The extent of desertification on Saudi Arabia

Desertification is the process that turns productive deserts into non-productive deserts as a result of poor land-management. Desertification reduces the ability of land to support life, affecting wild species, domestic animals, agricultural crops and humans. The reduction in plant cover that accompanies desertification leads to accelerated soil erosion by wind and water. South Africa is losing approximately 300–400 million tons of topsoil every year. As vegetation cover and soil layer are reduced, rain fall impact and run-off increases. This paper discusses the extent of desertification, its potential threat to sustained irrigated agriculture and possible measures adopted to control ongoing desertification processes to minimize the loss of agricultural productivity in an arid country such as the Kingdom of Saudi Arabia.     

腾格里沙漠西北缘湖泊沉积记录的全新世中期气候环境

通过腾格里沙漠西北缘青土湖沉积物粒度、有机碳、碳酸盐、碳氮比和有机质稳定同位素等多项沉积学气候代用指标及精确定年的综合分析,建立了该区域全新世中期9.0~3.0 cal.kaBP的古气候演化序列。结果表明,9~7.8 cal.kaBP期间,区域的水分条件和温度逐渐上升,植被状况好转,此阶段属于全新世早期向全新世暖湿期转变的过程;而在7.8~7.5 cal.kaBP期间出现了显著的百年尺度的干旱事件,沉积物主要以砂质沉积为主,此时湖泊生产力显著下降;全新世期间最为暖湿的气候适宜期出现在7.5~5.0 cal.kaBP;5.0 cal.kaBP以来,该区域出现了较为明显的干旱化趋势。     

Geology and rare-earth element geochemistry of highly evolved, molybdenite-bearing granitic plutons, Southeastern Desert, Egypt

The field relations, mineralogy, and major and trace elements (including REE analyses of whole-rock samples and minerals) of granites and their associated molybdenite uranium mineralized aplites in Southeastern Desert, Egypt, have been studied. The granites are leucocratic and mostly peraluminous in nature with muscovite increasing at the expense of biotite. The chemical and mineralogical characteristics of the granitic rocks indicate that their melts originated from the LILE-enriched mantle wedge by partial melting and are contaminated by crustal melts, followed by thermogravitational processes. Leucogranites with higher Na2O/K2O ratios from Um Dargag and Um Maiat crystallized under H2O-saturated equilibrium conditions in which the exsolved vapor continuously migrated away. The REE patterns of the granites studied are characterized by LREE enrichments and negative Eu anomalies. In comparison, the potassic aplites and the more sodic leucogranites are depleted in LREE, enriched in HREE and show more remarkable negative Eu anomalies. Allanite and monazite are the most important REE carriers in the granites. These minerals are strongly enriched in LREE, whereas fluorite and xenotime, which are more abundant in the aplites, are enriched in HREE. The average Lu/Ce ratio represents the fractionation trend with respect to HREE. It is 0.71 for radioactive fluorite, and it increases to 1.22 for non-radioactive fluorite. The high REE contents of molybdenite represent re-deposition of the mobilized Mo and REE. Due to the strong control of accessory minerals, the REEs are of limited use in petrogenetic modelling of highly evolved granitic systems.     

A study of the depth of weathering and its relationship to the mechanical properties of near-surface rocks in the Mojave Desert

Weathered granite extends 70 m deep at Hi Vista in the arid central Mojave Desert of southern California. The low strength of this granite is due to the alteration of biotite and chlorite montmorillonite. Deep weathering probably occurs in most granites, although we cannot rule out some anomalous mechanisms at Hi Vista. Geophysical instruments set in these slightly altered rocks are limited by the unstable behavior of the rocks. Thus, tectonic signals from instruments placed in shallow boreholes give vague results. Geophysical measurements of these weathered rocks resemble measurements of granitic rocks near major faults. The rheology of the rocks in which instruments are placed limits the useful sensitivity of the instruments.     

A note on the characteristics and possible origins of desert varnishes from Southeast Morocco. A reply to comments by R. I. Dorn

This reply to Dorn (1989) argues that a biotic origin is not the only way in which desert varnishes may be formed. It is suggested that a variety of initial conditions may result in similar end products. It is agreed that the dissimilar views of the authors may result from their different philosophies concerning the classification of Mn/Fe varnishes.     

Stable isotopic signatures of the modern land snail Eremina desertorum from a low-latitude (hot) dry desert—A study from the Petrified Forest,New Cairo,Egypt

This study was conducted on recent desert samples—including (1) soils, (2) plants, (3) the shell, and (4) organic matter from modern specimens of the land snail Eremina desertorum—which were collected at several altitudes (316–360 m above sea level) from a site in the New Cairo Petrified Forest. The soils and shell_{E. desertorum} were analyzed for carbonate composition and isotopic composition (δ¹⁸O, δ¹³C). The plants and organic matter_{E. desertorum} were analyzed for organic carbon content and δ¹³C. The soil carbonate, consisting of calcite plus minor dolomite, has δ¹⁸O values from −3.19 to −1.78‰ and δ¹³C values −1.79 to −0.27‰; covariance between the two values accords with arid climatic conditions. The local plants include C3 and C4 types, with the latter being dominant. Each type has distinctive bulk organic carbon δ¹³C values: −26.51 to −25.36‰ for C3-type, and −13.74 to −12.43‰ for C4-type plants.The carbonate of the shell_{E. desertorum} is composed of aragonite plus minor calcite, with relatively homogenous isotopic compositions (δ¹⁸O_mean = −0.28 ± 0.22‰; δ¹³C_mean = −4.46 ± 0.58‰). Most of the δ¹⁸O values (based on a model for oxygen isotope fractionation in an aragonite-water system) are consistent with evaporated water signatures. The organic matter_{E. desertorum} varies only slightly in bulk organic carbon δ¹³C values (−21.78 ± 1.20‰) and these values suggest that the snail consumed more of C3-type than C4-type plants. The overall offset in δ¹³C values (−17.32‰) observed between shell_{E. desertorum} carbonate and organic matter_{E. desertorum} exceeds the value expected for vegetation input, and implies that 30% of carbon in the shell_{E. desertorum} carbonate comes from the consumption of limestone material.     

Living where others dare not: Microhabitat distribution in Chorizanthe rigida,a serotinous desert annual

We studied the small scale distribution pattern of Chorizanthe rigida, a serotinous desert annual, in a shrub-mound-to-desert-pavement gradient. Two sites were established in the Mojave Desert and two sites where established in the Sonoran Desert. We placed transects along a soil surface gradient from shrub mounds to desert pavements, and measured land surface properties, soil characteristics, and C. rigida's small scale distribution. A Principal Components Analysis was done on the resulting site × soil variables matrix. C. rigida established preferentially in desert pavements outside shrub canopies. Pavements showed finer soil textures and higher electrical conductivities, while shrub mounds had sandy textures, low surface rockiness, higher nitrogen, and higher cover of annuals. We discuss a probable relationship between seed retention and the establishment on desert pavements: timing seed release allows this species to avoid wind- or animal-induced seed dispersal into shrub mounds and establish outside the nurse mounds, where most annual plants concentrate. The seeds encased in the serotinous involucres, released after rains, are dispersed by run-off into the desert pavements and transition zones where, once developed, the dead seed-retaining structures of adult C. rigida plants persist for years releasing seeds to subsequent rain events.     

Environmental effects on distributions of culturable soil oligotrophic bacteria along an elevational gradient in the Chihuahuan Desert

Oligotrophic bacteria have been isolated from many habitats, yet environmental regulation of their distributions in soil has not been elucidated. To address the issue of environmental influence upon oligotrophic distributions, Chihuahuan Desert soils were sampled from five sites along an elevational and vegetational gradient within Big Bend National Park during January and August of 2002 and 2003. Soils were diluted and plated on oligotrophic media, and plates were incubated at 15, 25, 35, 45 and 60 °C. Additionally, measurements of soil organic matter, pH, moisture, extractable nitrate, extractable ammonium and microbial biomass carbon were collected for each sample to relate oligotrophic bacterial distributions to soil nutrient and edaphic characteristics. Analysis of variance indicated significant site, season, incubation-temperature and interaction effects on total oligotroph numbers. Canonical correspondence analysis and multiple regressions indicated that all soil-chemistry variables significantly influenced discrete morphologies of oligotrophs. Oligotroph distributions were most congruent with soil-chemistry variation in three sites, whereas oligotrophic diversity in two sites did not adhere closely to measured environmental variables. While vegetation type may structure oligotroph communities at the two mid-elevational sites, abiotic constraints are drivers in low-desert sites.     

Malariological and sociological significance of ‘tanka’ and ‘beri’ in the Thar Desert, Western Rajasthan, India

Both ‘tanka’ and ‘beri’, the well-like structures made for storing drinking water fetched from distant places or superficially charged from the runoff water of the monsoon rains, are considered integral components of rural communities in the Thar Desert of Rajasthan State in north-western India.A. stephensi, the confirmed malaria vector in desert regions, breeds in these structures predominantly.