SAHARAN DUST INPUTS TO THE WESTERN MEDITERRANEAN SEA - DEPOSITIONAL PATTERNS, GEOCHEMISTRY AND SEDIMENTOLOGICAL IMPLICATIONS

Data are presented for a number of parameters for aerosols and rainwat ers collected at a station on Sardinia. The findings are interpreted w ith special reference to Saharan dusts, and are compared to other data on these dusts obtained from a variety of sites around the Mediterran ean Sea. At the Sardinia site the particle size distribution of the Sa haran outbreaks exhibits a bimodal structure, the two modes being betw een 2 and 4 mu m and 15 and 30 mu m. The presence of giant particles s trongly affects the deposition velocities of the Saharan aerosols. Sou rce markers for the Saharan dusts are palygorskite, kaolinite, calcite , dolomite and rounded quartz grains. The input of Saharan dust has im portant effects on the chemistry of the Mediterranean aerosols. These include: (i) increases in the atmospheric concentrations arid sea surf ace fluxes of crust-controlled trace metals (e.g. Al, Fe); (ii) decrea ses in the EFcrust values of non-crust-controlled trace metals (e.g. C u, Zn and Pb) in the aerosols, and (iii) changes in the solid stare sp eciation of Cu, Zn, and Pb, which decrease their solubilities in seawa ter. The Saharan dusts also affect the composition of rainwater by rai sing the pH, following the dissolution of calcium,and by decreasing th e solubility of trace metals such as Cu, Zn and Pb. Wet deposition con trols the flux of Saharan dust to the Mediterranean Sea, but dry depos ition can also be important. The dust transport occurs in the form of ''pulses'', and the annual dust flux can be controlled by a few episod es of Saharan outbreaks, e.g. sometimes a single outbreak can account for 40-80% of the flux. Saharan dust deposition fluxes range from 2 to 25 g m(-2) (average congruent to 10) in the west Mediterranean betwee n 39 degrees and 42 degrees N, from 6 to 46 g m(-2) (average congruent to 20) in the east Mediterranean, and from 0.4 to 1.0 g m(-2) over th e Alps on continental Europe. The present day Saharan dust fluxes (app roximate to 1 mg cm(-2) year(-1)) account for about 10-20% of the rece nt deep-sea sedimentation in the western Mediterranean (3-15 mg cm(-2) year(-1)). (C) 1997 Elsevier Science Ltd.