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International audience; Nowadays, many parts of the world are cultivated on terraces. These include Eastern Tigray (Ethiopia) and Southern Tunisia, where such terraces allow rain-fed agriculture without irrigation. The terraces of these two regions, however, have the particularity of being erected not on the slopes but in the valleys, which they cross perpendicularly to form flat cultivation areas and conserve water and soil on the plots.In Eastern Tigray, pre-Aksumite, Aksumite and post-Aksumite archaeological remains have been unearthed on the high points overlooking the terraced valleys. The cultural exchanges between these populations and the Sabaean populations of South Arabia, renowned for their hydro-agricultural works, are well documented today and lead to the hypothesis that terraces were ancient. Similarly, in Tunisia, the attested existence of hydro-agricultural developments from the Roman period in the region raises questions about the age and origin of the actually visible structures. A methodology combining geoarchaeology, geohistory and ethnogeomorphology allows apprehending these issues and estimating the period of installation of water and soil protection structures in the two studied areas, in a comparative perspective.Moreover, the contemporary evolution of these structures raises the question of their sustainability. In Eastern Tigray, agricultural terraces represent the main, if not the only means of subsistence for the populations. They are now subject to erosion. In Southern Tunisia, the recent rural exodus has exposed the structures to rapid evolution: destruction of low walls, loss of agricultural land by suffosion. Southern Tunisia thus allows a prospective reflection on the future of Tigray in the light of the ongoing rural exodus and questions the factors of regressive erosion in these two areas. More broadly, these two case studies can serve as analogues for reflection on other terraced areas, at different periods and in different regions.

International audience; In northern Ethiopia, the villages of Sewne and Wolwalo form the centre of two areas surveyed by the French archaeological mission in the Eastern Tigray which revealed pre-Aksumite, Aksumite and post-Aksumite settlements. Around Wolwalo, the study zone is on the Tigray plateau, at an altitude of 2700 m. Around Sewne, the site of Wakarida and the surveyed area extend on a topographic spur at the altitude of 2400 m, forming a transition zone between the plateau and the valleys plunging towards the Danakil Depression. The relations maintained by the sites of the two regions with the central powers during pre-Aksumite, Aksumite and post-Aksmuite periods are still unclear, as well as the relationship between the two spaces (spur/plateau), and ancient agricultural practices.Geographically, it is a margin area with a shifting relationship to political centrality, which can be apprehended on various spatio-temporal scales, thanks to an interdisciplinary approach, combining archaeology, geoarchaeology, geohistory and ethnogeomorphology. Our geographical analysis relies on the concepts of centrality, marginalization and mobility and is part of a diachronic reflection, essentially based on the modern and contemporary periods.During the modern period, this space is subject to erosion caused by anthropogenic activities from the 14th century onwards but is nevertheless absent from travellers' accounts and early cartographies. Marginalised at the regional scale and away from the main itineraries, the studied space remains locally active and continues developing. Today, the Sewne spur constitutes a central margin: its marginal location makes it a contact zone, an interface area, where the distance from the centre leads to the creation of a new centre. This area becomes a porous and dynamic borderland between sedentary agricultural practices and nomadic or semi-nomadic agro-pastoral practices of the neighbouring Afar populations. These contemporary data provide analogues allowing to question past periods through a regressive approach.

International audience

International audience

International audience; 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.

International audience

International audience

International audience

International audience; Les paysages actuels sont très fortement marqués par le couplage, à l'échelle historique, entre les fluctuations du climat et l'emprise des sociétés sur leurs territoires. L'objet de ce travail est l'étude de l'évolution historique de paysages de moyenne montagne dans un contexte de rupture tant climatique (fin du Petit Age Glaciaire) qu'anthropique (déprise rurale, actions de l'Etat contre l'érosion). Deux types de méthodes sont combinés afin de reconstituer précisément les changements hydroclimatiques et hydromorphologiques et l'évolution de l'occupation des sols : l'étude des archives historiques et l'utilisation de bio-indicateurs (dendrochronologie et lichénométrie). Ce travail contribue à la reconstitution des paléodynamiques récentes en se penchant sur la nature et la chronologie des changements fluviaux pendant la deuxième moitié du 19e siècle dans les hauts bassins des Alpes du sud et du sud-est du Massif Central (France). Il apporte des précisions nouvelles sur la transition entre le PAG et les conditions actuelles de l'évolution des versants et des lits torrentiels : l'identification d'une succession de phases dynamiques différentes (par exemple le rôle préparatoire de la sécheresse de 1830-1838 dans la crise torrentielle de 1855-1870) et un affinement de la chronologie (commencement très précoce de l'incision des lits). Les résultats mettent en évidence le rôle du climat (petites fluctuations hydroclimatiques), amplifié par une forte anthropisation, dans l'explication des rythmes de l'évolution des paysages et de la stabilisation relative de la fin du 19e siècle. Par ailleurs, le synchronisme de part et d'autre de la vallée du Rhône montre que ces évolutions se sont faites à l'échelle régionale.