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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 ∼400 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 eastern Algerian, Libyan, and western Egyptian lowlands located north of the central Saharan watershed (∼21°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 watershed. 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 recurred during insolation maxima throughout the last 3 million years. As would be expected, this orbital precession-scale mechanism is modulated on both short (∼100-kyr) and long (∼400-kyr) eccentricity time scales. We also observe a strong expression of the ∼41-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 observe a marked increase in sub-Milankovitch variability around the mid-Pleistocene transition (∼0.95 Ma), which suggests a link between millennial-scale climate variability, including monsoon dynamics, and the size of northern hemisphere ice sheets.

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Acknowledgements

We thank the staff from the ODP Core Repository in Bremen for technical assistance and hospitality during sampling. We also thank Catherine Kissel, Michael Sarnthein and two anonymous reviewers for their constructive comments that helped to improve the paper and Jean-Claude Duplessy for editorial handling. This work was financially supported by European Community TMR Network contract MAG-NET (ERBFMRXCT98-0247) and the UK NERC, and contributes to NERC project NER/B/S/2002/00268. Samples were provided by the ODP, which is sponsored by the US National Science Foundation and participating countries (including the UK) under management of Joint Oceanographic Institutions, Inc.

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Correspondence to A. P. Roberts.

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Larrasoaña, J.C., Roberts, A.P., Rohling, E.J. et al. Three million years of monsoon variability over the northern Sahara. Climate Dynamics 21, 689–698 (2003). https://doi.org/10.1007/s00382-003-0355-z

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Keywords

  • Isothermal Remanent Magnetization
  • Anhysteretic Remanent Magnetization
  • Saharan Dust
  • Dust Production
  • Ocean Drill Program Site