TRACE-METAL FRONTS IN WATERS OF THE CELTIC SEA

Two transects across the Celtic Sea and adjacent continental slope in August 1985 and August 1988 illustrate the importance of dynamic physi cal processes on dissolved trace metal distributions (Cd, Ni, Cu, Pb, Co, Fe, Mn). The influence of a coastal frontal zone can be seen in th e 1985 transect, while an intrusion of open ocean waters into the shel f environment is the main factor influencing the horizontal distributi on of trace metals in 1988. A third transect across the continental sl ope bordering the Porcupine Seabight (May 1984) reveals marked horizon tal fronts in the concentrations of Fe, Mn and Cu at the shelf edge (o ver a 21 km distance, concentrations fall from 3 to 1 nM for Fe, from 2 to 0.5 nM for Mn, from 2 to 1 nM for Cu) which are unaccompanied by any coincident hydrographic features. In transects 85 & 88, variations of metal concentrations versus salinity can be represented as two lin ear segments intersecting within the coastal frontal zone and at the l eading edge of the oceanic intrusion, respectively. We propose that be nthic inputs of metals to these waters are less important than previou sly assumed. By contrast, the remobilization of Fe, Mn and Cu from she lf edge sediments does provide the most plausible explanation for the presence of some unusual metal fronts observed at the Celtic shelf edg e in 1984. The 1984 transect indicates not only a common benthic sourc e of Fe, Mn and Cu in that particular area but also similarities in th eir cycling through the water column. These similarities are interpret ed in terms of the photoreductive dissolution of oxidized forms of Fe and Mn, coinciding with the enhanced organic complexation of Cu. None of the elements investigated shows any large scale perturbation of con centration by biological activity, even though nutrient concentrations are low where sigma(theta) is less than 27.00, i.e. in surface waters as well as landward of the isohaline coastal fronts. However, the 198 5 transect suggests that some oxidative removal of Mn is probably taki ng place throughout the Celtic Sea and that dissolved Co and Pb are re moved in places by uptake onto suspended solids.