BOUNDARY SCAVENGING AND DEEP-SEA SEDIMENT DATING: CONSTRAINTS FROM EXCESS Th-230 AND Pa-231

Apparent sedimentation rates derived from excess Th-230 and Pa-231 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 C-14 and partial derivative O-18) dating methods, which may be a consequence of generally higher accumulation rates of aluminosilicate and carbonate during the last glacial that diluted the excess Th-230 and Pa-231 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 Th-230 and Pa-231 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 Pa-231 during the last glacial may also have contributed to the lower apparent sediment accumulation rates based on excess Pa-231 for cores in ocean margin areas.