East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins

doi:10.1073/pnas.0703874104

East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins

^aDepartment of Earth Sciences, 204 Heroy Geology Laboratory, Syracuse University, Syracuse, NY 13244;
^cLarge Lakes Observatory and Department of Geological Sciences, University of Minnesota, Duluth, MN 55812;
^dDepartment of Geosciences, University of Arizona, Tucson, AZ 85721;
^eGraduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882;
^fDepartment of Geology and Environmental Science, University of Akron, Akron, OH 44325;
^gDepartment of Earth Science, University of Bergen, N-5007 Bergen, Norway;
^hMalawi Geological Survey Department, Zomba, Malawi;
ⁱGeological Survey Department of Ghana, Accra, Ghana;
^jDepartment of Earth and Environmental Sciences, University of Illinois, Chicago, IL 60607;
^kScottish Universities Environmental Research Centre, East Kilbride G75 0QF, Scotland; and
^lDepartment of Biology, University of Wisconsin-Eau Claire, Eau-Claire, WI 54702

Edited by William F. Ruddiman, University of Virginia, Charlottesville, VA, and accepted by the Editorial Board July 27, 2007 (received for review April 30, 2007)

Abstract

The environmental backdrop to the evolution and spread of early Homo sapiens in East Africa is known mainly from isolated outcrops and distant marine sediment cores. Here we present results from new scientific drill cores from Lake Malawi, the first long and continuous, high-fidelity records of tropical climate change from the continent itself. Our record shows periods of severe aridity between 135 and 75 thousand years (kyr) ago, when the lake's water volume was reduced by at least 95%. Surprisingly, these intervals of pronounced tropical African aridity in the early late-Pleistocene were much more severe than the Last Glacial Maximum (LGM), the period previously recognized as one of the most arid of the Quaternary. From these cores and from records from Lakes Tanganyika (East Africa) and Bosumtwi (West Africa), we document a major rise in water levels and a shift to more humid conditions over much of tropical Africa after ≈70 kyr ago. This transition to wetter, more stable conditions coincides with diminished orbital eccentricity, and a reduction in precession-dominated climatic extremes. The observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical Africa, but our records provide evidence for dramatically wetter conditions after 70 kyr ago. Such climate change may have stimulated the expansion and migrations of early modern human populations.

Footnotes

^bTo whom correspondence should be addressed. E-mail: cascholz{at}syr.edu
Author contributions: C.A.S., T.C.J., A.S.C., J.W.K., J.A.P., J.T.O., and M.R.T. designed research; C.A.S., T.C.J., A.S.C., J.W.K., E.T.B., L.K., P.Y.O.A., R.P.L., T.M.S., I.S.C., C.W.H., S.L.F., L.R.M., and K.R.B. performed research; C.A.S., T.C.J., A.S.C., J.W.K., J.A.P., E.T.B., R.P.L., S.L.F., L.R.M., J.G., and J.P. analyzed data; and C.A.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. W.F.R. is a guest editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/cgi/content/full/0703874104/DC1.
↵ m Brown ET, Johnson TC, Scholz CA, King J, Cohen AS (2006) Eos Trans AGU Fall Meeting Suppl 87:PP13A-1587 (abstr.).
Abbreviations:

LGM,

Last Glacial Maximum;

kyr,

thousand years;

TOC,

total organic carbon.
Freely available online through the PNAS open access option.