Flux of transuranium nuclides and chlorinated hydrocarbons in the northwestern Mediterranean

Abstract

The transuranium nuclides, plutonium and americium, and selected chlorinated hydrocarbon compounds have been measured in sediment trap samples, bottom sediments and waters. These measurements have been used to quantify vertical fluxes and elucidate particulate biogeochemical cycles, of these man-made contaminants in the northwestern Mediterranean. Sediment trap experiments in the Gulf of Lions (Lacaze-Duthiers Canyon) have shown that transuranic fluxes are considerably higher in the Gulf of Lions, than those which have been reported for the northeast Pacific; principally this is due to the relatively high mass fluxes in this region of the Mediterranean. Water, surface sediment and sediment trap data indicate that 241Am is being transported downward (via particle settling) more rapidly than 239+240 Pu. From direct flux measurement, residence times for 239+240 Pu and 241Am in the upper 300 m were computed to be 2.5 and 0.14 years, respectively. Comparison of 239+240 Pu concentrations in the water column, in 1986, with similar measurements made 5–11 years earlier suggests that fallout input to the Mediterranean has decreased by nearly a factor of 2 during the period 1975–1986. Fluxes of PCB, HCB, γ-HCH and DDT residues varied by as much as an order of magnitude, or more, both temporally and with depth. The average fluxes of most of the compounds in the Lacaze-Duthiers Canyon, during 1985–1986, were quite similar to those measured off the coast of Monaco several years earlier. Generally, the increased chlorinated hydrocarbon fluxes observed with depth were a direct result of the much higher mass fluxes which occurred at depth. Demonstrated differences in PCB concentrations in the settling particles, deposited surface sediments and overlying sediment floc indicate that simple bottom sediment resuspension is not a major contributor to the enhanced PCB flux noted at depth. The PCB concentration in sedimenting particulate material collected from deeper waters, is similar to previously analysed benthic flocs; it is likely to have been advected into the canyon with the nepheloid layer, which is a common oceanographic feature in this region.