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.

Activity of isotopes of U, Th and Pa versus depth in bottom sediments from Core ML1_66

A sediment core from the European Basin with alternation of biogenic calcareous oozes and terrigenous sediments is studied by several methods. Isotopic age is determined by 230Th-ex and 231Pa-ex and by the radiocarbon method. Surface water paleotemperatures are reconstructed from complexes of planktic foraminifera and oxygen isotope ratios in their shells, and the ratio of biogenic and terrigenous components are investigated. Stages 1-8 of the oxygen-isotope scale are identified. Fluctuations in paleooceanic conditions in the area of coring are discussed.

Thorium and Uranium analyses of sediment core GIK12310-4, NW African continental margin (Table 1,2)

The Th content of the sediment samples from "Meteor" core GIK12310-4 (3080 m water depth, off NW Africa) on a carbonate-free basis lies around the average of 12.4 ppm and is similar to the average content of the earth crust. On the contrary, uranium was found to be up to 3.5-fold enriched in the core section between 60 and 330 cm (within the Wuerm Glacial) where reducing conditions occur, due to deposition of authigenic uranium (9 µg/cm**2 1000 yrs.). The authigenic uranium content is correlated to the organic matter content (U/Corg ratio = 6 * 10**4). On the basis of the uranium content of the oxidized section uranium was split into a detritic and an authigenic component and the amount of supported ionium was calculated. From the profile of the specific Io-cxcess activity (= Io-total - Io-supported) with depth, average sedimentation rates of 3.3 ± 0.6 cm/1000 yrs. for the warmer stages and of 5 ± 1 cm/l000 yrs. for the cooler periods were estimated.

The Atlantic Meridional Overturning Circulation during the last glacial

We reconstruct the geometry and strength of the Atlantic Meridional Overturning Circulation during Heinrich Stadial 2 and three Greenland interstadials of the 20-50 ka period based on the comparison of new and published sedimentary 231Pa/230Th data with simulated sedimentary 231Pa/230Th. We show that the deep Atlantic circulation during these interstadials was very different from that of the Holocene. Northern-sourced waters likely circulated above 2500 m depth, with a flow rate lower than that of the present day North Atlantic Deep Water (NADW). Southern-sourced deep waters most probably flowed northwards below 4000 m depth into the North Atlantic basin, and then southwards as a return flow between 2500 and 4000 m depth. The flow rate of this southern-sourced deep water was likely larger than that of the modern Antarctic Bottom Water (AABW). Our results further show that during Heinrich Stadial 2, the deep Atlantic was probably directly affected by a southern-sourced water mass below 2500 m depth, while a slow southward flowing water mass originating from the North Atlantic likely influenced depths between 1500 and 2500 m down to the equator.