Boundary scavenging in the Pacific Ocean: a comparison of10Be and231Pa

Concentrations of U, Th,231Pa and10Be were measured in Holocene sediments from two cores collected off the west coast of South America, two cores from the East Pacific Rise, two from the equatorial Pacific and one from the south Pacific central gyre. Our results, together with data from 5 cores reported in the literature, show that boundary scavenging plays a major role in the removal of10Be from the Pacific Ocean. Deposition rates of10Be at three margin sites are more than an order of magnitude greater than at sites of red clay accumulation in the deep central Pacific. Deposition of231Pa is 4 to 5-fold greater at the margin sites. The residence time of10Be with respect to chemical scavenging, defined as its inventory in the water column divided by its rate of removal to the sediments, varies regionally from > 1000 years at the red-clay sites in the deep central Pacific to ∼ 100 years at the margin sites. Different factors control boundary scavenging of Pa and Be. For example, scavenging of231Pa is enhanced by metal-oxide coatings of particles, whereas this seems to have little influence on the scavenging of10Be.