Elemental and isotopic composition of surface soils from key Saharan dust sources

Abstract

Saharan dust contains significant amount of P, an important macronutrient to all living organisms, which has been shown to exert large effects on nearby and remote ecosystems located across the dust transport pathways. The biological effect of Saharan dust depends on the amount and nature of the P speciation of the dust. However, thus far relatively small numbers of samples from potential source areas (PSA) has been analyzed. Here we report the P speciation (resin-P, HCl-P, Fe-bound-P and organic-P), the δ18OP values, the elemental composition, and the 87Sr/86Sr and 143Nd/144Nd of the fine fraction and bulk soil from 5 important PSAs across Northern Africa. We found the HCl-P concentrations between different source areas were relatively constrained but that these concentrations were higher in the fine fraction, which here is used a surrogate for dust. The δ18OP values for soils from sand dunes varied from 15.0 to 21.4‰, which is in the range of phosphate minerals from sedimentary origin. The δ18OP values of soils from dry lakes were significantly higher (24.0–28.5‰), probably since their P is derived from fossilized plankton that lived in the lake as it dried up. The 87Sr/86Sr and εNd values ranged from 0.7219 to 0.7276 and −12.7 to −14.0 in eastern samples and from 0.7146 to 0.7185 and −11.9 to −13.4 in western samples, suggesting a different source for the siliciclastic material of eastern and western samples. Our analysis indicates that the δ18OP values are decoupled from the Sr and Nd isotopic systems. Together, the new chemical and isotope data are specific for different PSAs and thus are used for source apportionment purposes. Such data can be used to provide more accurate estimates of the flux of potentially bioavailable P to marine and terrestrial ecosystems. These estimates can be used in global climate models to determine the magnitude and distribution of P control on carbon uptake.