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Protactinium and thorium scavenging and advection in the Southern Ocean, Atlantic Sector

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Venchiarutti, C. , Stimac, I. and Rutgers v. d. Loeff, M. (2010): Protactinium and thorium scavenging and advection in the Southern Ocean, Atlantic Sector , 2010 Ocean Sciences Meeting, Portland, OregonFebruary 2010. Eos Trans. AGU, 91(26), Ocean Sci. Meet. Suppl., Abstract CO13A-02. .
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During the expedition ANTXXIV-3 (Feb-April 2008, GEOTRACES/IPY, RV Polarstern), Pa and Th isotopes (230Th, 232Th and 234Th) were determined for the first time in seawater and size-fractionated particle samples along the Zero-Meridian, in the Weddell Sea and Drake Passage. Pa and Th distributions in these different environments (shelf, open-ocean, ice formation areas ) are mainly controlled by particle flux and boundary scavenging but also by advection of water masses. Thus, at Drake Passage, using -as a first approach- a simple scavenging-mixing model, we show that the vertical distribution of 230Th is mainly driven by the upwelling of deep water from North to South whereas the 231Pa distribution only reflects this upwelling in the upper 1500 m of the water column. Below this depth, Pa is more influenced than Th by advection of deep water masses with different origins flowing across Drake Passage. Thus, in the northern part of Drake Passage, deep water with high 231Pa concentrations (suggesting reduced Pa scavenging during transport) was inferred to come from the Central Pacific (in agreement with CFC data) while in the southern part of Drake Passage, the observed low Pa concentrations were attributed to deep water coming from a distant source around the opal-rich Antarctic shelf (as inferred also from Hf isotopes) with strong Pa scavenging and a correspondingly low fractionation factor FTh/Pa<<10. In the Central Weddell Sea, enhanced Pa and Th concentrations strengthen the concept of a reduced scavenging of these radionuclides in this area. Hence, accumulated dissolved Pa and Th seem to exit the Weddell Sea to the north either via the Weddell Gyre (30-60 Sv) to be then incorporated into the ACC (140 Sv) toward the Zero-Meridian (as shown in the high radionuclide concentrations found close to Maud Rise), or flow westward with the deep waters along the Antarctic Peninsula through the Drake Passage. Enhanced dissolved 232Th concentrations are typically observed in the vicinity of the Antarctic shelf and Peninsula. Based on 230Th and 234Th in size-fractionated particles and using a reversible scavenging model, particle settling velocities, adsorption/desorption rates and when possible, aggregation/disaggregation rates, were investigated. This study pointed out that in this area, specific scavenging-mixing models should be applied to describe the radionuclide distributions and include advection of water masses from the open ocean toward the Antarctic shelf (which may contribute to boundary scavenging) or advection of water masses with different histories.

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