²³¹Pa, ²³⁰Th and ²³²Th were analyzed in unfiltered seawater samples (n =66) and suspended particles (n =19)collected in the Mediterranean Sea during the MedSeA-GA04-S cruise along the GEOTRACES section GA04S andused to investigate mechanisms controlling the distribution and fractionation of Pa and Th in an ocean marginenvironment. ²³¹Pa and ²³⁰Th are particle reactive radionuclides and are often used as tracers of processes suchas boundary scavenging, particle transport and ocean circulation. The depth profiles of total ²³¹Pa and ²³⁰Thconcentrations in the Mediterranean Sea displayed non-linear shapes. Higher total ²³²Th concentrations wereobserved at the straits and in deep waters pointing at lithogenic sources. Fractionation factors F_Th/Pa ranged from1.4 to 9. Application of a box-model illustrated that 94% of the ²³¹Pa and almost all of the ²³⁰Th (99.9%)produced in the Mediterranean Sea is removed to the sediment by scavenging. The negligible export of ²³⁰Th tothe Atlantic Ocean, leads to a reevaluation of the mean settling speed of the filtered particles, which is nowestimated to 500–1000 m/y. The low F_Th/Pa fractionation factors are attributed to the efficient scavenging andlack of transport of ²³¹Pa to the Atlantic Ocean.

²³¹Pa, ²³⁰Th and ²³²Th were analyzed in filtered seawater (n=70) and suspended particles (n=39) collectedalong a shelf-basin transect from the Barents shelf to the Makarov Basin in the Arctic Ocean during GEOTRACESsection GN04 in 2015. The distribution of dissolved ²³¹Pa and ²³⁰Th in the Arctic Ocean deviates from the linearincrease expected from reversible scavenging. Higher ²³²Th concentrations were observed at the shelf, slope andin surface waters in the deep basin, pointing at lithogenic sources. Fractionation factors (F_Th/Pa) observed at theNansen margin were higher compared to F_Th/Pa in the central Nansen Basin, possibly due to the residual occurrenceof hydrothermal particles in the deep central Nansen Basin. Application of a boundary scavengingmodel quantitatively accounts for the dissolved and particulate ²³⁰Th distributions in the Nansen Basin.Modelled dissolved ²³¹Pa distributions were largely overestimated, which was attributed to the absence of incorporation of water exchange with the Atlantic Ocean in the model. ²³¹Pa/²³⁰Th ratios of the suspended particlesof the Nansen Basin were below the ²³¹Pa/²³⁰Th production ratio, but top-core sediments of the Nansenmargin and slope have high ²³¹Pa/²³⁰Th-ratios, suggesting that scavenging along the Nansen margin partly actsas a sink for the missing Arctic ²³¹Pa.

The global mapping of the GEOTRACES program requires consistent and precise measurements of ²³²Th (pg/kg), ²³⁰Th and ²³¹Pa (fg/kg) in seawater and suspended particulate matter. Concentrations of particulate and dissolved ²³¹Pa, ²³⁰Th and ²³²Th were measured by four labs in samples collected at the Arctic crossover station in 2015. These samples were collected during two separate expeditions (HLY1502 GN01, Station 30 and PS94 GN04 ARK-XXIX/3, Station 101). Detailed descriptions of chemical procedures used by the participating labs are followed by a discussion focused on dissolved surface samples and particulate samples. Results demonstrated that participating labs can determine concentrations of dissolved ²³⁰Th and ²³¹Pa in deep water (<500 m depth) that are internally consistent within 4 % of the mean values. However, the analysis of radionuclide concentrations in suspended particles still needs improvement. The pre-concentration of particulate material used at LSCE was proven to be unsuccessful. Aliquots of particulate samples at St. 101 were re-measured and it was concluded that the absence of HF in the leaching solution was the cause for the previously underestimated particulate concentrations. Large blank contributions are still a problem, especially for measurements of particulate ²³¹Pa.