Date of this Version
In: ENCYCLOPEDIA OF QUATERNARY SCIENCE, Editor-in-Chief Scott A. Elias. Amsterdam, etc. (Elsevier, 2007), pp. 607-626.
Large dune fields, or sand seas, are landscapes often thought to be found only in deserts beneath the great, subtropical high-pressure zones, where subsiding air suppresses rainfall. Dune fields are also quite common in mid-latitude regions, to the north and south of subtropical deserts (Fig. 1). Two major characteristics distinguish many mid-latitude dune fields from the sand seas of lower latitudes. One difference is that many of those in mid-latitudes are in semiarid, rather than arid climates, and therefore are not presently active. In this article, ‘active’ refers to eolian sand bodies that are not covered with vegetation, and where particles are currently being transported by the wind. Evidence of active sand transport often takes the form of well-expressed sand ripples on a dune’s surface. Where sand is vegetated and particles are not being transported by the wind, a sand body is ‘stable.’
A second characteristic of mid-latitude dune fields is that many are situated near mountain ranges or at least areas of relatively high relief. Glaciation in high mountains reduces local bedrock to sand-sized particles, and additional reduction of rock to sand sizes takes place through colluvial and fluvial processes quickly in terrains of steep relief. Through these processes, new sediment becomes available to feed growing dune fields much more rapidly than in areas of low relief, which characterize lower-latitude regions.
Because of their occurrence in semiarid climates, mid-latitude dune fields are particularly sensitive indicators of shifts in the regional moisture balance. Sand dunes move only where there is wind, sandsized sediment, and a lack of vegetation cover. A shift to drier conditions may result in loss of vegetation cover and activation of sand by wind. A shift back to wetter conditions may stabilize dunes because of enhanced vegetation cover. The geologic record of this alternation of dry and moist climates tends to be preserved in many mid-latitude dune fields as eolian sand–paleosol sequences. In areas downslope of high sediment production, such as mountains, fresh sediment may accumulate in a dune field during periods of eolian activity, bury the pre-existing landscape, and preserve former surface soils as buried soils (or paleosols). In arid, lowlatitude environments of low relief, previously stabilized eolian sand may be continually reworked without addition of much new sand, and formerly stable surfaces, which would be marked by paleosols in semiarid climates, may not be preserved.
Throughout this article, we correlate eolian dunebuilding events, where they can be dated or where the age can be inferred, to the marine oxygen-isotope stratigraphic framework of Martinson et al. (1987). Where the abbreviation ‘MIS’ occurs, this refers to ‘marine isotope stage,’ using the numbering system and approximate ages in Martinson et al. (1987).