Palaeoenvironments and palaeohydrology of a tropical closed lake (Lake Asal, Djibouti) since 10,000 yr B.P.

Abstract

In eastern Africa (Republic of Djibouti), the hypersaline Lake Asal occupies an exceptional geological, topographical and hydrological situation at 155 m below sea level, and at 12 km from the sea. It lies in the youngest and the deepest basin in a chain of closed depressions of Central Afar, decreasing in elevation from west to east, and represents the hydrogeological low of the whole system. Presently, infiltration of marine water represents the major water input to the lake. Due to high evaporation conditions, halite precipitates. Halocene water level fluctuations (315 m), sedimentary conditions (from fresh-oligosaline waters to heavy brines), and palaeohydrology are reconstructed. Field observations are supported by 85 14C dates on inorganic carbonates, mollusc shells, stromatolites and hydrothermal travertines, stable isotope analyses on carbonates and sulphates, mineralogy, geochemistry, and studies of biological remains. Discussion of the water balance for the major stages is based on reasonable assumptions about Holocene conditions of precipitation and evaporation. After an arid terminal Pleistocene phase, marked by coarse deposits due to exceptional floods, the Holocene cycle begins with complex, strontium-rich carbonate phases (aragonite, high and low Mg-calcites, and dolomite). These carbonates could reflect the first underground marine supply, and waters concentrated through evaporation. A high lake level (ca 160 m.a.s.l.) from ca 8600 to 6000 yr. B.P. corresponds to the deposition of high Mg-calcite precipitated in a deep fresh-water lake. Lake Asal was draining the Awash river system (Ethiopian plateau and escarpment) through groundwater circulations. The terminal basin of the system was probably the present-day marine gulf of Ghoubbat-al-Kharab hydrologically separated from the open ocean by a sill at 4 m below sea level. Possibly due to fracture opening, a dramatic regression in the lake level (from + 160 to - 150 m) occurred between ca 6000 and 5000 yr B.P. The inputs of continental water were consequently overwhelmed by marine infiltrations. The carbonate phase exhibits clear resemblance with that of the transgressive episode. From 5000 yr B.P., Lake Asal experienced minor water-level fluctuations. Aragonite-rich carbonates precipitated, followed by calcium sulphates and halite. Stable isotopes of gypsum reflect its marine origin and various diagenetic processes. The Holocene evolution of Asal, which clearly differs from that of the neighbouring closed basins, is due to both its specific geological and hydrological setting, and to the regional climatic changes.