Contribution of Saharan dust to Mediterranean soils assessed by sequential extraction and Pb and Sr isotopes

Measurements of the concentrations and isotopic composition of Pb and Sr in Al-silicates and Fe-oxides of two soil profiles developed in the Guadalquivir River Basin, Andalusia, southwestern Spain are used in order to estimate the significance of local versus foreign sources. Lead concentrations are 10±6mg/kg in deep (more than 100cm) soil samples, and reach 30-60mg/kg near the surface. ²⁰⁶Pb/²⁰⁷Pb values range from 1.200 to 1.206 in deep samples and from 1.186 to 1.191 in the upper most samples. Sr concentrations range from 30 to 69mg/kg showing no systematic trend with depth. ⁸⁷Sr/⁸⁶Sr ratios range from 0.71753 to 0.72526, also with no systematic changes with depth in the Fe-oxide fraction, but with a systematic increase in values with depth in the Al-silicate fraction of one of the soil profiles. In all the samples the isotopic composition of Pb in the Al-Si fraction is slightly more radiogenic than its value in the Fe-oxide fraction. The isotopic composition of Sr exhibits the opposite behavior, and ⁸⁷Sr/⁸⁶Sr values in the Fe-oxide fraction are always higher than in the Al-silicate fraction.Based on our data we calculate that Al-silicates in the fine soil fraction (less than 5. μm) in both sites contain 33-86% of Saharan dust and that the Fe-oxides contain less Saharan dust and more material released by the weathering of basement, granitoid-type rocks. In the case of Pb a third source is present in the upper part of the soil, i.e., anthropogenic Pb probably added to the soil over millennia. The results of the present study suggest that Saharan dust makes up a significant fraction in Al-silicates and Fe-oxides of Mediterranean soils, but that local sources, both natural (rock-derived) and anthropogenic, are not trivial.