Abstract
Pollen and plant macroremains from the Saharan desert are used to reconstruct past vegetation changes. Pollen data are extremely scarce for pre-Holocene periods in one of the world's driest regions. The most detailed and continuous record of desert history is provided by marine cores off North Africa. The ODP 658 site off Mauritania shows that desert steppes began to spread around 2.5 million years ago, but the Djourab dunes in Chad show that arid environments existed as early as 7 Ma in a mosaic landscape comprising forests and grasslands. Over the last million years, the desert has undergone phases of contraction/extension in phase with glacial/interglacial alternations in the northern hemisphere. The best-documented phase is the Holocene, although here too, data are scarce and often discontinuous. Nevertheless, pollen data and plant macroremains show that tropical plants that were confined to the edge of the Gulf of Guinea during the last Ice Age migrated northwards, using migration corridors such as riverbanks and wetlands. At the height of the Holocene wet period, they reached as far north as 26°N. The result was a mosaic landscape of great biodiversity, with tropical plants in the wetter areas and desert or semi-desert plants elsewhere. These plants, now in distinct vegetation zones, occupied a common latitudinal band, especially between 10°N and 24N. Lake Yoa, in northern Chad, shows that the retreat of the " Green Sahara " began around 4,000 BC, while the modern desert environment was definitively established at 2,700 AEC.
