Scavenging of 230Th and 231Pa in the Western North Pacific: Implications for the 231Pa/230Th Ratio as a Palaeoproductivity Proxy

Abstract

Particle reactive elements introduced into the ocean in dissolves form are generally removed by scavenging onto settling particle. While the composition of settling particles mainly influence the part i t ioning of reactive chemical substances between particulate phase and dissolved form, the intensity of scavenging controlled by the flux of particulate matter through the water column. Fluxes and concentration of reactive chemical elements on particulate matter trend to be much greater in the ocean-margin and high latitude than those in the open ocean and oligotrophic regions, because of relatively high levels of primary productivity supported by land discharge and upwelling. This processes is generally realized as boundary scavenging. Science the first study of boundary scavenging focused on 226Ra-21~ disequilibrium in sea water, naturally occurring radionuclides are often used to study this process. All above, 23~ and 231pa are very good tracer to study those process (e.g. Anderson et al., 1994). Recently the 231pa/23~ ratio has been used as a palaeoproductivity proxy, because 231pa is more enriched in biogenic particles (Kumar et aI., 1993). Furthermore, the ratio is not a reliable indicator for the mass flux of particulate in regions, where the settling flux is dominated by biogenic opal, such as the Southern Ocean (Walter et al., 1997). In response to the questions: 'How much 23]pa is scavenged in the western North Pacific?', 'Which factor is controlled the scavenging of 231pa or the 231pa /23~ ratio?' and 'Can we use the 231pa/Z3~ ratio as a palaeoproductivity proxy in the western North Pacific?', we examined seasonal and regional change in the scavenging of 231pa and the 231pa/23~ ratio caused by primary productivity determining 23~ and 231pa in the settling particles, collected with time-series sediment traps at five stations in the western North Pacific. In this paper, we discuss lateral transport of fine-grained aluminum-rich particles in the western North Pacific influencing on the fluxes of 231pa and 23~ and their ratio.