I-n-Atei palaeolake documents past environmental changes in central Sahara at the time of the “Green Sahara”: Charcoal, carbon isotope and diatom records
Résumé
During the ‘Green Sahara event’, water bodies developed throughout the Sahara and Sahel, reflecting the enhanced influence of the Atlantic monsoon rainfall. Major lakes then dried out between 6.5 and 3.5 ka. This study investigates land cover change and lacustrine environment during the Holocene at I-n-Atei, Southern Algeria, a desert region lying in the hyperarid core of the Sahara. This site is remarkable by its extent (up to 80 km2) and by the exceptional preservation and thickness of the lacustrine deposits (7.2 m). I-n-Atei was a lake from 11 to 7.4 ka, then it dried out and left place to a swampy environment. Charcoal concentrations show that the surroundings of the lake were vegetated throughout the wet period with two short phases of possible vegetation deterioration associated with a lowering of the lake level at 9.3 and 8.2 ka, coeval with well-known dry events in the tropics. The stable carbon isotope record reflects the penetration of C4 herbaceous populations in replacement of the original C3, typical of the regional vegetation at the time of the maximum lake expansion. The δ13C of charcoals increase non-linearly with the 14C-based ages from − 24.5‰ to − 13.0‰ (V-PDB). Assuming that these extreme values sample both C3 and C4 plant end-members, mass balance calculations suggest that C3 were replaced by C4 plants according to an exponential decay law with a half-life (t1/2) of 850 ± 110 years. The replacement of C3 by C4 plants occurred in two main steps: a mixed C3–C4 vegetation of “wooded grassland” type was present from 10 ka to 8.4 ka while a C4 exclusive vegetation developed after 8.4 ka. After the end of the lacustrine phase a catastrophic event (flooding?) provoked the lifting of most of the lacustrine deposits and their re-deposition above the lacustrine sequence.