Morphological, pedological, and sedimentary evolution on the fringe of the southwestern European drylands during the Late Pleistocene and Holocene: Evidence of climate and land use changes

Late Quaternary alluvial sedimentary units in the Guadiana Menor River valley, a tributary of the Guadalquivir River in southern Spain are described based on geomorphological mapping, sedimentological analysis, palaeosol development, and radiocarbon chronology. Four infill units in the valleys were differentiated with surfaces at +45 (MU1), +70 (MU2), +8–6 (MU3), and +10m (MU4) above the riverbed. Each unit is capped by a palaeosol.Unit MU1 (Late Pleistocene: >43.5–34calkyrBP) consists of gravel and sand in channels and bars in braided river systems distally evolving to silty clays in the floodplain with vertic and clayic calcisols. It is capped by dark clays with gleysol development representing wetland environments in endorheic basins. Channelized gravels alternating with sandy silts make up unit MU2 (34–24calkyrBP), capped by a gypsisol and interpreted as a semi-arid alluvial fan distally evolving to playa-lake depositional systems. Unit MU3 (5600–2700calyrBP), extending along the valley bottom, is represented by gravels and sand distally evolving to a red and white laminated rhythmite of silt and clay deposited in the playa. Unit MU4 (600–700calyrBP) is scattered and represented by centimetre-thick gravels and sand sheet layers deposited by sheet flows in semi-arid constrained alluvial fans.Alternating intervals of alluvial aggradation (>43.5–24kyrBP, 5700–2700calyrBP, Late Middle Ages) and fluvial incision (24–6calkyrBP, 2700calyrBP to 1200AD, 1500AD to present) are interpreted as valley responses to climate variations and changes in land use. Two regional geomorphological changes are recorded in the sediments: (1) climate-induced aridification after 34calkyrBP with an arid climax after 24calkyrBP before the incision linked to the last glacial period, and (2) two human-forced aggradation stages (5600–2700calyrBP and 1200–1400AD) interrupting the Late Pleistocene to Early Holocene incisional trend.