Abstract
Large quantities of African dust are carried across the North Atlantic by Trade Winds every summer. The deposition of this dust has an impact on biogeochemical processes in the Tropical and Western Atlantic Ocean and Caribbean and it contributes to the formation of soils on Caribbean islands, the Bahamas, and the southeastern US. Here we report on a study of the temporal and spatial variability of the elemental composition of aerosol samples collected in the Trade Winds at Barbados and Miami over the summers of 2003 and 2004. Our objective is to identify characteristics that might serve as a useful tool to identify natural and anthropogenic sources of specific elements or element classes placing a special focus on dust-linked species. To this end we measured a large suite of elements: Al, As, Ba, Be, Cd, Ce, Co, Cr Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ho, La, Li, Lu, Nd, Ni, Pb, Pr, Rb, Sc, Sm, Sr, Th, Tl, Tm, U, V, Y, Yb and Zn. Most elements exhibited a surprising uniformity that is highly correlated with dust concentration as determined by aerosol filter ash residues and by Mn concentration, shown to be an excellent proxy for dust. The concentrations of most elements are very close to average upper crustal abundances. We measured the greatest enrichments and the largest variability for As, Cd, Cu, Cr, Ni, Pb, V, and Zn, elements known to have major anthropogenic sources. For some elements, most notably the Lanthanides, we found statistically significant differences between high-dust-load samples and low-load samples and also between individual dust peaks. However, the absolute differences were generally quite small. Consequently we feel that on a sample-by-sample basis the elemental composition of dust is unlikely to serve as useful indicator of source regions with the possible exception of the Lanthanides. The uniformity of dust composition suggests that a major fraction of the dust is either derived from regions having similar composition or from multiple different sources followed by mixing during transport.
