Atlantic Forcing of PersistentDrought in West AfricaT. M. Shanahan,1,2* J. T. Overpeck,1,3K. J. Anchukaitis,4J. W. Beck,5J. E. Cole,1D. L. Dettman,1J. A. Peck,6C. A. Scholz,7J. W. King8Although persistent drought in West Africa is well documented from the instrumental recordand has been primarily attributed to changing Atlantic sea surface temperatures, little is knownabout the length, severity, and origin of drought before the 20th century. We combinedgeomorphic, isotopic, and geochemical evidence from the sediments of Lake Bosumtwi, Ghana, toreconstruct natural variability in the African monsoon over the past three millennia. We findthat intervals of severe drought lasting for periods ranging from decades to centuries arecharacteristic of the monsoon and are linked to natural variations in Atlantic temperatures. Thusthe severe drought of recent decades is not anomalous in the context of the past three millennia,indicating that the monsoon is capable of longer and more severe future droughts.Beginning in the late 1960s, much of WestAfrica experienced severe drought, whichpeaked in the mid-1970s and lasted forseveral decades, displacing millions from sub-Saharan Africa (1). Since that time, concertedefforts by climatologists, geologists, and mod-elers have focused on understanding the factorscontrolling West African monsoon (WAM) varia-bility (2–6). Much of this research now impli-cates changing sea surface temperatures (SSTs)in generating long-lasting wet and dry periods,with positive land surface feedbacks exacerbat-ing these changes (3–5). However, it is uncertainwhether recent multidecadal drought is anoma-lous in the context of late Holocene climatevariability, because long instrumental records andhigh-resolution paleoclimate reconstructionsfrom the African tropics are lacking (5).We generated a near-annual record of WestAfrican hydrologic variability, using annuallylaminated sediment cores from Lake Bosumtwi,Ghana (6°30′N, 1°25′W) (7). Lake Bosumtwi isideally suited to reconstructing variations in theWAM because of its location in humid tropicalWest Africa and because its sediment laminationsprovide a high-resolution chronology for the pa-leoclimatic record (7, 8). Hydrologic changeswere reconstructed using oxygen isotopes ofauthigenic lake carbonate and micro–x-ray fluo-rescence (m-XRF) scanning of major elementconcentrations (7). Authigenic carbonate d18Oreflects that of Bosumtwi lake water, which,because the lake is a closed basin, is linked to thebalance between precipitation and evaporation.This interpretation is supported by the correlation(R49= 0.55, P < 0.001) between changes inprecipitation measured instrumentally and high-resolution (at 2- to 5-year sampling intervals)carbonate d18O variations in the sediment record(Fig. 1). Although the proxy data reproduce varia-tions in local precipitation on decadal time scales,the relationship is weaker on interannual time scales.These differences most likely reflect both the spa-tially heterogeneous nature of monsoon rainfall oninterannual time scales (8) and the practical diffi-culties in precisely subsampling laminae from highlyflocculant near-surface sediments. Although differ-ences exist in the magnitude and timing of low-frequency rainfall variability across West Africa,including an apparent delayed onset of late 20th-century drought near the coast and a variable dryperiod in 1941–1949, quite similar decadal varia-bility is apparent in instrumental records fromthroughout the monsoon region (fig. S1). Thisdemonstrates that local rainfall reconstructions,such as that from Lake Bosumtwi, may be usedto infer regional-scale changes in monsoon pre-cipitation on decadal and longer time scales.Variations in sediment elemental concentra-tions are interpreted as reflecting changes in theflux of terrigenous material to the lake during thesummer monsoon. This flux is controlled bythe size of the erodable catchment area, withhigher elemental concentrations occurring duringlow lake levels, when more of the crater is emer-gent and the erodable catchment area is larger.High-resolution (20- to 40-mm) XRF scanning ofintact sediment cores provides major elementconcentrations at subannual resolution, allowingthe reconstruction of annual to decadal climatevariability, which is not possible with d18O be-cause of variable carbonate preservation (7). Herewe focus on the first principal component (PC1)of the full suite of elemental data (Al, Si, K, Ca,Ti, Mn, and Fe) as a proxy for the terrigenous sedi-ment component of the record. The similaritiesbetween variations in carbonate d18O and PC1further support our interpretation of the XRF data asan indicator of lake status (supporting online text).Additional support for our interpretation of thegeochemical data comes from geological evidencefor past lake levels (9, 10). Low elemental concen-trations (more positive PC1) and more negativeisotopic signatures in the early part of the recordare consistent with evidence for mid-Holocenelake deposits up to 100 m above the modern lakesurface (9). Stromatolite-encrusted terraces at 20 to25 m above the modern lake level formed between1Department of Geosciences, University of Arizona, Tucson, AZ85721, USA.2Jackson School of Geosciences, University ofTexas–Austin, Austin, TX 78705, USA.3Institute for theEnvironment and Society and Department of Geosciences,University of Arizona, Tucson, AZ 85721, USA.4Lamont DohertyEarth Observatory of Columbia University, Palisades, NY 10964,USA.5Department of Physics, University of Arizona, Tucson, AZ85719, USA.6Department of Geology, University of Akron,Akron, OH 44325, USA.7Department of Earth Sciences, SyracuseUniversity, Syracuse, NY 13244, USA.8Graduate School ofOceanography, University of Rhode Island, Narragansett, RI02882, USA.*To whom correspondence should be addressed. E-mail:[email protected] AD1920 1930 1940 1950 1960 1970 1980 1990 2000Precipitation anomaly [1920-1999]-2-101234δ18ODS-5DS-20.00.51.01.52.02.53.0"Sahel drought"Sediment coreFig. 1. Proxy instrumental data comparison of rainfall variability in West Africa. Carbonate oxygenisotope data (circles and colored lines) from cores DS-2 and DS-5, placed on a laminae age modelplotted with normalized precipitation anomalies from the Kumasi meteorological station (blackline), 35 km northwest of the lake, are shown. More negative isotopic values occur during intervalswith high precipitation rates (r = 0.55, significant at 95%). Both