Oceanic 231Pa/230Th ratio influenced by particle composition and remineralization

We have investigated the role of particle compositions in affecting the distribution of 231Pa and 230Th in the ocean by separating sediments collected by sediment-traps and cores into three size fractions and analyzed for their excess 231Pa (231Paex), excess 230Th (230Thex) and major chemical components. The samples came from stations along 140°W in the equatorial Pacific and from the Blake Outer Ridge in the North Atlantic. Results indicate that aluminosilicates (`claysʹ) are the major carrier of 231Pa and 230Th, and the ratio 231Paex/230Thex in marine particulates decreases with increasing 232Th concentration. Model simulation shows that seafloor sediments and trap-collected particles have similar 231Paex/230Thex ratios in their biogenic components, which are very close to the seawater ratio but are about one order higher than the ratios in coexisting clays. Preferential scavenging of 230Th over 231Pa by clays is primarily responsible for the 231Pa¯230Th fractionation in the ocean. The fractionation factor between 230Th and 231Pa (FTh/Pa) varies from ~10 for aluminosilicates to ~1 for biogenic (232Th-free) particles. Changes in FTh/Pa as a function of particle composition exert primary controls on 231Paex/230Thex in marine particulates and bottom sediments and on the boundary scavenging of Pa. Remineralization of biogenic particles in the water column may result in a decrease of 231Paex/230Thex. Our observations place an important constraint on the use of sediment 231Paex/230Thex as a paleoproductivity proxy. Namely, in order to use down-core variations of 231Paex/230Thex to assess past changes in surface productivity, one must either verify or assume that the lithogenic particle flux and water-column particle remineralization have remained essentially constant at the core site.