Changing rainfall patterns in NW Africa since the Younger Dryas

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 precipitation 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 hydrology 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, combined with end-member modeling, show variability in sediment-transport mechanisms. Radiogenic isotopes 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 Sr-87/Sr-86 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 influence of this system reached to at least 26 degrees N 12 ka ago. During the mid-Holocene the boundary shifted further north, possibly reaching as far north as 29 degrees N. In the late Holocene the system evolved to a more southerly position that characterises the present