Constraints on deep water age and particle flux in the Equatorial and South Atlantic Ocean based on seawater 231Pa and 230Th data

Abstract

High-precision measurements of 231 Pa and 230Th in filtered seawater and suspended particulate matter samples are reported for the Equatorial and South Atlantic. Distributions of 231 Pa and 230Th clearly indicate the influence of advection, as evidenced by departures from scavenging models that predict a linear increase with depth for these tracers. Application of a scavenging-mixing model implies a deep water transit time of Ο60-100 years from the northern source water regions. The average particulate 231Pa/230Th activity ratio is 0.0498 ± 0.0160, a factor of Ο2 lower than the 231Pa/230Th production ratio of 0.093 and in agreement with reported excess 231Pa/230Th ratios of 0.06 ± 0.004 in Holocene sediments north of 50°S in the Atlantic. These water column data further suggest that lateral eddy diffusive transport combined with enhanced scavenging in high-particle flux marginal regions (boundary scavenging) is weakly expressed in the Atlantic. Particle fractionation of these tracers is also indicated by the elevated fractionation factors of FTh/Pa = 4.32-24.04 (ave. = 9.97 ± 4.98) compared to values of ˜1-4 in the Southern Ocean.