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Geomorphic Indicators of Holocene Winds in Egypt's Western Desert

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Geomorphic Indicators of Holocene Winds in Egypt's Western Desert Geomorphology 56 (2003) 155–166 www.elsevier.com/locate/geomorph Geomorphic indicators of Holocene winds in Egypt’s Western Desert Ian A. Brookes* Department of Geography, York University, Toronto, ON, Canada M3J 1P3 Received 1 July 2002; received in revised form 6 January 2003; accepted 14 January 2003 Abstract Geomorphic mapping of Egypt’s Western Desert from LANDSAT-MSS images reveals oriented aeolian landforms that record, in part, Holocene winds. Wind directions reconstructed from these landforms indicate the dominance of N–S airflow from 30jNto20jN, turning clockwise southward to NE–SW, conformable with modern circulation. A second direction appears over western Egypt, W between 30jN and 26jN, NW between 26jN and 20jN. Cross-cutting aeolian landforms show that W/ NW winds are older than the N/NE winds. Geomorphic evidence, abundant south to 26jN and less abundant to 20jN, also indicates that W and NW winds were early Holocene ‘palaeowesterlies’. Some evidence also indicates that they extended eastward to at least 30jE, perhaps to the Red Sea. These winds steered moist Atlantic/Mediterranean air masses to Egypt, sustaining early Holocene lakes and playas north of the limit of tropical monsoonal rainfall at 20jN. Upon aridification, beginning after 5 kyr BP, yardangs oriented west to east were eroded in early Holocene basinal sediments in western Egypt, indicating that these winds continued there for 1–2 kyr, until 3–4 kyr BP. Optically stimulated luminescence (OSL) ages of surface sand sheet in southern Egypt indicate that the present north–south winds were established ca. 3–4 kyr BP, at the same time as the northern savanna boundary was stabilized at its present position. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Egypt; Sahara; Holocene; Aeolian geomorphology; Palaeoclimate; Palaeowinds 1. Introduction Hassan, 1986; Brookes, 1989a; Haynes et al., 1989; Neumann, 1989; Kro¨pelin, 1993; Street-Perrott and Palaeoclimatic reconstructions in NE Africa for the Perrott, 1993; Pachur and Wu¨nnemann, 1996; Stokes period of the last glacial to the present have been based et al., 1998; Gasse, 2000, 2002; Hassan et al., 2001; on evidence from (i) lacustrine and aeolian sediments Swezey, 2001). Interpretations converge on a cold, dry, and their physical and chemical properties, (ii) pollen windy last glacial maximum (15–20 14C kyr BP), spectra and other palaeobiological indicators within changing through an erratic transition to a multiphase, these sediments, and (iii) archaeological remains. perhaps still cool, wetter, early Holocene (10–5 kyr Chronology has been supported by radiocarbon and BP), with pronounced arid intervals, the ‘‘African optically stimulated luminescence (OSL) ages (e.g., Humid Period’’ of DeMenocal et al. (2000), then to a drier and windier later Holocene (5 kyr BP to present). * Tel.: +1-416-265-8318. These empirical studies have spawned theoretical E-mail address: [email protected] (I.A. Brookes). research into climate change in North Africa, focussed 0169-555X/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-555X(03)00076-X 156 I.A. Brookes / Geomorphology 56 (2003) 155–166 on modelling of atmospheric and surface responses to arid core of the Western Desert is estimated at between orbitally forced insolation. The purpose has been to 2500 and 5000 mm (Shahin, 1985). Gasse (2002) simulate atmospheric processes and surface feedbacks gives an excellent summary of North African clima- responsible for the presence and character of Late tology with references. Pleistocene and Holocene lakes and playas in the Modern wind patterns in the Western Desert com- now hyperarid region of the Sahara and its arid borders prise two fields. A northern zone of variable winds (e.g., Kutzbach et al., 1993, 1996; Claussen and extends south from the Mediterranean coast, more Gayler, 1997; Kutzbach and Liu, 1997; Texier et al., westerly and stronger in winter (November to April), 1997; Ganapolski et al., 1998; de Noblet et al., 2000; bringing rain in cyclonic storms to about 25jN. In Doherty et al., 2000). Wind patterns, however, are not summer (May to October), weaker westerly winds usually considered in either empirical reconstructions reach only to about 30jN and yield no rainfall. Over or simulation models, and then only as output rather a more extensive southern zone of the Western Desert than input (e.g., Kutzbach et al., 1993). This paper between 30jN and 20jN, northerly winds dominate reports geomorphic evidence of wind patterns over an and bring no rainfall at either season. They strengthen area of Egypt encompassing 8j of latitude and longi- in winter, often meeting westerly winds along rain- tude, patterns that are, in part, of Holocene age, and bearing fronts which can affect any part of the desert at which provide input data to palaeoclimatic models. this season. Southward, these northerly winds veer northeasterly around the eastern limb of the subtropical anticyclone. 2. Study area The study area encompasses 70% of the Western 3. Methods Desert, about half of Egypt’s area (Fig. 1). Physio- graphically, the northern half of this desert is a low- The study area is covered by a set of 24 LANDSAT- relief, cuesta-form plateau developed across north- MSS images acquired from 1972 to 1976. These are dipping Palaeogene limestones, sloping south to north false-colour composites of bands 4, 5, and 7, processed for 550 km from f 500 to f 200 m elevation. Pro- at 1:250,000 scale by Earth Satellite Corporation longed wind erosion has formed fields of yardangs (ESC) and held at the National Air and Space Museum over much of it (Brookes, 2001). Its southern boundary (both in Washington, DC). Images were enhanced is a 200- to 300-m-high escarpment which overlooks a using GEOPIC, developed at ESC, a procedure which low-relief mosaic of plains, low cuestas, and isolated emphasized subtle tonal variations of surface materials hills at 100–250 m, developed on north-dipping and which revealed linear features, such as roads and Mesozoic sandstones extending over 500 km south- dunes, below the 79-m pixel resolution (El-Baz, ward into Sudan. The geomorphic evolution of this Centre for Remote Sensing, Boston University, written southern desert has been interpreted by Haynes (1982) communication, 2001). Some sense of resolution of and Maxwell and Haynes (2001). the images can be gained from the visibility of roads Climate in the study area is arid, with a large roughly 20 m wide (including bordering disturbed hyperarid core area where the few meteorological ground), and from visible barchans of comparable stations at oases record practically no rainfall (Fig. width that were visited in the field. 1). Over the wider region rain does fall, however, Aeolian erosional landforms mapped from these mainly from cyclonic winter storms, tracking SE images and recorded in the field for directional infor- across the Mediterranean Sea to meet westerly waves mation are discussed in the following. crossing the Sahara from the tropical Atlantic. A Aeolian erosional lineations (AELs):Theseare concise meteorological perspective on winter rainfall parallel, unstreamlined, ridge-trough sets, as well as across North Africa is given by Geb (2000). Rare fields of streamlined forms (yardangs). Those AELs summer rains mark exceptional northern excursions of capable of resolution on the LANDSAT images are all monsoonal convective systems, which are normally in bedrock, whereas smaller ones in unconsolidated restricted to south of 21jN. Evaporation in the hyper- sediments were recorded in the field. Because of the I.A. Brookes / Geomorphology 56 (2003) 155–166 157 Fig. 1. Egypt showing major physiographic features—plateaus, scarps (toothed lines), oasis depressions, dune fields (stippled), from El-Baz and Wolfe (1982); mean annual isohyets (mm) after Haynes (1987); rectangles (a–e) represent sample areas shown in Fig. 2a–e; latitude/longitude shown at 5j intervals; scale bar 200 km. 158 I.A. Brookes / Geomorphology 56 (2003) 155–166 time required to form these features in bedrock, bed- taken either from simple linear dunes, or from com- rock AELs are interpreted as of mainly pre-Holocene pound linear dunes only where they are parallel to age (Brookes, 2001). Within sets of bedrock AELs, nearby simple ones, which therefore allow only one however, features such as faceting of upwind faces can wind direction to be inferred. reflect more recent wind erosion. Also, bedrock AELs In western Egypt, where geomorphic features indi- are sometimes parallel to lineations in unconsolidated cate two palaeowind directions (W/NW and N/NE), Holocene sediments and can then be argued to reflect linear and transverse dunes are combined in fields at least some Holocene wind erosion. where transverse dunes perpendicular to the W/NW Aeolian scour zones: These are swaths of bedrock palaeowinds occur as ‘barbs’ attached to the flanks of or surficial deposits, < 10 to hundreds of kilometres linear dune ‘shafts’ lying parallel to N/NE palaeowinds long and < 1–5 km wide, where varnish and other (Sections 4.1.2–4.1.4 below). On the other hand, in patina has been erased or prevented from forming, and the central and eastern parts of the Western Desert, which therefore appear on the images lighter in colour where only one palaeowind direction is indicated, this than adjacent terrain, with sharp boundaries against it. compound dune form is absent and the two types, In the lee of cliffs, scour zones reflect wind accelerated transverse and linear, are less common and occur through ravines, but, where not topographically local- separately. ized, they reflect regional wind structure. These scour As with AELs, dunes (possibly) and ‘draa’ (defi- zones, too, probably record a longer period of wind nitely) represent a longer geomorphic history than the erosion than the last 10 kyr; but their parallelism with Holocene (Embabi, 1998). However, where smaller, mobile features such as small dunes, and with yard- more mobile, linear dunes and the mobile superstruc- angs in Holocene sediments, indicates that, in part, tures of larger forms are parallel to other mobile they reflect Holocene wind directions.
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