Elsevier

Geomorphology

Volume 56, Issues 1–2, 15 November 2003, Pages 155-166
Geomorphology

Geomorphic indicators of Holocene winds in Egypt's Western Desert

https://doi.org/10.1016/S0169-555X(03)00076-XGet rights and content

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 30°N to 20°N, turning clockwise southward to NE–SW, conformable with modern circulation. A second direction appears over western Egypt, W between 30°N and 26°N, NW between 26°N and 20°N. Cross-cutting aeolian landforms show that W/NW winds are older than the N/NE winds. Geomorphic evidence, abundant south to 26°N and less abundant to 20°N, also indicates that W and NW winds were early Holocene ‘palaeowesterlies’. Some evidence also indicates that they extended eastward to at least 30°E, 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 20°N. 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.

Introduction

Palaeoclimatic reconstructions in NE Africa for the period of the last glacial to the present have been based on evidence from (i) lacustrine and aeolian sediments and their physical and chemical properties, (ii) pollen spectra and other palaeobiological indicators within these sediments, and (iii) archaeological remains. Chronology has been supported by radiocarbon and optically stimulated luminescence (OSL) ages (e.g., Hassan, 1986, Brookes, 1989a, Haynes et al., 1989, Neumann, 1989, Kröpelin, 1993, Street-Perrott and Perrott, 1993, Pachur and Wünnemann, 1996, Stokes et al., 1998, Gasse, 2000, Gasse, 2002, Hassan et al., 2001, Swezey, 2001). Interpretations converge on a cold, dry, windy last glacial maximum (15–20 14C kyr BP), changing through an erratic transition to a multiphase, perhaps still cool, wetter, early Holocene (10–5 kyr BP), with pronounced arid intervals, the “African Humid Period” of DeMenocal et al. (2000), then to a drier and windier later Holocene (5 kyr BP to present).

These empirical studies have spawned theoretical research into climate change in North Africa, focussed on modelling of atmospheric and surface responses to orbitally forced insolation. The purpose has been to simulate atmospheric processes and surface feedbacks responsible for the presence and character of Late Pleistocene and Holocene lakes and playas in the now hyperarid region of the Sahara and its arid borders (e.g., Kutzbach et al., 1993, Kutzbach et al., 1996, Claussen and Gayler, 1997, Kutzbach and Liu, 1997, Texier et al., 1997, Ganapolski et al., 1998, de Noblet et al., 2000, Doherty et al., 2000). Wind patterns, however, are not usually considered in either empirical reconstructions or simulation models, and then only as output rather than input (e.g., Kutzbach et al., 1993). This paper reports geomorphic evidence of wind patterns over an area of Egypt encompassing 8° of latitude and longitude, patterns that are, in part, of Holocene age, and which provide input data to palaeoclimatic models.

Section snippets

Study area

The study area encompasses 70% of the Western Desert, about half of Egypt's area (Fig. 1). Physiographically, the northern half of this desert is a low-relief, cuesta-form plateau developed across north-dipping Palaeogene limestones, sloping south to north for 550 km from ∼500 to ∼200 m elevation. Prolonged wind erosion has formed fields of yardangs over much of it (Brookes, 2001). Its southern boundary is a 200- to 300-m-high escarpment which overlooks a low-relief mosaic of plains, low

Methods

The study area is covered by a set of 24 LANDSAT-MSS images acquired from 1972 to 1976. These are false-colour composites of bands 4, 5, and 7, processed at 1:250,000 scale by Earth Satellite Corporation (ESC) and held at the National Air and Space Museum (both in Washington, DC). Images were enhanced using GEOPIC, developed at ESC, a procedure which emphasized subtle tonal variations of surface materials and which revealed linear features, such as roads and dunes, below the 79-m pixel

Interpretation of sample areas

Sample geomorphic maps showing directional aeolian landforms are shown in Fig. 2a–e. Plateaus, scarps, mesas, valleys, and deflation basins have been added to indicate terrain influences on wind.

Wind fields

The reconstructed regional airflow pattern over the north and central Western Desert is dominated by N winds, and in the south by NE winds. A second wind field, however, emerges—W over NW Egypt, veering to NW over SW Egypt. The N/NE pattern is similar to the present-day circulation. As argued in the following section, this pattern ‘switched on’ in the last few millennia, but has also dominated much of the last 2.5 myr (Brookes, 2001). The W/NW pattern over western Egypt is argued below to

Conclusion

Geomorphic mapping of Egypt's Western Desert (20–30°N, 22–32°E) from LANDSAT-MSS images reveals two consistent orientations of aeolian landforms, which indicate Holocene wind directions, one from the W and NW and another from N and NE. The W/NW set are in evidence mostly over western Egypt: from the W over the northern part and from the NW over the southern part. Fragmentary evidence in north-central Egypt extends this westerly wind domain farther east to 30°E. The N/NE set is more extensive

Acknowledgements

Thanks to T.A. Maxwell, Center for Earth and Planetary Studies, U.S. National Air and Space Museum, for making the LANDSAT images available; to C.V. Haynes, Jr., University of Arizona, for an informal review; to C. King of the Cartography Unit at York University for the figures; to R.A. Marston for editorial work; and to two journal reviewers.

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