Kröpelin Wadi Bakht-Beitrag - Universität zu Köln 2005 AP 16 Wadi … · The detailed palaeoclimatic record of Wadi Bakht (Gilf Kebir, Southwest Egypt) and interpretation of associated

Geomorphological and palaeoclimatic framework 51

The geomorphological and palaeoclimatic frameworkof prehistoric occupation in the Wadi Bakht area

Stefan Kröpelin

Abstract

The detailed palaeoclimatic record of Wadi Bakht (Gilf Kebir, Southwest Egypt) and interpretation ofassociated floral and faunal remains suggest a hyperarid climate similar to the present until 9,300uncalibrated years bp (8,400 BC). The period between 9,300 and 5,400 bp (8,400 – 4,300 BC) witnesseda moderately arid summer-rain climate with estimated 100-150 mm of annual precipitation and aboutfour major rainfall events per century. With the unique climatic transition at 5,400 bp (4,300 BC), therainfall regime changed towards a drier winter-rain climate with estimated rainfall below 100 mm peryear that lasted until the return of extremely arid conditions at about 4,500 bp (3,300 BC). Newgeoarchaeological evidence underlines the crucial importance of seasonal rainfall distribution for thevegetation cover and thus for prehistoric settlement and land-use in arid regions.

With few exceptions, the playa lakes have dried out within several weeks or months after the rainyseasons or single major downpours. As a result, the water resources in Wadi Bakht have never beensufficient for permanent settlement in the connotation of years, decades or generations. It may thereforebe assumed that the seasonal, periodic or episodic inhabitants of the Gilf Kebir were based at JebelOuenat which lies 80 km of its southern tip and which provided permanent water points and lush valleyswith optimal conditions for permanent settlement.

Keywords: Eastern Sahara, climate change, Holocene, high-resolution palaeoclimatology, palaeomonsoon,playas, geoarchaeology.

1. The Gilf Kebir

The Gilf Kebir is a huge sandstone plateau in theremote southwest corner of Egypt [Fig. 1] (KEMALEL DINE HUSSEIN 1928). Crossed by the Tropicof Cancer, it represents the core of the EasternSahara which today is the largest hyperarid regionon earth receiving less than 2 mm of averageannual rainfall against a potential evaporation ofseveral metres (HENNING & FLOHN 1977). Theincreased aridity of this trade wind desert climatehalf-way between the Mediterranean winter rainfallzone in the north and the summer rainfall zoneof the wet-dry tropics in the south is related tothe cross circulation of the tropical easterly jet anda strong continentality. Consequently, the GilfKebir is virtually inanimate except for some

isolated localities with relict vegetation (BACK1981). Owing to the lack of any surface water orwater holes, the area has been void of any humansettlement for several millennia.

The Gilf Kebir (“big cliff” in Arabic) consistsof two flat-topped plateaus that are connected bya narrow bridge (EMBABI 2004). The northwestpart is also known as Abu Ras Plateau (KLITZSCH1979). The southeast plateau which is treated here,and which is now named Kemal el Din Plateau,has a surface of c. 5,800 km2, i.e. the size of theMediterranean island of Corsica. The height of theplateau surface declines from 1,050 m above sea-level at its southern tip to about 900 m a.s.l. in thenorthern part where it merges with the dunefields of the Great Sand Sea of Egypt. Boundedby steep to vertical cliffs with an approximate

52 Stefan Kröpelin

total frontage of 3,000 km, the plateau rises morethan 300 m above the surrounding desert plains.The geological structure of the southeast GilfKebir consists of horizontally bedded, Jurassicand Cretaceous sandstones, unconformablyoverlying Pre-Cambrian or Lower Palaeozoic for-mations (ISSAWI 1971. KLITZSCH & LIST, eds.,1978). Basaltic hills and lava flows, apparentlyassociated with the northwest-southeast, northeast-southwest and north-south striking fault system,indicate intrusive and effusive volcanic activity onthe plateau during the Upper Tertiary (FRANZ etal. 1987). Judging on their fresh morphology,some volcanic features possibly even date to theQuaternary (KRÖPELIN 1989).

The existing morphology of the southeastplateau with its box canyons and abrupt headcutsis controlled by the highly resistant ferruginousor silicified cap-rock (PEEL 1939). The initialfluvial dissection probably cut into overlying, lessresistant shales which were later completelystripped by deflation (HAYNES 1981). Otherprocesses such as subsurface drainage with

undercutting of cliffs and sapping have contributedto the formation of steep valley sides and heads(MAXWELL 1981), just as the aeolian activityduring the mainly arid conditions of theQuaternary overprinted the entire landscape(McCAULEY et al. 1981).

The southeast plateau is irregularly dissectedby up to 20 km long and up to 4 km wide canyon-like valleys which predominantly trend from westto east. The wadis are flat-floored with remnantsof drainage channels in their alluvial fill. As aconsequence of the nearly constant northerly tradewinds and an almost infinite source of sand inthe Great Sand Sea in the north, dune barriershave partly or completely blocked the uppercourses of the valleys running at right anglesduring the arid phases of the Quaternary. Thisspecific set of geomorphologic factors has per-mitted the accumulation and preservation of fine-grained playa (still-water) deposits during theintervening, relatively short humid periods of thePleistocene, and in particular during the early andmiddle Holocene.

Fig. 1 The southeastern partof the Gilf Kebir plateau withvalleys mentioned in text. Insetmap shows position in theWestern Desert of Egypt.(Apollo VII photograph)


2. Wadi Bakht

Wadi Bakht (“lucky valley” in Arabic; BAGNOLDet al. 1939) is one of the longest valleys of thesoutheast Gilf Kebir [Fig. 2]. 12.5 km below thevalley head, the typical sandy-pebbly wadi depositsgradually transform to a pelitic facies. This is thelocation of the pluvial playa which has beensingled out for its outstanding potential for thestudy of late prehistoric palaeoenvironments andcultural adaptation in the area (McHUGH 1980).Its unique palaeoclimatic archive and richarchaeological heritage has attracted scientificmissions since the 1970s (WENDORF & SCHILD1980. KUPER 1981. EL-BAZ & MAXWELL, eds.,1982. PACHUR & RÖPER 1984. KRÖPELIN 1987).

Playas which are not groundwater-fed providesome of the most reliable palaeoclimate archivesbecause their occurrence is solely caused by localrains and drainage, contrary to the palaeolakes inthe Sahara which are always dependent on specificconditions such as groundwater-buffering, theeffect of which is often difficult to assess. Theevidence from Wadi Bakht strongly suggests thatthe early- to mid-Holocene playas of the GilfKebir were such “dry playas”, i.e. temporarypools containing water for days, weeks or monthsonly.

The position of the playa and the thick valleyfill are due to the previous complete blockage ofthe valley by a still partly preserved fossil dunethe northern part of which is covered by a recentfalling dune [Fig. 3]. The longitudinal profile ofthe talweg is almost horizontal over 3 km abovethe dune barrier with a gradient of only 4.15 m/950 m, i.e. 0.0044 (KRÖPELIN 1989). This indicatesthat the local base level remained stable over along period

The playa sediments are exposed over an areaof at least 65,000 m2. Mud crack polygons ofdifferent orders with diameters of 1-10 m runthrough the playa surface and attest to the finaldesiccation. The northeast corner of the playa waslowered about 3 m by increased deflation causedby turbulences at the foot of the falling dune. Hereseveral metre-high yardangs are preserved atdifferent levels. They provide evidence of rootsand stems that indicate small trees and reed beltsof Phragmites communis along the former shore ofthe playas.

The central part of the playa is covered by a400 m2 sized, 3 cm thick layer of curled clay flakes,the remains of the last noteworthy rains whichprobably fell during the 1930s. Similar depositswere found 11 km northward, in the so-called“Winkelwadi”, the northern branch of WadiMaftuh (PACHUR & GABRIEL 1980). These claysediments are evidence of secular downpourswhich occur even during the present hyperaridclimate. Water columns of more than 2 m can beattained in such short-lived rain-pools (KRÖPELIN1989). Cemented-in petrol cans in contemporaryclay flake playas south of Wadi el Akhdar areproof of their recent age.

Prehistoric remains on the slopes of the blockingdune are remarkably rich for the Gilf Kebir area.The dense layer of countless stone artefacts hascontributed to its preservation. However, thequantities are not to be compared with theabundance of archaeological material on hun-dreds of huge dune habitats (“Siedeldünen” inGerman) along the Lower Wadi Howar inNorthwest Sudan that resulted from continuoussettlement over centuries or millennia (KRÖPELINin print).

3. Section Wadi Bakht 82/13

The palaeoclimatologically most significant playadeposits in the Gilf Kebir are exposed in anerosion gap [Fig. 4]. This minor channel incisionin the eastern edge of the playa deposits and theblocking dune forms a step-like gorge whichdescends 9 m from the horizontal playa surfaceto the level of the original valley floor over ahorizontal distance of 20 m only. At this site, thedune barrier was breached, thus terminating theformer local base level and re-establishing normalwadi flow down the valley.

Even after more than two decades of extensiveQuaternary-science field research, the geologicalmain section 82/13 in Wadi Bakht may still beconsidered the most indicative key section for theclimatic evolution of the central Eastern Saharaduring the past 12,000 years. It provides thethickest and most detailed sedimentary archive ofthe entire Western Desert of Egypt (KRÖPELIN1993a). Such palaeoclimatic data from remote,

54 Stefan Kröpelin

highly continental areas are essential to obtain acomprehensive understanding of past climatesystems, without which evaluation and calibrationof numeric climate models and reliable forecastingof future conditions are not possible. They alsoprovide the evidence on the climatic and ecologicalframework of prehistoric occupation in arid areasand enable the comparison of geological andarchaeological chronological data to reveal con-formities or discrepancies between climatic con-ditions and human agency.

The base of the section consists of wind-blownsands which formed the western slope of the fossilblocking dune. They consist of thinly laminatedcross-bedded medium sand with distinct grain-size differences between individual lamina whichpoint to varying wind speeds at the moment ofdeposition. Along the former shore, the aeoliansands are slightly consolidated by bonding agentsinfiltrated during the later lake phases [Fig. 5a].The dune sands are succeeded by reddish yellowsands which are referred to as “playa sands” inthe sense of sands that have been deposited inthe playa area (KRÖPELIN 1987). The medium tocoarse grained sands are well consolidated due toa pelitic matrix.

The following deposits are composed of thinalternating layers of siliceous mud and partlycemented playa sands. Microstratigraphical fea-tures and the thinness of the clayey-silty mudlayers indicate the event-controlled character ofthe pelitic layers as well as their generally shortduration of sedimentation [Fig. 5b]. Both themud deposits and the intermediate playa sandsare non-calcareous and biologically virtuallysterile, with the exception of rare unspecificgrass phytoliths. This is contrast to the carbonatic,highly fossiliferous sediments that are typicalof the permanent palaeolakes in the more south-erly regions of the Eastern Sahara (GABRIEL& KRÖPELIN 1983. KRÖPELIN & SOULIE-MÄR-SCHE 1991. KRÖPELIN 1999. HOELZMANN et al.2001).

Load casts are an important argument in favourof the playa sands having been deposited in a wetenvironment. The pressure marks are evidence ofthe plasticity of the still water-saturated peliticlayers during or shortly after sedimentation. Curledclay flakes, on the other hand, which would beevidence of complete desiccation, do not occur atall. Therefore, the playa sands can not beinterpreted as wind-blown sand sheets that were

Fig. 2 Wadi Bakht. View from the southern rim on the upper course with the residual blocking dune which caused theaccumulation of still-water deposits during the early and mid-Holocene. Note the almost horizontal playa surface beyond theformer dune barrier and the regular gradient below. Width of valley at the dune: 650 m, difference in elevation between sandstoneplateau and playa surface: 110 m. Trucks for scale.


Fig. 3 The recent sharp-crested falling dune below the northern rim is superposed on the ancient blocking dune; elsewhere,its upper parts have been deflated. The site of section Wadi Bakht 82/13 is located in the centre. The light patch on the playasurface results from thin curled clay flakes, which probably date from the last rains worth mentioning during the 1930s.

Fig. 4 Wadi Bakht, site of section 82/13. The gorge exposing the outstanding natural outcrop was caused by the breachingof the ancient dune barrier at about 4800 bp (3300 BC) and subsequent fluvial erosion. The playa sediments dip up-valley nearthe former shore of the playa lakes and are more than 8 m thick here.

56 Stefan Kröpelin

Fig. 5a Close-up of the aeolian layersof the Terminal Pleistocene blockingdune showing calcareous stem and rootcasts of reed and termite burrow thatdate from the early Holocene. Thefinely laminated structure is partlydisturbed by bioturbational features.Scale: 10 cm.

a

b Fig. 5b Detail of playa layers, each ofwhich records a single major rainfallevent. Load casts at the basal sides ofthe intermediate sandy layers indicatethat they have been deposited on thestill water-saturated mud layers. Scale:10 cm.

Fig. 6 Section Wadi Bakht 82/13 withuncalibrated charcoal radiocarbon datesand main stratigraphic units.


deposited after total desiccation of the playa lakes.Even close to the former lake shore, the overallthickness of the playa strata exceeds 8 m [Fig. 6].These pelitic layers occur 89 times within thesection and have an average thickness of only14 mm, disregarding the singular, more than 1 mthick top layer [Fig. 7].

The rain pools or playa lakes in which the fineparticles of the incoming were deposited understill-water sedimentary environments were fedexclusively by local rainfall and run-off (KRÖPELIN1987. 1989). These were ephemeral lakes thatcontained water for weeks or months at most.Evidence for this interpretation includes sedimen-tological parameters such as the thinness of themud layers, the lack of any aquatic organisms orplant remains requiring permanent water, theextremely high rates of potential evaporation atthis geographical position, and the exclusion ofgroundwater-buffering because of the enormousdepth of the regional aquifer. According to thetopography of the valleys and information fromthe bore hole at “Eight Bells” 40 km southwest– the only drill site in the Gilf Kebir region –, theregional groundwater level appears to lie severalhundred meters below the playa surface.

4. Chronology

In agreement with all available palaeoclimate data,it is beyond any doubt that at least the lower partsof the dune which hydrologically blocked the entirevalley of Wadi Bakht can be attributed to theterminal Pleistocene hyperarid phase, even if thereare still no direct thermoluminescence dates.

Considering the onset of playa sedimentationin the neighbouring Wadi el Akhdar (KRÖPELIN1989) and the general onset of humid conditionsover the entire Eastern Sahara at approximately9,300 bp (8,400 BC – KRÖPELIN 1993b. HAYNES2001) there are strong arguments to place thebeginning of aquatic sedimentation in Wadi Bakhtto the same stage. It is obvious that the earliestplaya sediments must have been deposited in thecentral and deepest part of the playa basin.Therefore they are not exposed in the naturaloutcrop which is situated at least several deca-metres east of the presumed centre. This inferenceis in accordance with the estimated thickness of

the hidden sediments in view of the swiftlydecreasing dip of the strata towards the bottomof the playa basin, the mapped extent of the playasurface, and the average sedimentation rate.

Tiny pieces of charcoal can be observed in manyof the exposed pelitic layers. They are from formerhuman activities at the lake shore, or from wind-blown burned material resulting from natural oranthropogenic shrub or grass fires on the sur-rounding plateau or in the valley. Due to the lackof appropriate charcoal samples and restraints indating techniques on small quantities of carbonby conventional methods, the first establishedchronology included long chronological gaps andhigh sigma values (KRÖPELIN 1987. 1989). NewAccelerator Mass Spectrometry (AMS) dates ofvery small charcoal fragments from the mostinteresting stratigraphical positions substantiallyimproved the temporal resolution in the middlepart and particularly the top of the section 82/13 (LINSTÄDTER & KRÖPELIN 2004).

Many more and bigger charcoal samples couldbe retrieved for high-resolution dating at sub-century or even decadal scale by gradually cuttingback the exposed section by aid of a mechanicalexcavator. This would have meant, however, thepartial destruction of this outstanding locality,which is a candidate for the nomination as a“Geological Monument” in the Global IndicativeList of Geologically Relevant Sites of UNESCO(KRÖPELIN 1996). The location may even becomeone of the highlights of a proposed protected areawithin UNESCO’s recent “GeoParks” initiative(KRÖPELIN 2000). For all future geological workin fragile sites like the Wadi Bakht, it will beimportant to find a well-balanced compromisebetween scientific interest and the preservation ofnatural heritage.

Lacking any stratigraphical unconformities, thesedimentary sequence of playa section 82/13 hasrecorded every major rainfall event in the GilfKebir during the entire early and mid-Holocene,i.e. over a period of 5,500 calendar years (KRÖPE-LIN 1989). The AMS date of 5,405 ± 75 bp (4,210 ±110 BC; Erl-2873) marks the onset of the compact,120 cm thick near-top layer, that differs from themuch thinner layers below because of its thicknessand the lack of interlayering sand. This sharp

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boundary clearly indicates a major change in theprecipitation regime (KRÖPELIN 1987).

The definite end of playa-type accumulationcan be put to about 4,800 bp (3,300 BC). Thisestimate is based on charcoal samples from thethree most recent hearths on the playa surface,

located very close to the section, that yielded datesof 4,880 ± 390 bp (3,590 ± 480 BC; KN-3182), 4,820± 60 bp (3,590 ± 70 BC; KN-3098) and 4,770 ±130 bp (3,530 ± 140 BC; KN-3184) (KUPER 1989.KRÖPELIN 1989. See also LINSTÄDTER Feuerstel-len, this volume).

Fig. 7 Section Wadi Bakht 82/13 during sampling in January 1982 (left) and its stratigraphy with radiocarbon dates and identifiedplant remains (right). Black signatures designate playa mud deposits, dotted patterns indicate sandy layers.

Z = Holzkohlen von Ziziphus sp.T = Holzkohlen von Tamarix sp.M = Holzkohlen von Maerua crassifolia

5405 ± 75

6200 ± 10006080 ± 420

6870 ± 65

7145 ± 1007405 ± 125

7980 ± 90

8200 ± 500

Wadi Bakht 82/13

Gilf Kebir


5. Artefact and plant content

Stone artefacts, mainly unmodified flakes, havebeen retrieved in section Wadi Bakht 82/13 andadjacent strata down to the base at 8 m depth. Ofparticular interest are fragments of pottery atdepths up to 5 m (KRÖPELIN 1989). They indicatethe presence of humans around the playathroughout the millennia which is in agreementwith the occupation history of the neighbouringWadi el Akhdar (see Fig. 1), where similar pottery(KUPER 1981) has been dated by associated charcoalto 7,670 ± 75 bp (6,520 ± 70 BC; KN-2934) and7,700 ± 60 bp (6,530 ± 70 BC; KN-2878. The freshedges of most of the lithic artefacts from section82/13 indicate their short duration of exposureafter production and their fast deposition withinthe playa mud.

Owing to the absence of pollen, diatoms andidentifiable opal phytoliths, small pieces of charcoalprovide the essential information on the pastwooden vegetation in Wadi Bakht. Even if mostsamples of the recent collection from section 82/13 can not be identified to the species because oftheir very small size (<< 1 mm), there is evidenceof Tamarix sp., Ziziphus sp. and Maerua crassifoliaat different levels (determination: St. Nußbaum/Köln). These species fit into the taxonomic co-position of earlier anthracological identifications(NEUMANN 1989). Remarkably, Tamarix is foundthroughout the section from a depth of 700 cm (c.8,200 bp / 7,200 BC) up to 160 cm below the top(c. 5,700 bp / 4,600 BC; see Fig. 7). The new finddates back to c. 6,900 bp (5,750 BC) while thepreviously undated find of Ziziphus can now beput to about 5,500 bp (4,400 BC) based on theirstratigraphical position.

Tamarix is typical for saline positions on clayeysoils and close to open water where it usuallyinhabits the banks. Tamarix articulata which cansurvive periods of drought is the most likelyspecies for the locality. Today, the sahelian speciesZiziphus mauritiana densely covers the wadichannels of the Ennedi plateau in Northeast-Chad. This remote plateau situated about 700 kmto the southwest still receives monsoonal summerrains of 50-150 mm/year and morphologicallyresembles the Gilf Kebir. It may therefore beconsidered a “living Gilf Kebir” and serve as the

best modern analogue to the early to mid-Holoceneclimatic and environmental conditions in southwestEgypt (GEORGE & KRÖPELIN 2000).

The joint occurrence of Ziziphus, Maerua andTamarix in 380-430 cm depth indicates somewhatwetter conditions around 6,870 ± 65 bp (5,750 ±60 BC; Erl-2874). Maerua crassifolia probably haspopulated the slopes in low density (comm. St.Nußbaum/Köln). In this context, however, obser-vations in the desert south of Cairo with anannual precipitation of 25 mm have to be takeninto account (WALTER 1979). Given that 40 % ofthe rainfall there collects in the deepest parts ofthe terrain which make up about 2 % of the area,the plants at these privileged sites receive thesame amount of water as those on the plain wouldonly obtain with a rainfall of 500 mm/year. Thefact that the vegetation on the bottom of WadiBakht was rather limited thus indirectly suggeststhat the wooden plant cover on the surroundingplateau was very sparse.

6. Palaeoclimatic inferences

The stratigraphic and sedimentological evidencefrom section Wadi Bakht 82/13 in combinationwith the previously reported conventional radio-carbon dates on charcoal has been used to developthe following climatic model (KRÖPELIN 1987.1989). Until about 9,300 bp (8,400 BC) a hyperaridclimate, similar to the present one, must haveprevailed because of the complete lack of anysedimentological or biological indicators. Theperiod between 9,300 bp (8,400 BC) and approxi-mately 5,400 bp (4,300 BC) had an arid climatewith rare heavy rainfalls (on an average fourevents per 100 years) enabling incipient soil forma-tion and sparse plant growth in the areas aroundthe temporary rain pools. Between 5,400 and c.4,500 bp (4,300 – 3,300 BC) conditions tendedtoward moderate aridity, in agreement with theoccurrence of slightly more demanding tropicalplant species (NEUMANN 1989) and the apparentmain phase of Neolithic settlement in the GilfKebir (KUPER 1988. SCHÖN 1996).

In a palaeoclimatic interpretation of the thindiscontinuous pelitic layers in the lower parts ofthe section 82/13 and the thick continuous pelitic

60 Stefan Kröpelin

layer in the top, it had been concluded that aregime of secular monsoonal-convective summerrains triggered by exceptional northward surgesof the surface intertropical convergence zoneprevailed from about 9,300 – 5,400 bp (8,400 –4,300 BC). From 5,400 – 4,500 bp (4,300 – 3,300 BC),this phase was succeeded by a west-wind inducedtype of climate with occasional winter rainfallwith steady rains. At about 4,800 bp (3,600 BC) thedeposition of playa sediments stopped. Thissuggests that the dune blocking Wadi Bakht wasbreached at that time owing to exceptionally highwater pressure or level pointing to a rainfalloptimum, and/or a unique millennial rainfallevent (KRÖPELIN 1987. 1989).

The new evidence of Ziziphus, Maerua andTamarix, as already mentioned above, at around6,870 ± 65 bp (5,750 ± 60 BC; Erl-2874) points to aslightly more humid episode and might correspondto isotopic data on increased rainfall at that periodfrom Northwest Sudan (RODRIGUES et al. 2000).

Most crucial is the new AMS date of 5,405 ± 75 bp(4,210 ± 110 BC; Erl-2873) that signifies the time ofthe only major change in the climatic system tobe deciphered from the stratigraphic record ofWadi Bakht. Exclusively at this time, there occurreda sharp change in the depositional system that isthought to mark the transition from an Africanmonsoonal type of climate to a Mediterraneanwinter rainfall pattern with quantitatively loweramounts but more continuous rainfall duringwinter. A direct correlation of the 5,400 bp(4,500 BC) transition in the Gilf Kebir with thelowlands of Southwest Egypt is not viable becauseof the lack of comparable climate archives outsideof the groundwater-supported oases that are notsuitable for comparisons with the exclusively rain-fed “dry playa” of Wadi Bakht (KUPER 1989.NICOLL 2001. SCHILD & WENDORF 2001. WEN-DORF & SCHILD 2001. GEHLEN et al. 2002). Ageneral climatic instability around 6,000 bp(4,900 BC; i.e. over a period of ± 500 years), how-ever, has also been stated for other regions ofAfrica and the Sahara (e.g. HOELZMANN et al.1998. GUO et al. 2000).

Regarding the precipitation rates, stratigraphicand archaeobotanical analyses of the playa depositslead to the conclusion that the climate in the

central part of the Eastern Sahara has beenrelatively arid during the entire Holocenecompared to the western and central Sahara at thesame latitude. Even for the humid optimum of theearly Holocene, the best-estimate maximumprecipitation ranges between 100-150 mm per year(KRÖPELIN 1987. PETERS 1988. NEUMANN 1989).Nevertheless do these low figures of rainfallequate to more than 50 times the present rates ofprecipitation – a ratio which is in line with theone established for the entire Saharan belt alongthe Tropic of Cancer with a decreasing gradientfrom Mauritania to the Nile (PETIT-MAIRE &KRÖPELIN 1991). They also match with the well-established south-north gradient of decreasingprecipitation that is supported by extensiveevidence between the Wadi Howar at 17° N andthe Gilf Kebir (HAYNES 1987. NEUMANN 1989.KRÖPELIN 1993b). In any case did the moderatelymore humid climatic conditions allow for settle-ment, gathering activities, nomadic cattle-keepingand far-reaching cultural contacts of prehistoricpeople, and for faunal migrations (HASSAN 1986.KUPER 1995. KUPER & KRÖPELIN in prep.).

The earlier notion of much more pronouncedpluvials with annual rainfall of up to 600-800 mmin the Western Desert of Egypt adopted by someauthors (McHUGH 1974. WENDORF et al. 1987) hastherefore been an overstatement. Such maximumestimates which were solely based on the inter-pretation of faunal remains or rock art are notnecessary to explain the known facts establishedfor the late Quaternary wet phases in the region(KRÖPELIN & PACHUR 1991).

Modern weather sequences in desert regionsmay help to explain the climatic conditions in thecentral Eastern Sahara at the end of the postglacialhumid period (GEB 2000). Even if many of thewaves and cyclones are weak or not evendeveloped compared to the mid-Holocene condi-tions because of insufficient humidity at theground and in the lower troposphere, the dynamicsof present-day weather sequences are apparentlysimilar to those of the middle Holocene. They arecontrolled by(a) a temporary northward extension of monsoonal

rain,(b) inter-seasonal rain period at the Red Sea, and(c) an advance of winter rains as far south as

latitude 20° N.


Such a combination of tropical and non-tropicalweather mechanisms may have resulted in apartial overlapping of areas with both summerand winter rain (GEB 2000). Winter precipitationat that time was presumably of greater extent,abundance and duration, due to increased interiormoisture sources. In-situ evaporated moisture waspartially recycled to the ground and not completelyblown out to the Atlantic as today. Such sourcesof moisture, advection of water vapour from thesouth, and advection and production of upperwaves or vorticity are major factors.

There is general agreement that arid to hyperaridconditions in the Western Desert of Egypt beganaround 4,500 bp (3,300 BC – RITCHIE et al. 1985.PACHUR & KRÖPELIN 1989. KUPER 2002). Thisdesiccation of Southwest Egypt, the highlycontinental centre of the Eastern Sahara, has beenan irreversible process. It was the beginning of thecontinuous southward shift of the desert boundarywhich roughly corresponds to the 100 mm isohyets,progressing with an average rate of about 35 kmper 100 years or 1° latitude per 300 years (HAYNES1987. KRÖPELIN 1993b).

There was no ecologically significant revival ofthe rains over the Egyptian desert after the endof the “Neolithic” wet phase. The Nile valley wassettled during the Predynastic of Egypt, an intervalbetween c. 4,000 and 3,000 BC, by refugees fromthe Eastern Sahara fleeing from mid-Holocenedroughts; this was the very time when the identityof Egyptian society was forged (HASSAN 1988.KUPER 2002). The coincidence of the desert landsbecoming uninhabitable, and the forming of thePharaonic Nile culture, now confirmed by geo-archaeological field evidence, has been a much-discussed historical and philosophical issue(BUTZER 1959).

7. Geoarchaeological implications

From a geoarchaeological perspective, some of themajor questions concern the hydrologic and en-vironmental conditions for prehistoric settlementduring different occupation phases and possibleclimatic explanations for changes in land-use andhumanactivities. Even if there is indication of ab-solute high-stands with a maximum water depth

of 7 m in the deepest part of the dune-blockedbasin and water volumes of several 100,000 m3,the large majority of the playa lakes has beenmuch shallower and less voluminous. Consideringthe potential evaporation rates in the range ofseveral metres per year, the playa lakes must havedried out within several weeks or months afterthe rainy season or single rainfall events as it isthe case today in the rain-fed pools of NorthernKordofan in Sudan or the Ennedi region ofNortheast-Chad that receive comparable amountsof precipitation. Therefore, the water resources inWadi Bakht have never been sufficient for per-manent settlement in the connotation of years ordecades or even generations with a few possibleexceptions that may have occurred during themid-Holocene playa phase when some open watermay have persisted until the onset of the rainsin the following year.

Nevertheless, it cannot be totally excluded thatthere were some small rock pools or gueltas inthe Gilf Kebir at the shady steep-sided heads ofsecondary canyons that provided limited quantitiesof water all over some of the rainier years. Suchpotential localities have been observed in one ortwo amphitheatre-like settings in the northernbranch of Wadi Maftuh (Winkelwadi). However,no such sites have been located in the Wadi Bakhtor its tributary valleys. As a conclusion, it is mostlikely that settlement during the entire early andmid-Holocene, or what has been called the “Neo-lithic wet phase”, was possible only temporarilyduring summer and autumn in the early Holocene,and during the winter months during the middleHolocene.

Because of the geological and hydrologicalpreconditions which prevented water storage insubsurface layers of the playa or on the plateau,there were also little chances to bridge the seasonalor episodic occupation from one year to the otherby the use of wells. Accordingly, no signs of wellconstructions have been observed over the entireWadi Bakht area.

The unsuitable hydrological conditions for long-term stationary stays in Wadi Bakht also applyto the other valleys of the southeast Gilf Kebir.They differ from the permanent availability ofwater in the neighbouring Jebel Ouenat some 80km to the south. There, two permanent springs

62 Stefan Kröpelin

exist to the present day due to the inselberg’salmost 1000 m higher elevation which attractsmore rainfall and a hydrogeologically supportiveplutonic structure which prevents infiltration,accelerates run-off and provides for pools deeplyhidden below giant granite boulders that sig-nificantly reduce the evaporation. It may thereforebe assumed that the seasonal, periodic or episodicinhabitants of the Gilf Kebir were based at JebelOuenat (22° N – 25° E) with its permanent waterpoints and lush valleys. Especially Karkur Talhin its northeast part which drains towards the80 km distant southern tip of the Gilf Kebir musthave provided optimal conditions for permanentsettlement all over the early and mid-Holocene,as is well documented by the abundant rock artthere (WINKLER 1939. RHOTERT 1952).

A remarkable fact of the prehistoric occupationof the Gilf Kebir is the existence of a Late Neo-lithic in the 4th and 5th millennium BC. Duringthat period, which was already marked byincreased aridity, human settlement elsewhere inthe Egyptian part of the Eastern Sahara hadalready moved to the oasis depressions of Kharga,Dakhla, Farafra and Bahariya, or to the Nile ornorthern Sudan (KUPER 1988. KUPER & KRÖPELINin prep.). In the southeast Gilf Kebir, however, theupper reaches of Wadi Bakht and neighbouringWadi el Akhdar were still used by prehistoricman, apparently even more intensely than duringthe generally more favourable early Holocene(SCHÖN 1996).

Despite their poor preservation, the faunalinventories of Wadi Bakht show a clear differencebetween these phases (PETERS 1988. VAN NEER& UERPMANN 1989). While the faunal remainsfrom the earlier phase are restricted to wild gamesuch as antelope and gazelle, the later phase alsoincludes sheep or goat and cattle. This fact, inconnection with archaeological indications, led tothe inference that the first inhabitants were hunter-gatherers, and that the later ones were pastoralgroups, for the needs of which the altered con-ditions on the plateau provided a suitable habitat(LINSTÄDTER & KRÖPELIN 2004).

An expansion of the settlement to the plateauduring a phase of general climatic deteriorationin the Eastern Sahara due to the retreatingmonsoonal rains (HAYNES 1987. KRÖPELIN 1987.

PACHUR & KRÖPELIN 1989) seems surprising atfirst glance but may be explained by a model ofclimatic control that is supported by newchronological and archaeological evidence(LINSTÄDTER & KRÖPELIN 2004). Apparently, thequantitatively more significant monsoonal summerrains that were characteristic for the early Holocene(9,300 – 5,500 bp / 8,400 – 4,400 BC) typically fellduring the day-time and resulted in lower amountsof grass growth on the plateau than the presumedwinter rains characteristic for the terminal phaseof the Holocene pluvial (5,500 – 4,800 bp / 4,400 –3,500 BC).

The two precipitation regimes – the short butmore violent downpours of monsoon-driven sum-mer rains, and the rather continuous winter-rains– certainly had a different effect upon surface run-off, evaporation and soil infiltration of the water.The winter precipitation, which typically fell atnight because of the nocturnal temperaturereduction, was subjected to clearly lower evapora-tion rates than the monsoonal summer rains andresulted in a better moisture penetration of thesoil. The steady winter rains also had substantiallylower surface run-off rates. The combination ofthese factors seems to have had a decisive impacton the amount of water available for the plantsand especially on the growth of grass. Accordingly,the total annual precipitation seems to have beenless important than its temporal distribution,particularly for the exploitable grasslands on theextended plateau surfaces of the Gilf Kebir.Concerning the growth of grass, the orthic solon-chaks on the level ground of the plateau (ALAILY1993) apparently responded more favourably tothe winter rains than the cambic arenosols ortakyric yermosols on the valley floor that werederived from fluvial sands and debris, or from thepelitic playa deposits.

As a conclusion, the role of seasonal rainfalldistribution on cultural landscapes in arid regionsseems to have been more significant than the sumof annual precipitation. This assumption involvesan aspect neglected in existing numeric models ofsubtropical palaeoclimates. The unique climatictransition at about 5,500 bp (4,400 BC) appears tohave induced a fundamental environmental changethat resulted in different patterns of humanbehaviour, economy and land-use in the canyon-


like valleys and on the surrounding plains on theplateau.

Despite a marked trend towards increasingaridity in the region, the mid-Holocene winterrainfall pattern has apparently produced morefavourable conditions than the monsoonal shortsummer precipitation did during the precedingmillennia. It facilitated the use of the plateau forgroups with pastoral nomadic economies, untileven these last retreat areas had to be given upowing to the final desiccation of the EasternSahara around 4,500 bp (3,300 BC).

Acknowledgements

To name but a few, I thank my long-time friend and desertcompanion Rudolph Kuper, the initiator and director of theprojects in Egypt within the research centres B.O.S. (1980-1992)and SFB 389 ACACIA (1995-2007), for his enduring organi-zational, logistic and scientific support, Baldur Gabriel andKristin Heller for the sampling of the charcoal remains in 1999,Stefanie Nußbaum for their identification, and Hans Back withwhom I reached the Gilf Kebir for the first time some 25 yearsago. Field seasons since 1980 were undertaken in fruitfulcollaboration with the Egyptian Supreme Council of Antiquitiesand the General Petroleum Company of Egypt.

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Documents

Kröpelin Wadi Bakht-Beitrag - Universität zu Köln 2005 AP 16 Wadi … · The detailed palaeoclimatic record of Wadi Bakht (Gilf Kebir, Southwest Egypt) and interpretation of associated