Sedimentary fillings of mountainous valleys as evidences of climatic and anthropogenic changes around Wakarida's archaeological site (Tigray, northern Ethiopia)
Résumé
Well known in West Africa, the African Humid Period (AHP) can also be spotted in sedimentary accumulations in East Africa. The earlier deposits (7th millennium BCE), found in valleys around the axumite (1st-6th Century CE) site of Wakarida (northern Tigray, Ethiopia), seem to correspond to the wet conditions of the end of the AHP. Ages range from the 7th millennium BCE to the 17th Century CE and analyses show variations in depositional environments. Gaps in the dating in Wakarida could correspond to drier spells, identified by Dramis et al. (2003) in other Ethiopian places. At the turn of the 3rd/2nd millennium BCE, the aridification of the climate, pointed by Pietsch and Machado (2014) in Yeha, is sensible through the accumulation of silts, interbedded with gravels. This evolution towards a drier climate has consequences for the vegetation cover and the erodibility of the hillsides. At the end of the AHP, the sediment accumulations are mainly fine-grained, testifying of low competence processes. Nevertheless, coarse beds (with pebbles and gravels) show the occurrence of higher energy events and/or higher sensitivity of the drainage basins to erosive processes. Those can be attributed to climatic changes but they can also be the results of a progressive opening of the vegetation by a growing population. It is not easy to determine whether those changes are the result of climatic, anthropogenic or mixed processes. Sedimentary records dating from the pre-axumite period (800-0 BCE) could indicate the beginning of the clearing of the vegetation by population living in or around the archaeological site, even though no archaeological evidence of cultivation has been found in the excavations. Climatic and anthropogenic processes continue to combine in more recent periods, with two wetter spells leading to sedimentary accumulations in the 8th Century BCE and around the 12th-14th and 15th-17th Centuries CE. After the 17th Century, we lose track of accumulations processes and ablation seems to take place, leading to the erosional forms that we can see nowadays. At present time, the area around Wakarida seems to meet an unprecedented phase of incision and sedimentary crisis, which results in important headward erosion in the valleys’ sedimentary fillings and endangers food security in this dry and highly populated area. This chronostratigraphic work has been conducted within a five-kilometre radius study area around the archaeological site and relied on the interpretation and sampling of sixteen sections, laser granulometry and C14 dating of charcoal samples. Working at the scale of small watersheds and multiplying the number of studied sites allows to understand the functioning of the catchment on long time and to identify and integrate some controlling factors like the organisation of the basin or the connectivity between upstream and downstream parts of the valley.