228Ra226Ra activity ratio in oceanic settling particles: implications regarding the use of barium as a proxy for paleoproductivity reconstruction

Activities of the natural radionuclides 226Ra (T12 =1602 years) and 22sRa (T12 =5.75 years) in particulate matter are reported from sediment traps deployed in the tropical northeast Atlantic. 228Ra226Ra activity ratios in settling particles are compared to those found in the water column in order to specify the origin of radium incorporated in particulate matter, and consequently that of barium, since the two elements are known to display close geochemical behaviours in the oceans. Whereas 228Ra226Ra activity ratios in the water column display very large variations with depth, they remain nearly constant in particles, with values close to those found in the upper 250 m. These results clearly demonstrate that particles acquire their radium and, by inference, their barium, in the upper water column and that there is very little exchange with the dissolved phase as they settle to the bottom.