycles, than South of it. Distributions of dissolved and total 231Paxs also reflect the influence of the North-South upwelling but somehow this effect appears to be limited to the upper 1500 m depth of the water column. Below this depth, 231Paxs vertical profiles exhibit contrasted concentrations, with some high dissolved activities in the deep water of the stations in the northern part of the ACC and not South of the ACC. These N-S differences in dissolved 231Paxs were attributed to the different origins and scavenging history of the deep Pacific waters flowing across Drake Passage. Here at North, radionuclides-rich deep water originates from the Central Pacific, while at South, deep water derives from the Southern Pacific in which the observed low radionuclides concentrations are attributed to high opal abundance. South of the Drake Passage, high dissolved and particulate activities of 230Th and 232Th confirmed the intrusion of 230Th-rich Weddell Sea Deep Water (WSDW) close to the Antarctic Peninsula.