231Pa and 230Th in the Arctic Ocean: Implications for boundary scavenging and 231Pa-230Th fractionation in the Eurasian Basin

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