The southern coastline of South Africa exhibits extensive aeolianites and spectacular barrier dunes; the ages and palaeoenvironmental significance of which have, until recently, received limited attention. We present preliminary results for samples taken from a single 6.5 m section in the seaward barrier dune near to the town of Wilderness. The reliability of these ages has been investigated using a series of dose recovery and pulsed irradiation experiments, as well as through analyses of single aliquot and single grain De distributions. Results indicate that the SAR protocol is providing accurate and precise De determinations and that adequate sensitivity correction is being made. The application of pulsed irradiations does not result in significant changes in the dose–response curves or averaged De's. Single grain De distributions are broad, with significant levels of overdispersion, which may reflect beta dose heterogeneity effects.
Peaks in aeolian activity date to 157–154 and 140–125 ka, with a slower rate of deposition recorded from 115 to 85 ka. Brief phases at 40 and 21 ka are also identified. Some ages, notably those from OIS 3 and OIS 2, are surprising from a geomorphological perspective, given the likely distance of the dune from the shoreline at such times, although they are not inconsistent with evidence from the east coast of South Africa. The results reveal multiple phases of Late Quaternary barrier dune accumulation, and suggest that aeolian activity in this region has been episodic and potentially more complex than previously thought. 相似文献
Longitudinal dunes are the most widespread dune types in the world sand seas but comprehensive study on the sand surface stability
is scarce. The southern part of Gürbantünggüt Desert is mainly covered by longitudinal dune in which fixed and semi-fixed
dunes occupy over 80% of the total area. Systematic analysis on the climatic conditions, the soil moisture and vegetation
distributions, and the sand surface activities showed that the fixed and semi-fixed dunes are in a comprehensive low-energy
wind environment. Snow cover and frozen soil provide a good protection to the ground surface in winter. The temporal distribution
of precipitation and corresponding variation of temperature create a favorable condition for the desert plants growth, especially
for the ephemeral plants. The occurrence of effective winds for sand moving in April to June coincides with the stage of relatively
wet sand surface and good vegetation cover, which effectively keep the sand surface stable at the interdune and the plinth
of the dunes. Activity sand surface appears only at the crest and the upper part of the sand dunes. 相似文献
This paper considers the nature, classification and possible origin of a variety of desert sand dunes which occur in association with ephemeral drainage features in the south-west Kalahari. The dune forms, which we term valley-marginal dunes, are found at the top of valley flanks, and are narrow, elongate forms with arcuate and straight planform elements which mirror the course of extant and ancient channels. Following a discussion of the major schemes used to classify different dune types, we describe the morphology and situation of these valley-marginal dunes. We suggest that whilst the basic morphometry of these dunes is similar to many other dune types, the topographic setting in which they occur is relatively unusual. We conclude that valley-marginal dunes are a previously unrecognized landform which cannot be fitted into current classificatory schemes. It is suggested that one reason for this may be the lack of an explicit temporal dimension in most dune classifications. 相似文献
Based on systematically monitoring plants on dune ridges in the southern part of the Gurbantunggut Desert in 2002, this paper, from the angle of dune stabilization by vegetation,describes the temporal and spatial distribution patterns of ephemeral plants on isolated sand dunes,analyses the natural invasion processes of ephemeral plants on human-disturbed sand surface and expounds the importance of ephemeral plants in stabilizing sand dune surface. A total of 45 plant species were identified in the study area, 29 of which are ephemeral plants. Ephemeral plants sprouted in early April and completed their life-circle within about two months. Just as aeolian sand activities came to the strongest stage from April to June in desert regions of northern Xinjiang, the total coverage of trees, shrubs and herbs of long vegetational period on most dune ridges was less than 10%, while the mean coverage of ephemeral plants reached 13.9% in April, 40.2% in May and 14.1% in June. Therefore ephemeral plants acted as the major contributor to dune surface stabilization in the Gurbantunggut Desert.Investigations of vegetation restoration on engineering-disturbed dune surface show that ephemeral plants first recolonized the disturbed dune surface. 相似文献
Grain flows are an integral part of sand dune migration; they are a direct response to the local wind regime and reflect complex interactions between localized over‐steepening of a dune slipface and complex turbulent airflow on the lee slope. Grain flows are primarily responsible for delivering sediment to the base of a dune, thus driving slipface advancement; yet, there are few constraints on their morphological and spatial characteristics or the amount of sediment that is redistributed by these flows. Using a combination of high‐resolution terrestrial laser scanning and video recordings, four distinct grain‐flow types are identified based on morphology and area on a dune slipface. Grain‐flow morphologies range from small, superficial flows to larger flows that affect greater portions of the slipface, moving significant amounts of sediment. Detailed field observations are presented of the dynamics of lee slopes, including measurements of the initiation location, thickness, magnitude and frequency statistics of grain flows, as well as volume estimates of redistributed sediment for each grain flow observed. High‐resolution laser scans enable accurate quantification of bulk sediment transfer from individual grain flows and can be used to study grain flows in a variety of environments. A categorization of grain‐flow morphologies is presented that links styles of flows with wind strength and direction, turbulent airflow, sediment deposition and environment. 相似文献