Variation of soluble and insoluble calcium in red rains related to dust sources and transport patterns from North Africa to northeastern Spain

Abstract

We use the chemical composition of African dust delivered by red rains at a rural site in northeastern Spain (Montseny, 41°46′N, 2°21′E) to describe its relationship with the possible provenance areas and the processes occurring during transport. To this end, we obtained the red rain insoluble composition for the major elements (Al, Fe, Ca, Mg, K, P, Ti, and Na) in 30 filters, the 210Pb concentration in 23 filters, and the soluble cation concentrations (Na, K, Ca, and Mg) in 28 coincident red rain samples. These samples comprised most major events occurring at the site from 1983 to 2002. On the basis of back trajectories and satellite images, a distinction has been made between an eastern and western air mass flux with respect to 0° Greenwich for the analyzed samples. Principal component and ANOVA analyses between the two provenance groups have shown striking differences in the insoluble phase, with eastern samples being significantly richer in insoluble Ca, Mg, and Sr compared to western samples. Conversely, western samples had significantly higher concentrations of insoluble Al, Fe, K, V, and 210Pb than eastern samples. Therefore, in the insoluble phase, the ratios of various elements to Ca were significantly higher in western provenances. However, these differences disappeared when considering bulk Ca ratios (bulk Ca = insoluble + soluble Ca). Neither of the ratios Fe/Al and Ti/Fe showed significant differences. This lack of differences is interpreted in view of a similar carbonated lithology broadly underlying both areas. The difference in insoluble Ca with respect to total Ca between provenances (Cainsoluble/Catotal = 0.10 and 0.70 for western and eastern trajectories, respectively) is interpreted as a difference in calcite dissolution during transport. Evidence from 210Pb data and from the length of the back trajectories indicates that western trajectories covered a longer distance than the eastern ones; their higher soluble Ca could be due to (1) higher calcite dissolution due to longer contact with wet fronts from the Atlantic and (2) particle segregation during transport, with finer (carbonate) particles more prone to dissolution due to a higher surface to volume ratio.