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

Abstract

231Pa, 230Th and 232Th 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 231Pa and 230Th in the Arctic Ocean deviates from the linear increase expected from reversible scavenging. Higher 232Th concentrations were observed at the shelf, slope and in surface waters in the deep basin, pointing at lithogenic sources. Fractionation factors (FTh/Pa) observed at the Nansen margin were higher compared to FTh/Pa 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 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 particles of the Nansen Basin were below the 231Pa/230Th production ratio, but top-core sediments of the Nansen margin and slope have high 231Pa/230Th-ratios, suggesting that scavenging along the Nansen margin partly acts as a sink for the missing Arctic 231Pa.