Three million years of monsoon variability over the northern Sahara

Abstract

We present a 3 million year record of aeolian dust supply into the eastern Mediterranean Sea, based on hematite contents derived from magnetic properties of sediments from Ocean Drilling Program Site 967. Our record has an average temporal resolution of 3400 years. Geochemical data validate this record of hematite content as a proxy for the supply of aeolian dust from the Sahara. We deduce that the aeolian hematite in eastern Mediterranean sediments derives from the east- ern Algerian, Libyan, and western Egyptian lowlands located north of the central Saharan watershed (321� N). In corroboration of earlier work, we relate dust flux minima to penetration of the African summer monsoon front to the north of the central Saharan wa- tershed. This would have enhanced soil humidity and vegetation cover in the source regions, in agreement with results from ''green Sahara'' climate models. Our results indicate that this northward monsoon penetration re- curred during insolation maxima throughout the last 3 million years. As would be expected, this orbital pre- cession-scale mechanism is modulated on both short (3100-kyr) and long (3400-kyr) eccentricity time scales. We also observe a strong expression of the 341- kyr (obliquity) cycle, which we discuss in terms of high- and low-latitude mechanisms that involve Southern Hemisphere meridional temperature contrasts and shifts in the latitudes of the tropics, respectively. We also ob- serve a marked increase in sub-Milankovitch variability around the mid-Pleistocene transition (30.95 Ma), which suggests a link between millennial-scale climate variability, including monsoon dynamics, and the size of northern hemisphere ice sheets.