No abstract

The purpose of this paper is to investigate the distribution and the relative importance of processes that affect mercury distribution in the water column at the shelf edge of the Celtic Sea, on the western European continental margin. The water column, down to 4500 m, was sampled during two cruises, one in winter 1994 and the other in June 1995, on eight stations. Total mercury concentrations ranged from 0.29 to 9.37 pM with the extreme values in the first 200 m. The highly variable concentrations in the surface layers are attributed to intense atmospheric-sea water exchange processes. The low concentrations encountered in the shelf water prelude any significant influence of continental waters in this area. In the mixed layer, biological uptake and regenerative processes appear to dominate in controlling the vertical mercury distribution. The intermediate and deep-water mercury distribution is mainly governed by the hydrographic features. In summer, a mercury enrichment was observed in the upwelling zone of the shelf break. Mean mercury concentration in the Lower Deep Water (LDW) was typically 1.96±0.23 pM and in the Labrador Sea Water (LSW) 1.97±0.63 pM. The Mediterranean Intermediate Water (MIW) revealed similar levels, with a mean mercury concentration of 2.04±0.84 pM. Between LDW and LSW, the Iceland-Scotland Overflow Water (SOW) exhibited higher concentrations (2.76±0.35 pM). The upper part of the North East Atlantic Central Water (NEADW) and lower part of the North Atlantic Deep Water (NEACW) were characterized by marked peaks of concentration, which are interpreted in term of atmospheric influence and particle regeneration.

Two cores were sampled in the Bay of Biscay: one a few miles off the Gironde estuary (pointe de la Coubre), the other near the Spanish border (Capbreton canyon) to study past variations in sediment contamination. Radiochronology (210Pb, 137Cs) and total metal determinations by ICP-MS allowed the study of time trends for metal concentrations in sediment. Off the Gironde, during the time interval recorded by the core (1958-1999), all metal concentrations remained less than OSPAR background values. Some variations occurred along the Cd and Zn profiles, which may correspond to a very attenuated signal from the Gironde outputs. This suggests that this area was not submitted to any significant anthropogenic input of metals between 1958 and 1999. In the canyon of Capbreton, the undisturbed part of the core recorded the influence of surface deposition between 1977 and 1999. From as early as 1977 high lead and mercury concentrations were noticed. Mercury concentrations increased until 1992, then decreased until 1999. This marks a drop of inputs in 1992, but the final concentration remained elevated. Lead had important concentrations (more than twice the OSPAR reference) all along the core and began to decrease only in the upper level of the unmixed layer. This shows a late (near 1998) drop of lead inputs. Other metals (Cd, Cu, Zn, Ni, Cr) stayed at a much lower level: less than the OSPAR reference for Cu, Cr and Ni or not far from the OSPAR reference for Cd and Zn.