Accumulation rates and mixing processes in near-surface North Atlantic sediments: Evidence from C-14 and Pu-239,240 downcore profiles

The vertical distributions of C-14 and Pu-239,240 activities and C-14 ages of sediments collected in N-Atlantic areas are analyzed. In NE-Atlantic sediments, radiocarbon mixed layer ages and depths range from 2500 to 5500 years and from 6.5 to 7.3 cm, respectively, while the vertical profiles of Pu-239,240 indicate, for the NW-Atlantic sites, mixing depths of only 1–2cm. In the NE-Atlantic sites accumulation rates, over a period of 10 kyr, range from 1.4 to 3.7 cm kyr −1 and fluxes of material from 9.2 to 24.1 g m −2 yr −1, with a marked difference between samples collected from plain (highest values) and hill (lowest values) sites, related to downslope transport of sediment. The inventories of Pu range from 9.0 to 25.2 Bq m −2 for NE-Atlantic sites and from 0.3 to 1.1 Bq m −2 for NW-Atlantic sites, probably reflecting different primary production rates in the two areas. Numerical simulations of the mixing processes of plutonium have been performed by using both a continuous and a pulse input flux. The estimated values of D B range from 0.1 to 1.0 cm 2 yr −1 for the NE-Atlantic sites, while they are in the order of 0.01 cm 2 yr −1 for the NW-Atlantic sites.