Advection and scavenging: Effects on 230Th and 231Pa distribution off Southwest Africa

Abstract

We investigated the controls of hydrography and of scavenging on the distribution of the particle reactive radionuclides 231Pa and 230Th in the water column and in surface sediments off Southwest Africa (Angola and Cape basins). Based on a vertical section of total 230Th ex concentrations in the water column we show that small differences in the salinity between the North Atlantic Deep Water (NADW) in the Angola Basin and the NADW in the Cape Basin as well as the advection of NADW associated with the Namib Col Current are reflected in total 230Th ex concentrations. These variable total concentrations are believed to reflect the flow path and mixing history of NADW with the NADW in the Angola Basin being relatively older and 230Th enriched compared to the NADW in the Cape Basin. In the area investigated we found high 231Pa ex/ 230Th ex ratios ( 231Pa ex/ 230Th ex > 0.093) in surface sediments at the continental margin and lower ones ( 231Pa ex/ 230Th ex < 0.093) in the open ocean. Such a distribution is normally interpreted to result from high particle flux at ocean margins (boundary scavenging). However, the lack of any significant depletion of dissolved 230Th and 231Pa in the water column does not indicate extensive scavenging at the continental margin. High 231Pa ex/ 230Th ex ratios are constrained to shallow waters depths (< 2000 m) only and coincide with low fractionation between 231Pa and 230Th indicating that preferential scavenging of 231Pa on opal may have caused high 231Pa ex/ 230Th ex ratios in the sediments. The observed close negative correlation ( r 2 = 0.82) between 231Pa ex/ 230Th ex ratios in sediments and water depths is believed to reflect changes in the particle composition, i.e. a decrease in opal content with water depth. In the Angola and Cape basins the total 231Pa ex concentrations in NADW were the highest observed so far in the Atlantic Ocean, and they are attributed to the meridional export of 231Pa from the North Atlantic. This caused the average dissolved 231Pa/ 230Th in the Southeast Atlantic to be about a factor 2 higher when compared to the North Atlantic (Labrador Sea). These differences in the dissolved 231Pa/ 230Th were not reflected in 231Pa/ 230Th ratios of surface sediments because the fractionation is lower in the Labrador Sea compared to the Southeast Atlantic, i.e. fractionation counteracts changes in the dissolved 231Pa/ 230Th. This suggests that fractionation is more important for the determination of 231Pa ex/ 230Th ex ratios in sediments than the meridional export of 231Pa from the North Atlantic.