Changing rainfall patterns in NW Africa since the Younger Dryas Inka Meyer a,⇑ , Gareth R. Davies b,1 , Christoph Vogt a,c,2 , Holger Kuhlmann a,3 , Jan-Berend W. Stuut a,d,4 a MARUM – Center for Marine Environmental Sciences, Leobener Straße, 28359 Bremen, Germany b Deep Earth and Planetary Science Cluster, VU University Amsterdam, De Boelelaan 1105, 1081 HV Amsterdam, The Netherlands c Crystallography/ZEKAM, Geosciences, University of Bremen, Klagenfurter Straße, 28359 Bremen, Germany d NIOZ – Royal Netherlands Institute for Sea Research, 1790 AB Den Burg, The Netherlands article info Article history: Received 23 June 2012 Revised 20 March 2013 Accepted 20 March 2013 Available online xxxx Keywords: Holocene NW African climate Latitudinal shift Grain size Radiogenic isotopes abstract Currently, two climate systems dominate the environmental conditions in NW Africa; the Mediterranean climate, with winter rains in the north, and the NW African monsoonal climate with summer precipita- tion in the south. These climate regimes are separated by the Saharan Desert. Previous studies indicated past latitudinal movements of the boundary between these climatic systems, causing changes in hydrol- ogy over the area. In the arid setting of NW Africa possible future changes in hydrological systematics will have a tremendous impact on the environment and human welfare. Thus, detailed understanding of past wet/dry alterations is of great importance. Here we present new data about the latitudinal shifts of the transition zone between the prevailing NW African rainfall patterns over the last 12 ka. We investigated the terrigenous fraction of marine cores retrieved offshore NW Africa. Grain-size measurements, com- bined with end-member modeling, show variability in sediment-transport mechanisms. Radiogenic iso- topes combined with trace element data show contrasting hydrological conditions in northern versus southern sediment records, indicating a shift of the climate systems during the Holocene. Higher 87 Sr/ 86 Sr and lower Rb/Sr ratios in the north point to an increased influence of chemical weathering due to the greater impact of the North Atlantic climate system during the Younger Dryas. We propose that the influ- ence of this system reached to at least 26 °N 12 ka ago. During the mid-Holocene the boundary shifted further north, possibly reaching as far north as 29 °N. In the late Holocene the system evolved to a more southerly position that characterises the present. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The modern rainfall over NW Africa is influenced by two cli- mate systems, separated by the Saharan Desert (Nicholson, 2000). The northernmost part of the continent is dominated by the Mediterranean climate system, which is related to the North Atlantic climate regime, where the west–east migration of cy- clones causes winter rain (Nicholson and Flohn, 1980). South of 20–24°N the NW African monsoonal system provides precipitation during summer (Knippertz et al., 2003). Present-day distribution of rainfall and contrasting rainfall patterns over the year for the northern and southern part of the study area are shown in Fig. 1. The main climatic driver of the region is the tropical rainbelt, which shows a seasonal variability. During boreal winter the core of the rainbelt is located at 5°N, whereas during boreal summer it lies at 20 °N, following the seasonal migration of maximum inso- lation (Nicholson, 2000). It has been shown in various studies that the latitudinal position of the tropical rainbelt has changed over geological time scales, either by latitudinal movements (Gasse et al., 1990; Gasse and Van Campo, 1994) or through expansion and contraction of the rainbelt (Collins et al., 2011) causing varia- tions in the hydrological conditions on land. Grain-size measurements combined with radiogenic isotope data as well as element intensities indicate that a latitudinal shift of the NW African climate systems may cause changes in hydrolog- ical conditions during the Holocene as well as differences in hydro- logical conditions between the northern and southern part of NW Africa (Meyer et al., 2011; Kuhlmann et al., 2004). In order to gain further information about the detailed extent of the latitudinal shift of the transition zone between the Mediterranean climate system in the north and the monsoonal climate system in the south the existing record was extended in this study by additional sediment cores from offshore Morocco, Western Sahara and Mau- ritania (Fig. 1). Existing results record a different pattern in conti- nental weathering regime at 29 °N compared to results obtained south of 23 °N(Meyer et al., 2011). To verify that there was a 1875-9637/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aeolia.2013.03.003 ⇑ Corresponding author. Tel.: +49 421 218 65518; fax: +49 421 218 65505. E-mail addresses: minka@marum.de (I. Meyer), gareth.davies@falw.vu.nl (G.R. Davies), cvogt@uni-bremen.de (C. Vogt), kuhlma@marum.de (H. Kuhlmann), jbstuut@nioz.nl (Jan-Berend W. Stuut). 1 Tel.: +31 20 59 87329; fax: +31 20 59 89942. 2 Tel.: +49 421 218 65161; fax: +49 421 218 65189. 3 Tel.: +49 421 218 65673; fax: +49 421 218 9865673. 4 Tel.: +31 222 369 405; fax: +31 222 369 674. Aeolian Research xxx (2013) xxx–xxx Contents lists available at SciVerse ScienceDirect Aeolian Research journal homepage: www.elsevier.com/locate/aeolia Please cite this article in press as: Meyer, I., et al. Changing rainfall patterns in NW Africa since the Younger Dryas. Aeolian Research (2013), http:// dx.doi.org/10.1016/j.aeolia.2013.03.003