Natural dynamic vs anthropogenic transformations of the AlUla oasis, NW Arabia, during the Holocene: a combination of geoarchaeology and geomatic approaches

Oases are man-made environment in response to hydro-climatic constraints of dryland. They form landscapes of natural origin that have been modified and exploited by the agro-pastoral practices of human societies. The oasis of AlUla, in Northwestern Saudi Arabia, is settled at least since the beginning of the Bronze Age. This oasis is located at the foot of the Harrat al-Uwayrid formation which separates the sandy deserts of northern Arabia from the Red Sea. AlUla has a long history of occupation, notably through the development of major archaeological sites such as the Late Bronze Age site of Dadan, the Nabataean site of Hegra, and the old towns of AlUla and Al Mabyat from the Islamic period. Recent research has also revealed the development of agro-pastoral activities since the Neolithic and hydroagricultural development from the Late Bronze Age onwards.

The aim of this research is to reconstruct the landscape of the AlUla oasis and thus to find out in what environmental framework these agro-pastoral societies developed. It also aims to study the role of climatic changes in the natural dynamics that have formed the geomorphology of the oasis (aeolian, fluvial, slopes processes and formations) and to determine the impact of human development. These objectives are to provide answers about the understanding of the current organization of geomorphological objects and their evolution/transformation through the Holocene and thus the interactions between societies and their environment that have produced them. This work has also enabled us to produce quantitative data on the volumes of earth moved/excavated since the rampant urbanization of the oasis, and to identify the practices (quarrying vs. levelling) and geomorphological environment most affected by these earthworks.

To achieve these objectives, we have produced a diachronic geomorphological map covering the AlUla oasis and its margins, with the aim of tracing the natural history of the oasis from its current state back to the Neolithic/Early Holocene. This realization of this map is based on a combination of geomatics methods, using a DEM produced by LiDAR data (2018, 40 cm accuracy), orthophotographs (2018, 10 cm accuracy), remote sensing with satellite images (1965-2024 Google, Bing, Corona), geological data (1:500,000) and fieldwork in order to inventory landforms and determined their organization and their chronology (C14, OSL).

Our results show a long-term trend towards aridification since the second half of the Holocene and an increase in human pressure since the Bronze Age. This last observation result from the initiation and expansion of agricultural practices supported by earthworks which have led to the development of levees along wadis, agricultural terraces and anthroposols. These anthropogenic forms associated with numerous excavations have greatly modified the initial topography and therefore the geomorphology of the oasis, from the Bronze age but with an astonishing and constant acceleration over the last thirty years. This demonstrates that the natural dynamics which have prevailed during the Holocene are progressively replaced by the impact of human societies, acting as an agent of erosion in the oasis environment in NW Arabia during the Anthropocene.