Weathering in the central Namib Desert,Namibia: Controls,processes and implications

The hyper-arid Namib Desert is an ancient desert of great lithological diversity. Weathering plays a key role in landscape evolution in the Namib which, in spite of its aridity, has a number of sources of moisture – rainfall, fogs, dews and groundwater seepage – which enhance weathering. Among the weathering processes that have been the subject of recent study in the central Namib are salt, thermal and lichen weathering which, in often complex associations of processes, contribute to the array of small scale weathering features found on marble and granite outcrops here. At the landscape scale weathering in the central Namib is also highly interrelated with erosion and the development of geochemical sediments (calcrete, gypcrete, tufas etc), whilst weathering ‘hotspots’ are thought to be important sources of fine sediment production. Over the long term weathering has played important roles in landscape evolution here which, despite localized weathering hotspots, has been very slow for much of the Cenozoic.     

Monitoring of rapid salt weathering in the central Namib Desert using limestone blocks

Pre-weighed blocks of a Jurassic Limestone were exposed on the ground surface in the coastal Namib Desert for a period of 2 years. The environment is both salty and foggy. Some of the blocks suffered extensive disintegration, and laboratory analyses (including geochemistry, XRD and SEM) indicate that the weathered samples have a high halite (sodium chloride) content. Cycles of wetting and drying associated with the frequent fog events of the area cause cycles of halite crystallization. Rocks exposed at the surface absorb salts from the surrounding desert surface and then disintegrate, contributing to planation of the landscape.     

Psammophily in Namib Desert spiders

A community of psammophilous spiders was studied in the central Namib Desert. Of 20 species, 85% were cursorial. Complex setae, claws or keels facilitate digging, sweeping, carrying, and swimming in or walking on sand, while long spigots enable spiders to bind sand with silk when burrowing down to depths with amenable microclimate (10–120 cm). Sand is excavated either by sweeping it up an incline or carrying it vertically up. Other characteristics of dune spiders were large size, polyphagy, low metabolism, longevity, brood care and seasonal foraging and breeding patterns. The large, dominant heteropodid,Leucorchestris, may determine many characteristics of the Namib arthropod community.     

The respiratory environment of the Namib Desert Golden Mole

We measured the partial pressure of oxygen (PO₂) in the interstitial gas surrounding the sand-swimming Namib moleEremitalpa granti namibensis. At a sand temperature of 26 °C, which produced a nearly maximal rate of oxygen consumption, thePO₂near the noses of the animals averaged only 0·9 kPa (6·7 Torr) below the level in the free atmosphere. High oxygen availability was a result of the notably low metabolic rate in the 20 g mammals and the dry, porous and metabolically inactive nature of dune sand. A mathematical model indicated that normal mammals weighing 200 g or more could comfortably exist completely encased in dune sand. We concluded that the moles' small size and low metabolic rate are not adaptations to hypoxia or hypercapnia underground but are probably related to low food availability and the energetic cost of foraging in their desert environment.     

Ecology and spatial patterns of large-scale vegetation units within the central Namib Desert

This article offers a review of published knowledge and a new state-of-the-art analysis regarding the floristic composition, the functional composition and the plant communities found in the central Namib Desert. At the same time, this paper contributes to the understanding of the relationship between the plant species composition of the central Namib Desert and the prevailing environmental gradients, with an emphasis on diversity and ecology in space and time. This article builds on three thematic foci. The first focus (1) lies on the present knowledge of the composition and the characteristics of the flora. A comprehensive floristic database has been compiled based on all available sources. A second focus (2) lies on the characterization and spatial distribution of the vegetation units. Therefore, we created a new vegetation classification based on a unique vegetation-plot database (http://www.givd.info/ID/AF-00-007) and additional data summing up to 2000 relevés, resulting in 21 large-scale vegetation classes. Using a supervised classification approach based on the vegetation classification, remote sensing and environmental data, we were able to produce a new vegetation map of the Central Namib. This was updated using expert knowledge, field visits and through manual preprocessing. With the third focus (3) we explore the spatial patterns of the previous foci and discuss their relation to environmental parameters and gradients.     

Rainfall regulates decomposition of buried cellulose in the Namib Desert

Because of the aridity of the Namib Sand Sea, it has long been assumed that decomposition of buried plant material was largely independent of rainfall. Losses were attributed to consumption by detritivores that forage year-round. Moisture-limited micro-organisms were reported to occur in low densities in Namib sands, supporting the assumption that rainfall was insignificant in regulating decomposition. Observations of abundant macrofungal fruiting from buried plant material and herbivore dung, following a 12 mm rain, suggested the importance of rain-induced decomposition had been underestimated. We used cellulose substrates to compare material loss during dry periods and following differing amounts of rain. Strips of cotton cloth and filter paper, buried at 10 cm depths at five disjunct locations, were sequentially removed over 10 months. A period of at least 170 days elapsed before rains, ranging from 4–46 mm, fell at all locations. Material loss during the dry period averaged 8·2% (range 0–16·7%), and was attributed to macrodetritivore consumption. In marked contrast, an average of 84·1% of the material (range 64·7–97·2%) was lost following rains greater than 9 mm. Wet substrates were heavily colonized by fungi, and termites and tenebrionid beetle larvae were observed feeding on fungus-colonized substrates. Stepwise regression analysis revealed that rainfall, rather than duration of burial, was the primary factor determining substrate loss in the Namib Sand Sea. Although rain events are infrequent and ensuing periods of moist soil are brief, substrate loss following rains is highly significant relative to that occurring in the absence of rain. In contrast to more mesic deserts, rainfall is an important trigger of decomposition in the Namib Sand Sea where soils are too dry to support significant decomposition, except when episodically moistened by rain.     

The Evolution of the Morphological Framework of the Central Namib Desert, Namibia, Since the Early Cretaceous

The Central Namib Desert in Namibia is a hyper-arid area which was greatly affected by tectonic changes in the Early Cretaceous, associated with the opening up of the South Atlantic Ocean, continental fragmentation of West Gondwanaland and the movement of a major mantle plume (the Tristan Plume). These events led to the formation of a range of subvolcanic complexes – the so-called Damaraland Complexes – and to the deposition of flood basalts – the Etendeka Lavas. The Damaraland Complexes include some striking inselberg features of great size, including Erongo, Brandberg and Spitzkoppe. The Great Escarpment, which bounds the Central Namib to landward, is of uncertain age, but it appears to have experienced a substantial degree of erosion by the Late Cretaceous. The feature is rather less well developed and persistent in the Central Namib than elsewhere in southern Africa. It is probable that the Namib has been dry for much of the last 130 Ma, and there is evidence for aridity in the early Cretaceous and in the mid-Tertiary.     

The nature of deep crustal structures in the Mojave Desert, California

Summary. The character of multi-offset reflections from the deep crust in the Mojave Desert are examined to reveal the physical nature of the reflecting structures. We focus on distinguishing classical abrupt discontinuities, such as traditional models of the Conrad and Moho boundaries, from more unusual structures. Finite-difference modeling and simple interference relations show that pre-critical reflections exhibiting an increase in peak frequency with offset arise from thinly-layered horizontal structures, while reflections from step discontinuities show no change in frequency with offset. In the deep crust thin layers may result from sill intrusion or fault motion.
The sense of changes in Poisson's ratio and the relative strength of density changes determine whether reflection amplitudes will increase or decrease with offset. A simple linear regression on pre-critical reflection amplitudes against offset is adequate to separate reflections arising from increases in Poisson's ratio from those arising from decreases in Poisson's ratio and/or density changes. The latter condition may be the result of strong anisotropy or the presence of pore fluid. Comparisons of the properties of major deep reflectors across the Mojave Desert suggest that the effects of tectonic motion and fluid injection have penetrated all levels of the crust.     

Mysterious circles in the Namib Desert: review of hypotheses on their origin

Circular, slightly concave depressions, devoid of vegetation and often surrounded by a fringe of tall grasses occur in a broken belt in the pro-Namib zone of the west coast of southern Africa. Several hypotheses have been proposed to explain the origin of these so-called fairy circles. The most important of these relate to areas of localized radioactivity; termite activity; or allelopathic compounds released by dead Euphorbia damarana plants. No evidence of increased radioactivity could be detected in soil samples collected from these sites. Although termites occur at these localities no evidence was found to link termite activity directly to the formation of barren patches. Bioassays conducted on soil collected from the centre of the barren patch clearly demonstrated an inhibition of plant growth, while soil collected from the edge of the barren patch had a stimulatory effect on plant growth. No indication of growth inhibition was found in soil collected beneath E. damarana plants. At this stage none of the proposed hypotheses can satisfactorily explain the origin of the fairy circles.     

古尔班通古特沙漠腹地土壤水分动态

采用土钻法对古尔班通古特沙漠腹地典型自然沙垄剖面土壤含水量进行长期监测.采取多重比较(LSD)、典型相关分析(MANOVA)、并在国内第一次采用季节交乘趋势等统计方法,对土壤水分与环境因子的关系进行分析,研究结果表明:土壤含水量时间变化规律分为补给期、失水期和冻结滞水期.土壤含水量均值为补给期>冻结滞水期>失水期,变异系数为补给期>冻结滞水期>失水期;土壤含水量空间变化规律表现为:阴阳坡间具有极显著差异(P<0.01,F=0.002 9).垄间分别与坡中、垄顶具有显著性差异(P<0.05).垂直变化分为活跃层、过渡层和稳定层,活跃层与其他土层间均有显著性差异(P<0.05),土壤含水量均值为阴坡>阳坡,垄顶>垄间>坡中.活跃层>过渡层和稳定层;变异系数为阴坡>阳坡,垄间>坡中>垄顶,活跃层>过渡层>稳定层;降雨量和温度是影响研究区土壤水分时间分布变化的重要影响因子,同时也是土壤含水量变化的决定性影响因子,与土壤含水量具有显著相关性(P<0.05,F=0.02).     

The role of dew as a moisture source for sand microbiotic crusts in the Negev Desert, Israel

Dew and fog were measured periodically during the years 1992–1995 at the Hallamish dune field, western Negev Desert (average annual rain precipitation ca. 95 mm). A major portion of the dunes are covered by cyanobacterial and moss-dominated microbiotic crusts. Chlorophyll- a measurements and microscopic examination were carried out in order to detect the possible changes in crust biomass and phenological changes of the mosses following the occurrence of dew and fog. Dew and/or fog were present in over 50% of the late summer and fall mornings with an average yield of 0·1 mm on cloth attached to glass plates. This corresponds to ca. 0·03 mm for the upper 0·5 cm of the crust, and is less than the 0·1 mm threshold which implies liquid water and which was also found necessary for net photosynthesis of the cyanobacterial crust. This threshold was met only during heavy dew or fog events, during which 10–40% of the crust surface area was moistened. It was estimated that dew and fog could account for upto 3·2–9·4% of the total annual time of crust wetness and consequently activity due to liquid water. Nevertheless, microscopic examination of the dominant moss species at Nizzana, Bryum dunense, disclosed phenological changes, mainly resulting in growth of vegetative reproduction organs (bulbils), and sexual organs, gametangia (antheridia and archegonia). B.dunense may thus be adapted to the utilization of dew and fog precipitation for the production of nearly mature sexual organs. The moss may thus be capable of already utilizing the first winter rains for fertilization and sporophyte formation.     

A ‘mathematical’ spider living on gravel plains of the Namib Desert

In certain areas of the Namib gravel plains a new Ariadna sp. was discovered. It rims its vertical burrow with a stone circle, made on average of seven quartz stones. Some mathematical relationships between hole diameter, stone sizes and weight, and animal size were studied. A correlation was found, and stone selection by this spider postulated. Different hypotheses on the adaptive value of stone circle are suggested.     

The nature of the Precambrian-Tertiary boundary at Renardodden, Bellsund, Svalbard

The previous literature on the occurrence of Tertiary strata at Renardodden provides contradictory information about the primary versus tectonic boundary with the Precambrian basement. Tertiary sandstones and shales overlie unconformably the basement rocks, which have been resedimented as boulder conglomerates prior to Caledonian metamorphism and weathered prior to the deposition of the Tertiary strata. The boundary relations are complicated by a repeatedly active fault system that may form part of the Inner Hornsund Fault Zone.     

Seed reserves in the central Monte Desert, Argentina: implications for granivory

It has been suggested that lower rates of granivory in South American warm deserts compared to their North American counterparts arose from a reduction of seed reserves in the former owing to the extinction of argyrolagid marsupials. We measured seed reserves in two habitats of the central Monte Desert in an attempt to detect such seed decline, but to no avail. After moderate rainfall, maximum seed standing crops reached 16,000 and 23,000 seeds m⁻²in shrublands and open forests, respectively. Under the canopy of trees and shrubs there were 19,000 and 37,000 seeds m⁻²; whereas in exposed areas there werec. 10,000 seeds m⁻²in both habitats. Seed banks in other South American semi-arid areas showed similar values. Total grass seeds as well as those presumably preferred by ants also seem to be similar in both continents. Hence, granivory in South America, as has been already reported for Australia, is lower than in North America in spite of the great similarity of seed bank sizes. Moreover, argyrolagids were unlikely seed-hoarding granivores, therefore some other reason than argyrolagid extinction should be sought to explain the lack of specialized seed-eating mammals, and the smaller overall seed consumption rates in South American deserts.     

古尔班通古特沙漠人工林土壤水分及其影响因素

通过2004年2～12月对古尔班通古特沙漠南缘2年生人工梭梭和沙拐枣混交林内3条典型坡面土壤水分进行了系统监测与研究。结果表明:人工林土壤水分状况受季节性降水、人工林种植密度、地形因子的影响而出现明显的时空分异。在空间上将土壤水分变化划分为三层,0～30cm为活跃层,30～60 cm为次活跃层,60～120 cm为相对稳定层。通过主成分分析,得出地形因子、种植密度对0～30 cm土层土壤水分的影响次序为坡度>坡向>种植密度>高程,30～60 cm土层为坡向>种植密度>坡度>高程,60～120 cm土层为种植密度>高程>坡度>坡向。     

Hydrologic controls of physical and ecological processes in Namib Desert ephemeral rivers: Implications for conservation and management

Ephemeral rivers have been largely excluded from previous attempts to classify global hydrologic regimes, or to assess the role of hydrologic characteristics in regulating ecological processes and patterns in fluvial ecosystems. The Namib Desert's ephemeral rivers are amongst the most hydrologically variable fluvial systems yet described. The coefficient of variation for mean annual runoff (CV_MAR) among 28 stations, representing 7 Namib rivers, averages 1.55, compared with a global mean of ∼0.45. Distinct curvilinear relationships exist among hydrologic characters and longitudinal position along the rivers. In particular, peak discharge, annual flow volume, and days of flow per annum exhibit a marked decline in the lower reaches of the rivers, after a mid-catchment peak. These longitudinal gradients exert strong controls over the composition of vegetation, invertebrate, and fungal communities; the availability and structure of various micro-habitats; and the rates of ecological processes such as decomposition. Flood pulses, although variable in their timing and magnitude, play a critical role in regulating organic matter transport and deposition, and primary and secondary production. Despite the tolerance of the biota to harsh and variable abiotic conditions, these ecosystems are highly sensitive to hydrologic alterations as water is acutely limiting for many organisms and ecological processes. Accordingly, alterations to flow regimes can quickly degrade ecological integrity. Managers must seek to maintain existing hydrologic regimes, a challenge amplified by increasing human water demand and changing regional climates.     

中国古尔班通古特沙漠东部中心地区植物分布与植物—土壤关系(英文)

The distribution of plant species and relationships between species and soil factors in the east central part of Gurbantunggut Desert was studied to provide more insight into the flora and determine differences in vegetation across various parts of the desert. Two-way Cluster Analysis showed that the vegetation in the area could be divided into three groups, the first group was dominated by the shrub species, Ephedra przewalskii and the grass species, Carex physodes mainly in areas of flat grounds and gentle slopes; the second group was dominated by C. physodes, Artemisia songorica and A. xerophytica mainly on the slope of sand dunes and the third group was dominated by the shrub species, Haloxylon persicum mainly on the top of sand dunes. There was no difference in plant density between Groups 1 and 2 but there was a significant decrease in Group 3. Soil water under vegetation Group 3 was much lower than that in the other two groups at all soil depths. The EC, organic matter, total P and soluble Na, Ca and Mg varied very similarly with soil water. Canonical correspondence analysis(CCA) satisfactorily assessed the species-soil relations in the area. The distribution of plant species was strongly correlated with the soil factors of water content, organic matter, EC and nutrients. The variations in species occurrence explained by the three CCA axes were about 70%, indicating that some explanatory site variables may exist outside our studied parameters. Soil texture is suggested to be included in future studies to improve the explanation of CCA.     

Distribution of plant species and species-soil relationship in the east central Gurbantunggut Desert,China

The distribution of plant species and relationships between species and soil factors in the east central part of Gurbantunggut Desert was studied to provide more insight into the flora and determine differences in vegetation across various parts of the desert. Two-way Cluster Analysis showed that the vegetation in the area could be divided into three groups, the first group was dominated by the shrub species, Ephedra przewalskii and the grass species, Carex physodes mainly in areas of flat grounds and gentle slopes; the second group was dominated by C. physodes, Artemisia songorica and A. xerophytica mainly on the slope of sand dunes and the third group was dominated by the shrub species, Haloxylon persicum mainly on the top of sand dunes. There was no difference in plant density between Groups 1 and 2 but there was a significant decrease in Group 3. Soil water under vegetation Group 3 was much lower than that in the other two groups at all soil depths. The EC, organic matter, total P and soluble Na, Ca and Mg varied very similarly with soil water. Canonical correspondence analysis (CCA) satisfactorily assessed the species-soil relations in the area. The distribution of plant species was strongly correlated with the soil factors of water content, organic matter, EC and nutrients. The variations in species occurrence explained by the three CCA axes were about 70%, indicating that some explanatory site variables may exist outside our studied parameters. Soil texture is suggested to be included in future studies to improve the explanation of CCA.     

Movement of a small slipfaceless dome dune in the Namib Sand Sea, Namibia

The migration of a small slipfaceless dome dune close to the northern edge of the Namib Sand Sea has been measured by topographic survey. The dune dimensions are 45-m wide and 1-m high with a volume of 551 m³ that has been calculated as the difference between the dune's surface elevation and an interdune surface extrapolated from measurements around the edge of the dune. The migration direction, 64°, and distance moved, 90 m, are measured against stakes set out in 1976. The dune has moved about 90 m between 1976 and 1999. This is an average linear migration rate of around 4 m year⁻¹, and is equivalent to an annual sand transport rate of about 1.2 tonnes m⁻¹ year⁻¹. The calculated total potential sand flow in this part of the Namib Desert is 119 tonnes m⁻¹ year⁻¹, and the resultant potential sand flow is 63 tonnes m⁻¹ year⁻¹. The dune migration is therefore about 1% of the total potential sandflow and 2% of the resultant indicating that dune migration is only a small part of total potential sand transport. The results suggest that small slipfaceless dome dunes are very inefficient at trapping sand, and that winds blowing across the interdune in this area are undersaturated with sand.     

Casting new light on late Quaternary environmental and palaeohydrological change in the Namib Desert: A review of the application of optically stimulated luminescence in the region

The application of optically stimulated luminescence (OSL) dating in the Namib Desert is casting new light on late Quaternary environments. OSL has been applied to: (i) complex linear dunes, alongside ground penetrating radar stratigraphy in order to establish dune migration rates, (ii) fluvial lithofacies associations that distinguish between flood deposits and river end points, in order to constrain the timing of periods of higher discharge and conditions relatively drier than present and (iii) aeolian sand interbedded with carbonate deposits in order to provide chronologies for water-lain interdune sediments. We present and review the contribution of these data to enhancing reconstructions of the palaeoenvironments and palaeohydrology of the west coast of Namibia, particularly the increased confidence in interpretations provided by lithofacies analysis of the river deposits. This includes major silt deposits, which have had a contested palaeohydrological interpretation, such as the Kuiseb River Homeb Silts. We conclude that OSL should remain a key chronological technique to further elucidate the palaeoenvironmental history of southern Africa.