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New water column profiles of dissolved thorium and protactinium in the western North-Atlantic

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Kretschmer, S. , Masque, P. , Geibert, W. and Rutgers v. d. Loeff, M. (2012): New water column profiles of dissolved thorium and protactinium in the western North-Atlantic , Goldschmidt Conference, Montréal, Canada, 24 June 2012 - 29 June 2012 .
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Abstract:

The strength of the Atlantic Meridional Overturning Circulation (AMOC) is a major factor in the global thermohaline circulation system. Because of this central role of AMOC in the global climate system, it is important to have reliable tools to reconstruct the strength of deep water ventilation in general and of AMOC in particular during major phases of past climate variability. The basin-wide Atlantic 231Pa/230Th signature in sediment archives has been identified as a powerful tracer for the intensity of the AMOC in the past. The information on ocean circulation arises from an overall higher export rate of 231Pa over 230Th within NADW from the entire Atlantic basin to the Southern Ocean due to the somewhat lower particle reactivity of 231Pa. A fundamental complication for reliable applicability of this tool arises from the fact that there is a severe lack of data of these isotopes in the critical areas of the modern AMOC. Only very few 230Th and 231Pa profiles in the Labrador Sea, in the NE Atlantic, and in the tropical and South Atlantic have been published [1, 2, 3, 4]. To remove this gap, 14 water column profiles were sampled under trace metal clean conditions during the expedition of RV Pelagia (GEOTRACES Cruise GA02) on a transect following the North Atlantic deep water (NADW). At all stations the deepest sample was collected within the bottom nepheloid layer, providing information on the latest stage of signal development in the water column. Here we will present the first results on 5 profiles of dissolved 230Th and 231Pa along NADW between 15 and 55°N including also the crossover station Bermuda Atlantic Time-Series (BATS). With this study we aim to provide missing information of the factors controlling signal generation in order to answer the questions: What is the 231Pa/230Th isotope composition of the main water masses of the AMOC and how do the 230Th and 231Pa activities in NADW evolve on its way south and east? Can they be explained by ventilation or are there other controlling factors to identify such as the composition of suspended particles? [1] Moran et al. (2002) Earth Planet. Sci. Lett. 203, 999-1014. [2] Moran et al. (1997) Earth Planet. Sci. Lett. 150, 151-160. [3] Moran et al. (1995) Geophys. Res. Lett. 22, 2589-2592. [4] Vogler et al. (1998) Earth Planet. Sci. Lett. 156, 61-74

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