Boundary Scavenging and Deep‐Sea Sediment Dating: Constraints from Excess 230Th and 231Pa

Abstract

Apparent sedimentation rates derived from excess 230Th and 231Pa in deep‐sea sediments accumulated during the last 25,000 years in the Pacific Ocean are mostly lower than the average rates determined by other independent (such as 14C and ∂18O) dating methods, which may be a consequence of generally higher accumulation rates of aluminosilicate and carbonate during the last glacial that diluted the excess 230Th and 231Pa activities in glacial sediments. The pattern of enhanced scavenging of nuclides in ocean margin areas (i.e., boundary scavenging) also changed from the glacial to the Holocene. Fluxes of 230Th and 231Pa throughout the ocean varied over time responding to changes in the nature and intensity of boundary scavenging which, in turn, were influenced by changes in particle flux and particle composition. The lower intensity of boundary scavenging of 231Pa during the last glacial may also have contributed to the lower apparent sediment accumulation rates based on excess 231Pa for cores in ocean margin areas.