THE CHEMICAL-COMPOSITION OF DUST TRANSPORTED IN RED RAINS - ITS CONTRIBUTION TO THE BIOGEOCHEMICAL CYCLE OF A HOLM OAK FOREST IN CATALONIA (SPAIN)

The chemistry of North African dust reaching NE Spain with red rains i s here described to (1) characterize the red dust elemental compositio n, (2) analyze the relative contribution of dissolved and particulate forms to the total element inputs for the period 1983-1994, and (3) st udy the role of the particulate and dissolved inputs in red rains to t he forest nutrient cycle. Five dust samples, obtained from the filtrat ion of five red rain events, have been considered as representative of the dust reaching NE Spain as they include dust from the main source regions in North Africa. Enrichment factors were lower than four for a ll elements and samples, indicating the crustal character of the red d usts and their scarce mixing with anthropogenic pollutants. Back traje ctory analysis of the red rain air masses computed at various isentrop ic surfaces showed northward fluxes at all altitudes in four out of fi ve events. The remaining event, which presented a lower layer from Eur opean origin contacting upper layers of North African provenance, bad higher trace metal concentrations in the dust (still lower than report ed values for the circum-Mediterranean area) and higher S and N concen trations in dissolved form. The occurrence of red rains introduced hig h interannual variability in the input fluxes for the major elements. Phosphorus inputs occurred mostly in particulate form linked to red du st deposition. Red dust particulate inputs were also important for Ki and Mg2+. For Ca2+, dissolved inputs in red rains equalled and sometim es overuled particulate inputs in red dust, due to the calcite dissolu tion. Sodium and S inputs in red rains were mostly in dissolved form. The amounts of base cations delivered by red rains are important contr ibutors to the helm oak forest needs at Montseny, by providing 27% of K+, 45% of Ca2+ and 84% of Mg2+ fluxes needed for the above ground bio mass annual increment. For Ca2+ this is specially relevant because of the calcium-poor lithology of the site. (C) 1997. Published by Elsevie r Science Ltd.