Processes affecting particulate trace metals in the sea surface microlayer

Concentrations of particulate Fe, Mn, Ni, Cu, Zn, Cd and Pb have been measured in surface microlayer and subsurface seawater samples collected in the North Sea adjacent to the East Anglian coast, in an area subject to a considerable fluvial input of clay minerals. The results are interpreted by estimating the magnitudes of different processes affecting particulate matter in the microlayer: atmospheric deposition, Brownian diffusion, gravitational settling, bubble flotation and mixing. Both Fe and Mn are strongly depleted in the microlayer, evidently as a result of gravitational settling of Fe- and Mn-bearing mineral particles out of the microlayer. These particles are mixed into the surface region from the water column beneath. Microlayer enrichment of Cu, Zn and Pb was also observed and probably results from flotation of particles attached to rising bubbles. In one set of samples, however, the marked enrichment of these elements, as well as Ni, may result instead from deposition of particles from the atmosphere directly onto the water surface.