GEOLOGY, June 2007 555 Geology, June 2007; v. 35; no. 6; p. 555–558; doi: 10.1130/G23369A.1; 3 figures; 1 table. © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. INTRODUCTION Linear dunes are the most widespread type of desert sand dune (Lancaster, 1995), but they are rarely recognized in the geologic record (Rubin and Hunter, 1985). It has been argued that large linear dunes are relics of a cooler, drier, and windier climate during the Last Glacial Maxi- mum, a hypothesis supported by luminescence dating of linear dunes in many areas (Lancaster, 2007) and by the considerable inertia of large dunes, which require thousands of years to respond to changes in wind regime (Warren and Allison, 1998). In addition, the large number of endemic species within the Namib Sand Sea has been given as evidence for a long and continuous period of hyperarid conditions within Namibia (Ward et al., 1983) and therefore potentially very old dunes. These ideas gave rise to the hypoth- esis that, although the linear dunes of the Namib Sand Sea are currently active, they should have some older, Pleistocene core. Livingstone (1989), however, questioned the “relic” dune hypothesis, suggesting that, given sufficient time, contempo- rary dune processes were capable of creating the present form of the dunes, although there were no techniques to determine the age or structure of large dunes at that time. The two problems of determining dune age and sedimentary structure have now been solved using optically stimulated luminescence (OSL) dating and ground-penetrating radar (GPR) respectively. GPR exploits changes in permit- tivity within the dune sediments to image dune sedimentary structures (Van Dam et al., 2003) and has been shown to work very well in eolian sands (Bristow et al., 2000, 2005). We used GPR to image the sedimentary structures and stra- tigraphy of the dune. Based upon stratigraphic interpretation of the GPR profiles, sample loca- tions for dating were selected and boreholes were drilled into the dune to obtain samples. We used OSL dating to determine when sand was last exposed to daylight and therefore how long it has been buried within a dune. When used together, these two techniques offer a powerful tool for investigation of sand accumulation and dune migration rates (Bristow et al., 2005). THE STUDY AREA The Namib Sand Sea is dominated by large, north-south–trending, complex, linear dunes (Lancaster, 1989), the orientation of which is controlled by a bimodal wind regime with a south-southwesterly wind blowing inland from the South Atlantic Ocean and an easterly “berg” wind that sweeps down the escarpment from the interior. The study dune is located close to the northern edge of the Namib Sand Sea, 7 km southeast of Gobabeb and the valley of the ephemeral Kuiseb River (Fig. 1). This dune has been surveyed repeatedly (Livingstone, 1993, 2003) to monitor annual to decadal changes in morphology. In the study area, the dune is ~70 m high and up to 600 m wide, decreasing in size toward the north. The dune has a sinu- ous crest and superimposed transverse dunes on both flanks. The dune crest shifts back and forth by ~15 m each year in response to the season- ally bimodal wind regime (Livingstone, 1989). The profile of the dune changes due to migration of superimposed dunes along the dune flanks, but the base of the dune shows no detectable lat- eral migration over a 30 yr period (Livingstone, 2003). The section surveyed in this study is located between sites 1 and 2 of Livingstone. METHODS We carried out topographic surveys and col- lected over 4 km of GPR profiles across the dune. We obtained GPR data using a Pulse EKKO 100 with a 1000 V transmitter and 100 MHz antennae spaced 1 m apart. We collected data every 0.5 m Age and dynamics of linear dunes in the Namib Desert C.S. Bristow School of Earth Science, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK G.A.T. Duller Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Ceredigion SY23 3DB, UK N. Lancaster Desert Research Institute, 2215 Raggio Parkway, Reno, Nevada 89512, USA ABSTRACT Ground-penetrating radar and luminescence dating studies of a large, complex, linear dune in the northern part of the Namib Sand Sea provide new information on the age and inter- nal sedimentary structures of these dunes, with important implications for interpretations of paleoclimates and the rock record of eolian sandstones. The dune is a composite feature formed during several episodes of construction, including a hiatus of almost 2000 yr. The oldest sands within the dune are 5700 yr old, indicating complete turnover of sand during the Holocene. The dune has moved laterally by ~300 m during the past 2500 yr, proving lateral migration of a large linear dune. Dune construction has been affected by climate change, and we attribute the hiatus to increased rainfall and vegetation, which largely halted sand move- ment and dune building in the Namib Desert during the middle Holocene. Keywords: linear dunes, Namib Desert, ground-penetrating radar, optically stimulated lumines- cence dating. Line of section N 1 km 200km Enlarged area Tropic of Capricorn Namibia 16°E 28° Kuiseb River Namib Sand Sea 22°S Figure 1. Location map showing the study site and the position of the linear dune in the northern part of the Namib Sand Sea.