Late Holocene water level fluctuations of Lake Afourgagh (Middle-Atlas Mountains, Morocco) inferred from charophyte remains
Abstract
Water level fluctuations of Lake Afourgagh (Middle-Atlas Mountains, Morocco) over the last 2500 years (Late Holocene) have been reconstructed using charophyte remains in the lake sediment archives. The study involved 22 pits (1-3 m deep) dug along a transect across the lake shore terrace. Biogenic activity appears to be a dominant contributor to the accumulation of the lake sediments, as the thickest deposits are tufas composed of charophyte-encrusted stems with numerous gyrogonites (termed "charophyte tufas"). The gyrogonites were identified as Chara aspera and Chara hispida remains. The spatial distribution of these two species, measured vertically within a single core and horizontally along the shore terrace, varies according to bathymetry. Shallow proximal environments are characterized by the development of C. aspera, whereas C. hispida remains are mostly observed in distal sediments. Moreover, according to their ecological characteristics, the C. hispida belt migrated proximally when the water level rose, whereas the C. aspera belt migrated distally when the water level fell. These trends were observed successively in a single charophyte tufa layer which records the rise, highstand, and fall of lake level in a single depositional sequence. The late Holocene palaeobathymetry of Lake Afourgagh was inferred by comparing data on charophyte remains and deposit geometry. Data provide evidence for four major low lake level phases (estimated in terms of amplitude) which induced the development of palaeosols in proximal settings at 2191 cal. BP, 1769 BP, 1515 cal. BP and 1062 cal. BP. These results confirm the accuracy of charophyte-based palaeobathymetry studies.
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