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Distribution des radionucléides naturels ( 226Ra et 227Ac) le long de la section GA01 dans l’Atlantique Nord

Abstract : The Subpolar North Atlantic, being a region of deep water mass formation, is a key region for the global thermohaline circulation and is thus sensitive to climate change. In this context, the GEOVIDE cruise was carried out between Portugal, Greenland and Newfoundland as part of the GEOTRACES program (GA01; May 15 to June 30, 2014—R/V Pourquoi Pas?). This cruise, which crossed different topographic features and biogeochemical regions, provided a unique framework for the study of i) ocean mixing, ii) transport of water masses and iii) inputs of chemical elements released by the sediments deposited onto continental margins. Naturally occurring radionuclides is used as tracers: radium-226 (226Ra, t1/2 = 1602 y) and actinium-227 (227Ac, t1/2 = 21.7 y). Due to their different half-lives, these radionuclides behave differently in the ocean and can be used to study these various processes. First, a detailed section of dissolved 226Ra activities associated with dissolved barium (Ba) concentration is reported. Because 226Ra and Ba have been widely used as tracers of water masses and ocean mixing, their behavior in this crucial region is investigated more thoroughly. The use of the 226Ra/Ba ratio as a chronometer of the global circulation is evaluated using an optimum multiparameter analysis. Results show that 226Ra and Ba distributions mainly result from conservative mixing, notably at intermediate depths (i.e., away from the ocean interfaces). 226Ra and Ba can thus be considered as conservative tracers of water mass transport in the ocean interior at basin scales. However, at the ocean boundaries 226Ra and Ba displayed non-conservative behaviors due to sedimentary, river and possibly hydrothermal inputs. Suspended particles that form in the upper water column which settle to the seafloor may also impact the distributions of 226Ra and Ba. Second, the behavior of 227Ac in the North Atlantic is investigated. 227Ac is primarily released by deep-sea sediments. Due to the low abundance of 227Ac in seawater, the analysis of this radionuclide requires the use of high-sensitivity, low-background instruments and the collection of large volume samples. In this work, an evaluation of an in situ pre-concentration technique that relies on cartridges impregnated with manganese oxide is presented. This thesis also provides one of the first sections of 227Ac in the world ocean, which is compared to the section of dissolved protactinium-231, the mother nuclide of 227Ac. The 227Ac distribution highlighted conservative behavior in the ocean interior with an excess of 227Ac near the seafloor, reflecting external inputs such as diffusion from sediment and inputs from nepheloid layers at the ocean boundaries. The behavior of 227Ac is more complicated than previous studies have shown; additional parameters are required in order to use 227Ac as a vertical mixing tracer.
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Submitted on : Friday, January 24, 2020 - 2:59:07 PM
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  • HAL Id : tel-02454460, version 1

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Emilie Le Roy. Distribution des radionucléides naturels ( 226Ra et 227Ac) le long de la section GA01 dans l’Atlantique Nord. Océanographie. Université de Toulouse III – Paul Sabatier, 2019. Français. ⟨tel-02454460⟩

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