Long- to short-term environmental changes in the Bao Bolon River valley (Senegal). A geoarchaeological perspective on a World Heritage site (Wanar).

In the Sudano-Sahelian context, where strong seasonal contrasts characterize the climatic and hydrological regimes, palaeoenvironmental studies are faced with a real challenge, because the exacerbated hydro-sedimentary functioning generally limits the conservation of quality records. In the middle valley of the Bao Bolon River (Region of Kaffrine, Central Senegal), fluvial sediments constitute useful archives for characterizing past hydrosystems, their paleoenvironments and their hydro-climatic changes. These archives can also record the contemporary landscape dynamics of ancient societies. The region is characterised by human presence for at least two millennia, as evidenced by the megalithic World Heritage Site of Wanar. This megalithic complex has been the subject of archaeological excavations since 2005, recently coupled with geoarchaeological analyses (field geomorphology and soil analysis, chronostratigraphy, sedimentology, geochemistry, organic matter analysis, bio-indicators, fire signal) in order to reconstruct the evolution of the environment at different time scales under natural and anthropogenic forcing. The numerous profiles studied (cores, sections, transects) reveal a richness and consistency of fluvial sedimentary records at the Holocene scale, rare in West Africa.

At the Holocene scale, regional climatic fluctuations explain the construction of a chronostratigraphic sequence marked by about ten phases of sedimentary aggradation, separated by periods of crisis, even erosion. The last two millennia, which have seen in particular the development of Megalithism, are fairly well documented and make it possible to cross hydro-sedimentary dynamics and anthropogenic action. The periods of funerary activity on the site of Wanar (10th-13th centuries) witnessed significant erosion in the watershed near the site. During the 20th century, the integration of this region into the “Groundnut Basin” of Senegal, with a transition towards increasingly intensive agriculture, led to a drastic change in environmental dynamics by an exacerbation of hydro-geomorphological processes: colluviation and slope-wash, gullying and filling of valley bottoms.

This coupling of the long and short time scales clearly accounts for a gradual transition from a Holocene operating logic to an Anthropocene model.