Palaeoclimatic evidence from sand seas
Abstract Sand seas can be an important source of information on the Quaternary history of many deserts. Because sand seas appear to react sensitivity to climatic changes, the quality of palaeoenvironmental information obtained from them is frequently much better than that from adjacent rocky desert areas. Patterns of dune alignments may be used to reconstruct the wind directions that formed currently inactive or fixed dunes and thus can provide information on palaeocirculation patterns. The climatic conditions under which these dunes formed can be examined using palaeoclimatic models to explore the combinations of wind velocity, rainfall and evaporation rates required to reactivate them. Choice of appropriate values for these parameters may be hampered by the problems of dating episodes of dune activity. In many sand seas, the best palaeoclimatic information comes from studies of interdune lacustrine or playa deposits, which may reflect increased local rainfall or higher groundwater discharge, or greater penetration of the sand sea by rivers with headwaters outside the desert. Caution should be taken in the interpretation of such deposits, for juxtaposition of aquatic and aeolian environments can occur even in modern hyperarid and seas. The record of climatic changes preserved in the deposits of modern sand seas suggests that periods of dune formation may have been interrupted by long periods of stability, which may also have included deposition of interdune lacustrine deposits. For example, within the past 40,000 years, active dune formation in many sand seas appears to have been restricted to the interval between about 20,000 and 12,000 yr B.P., and to the late Holocene (approximately 1 4 to 1 3 rd of the time).