Abstract

The Liwa region of the United Arab Emirates is one of the most distinctive geomorphological features of the Rub Al Khali desert of southern Arabia. Characterized by a sharp crescentic boundary coincident with a N-S elevation drop of ∼65 m with the transverse ridges in Al Qâfa to the north, Liwa is an area of some of the world's largest megabarchan dunes. Deep drilling of continuous cores has provided an opportunity to observe the internal structure and age of these two ergs. Subsurface stratigraphy is complex, reflecting rapid facies transitions between dune and interdune sub-environments. Most of the cored sediment is dominated by cross-bedded and structureless sand units of inferred eolian (transverse dune) depositional origin. A total of 56 optical ages for sand-sized quartz grains extracted from these cores provide a chronological framework for deposition of the late Quaternary ergs. A marine isotope stage (MIS) 5 erg is identified north of the Liwa crescent in the Al Qâfa region, which reaches vertical thicknesses >100 m. A MIS 1 erg in the form of megabarchan dunes is recorded south of the Liwa crescent. This was deposited on a pre–MIS 5 land surface, since ca. 6 ka, over a period of just a few thousand years. Contemporary bypassing of eolian sands via superimposed dunes in both Al Qâfa and Liwa appears to be in equilibrium with the current wind regime. A consideration of a variety of factors that control the availability, mobilization, and preservation of eolian sediments and resulting bedforms leads us to infer that the system is not sediment-supply or transport limited. Instead, the system is preservation limited, being controlled by a correlated combination of sea level and precipitation. Both of these factors are strongly linked to global climate variations in the eccentricity (ca. 100 ka) band. Paradoxically, the bulk of the preserved record of eolian activity in the southern Arabian Peninsula occurs within relatively humid interglacial phases rather than arid intervals. Evidence of eolian deposition during arid phases may not for the most part be preserved in large areas of the sand sea. Observed contrasts in the preserved record of eolian activity between Al Qâfa and Liwa, and the Wahiba Sand Sea, may in part relate to contrasting eolian bedform morphology.

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