Boundary scavenging at the East Atlantic margin does not negate use of Pa-231/Th-230 to trace Atlantic overturning

Abstract

The231Pa/230Th method is a promising tool to reconstruct Ocean circulation over the past Glacial-Interglacial cycle. However, marine particle flux may constrain the applicability of this ratio as a direct quantitative proxy for the strength of Atlantic Meridional Overturning Circulation (AMOC) by influencing the fractionation between the in situ produced231Pa and230Th in ocean water. Here we present231Pa/230Th down-core profiles from high particle flux areas off Namibia and Senegal covering the past ~35ka. The231Pa/230Th profiles at these sites show very different responses to temporal variations of particle fluxes and to changes in water masses. Our results show that sedimentary231Pa/230Th in the Eastern Atlantic margin is linked to particle flux, but controlled primarily by the mode of the AMOC. Our data suggest that during the past ~30ka the high productivity Eastern margin was not capable of importing and storing significant amounts of231Pa from the open Ocean. Consequently, the applicability of the231Pa/230Th proxy to reconstruct past ocean circulation is not hampered by this potentially additional231Pa sink.