The Sahara Desert is the largest source of mineral dust in the world. Emissions of African dust increased sharply in the early 1970s (ref. 2), a change that has been attributed mainly to drought in the Sahara/Sahel region caused by changes in the global distribution of sea surface temperature. The human contribution to land degradation and dust mobilization in this region remains poorly understood, owing to the paucity of data that would allow the identification of long-term trends in desertification. Direct measurements of airborne African dust concentrations only became available in the mid-1960s from a station on Barbados and subsequently from satellite imagery since the late 1970s: they do not cover the onset of commercial agriculture in the Sahel region approximately 170 years ago. Here we construct a 3,200-year record of dust deposition off northwest Africa by investigating the chemistry and grain-size distribution of terrigenous sediments deposited at a marine site located directly under the West African dust plume. With the help of our dust record and a proxy record for West African precipitation we find that, on the century scale, dust deposition is related to precipitation in tropical West Africa until the seventeenth century. At the beginning of the nineteenth century, a sharp increase in dust deposition parallels the advent of commercial agriculture in the Sahel region. Our findings suggest that human-induced dust emissions from the Sahel region have contributed to the atmospheric dust load for about 200 years.

[1] Five sediment cores from offshore NW Africa were analyzed for strontium and neodymium isotope ratios to reconstruct temporal variations in continental weathering regimes. Sediments were taken from three time slices with well-known and distinctive environmental conditions: present-day (dry and warm), ∼6 ka (wet and warm), and ∼12 ka (dry and cold). Terrigenous sediment samples were split into two size fractions to distinguish between the two dominant transport mechanisms offshore NW Africa: fluvial (0-10 mm) and aeolian (10-40 mm). Sr isotope data record evidence of marked grain size control with higher isotopic ratios in the fine fraction. In contrast, " Nd values are largely unaffected by grain size. Minor variability in Nd isotope data at each sampling site indicates near constant sources of terrigenous matter over the last ∼12 ka. Variations in Sr isotope ratios are interpreted to reflect major changes in the evaporation-precipitation balance. We suggest that the Sr-Nd isotope data record a latitudinal shift of the northern limit of the African rainbelt and associated wind systems causing changes in the humidity and rate of chemical weathering over NW Africa. While hyperarid conditions prevailed ∼12 ka, more humid conditions and intensified monsoonal rainfall at ∼6 ka resulted in greater breakdown of easily weathered K-bearing phases and increased 87 Sr/ 86 Sr in the detritus. In late Holocene times the monsoonal circulation diminished resulting in a return to arid conditions. Our results clearly show that it is of vital importance in paleoenvironmental studies to carry out isotopic analyses on individual sediment fractions that were carried to the studied deposition site by distinct sediment transport mechanisms. If isotopic analyses are carried out on bulk sediments, the observed variability in isotopic values most likely represents changes in the particle size and mixing proportions of the sediment subpopulations.

Mineral dust deposits were collected at Mbour, Senegal, throughout the spring of 2006 and especially during the well‐documented March 7–13 large Saharan dust outbreak. During this 7‐day period, significant changes in mass flux, grain‐size, clay mineralogy and Sr and Nd isotopic compositions were recorded, indicating distinct provenances for the dust transported and deposited during and outside the event. All these terrigenous proxies, as well as freshwater diatom taxa, also showed significant temporal variations during the outbreak, implying contributions from at least two different provenance regions. Tri‐dimensional back‐trajectories and satellite imaging enabled us to link those distinct signatures to regions increasingly to the southeast within a large area covering Mauritania, Mali and southern Algeria, identified by the Total Ozone Mapping Spectrometer (TOMS) as the main source of the prominent winter/spring plume over the tropical Atlantic. The multiproxy characterization of the March 7–13 dust fall therefore enables us to typify the terrigenous signature of two different regions supplying dust off West Africa, and provide valuable clues for the interpretation of Northeastern Tropical Atlantic Ocean dust sedimentary records in terms of changes in provenance regions and transport systems. Additionally, because dust deposition data are scarce, flux and grain size data obtained in this study, among other parameters such as clay assemblages, provide important constraints for atmospheric transport models and dust deposition budget estimates in this area.

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