Late Holocene morphosedimentary and palaeoenvironmental study of the Middle Drâa river basin (southeastern Morocco)

Abstract

The alluvial filling of the middle basin of the Drâa River (southeastern Morocco) has recorded hydrogeomorphological responses to environmental changes. A systematic study of the Holocene fluvial terraces along an upstream-downstream section of about a hundred kilometers, combined with a sedimentary multiproxy study, was carried out on the alluvial archives of the Drâa's river. The stratigraphic field studies, combined with granulometric and geochemical X-ray Fluorescence analyses and radiocarbon dating, enable us to propose, for the first time, a reconstruction of the major stages in the morpho-hydrodynamic and paleo-environmental evolution of the middle Drâa river over the past 3500 years. Our results highlight six morpho-hydrodynamic and paleoenvironmental phases. Between 3500–2700 and 1800–1600 cal BP, the Drâa river was highly active, associated with torrential activity, reflecting arid climatic conditions. The lack of sedimentary record observed between 2800 and 2350 cal BP points either to a phase of erosion, or to a minimal and discontinuous activity that left no traces in the studied archives. The periods 2350–1800 (Roman Warm Period) and 1600–550 cal BP (Medieval Climate Anomaly) are characterized by strong fine and more regular alluviation punctuated by episodes of low energy of the Drâa floodplain, sometimes favoring fluviosols development that showed similar characteristics in the three outcrops studied. Finally, from 550 cal BP the fluvial records generally shows signs of anthropization, marked by the formation of anthrosols characteristic of fluvial oasis construction, and associated with the presence of ceramics and hearths, in which eolization features are frequent. The comparison of Drâa evolution in a broader paleohydrological and climatic context, integrating other Moroccan studies and regional data, shows solid connection with the river, lakes, and marine archives from the southern Mediterranean to the Western Sahelian steppe zones, revealing a clear response of this large hydrosystem to regional climatic variations.