Record of Holocene aridification (6000–7000 BP) in Egypt (NE Africa): Authigenic carbonate minerals from laminated sediments in Lake Qarun

Abstract

The origin and climate implications of authigenic carbonate minerals from Lake Qarun, Egypt, were examined in relation to the mid–late Holocene climate changes. These minerals occur as very thin (0.5–3 mm), very fine, well-sorted, white to pale yellow aragonite or calcite laminae intercalated with olive green clayey silt with sharp and horizontal contacts. Under the scanning electron microscope (SEM), aragonite appears as dense euhedral and ellipsoidal (rice-grained) crystals that range in length from 0.5 to 1 micron and in width from 0.2 to 0.5 micron. The calcite crystals are stubby, equant or blocky polyhedra of about 5 microns in length. δ 18O values range between 2.7‰ and 3.7‰ for the calcite and between 2.1‰ and 6‰ for the aragonite. δ 13C values range between 0.4‰ and 1‰ for the calcite and between 0.5‰ and 3.4‰ for the aragonite. A very strong positive correlation is observed between carbon and oxygen isotopes for the aragonite ( r 2 = 0.9) and a more moderate positive correlation with calcite ( r 2 = 0.6). The complete absence of biological and other no-carbonate grains, the sharp contacts of laminae basal, and the occurrence of calcite and aragonite as individual euhedral crystals, rather than clusters of grain aggregation, indicate that aragonite and calcite layers represent rapid inorganic precipitation from the lake water (endogenic). The positive values of δ 18O and δ 13C and the strong positive correlation between δ 18O and δ 13C suggest prevalence of a low lake level, saline, and dry climatic conditions during their formation. The change from calcite to aragonite indicates a change in the formational conditions from less arid conditions in the case of calcite to severe arid conditions in the case of aragonite. The relative abundance of diatoms in calcite-rich laminae and the less strong positive correlation between δ 18O and δ 13C of calcite support this interpretation. The arid conditions in which the carbonate minerals were formed could correspond to the aridification period of the North East Africa between ca 6000–8500 BP due to the north-south shifts of the Intertropical Convergence Zone (ITCZ) that control the evaporation/precipitation balance over North East Africa and/or to dry climate and desert environment conditions in Egypt at the same period. These results are also consistent with the aridification conditions during the mid-late Holocene, suggesting its global trend.