A detailed reconstruction of the Holocene palaeoenvironments and palaeohydrology is presented for wwamp and lacustrine calcareous sediments which lie in closed depressions along the northern margin of the Great Western Erg (Algeria). It is based on a multidisciplinary approach using numerous techniques. The chronological framework is established on the basis of 23 14C dates (on inorganic calcite and mollusc shells) ranging from ca 9300 to 3000 yr B.P. (and on an isolated 14C age of ca 10,900 yr B.P.). The significance of various environmental markers is discussed. Biological remains are abundant and diversified (about 150 taxa including diatoms, chrysophytes, charophytes, ostracods, molluscs and foraminifera). They provide information on the water chemistry (total salinity, ionic ratio, pH, and partial pressure of carbon dioxide), and on the habitats (water depth, development of aquatic plants, ephemerality, etc). Emphasis is given to the total salinity. Individual organisms and palaeobiological assemblages are ordered as a function of their salt requirements and tolerances. Calibration based on modern diatom samples allows a tentative estimate of the palaeosalinities which range from fresh to marine-like, and even hypersaline waters. Attention is drawn to the possible colonization of "marine" organisms in continental basins without any connection with the sea. Stable isotope analyses have been carried out on both inorganic and biogenic (mollusc and ostracod shells, charophyte gyronites) carbonates. The 18O contents demonstrate wide changes in the hydrological balance. Fluctuations in 13C contents inform on water mixing, residence time, and on the importance of biological effects on total dissolved inorganic carbon. Geochemical analyses were performed on inorganic calcite and on ostracod shells. High contents in Fe and Mn indicate reducing environments during stages of high salinity. Sr/Ca and Mg/Ca are used as salinity indicators. Zn/Mg ratios suggest that the major water supply was the aquifer of the Great Western Erg, today down to about - 50 m, rather than surface waters from the Atlas mountains. Salinities resulted from the balance between inputs from the water-table, evaporation rate, leaching of pre-existing salt crusts, and seepage through the lake bottom. A reconstruction of the evolution of the palaeosystems through time and space has been attempted, with special attention to the instability of the water and ionic balances for different time scales. Short term fluctuations occurring during a single hydrological cycle (annual or seasonal) are deduced from the apparent discrepancies between the various indicators found in a given sample. Long term (102-103 years) changes confirm the existence of a climatic phase being wetter than today from at least ca 9300 (and may be from ca 10,900 yr B.P.) to 3000 yr B.P. For the first time, the salinity deduced from diatoms and the 18O content of the water deduced from that of the carbonate (at 25°C) are used simultaneously to model the hydrological history of the basins and to estimate the relative humidity of the atmosphere during the Holocene. Major environmental trends recorded from ca 9300 to 3000 yr B.P. do not show a clear synchronims between the different profiles studied. Therefore, they cannot be interpreted in terms of palaeoclimate, but instead demonstrate the importance of local hydrology in the evolution of closed basins associated with groundwater, a common situation in arid and semi-arid zones. © 1987.
