Holocene environmental changes in the Gebel Umm Hammad, Eastern Desert, Egypt

Gebel Umm Hammad in the Red Sea Mountains east of Quseir, Egypt, today enjoys small but irregular amounts of winter rain, enabling the widening of joint controlled openings in the Thebes Limestone. Cavities are especially affected by flaking, while rock fragmentation is more active on the outside. The sedimentological and botanical study of fan deposits at the outlet of a karstic shaft in the Tree Shelter showed the local Holocene environmental evolution. Three periods of different degree of aridity can be considered: (i) Before 8120 +/- 45 BP (UtC-5389), bedload aggradation points to rare but occasionally heavy rains, lasting for several hours, attaining intensities of more than 76 mm/h and covering some 20 km(2). Wadi flash floods occasionally attained bankfull stage. (ii) Since 8120 +/- 45 BP (UtC-5389), such heavy rains have not occurred in the Egyptian Red Sea Mountains. Instead, a more moderate but maybe wetter precipitation regime was established. The karstic shafts were active, and there was water and life in the desert. Two humid pulses can be distinguished within this period. The first occurs at +/-8000 BP, the second between 6630 +/- 45 (GrN-22560) and 6770 +/- 60 BP (GrN-22562). (iii) After the last wet culmination, there was a gradual shift to drier conditions. Shortly after +/-5000 BP, modem climatic conditions are believed to have been attained. Today, the occasional rain storms are less heavy than before +/- 8000 BP. Bankfull stage river floods do not occur. Instead, secondary channels are eroded in the wadi beds. The general arid character during the whole period and the inherent local and temporal variations in precipitation patterns might explain apparent aberrations between the palaeoenvironmental evolution of the Tree Shelter site and other remote study areas in Egypt and Sudan. (C) 1999 Elsevier Science B.V. All rights reserved.