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

Abstract

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 ( 231Pa ex), excess 230Th ( 230Th ex) 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 (`clays') are the major carrier of 231Pa and 230Th, and the ratio 231Pa ex/ 230Th ex in marine particulates decreases with increasing 232Th concentration. Model simulation shows that sea-floor sediments and trap-collected particles have similar 231Pa ex/ 230Th ex 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 ( F Th/Pa) varies from ∼10 for aluminosilicates to ∼1 for biogenic ( 232Th-free) particles. Changes in F Th/Pa as a function of particle composition exert primary controls on 231Pa ex/ 230Th ex 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 231Pa ex/ 230Th ex. Our observations place an important constraint on the use of sediment 231Pa ex/ 230Th ex as a paleoproductivity proxy. Namely, in order to use down-core variations of 231Pa ex/ 230Th ex 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.