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- Gdaniec, Sandra, 1988-
(författare)
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231Pa, 230Th and 232Th as tracers of deep water circulation and particle transport : Insights from the Mediterranean Sea and the Arctic Ocean
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The naturally occurring U and Th-series radionuclides have shown to have a considerable importance for the understanding of biogeochemical processes on Earth and in the ocean. In this thesis, the isotopes 230-thorium (230Th), 232-thorium (232Th) and 231-protactinium (231Pa) are used as tracers of the transport and scavenging of marine particles and water circulation. Pa and Th are particle reactive elements, which makes the production, transport and distribution of Pa and Th key factors for our understanding of the origin, fate and distribution of marine particles in the oceans. This thesis explores the distribution of 231Pa, 230Th and 232Th in two different ocean continental margin environments. In particular, the relative influence of water circulation and particles on the 231Pa, 230Th and 232Th distributions in the Arctic Ocean and Mediterranean Sea was investigated. 231Pa, 230Th and 232Th were analyzed in particles and seawater collected in the Mediterranean Sea during the MedSeA-GA04-S cruise along the GEOTRACES section GA04S and in the Arctic Ocean during the PS94 GN04 ARK-XXIX/3 along the GEOTRACES section GN04.One of the important findings of this thesis was the low fractionation between 231Pa and 230Th in the Mediterranean Sea, contrasting what is observed in the open ocean. Additionally, the observed depth profiles of Pa-Th allowed the identification of deep water convection and ventilation in the Western and Eastern Basins, respectively. Moreover, the particle settling speed was reevaluated to ~500 – 1000 m/y.In the Arctic Ocean, scavenging onto particles derived from hydrothermal activity was producing relatively low F-factors (FTh/Pa ~ 10), while higher values were observed in deep waters (FTh/Pa ~ 20). Additionally, the hydrothermal particles in the Nansen interior produce lower FTh/Pa values compared to FTh/Pa observed at the Nansen continental margin. Application of a boundary scavenging model revealed the importance of 230Th scavenging at the continental margin along the Nansen Basin, hereafter the Nansen margin, and advocate for the advection of 231Pa into the Atlantic Ocean. As the ocean margin was included in this model, a particle settling speed of 600 m/y was obtained at the Nansen margin.Moreover, this thesis includes an inter-comparison of dissolved and particulate 231Pa, 230Th and 232Th measurements between four laboratories of the GEOTRACES community. This comparison was conducted to provide detailed descriptions of various chemical procedures used for Pa-Th analysis and to provide a measure of consistency between the laboratories. Results demonstrated that participating labs can determine concentrations of dissolved 230Th and 231Pa in deep water (below 500 m depth) that are internally consistent within 4 % of the mean values. Analysis of particulate 231Pa, 230Th and 232Th allowed the highlighting of an incomplete Pa dissolution problem with our initial leaching procedure, a problem solved by measuring aliquots of particulate samples at two labs. However, in the present work, consistent particulate 231Pa concentrations as low as ~ 0.002 fg/kg were obtained. Overall, it suggests an improvement of the results consistency compared to the previous GEOTRACES intercalibration exercise.
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| 2. |
- Gdaniec, Sandra, 1988-
(författare)
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231Pa and Th isotopes as tracers of deep water ventilation and scavenging in the Mediterranean Sea
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The naturally occurring isotopes 231Pa and 230Th are used as tracers of marine biogeochemical processes. They are both produced from the radioactive decay of their uniformly distributed uranium parents (235U and 234U) in seawater. After production, 231Pa and 230Th are removed by adsorption onto settling particles (scavenging) and subsequently buried in marine sediments. 230Th is more particle reactive compared to 231Pa. Consequently, 230Th will be removed from the open ocean by adsorption onto settling particles, while 231Pa tend to be laterally transported by currents and removed by scavenging in areas of high particle flux (e.g. ocean margins). The primordial 232Th indicates lithogenic supply via rivers and resuspension of sediments, which provides additional information about processes involved in the cycling of particle reactive elements in the ocean. The preferential deposition of particle reactive elements at ocean margins (boundary scavenging) has important implications for our understanding of the distribution and dispersion of micronutrients (e.g. iron) and pollutants in the ocean. It is therefore valuable to understand the nature of boundary scavenging processes in order to evaluate the relative contribution of circulation and scavenging behaviors.The major characteristics of thermohaline circulation in the Mediterranean are well known and have been studied for decades. This sea is an almost land-locked area, where limited water-exchange with the Atlantic Ocean only occurs through the Strait of Gibraltar. Therefore, this marginal sea is often referred to as a “miniature ocean” suitable as a “laboratory” for marine environmental research. In this licentiate thesis, distributions of 231Pa, 230Th and 232Th in seawater and marine particles collected during the GEOTRACES MedSeA-GA04-S cruise in 2013 are presented. Observed nuclide distributions indicate the impact of deep water formation processes, where observed differences can be linked to the type of deep water formation process that occurs in respective basin. Essentially all in-situ produced 230Th is buried in Mediterranean Sea sediments. Despite lower affinity of 231Pa for marine particles, most 231Pa is also scavenged and deposited in Mediterranean Sea sediments. The efficient scavenging of 231Pa produces a relatively low fractionation between 231Pa and 230Th in terms of the fractionation factor FTh/Pa. This licentiate thesis presents a summary of the methods used for the analysis of 231Pa and Th-isotopes with details on the exchange chromatography method and the treatment of mass spectrometric data. The study of 231Pa, 230Th and 232Th in the Mediterranean Sea has important implications for our understanding of processes that control their water column distributions and how their behavior can be utilized to trace chemical flux in modern and past ocean environments.
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