Fluxes of 226Ra and barium in the Pacific Ocean: The importance of boundary processes

The 226 Ra/Ba ratio in particles collected in sediment traps decreases with depth. Fluxes of Ba and Al increase with depth. These observations cannot be explained by simple vertical processes. Instead they require a component to the deep flux which originates at the ocean margins. We suggest that the deep traps are enriched in Ba through the advection of barite originally precipitated in biologically productive regions of the ocean margin. This source is less important for 226 Ra because some is lost by radioactive decay between the precipitation of barite on the margin and outward transport. The boundary flux is an important source of material to the deep sea. From 226 Ra fluxes in the upper 1000 m of the water column, we estimate that the residence time of 226 Ra in the upper ocean is 400 years. The removal of 226 Ra in the particle flux is approximately balanced by the upwelling of waters enriched in 226 Ra.