A LONG-TERM QUATERNARY RECORD OF DUAL-SOURCE AFRICAN DUST INPUTS TO SOILS ON THE CANARY ISLANDS, SPAIN

It is well known that African dust is transported seasonally to the Atlantic Ocean at present. What is less certain is how long this process has been going on and whether multiple dust sources in Africa can be identified. On the Canary Islands off the west coast of Africa, we studied sections of carbonate-rich dune sand (carbonate sand=70-95%; basaltic sand=5-30%) on Lanzarote (14 meters thick; 11 paleosols) and Fuerteventura (8 meters thick; 5 paleosols). Paleosols are distinguished from unaltered carbonate dune sand by their pedogenic structure and color, lower carbonate content, and higher silicate clay and fine silt contents. Radiocarbon ages of land snails indicate that all but the youngest paleosols (which are Holocene) are older than ~40,000-50,000 yr. Silt-sized quartz and immobile trace elements (Sc-Th-Ta; Cr-Hf-Th; La/Yb-Eu/Eu*) show that paleosols are derived dominantly from African dust rather than chemical weathering of basaltic sands in the dunes. Furthermore, clay mineralogy shows the presence of smectite, mica, and kaolinite in the paleosols. Mica (absent in basalt) is likely derived from mica-rich dust sources in the Sahara. Both smectite and kaolinite could be derived from in situ chemical weathering, but pedogenic kaolinite is unlikely in this arid environment, where even carbonate leaching is minimal. Thus, we interpret kaolinite to be derived from dust sources in the Sahel (kaolinite-rich Alfisols). Our results indicate that dust transport from Africa to the Atlantic Ocean has been ongoing for much of the Quaternary and that sources have included both the Sahara and Sahel regions of Africa.