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 (F_Th/Pa ~ 10), while higher values were observed in deep waters (F_Th/Pa ~ 20). Additionally, the hydrothermal particles in the Nansen interior produce lower F_Th/Pa values compared to F_Th/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.