Saharan dust inputs to the western Mediterranean Sea: depositional patterns, geochemistry and sedimentological implications

Abstract

Data are presented for a number of parameters for aerosols and rainwaters collected at a station on Sardinia. The findings are interpreted with special reference to Saharan dusts, and are compared to other data on these dusts obtained from a variety of sites around the Mediterranean Sea. At the Sardinia site the particle size distribution of the Saharan outbreaks exhibits a bimodal structure, the two modes being between 2 and 4 μm and 15 and 30 μm. The presence of giant particles strongly affects the deposition velocities of the Saharan aerosols. Source markers for the Saharan dusts are palygorskite, kaolinite, calcite, dolomite and rounded quartz grains.The input of Saharan dust has important effects on the chemistry of the Mediterranean aerosols. These include: (i) increases in the atmospheric concentrations and sea surface fluxes of crust-controlled trace metals (e.g. Al, Fe); (ii) decreases in the EFcrust values of non-crust-controlled trace metals (e.g. Cu, Zn and Pb) in the aerosols, and (iii) changes in the solid state speciation of Cu, Zn, and Pb, which decrease their solubilities in seawater. The Saharan dusts also affect the composition of rainwater by raising the pH, following the dissolution of calcium, and by decreasing the solubility of trace metals such as Cu, Zn and Pb.Wet deposition controls the flux of Saharan dust to the Mediterranean Sea, but dry deposition can also be important. The dust transport occurs in the form of “pulses”, and the annual dust flux can be controlled by a few episodes 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 ≅10) in the west Mediterranean between 39° and 42°N, from 6 to 46 g m−2 (average ≅20) in the east Mediterranean, and from 0.4 to 1.0 g m−2 over the Alps on continental Europe. The present day Saharan dust fluxes (≈ 1 mg cm−2 year−1) account for about 10–20% of the recent deep-sea sedimentation in the western Mediterranean (3–15 mg cm−2 year−1).