231Pa, 230Th and 232Th as tracers of deep water circulation and particle transport : Insights from the Mediterranean Sea and the Arctic Ocean

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.geotracesMedSe