Gravel Plains Research Articles

Modeling terminal Pleistocene and Holocene forager population increase and environmental change in the Central Namib desert, Namibia

This paper examines the interplay of environmental change and human demographic shifts among late Pleistocene and Holocene populations in the hyper-arid context of the Central Namib Gravel plains, Western Namibia. This paper applies a structural equation modeling (SEM) approach to arid-zone hunter-gatherer mobility patterns and settlement systems based on variability among ethnographically and historically observed groups. It then applies this model in examining issues related to the Middle-to-Later Stone Age transition at the sites of Erb Tanks and the Mirabib rock shelter. This model suggests that, among modern arid-zone foragers, population density is by far the strongest factor influencing mobility and settlement systems, with higher population densities favoring longer residential site usages and shorter residential moves within smaller territories. In contrast, environmental variables having to do with annual rainfall, rainfall seasonality, and effective temperature affect mobility and settlement systems in ambiguous and statistically insignificant ways. This paper closes by arguing that regional-scale population increases signaled by various features of Later Stone Age archaeological patterning led to local population expansions in the Central Namib Desert, and that this in turn caused a range of recognizable cultural shifts that were strongly linked with adaptations to aridity.

Fractional solubility of iron in mineral dust aerosols over coastal Namibia: a link to marine biogenic emissions?

Abstract. This paper presents the first investigation of the solubility of iron in mineral dust aerosols collected at the Henties Bay Aerosol Observatory (HBAO), in Namibia, from April to December 2017. During the study period, 10 intense dust events occurred. Elemental iron reached peak concentrations as high as 1.5 µg m−3, significantly higher than background levels. These events are attributed to wind erosion of natural soils from the surrounding gravel plains of the Namib desert. The composition of the sampled dust is found to be overall similar to that of aerosols from northern Africa but is characterized by persistent and high concentrations of fluorine which are attributed to local fugitive dust. The fractional solubility of Fe (%SFe) for both the identified dust episodes and background conditions ranged between 1.3 % and 20 % and averaged at 7.9 % (±4.1 %) and 6.8 (±3.3 %), respectively. Even under background conditions, the %SFe was correlated with that of Al and Si. The solubility was lower between June and August and increased from September onwards during the austral spring. The relation to measured concentrations of particulate MSA (methane sulfonic acid), solar irradiance, and wind speed suggests a possible two-way interaction whereby marine biogenic emissions from the coastal Benguela upwelling to the atmosphere would increase the solubility of iron-bearing dust according to the photo-reduction processes. This first investigation points to the western coast of southern Africa as a complex environment with multiple processes and active exchanges between the atmosphere and the Atlantic Ocean, requiring further research.

Mapping Changes in Fractional Vegetation Cover on the Namib Gravel Plains with Satellite-Retrieved Land Surface Emissivity Data

Monitoring changes in vegetation cover over time is crucial for understanding the spatial distribution of rainfall, as well as the dynamics of plants and animals in the Namib desert. Traditional vegetation indices have limitations in capturing changes in vegetation cover within water-limited ecosystems like the Namib gravel plains. Spectral emissivity derived from thermal infrared remote sensing has recently emerged as a promising tool for distinguishing between bare ground and non-green vegetation in arid environments. This study investigates the potential of satellite-derived emissivities for mapping changes in fractional vegetation cover across the Namib gravel plains. Analyzing Moderate Resolution Imaging Spectroradiometer (MODIS) band 29 (λ = 8.55 µm) emissivity time series from 2001 to 2021, our findings demonstrate the ability of both Normalized Difference Vegetation Index (NDVI) and emissivity to detect sudden vegetation growth on the gravel plains. Emissivity additionally allows monitoring the extent of desiccated grass over several years after a rainfall event. Our results support a relationship between the change in fractional vegetation cover, the amount of rainfall and emissivity change magnitude. Information from NDVI and emissivity therefore provide complementary information for assessing vegetation in arid environments.

Chemical Characteristics-Based Evolution of Groundwater in Tailan River Basin, Xinjiang, China

Groundwater plays an important role in ecological environment protection in arid and semi-arid areas. Therefore, understanding the characteristics of hydrochemical evolution is of great significance for the sustainable use of groundwater in the area of the Tailan River Basin. The Tailan River Basin is located in an arid, ecologically sensitive area in western China. In this study, we collected 42 groups of representative water samples from the Tailan River Basin and analyzed the chemical distribution in the groundwater using mathematical statistics, Piper and Gibbs diagrams, ion ratio analysis, and hydrogeochemical simulation methods. We also discussed the water–rock interactions in the groundwater hydrochemical evolutionary process. The results were as follows: (1) The chemical types of groundwater changed from HCO3·SO4-Ca·Na to SO4·Cl-Na·Ca, Cl·SO4-Na, and Cl-Na, and the total dissolved solids content increased from less than 1 g/L to more than 40 g/L from the gravel plain to the fine soil plain. (2) The Gibbs diagram, the ion ratio analysis, and the saturation index showed that the groundwater chemical characteristics in the study area were mainly controlled by water–rock interactions, as well as evaporation and concentration. Along the runoff of groundwater, halite and gypsum were dissolved. Nevertheless, dolomite and calcite precipitated. The relationship between the chlor-alkali index and [(Na+ + K+)-Cl−] and [(Ca2+ + Mg2+)-(HCO3− + SO42−)] indicated that cation exchange also affects the chemical composition of groundwater in the area. (3) Through reverse hydrogeochemical simulation, the main water–rock effect of the groundwater runoff process revealed by qualitative analysis was quantitatively verified.

Chemical Composition and in vitro Biological Evaluation of Essential Oil of Euphorbia larica Boiss from Northern Oman

Euphorbia larica Boiss is a wild plant of Oman that grows in gravel plains, mountain areas, desert and forest edges. Traditionally, Omani herbalists use the Euphorbia larica plant extract, resins, latex and juice to treat a wide range of ailments such as wounds, burns, insect bites, intestinal parasites, gonorrhea, eye infections, migraines and warts. In an attempt to scientifically validate the traditional uses and to unlock its hidden therapeutic potential, in this work, the chemical composition of E. larica essential oil (ELEO) along with its potential antioxidant, antimicrobial and hemolytic activities is studied. In a GC-MS analysis of the extracted ELEO, 29 chemicals were found, with α-pinene (27.36%) and limonene (11.5%) being the two most abundant volatile components. ELEO exhibited moderate inhibition of DPPH radicals (17.74-40.96%; IC50 = 48.97 μg/mL). It showed antimicrobial activity against S. aureus, B. subtilis, S. pyogenes, E. coli, P. aeruginosa and P. vulgaris but was ineffective against K. pneumoniae bacterial strain however, exhibited the maximum antibacterial activity against S. aureus (11 ± 1.3 and 15 ± 2.3 mm at 5 and 10 μL concentrations, respectively) comparable to positive antibiotic ampicillin (15 mm at 5 μg). Moreover, ELEO exhibited a very low hemolytic activity (0.12 to 1.1% at doses of 50-500 μg) suggesting it to be non-toxic to human or animal cells. Thus, it can be concluded that E. larica plant holds significant potential as a source of bioactive agents for the development of novel therapeutics.

Nematode communities indicate diverse soil functioning across a fog gradient in the Namib Desert gravel plains.

Soil nematodes are fundamentally aquatic animals, requiring water to move, feed, and reproduce. Nonetheless, they are ubiquitous in desert soils because they can enter an anhydrobiotic state that allows them to persist when water is biologically unavailable. In the hyper‐arid Namib Desert of Namibia, rain is rare, but fog routinely moves inland from the coast and supports plant and animal life. Very little is understood about how this fog may affect soil organisms. We investigated the role of fog moisture in the ecology of free‐living, soil nematodes across an 87‐km fog gradient in the gravel plains of the Namib Desert. We found that nematodes emerged from anhydrobiosis and became active during a fog event, suggesting that they can utilize fog moisture to survive. Nematode abundance did not differ significantly across the fog gradient and was similar under shrubs and in interplant spaces. Interplant soils harbor biological soil crusts that may sustain nematode communities. As fog declined along the gradient, nematode diversity increased in interplant soils. In areas where fog is rare, sporadic rainfall events can stimulate the germination and growth of desert ephemerals that may have a lasting effect on nematode diversity. In a 30‐day incubation experiment, nematode abundance increased when soils were amended with water and organic matter. However, these responses were not evident in field samples, which show no correlations among nematode abundance, location in the fog gradient, and soil organic matter content. Soil nematodes are found throughout the Namib Desert gravel plains under a variety of conditions. Although shown to be moisture‐ and organic matter‐limited and able to use moisture from the fog for activity, variation in fog frequency and soil organic matter across this unique ecosystem may be biologically irrelevant to soil nematodes in situ.

Respiratory Strategies in Relation to Ecology and Behaviour in Three Diurnal Namib Desert Tenebrionid Beetles.

Simple SummaryThe Namib Desert has a large diversity of darkling beetle species. All these beetles are flightless and feed on plant detritus. The aim of this study was to investigate whether the respiration strategies used by these beetles are linked to their ecology and behaviour. Three beetle species, which are all active during the day, in direct sunshine, running across the sand surface, but found in different parts of the desert, were chosen for this study. All three beetle species used intermittent breathing. They held their breath for several minutes and released CO2 in pulses. The large beetle species, which runs rapidly on the dune slip face when air temperatures are high, used evaporative cooling to prevent over-heating. The water used in the cooling comes from their respiratory surfaces and is replenished from metabolising food and drinking water droplets left on vegetation after a fog event. The two smaller beetle species, which inhabit the gravel plains, limit the area of the respiratory surface exposed to the atmosphere, which reduces body water loss. These two beetle species are not known to drink water, and thus have a greater need to conserve their body water.The respiratory physiology of three diurnal ultraxerophilous tenebrionid beetles inhabiting either the dune slipface or gravel plain in the Namib Desert was investigated. The role of the mesothoracic spiracles and subelytral cavity in gas exchange was determined by flow-through respirometry. All three species exhibited the discontinuous gas exchange cycles with a distinct convection based flutter period and similar mass specific metabolic rates. There was variation in their respiration mechanics that related to the ecology of the species. The largest beetle species, Onymacris plana, living on the dune slipface, has a leaky subelytral cavity and used all its spiracles for gas exchange. Thus, it could use evaporative cooling from its respiratory surface. This species is a fog harvester as well as able to replenish water through metabolising fats while running rapidly. The two smaller species inhabiting the gravel plains, Metriopus depressus and Zophosis amabilis, used the mesothoracic spiracles almost exclusively for gas exchange as well as increasing the proportional length of the flutter period to reduce respiratory water loss. Neither species have been reported to drink water droplets, and thus conserving respiratory water would allow them to be active longer.

Long-Term Population Dynamics of Namib Desert Tenebrionid Beetles Reveal Complex Relationships to Pulse-Reserve Conditions.

Simple SummaryRain seldom falls in the extremely arid Namib Desert in Namibia, but when a certain amount falls, it causes seeds to germinate, grass to grow and seed, dry, and turn to litter that gradually decomposes over the years. It is thought that such periodic flushes and gradual decay are fundamental to the functioning of the animal populations of deserts. This notion was tested with litter-consuming darkling beetles, of which many species occur in the Namib. Beetles were trapped in buckets buried at ground level, identified, counted, and released. The numbers of most species changed with the quantity of litter, but some mainly fed on green grass and disappeared when this dried, while other species depended on the availability of moisture during winter. Several species required unusually heavy rainfalls to gradually increase their populations, while others the opposite, declining when wet, thriving when dry. All 26 beetle species experienced periods when their numbers were extremely low, but all had the capacity for a few remaining individuals to repopulate the area in good times. The remarkably different relationships of these beetles to common resources, litter, and moisture, explain how so many species can exist side by side in such a dry environment.Noy-Meir’s paradigm concerning desert populations being predictably tied to unpredictable productivity pulses was tested by examining abundance trends of 26 species of flightless detritivorous tenebrionid beetles (Coleoptera, Tenebrionidae) in the hyper-arid Namib Desert (MAP = 25 mm). Over 45 years, tenebrionids were continuously pitfall trapped on a gravel plain. Species were categorised according to how their populations increased after 22 effective rainfall events (>11 mm in a week), and declined with decreasing detritus reserves (97.7–0.2 g m−2), while sustained by nonrainfall moisture. Six patterns of population variation were recognised: (a) increases triggered by effective summer rainfalls, tracking detritus over time (five species, 41% abundance); (b) irrupting upon summer rainfalls, crashing a year later (three, 18%); (c) increasing gradually after series of heavy (>40 mm) rainfall years, declining over the next decade (eight, 15%); (d) triggered by winter rainfall, population fluctuating moderately (two, 20%); (e) increasing during dry years, declining during wet (one, 0.4%); (f) erratic range expansions following heavy rain (seven, 5%). All species experienced population bottlenecks during a decade of scant reserves, followed by the community cycling back to its earlier composition after 30 years. By responding selectively to alternative configurations of resources, Namib tenebrionids showed temporal patterns and magnitudes of population fluctuation more diverse than predicted by Noy-Meir’s original model, underpinning high species diversity.

Long-term column-averaged greenhouse gas observations using a COCCON spectrometer at the high-surface-albedo site in Gobabeb, Namibia

Abstract. In this study, we present column-averaged dry-air mole fractions of CO2 (XCO2), CH4 (XCH4) and CO (XCO) from a recently established measurement site in Gobabeb, Namibia. Gobabeb is a hyperarid desert site at the sharp transition zone between the sand desert and the gravel plains, offering unique characteristics with respect to surface albedo properties. Measurements started in January 2015 and are performed utilizing a ground-based Fourier transform infrared (FTIR) EM27/SUN spectrometer of the COllaborative Carbon Column Observing Network (COCCON). Gobabeb is the first measurement site observing XCO2 and XCH4 on the African mainland and improves the global coverage of ground-based remote-sensing sites. In order to achieve the high level of precision and accuracy necessary for meaningful greenhouse gas observations, we performed calibration measurements for 8 d between November 2015 and March 2016 with the COCCON reference EM27/SUN spectrometer operated at the Karlsruhe Institute of Technology. We derived scaling factors for XCO2, XCH4 and XCO with respect to the reference instrument that are close to 1.0. We compare the results obtained in Gobabeb to measurements from the Total Carbon Column Observing Network (TCCON) sites at Réunion Island and Lauder. We choose these TCCON sites because, while 4000 km apart, the instruments at Gobabeb and Réunion Island operate at roughly the same latitude. The Lauder station is the southernmost TCCON station and functions as a background site without a pronounced XCO2 seasonal cycle. We find a good agreement for the absolute Xgas values, apart from an expected XCH4 offset between Gobabeb and Lauder due to significantly different tropopause height, as well as representative intraday variability between TCCON and COCCON. Together with the absence of long-term drifts, this highlights the quality of the COCCON measurements. In the southern hemispheric summer, we observe lower XCO2 values at Gobabeb compared to the TCCON stations, likely due to the influence of the African biosphere. We performed coincident measurements with the Greenhouse Gases Observing Satellite (GOSAT), where GOSAT observed three nearby specific observation points, over the sand desert south of the station, directly over Gobabeb and over the gravel plains to the north. GOSAT H-gain XCO2 and XCH4 agree with the EM27/SUN measurements within the 1σ uncertainty limit. The number of coincident soundings is limited, but we confirm a bias of 1.2–2.6 ppm between GOSAT M-gain and H-gain XCO2 retrievals depending on the target point. This is in agreement with results reported by a previous study and the GOSAT validation team. We also report a bias of 5.9–9.8 ppb between GOSAT M-gain and H-gain XCH4 measurements which is within the range given by the GOSAT validation team. Finally, we use the COCCON measurements to evaluate inversion-optimized CAMS model data. For XCO2, we find high biases of 0.9 ± 0.5 ppm for the Orbiting Carbon Observatory-2 (OCO-2) assimilated product and 1.1 ± 0.6 ppm for the in situ-driven product with R2 > 0.9 in both cases. These biases are comparable to reported offsets between the model and TCCON data. The OCO-2 assimilated model product is able to reproduce the drawdown of XCO2 observed by the COCCON instrument at the beginning of 2017, as opposed to the in situ-optimized product. Also, for XCH4, the observed biases are in line with prior model comparisons with TCCON.

Regional analysis of evapotranspiration changes in an arid river basin using satellite observations

This study employed the Surface Energy Balance System (SEBS) algorithm to determine the actual evapotranspiration (ET) trends of the piedmont plain region of the Golmud River Basin between 2001 and 2016 and the effects of climate change and human activities on ET. The results indicate that the regional ET increased in the study area from 2001 to 2016. However, ET trends exhibited no change in most parts (68.58%) of the study area over the 16 years, but increased significantly in saline ponds and areas with vegetation cover, whereas no significant trends were observed in saline marshes or the piedmont Gobi gravel plain. The ET trend was closely related to the land-cover changes caused by human activities in the Golmud River Basin. During the study period, saline pond areas expanded from 50.57 to 257.85 km2 due to potash fertilizer production, and the area of farmland increased from 28.71 to 62.91 km2 and these changes contributed greatly to ET changes. Also, groundwater exploitation for potash fertilizer production and irrigation were correlated with ET.

Spatial variability of the groundwater exploitation potential in an arid alluvial-diluvial plain using GIS-based Dempster-Shafer theory

The spatial distribution of groundwater exploitation potential is crucial to the sustainable management of groundwater resources in arid regions. In this study, an effective approach is developed using a geographical information system (GIS) associated with the Dempster-Shafer (D-S) theory to explore the groundwater exploitation potential based on eight factors (i.e., slope, topography, distance to a river, canal density, lithology, land use, soil type, and groundwater depth)based on pumping test data. Different groundwater exploitation potential zones from the output map are classified as follows: very high (0.67–0.96, 8% of the total area), high (0.45–0.67, 8%), moderate (0.27–0.45, 14%), low (0.13–0.27, 24%), and very low (0.02–0.13, 46%), in the Hotan River basin of the Kunlun Mountains, China. The groundwater exploitation potential is highest in a gravel plain in the piedmont and gradually decreases from the piedmont to the desert. The predicted spatial variability of groundwater exploitation potential can explain 71% of the measured pumping test data. The uncertainty of the groundwater exploitation potential map ranges from <1 to 25.5% across the basin. The Dempster-Shafer model can reflect the combined influence of various factors affecting the groundwater exploitation potential, and the system uncertainty is within the belief functions. Based on the developed method, decision-makers are provided with an effective and reliable tool for groundwater exploitation and sustainable management.

Physiological responses of the xerohalophyte Suaeda vermiculata to salinity in its hyper-arid environment

Few plants can survive and grow equally well in salty and salt-free substrates (i.e., habitat-indifferent halophytes). Such plants provide a good opportunity to understand physiological and biochemical mechanisms underlying salinity tolerance. In this study, we investigated the environmental salinity impacts on several physiological and biochemical features of Suaeda vermiculata, a habitat-indifferent halophyte. Samples of different organs were collected from S. vermiculata from both a highly saline marsh habitat (HSMH) and non-salty gravel plain (NSGP) for the determination of the following physiological and bio-chemical features: chlorophyll and carotenoids, proline, malondialdehyde (MDA), and hydrogen peroxide (H2O2), antioxidant enzymes (Catalase, CAT; guaiacol peroxidase, GPX; Ascorbate peroxidase, APX) activities. Elemental compositions in soil and plant samples from both habitats were also assessed. Results showed that plants from HSMH had significantly lower values of chlorophyll a, b, carotenoids and leaf biomass, compared to those from NSGP. Roots from HSMH attained higher levels of antioxidant enzymes (CAT, GPX, APX) and lower values of reactive oxygen species (MDA and H2O2), indicating that the enzymes are more likely scavenging the reactive oxygen species (ROS). The enzyme activities and ROS levels were much lower in the shoots of both HSMH and NSGP than in roots. Accumulation of sodium was higher in leaves and shoots than roots of S. vermiculata. This study indicates that Suaeda vermiculata is a salt tolerant plant with adaptations to different environments through down-regulation of different biochemical and physiological features to avoid oxidative stress.

Assessment of the heterogeneity of hydraulic properties in gravelly outwash plains: a regionally scaled sedimentological analysis in the Munich gravel plain, Germany

The favorable overall conditions for the utilization of groundwater in fluvioglacial aquifers are impacted by significant heterogeneity in the hydraulic conductivity, which is related to small-scale facies changes. Knowledge of the spatial distribution of hydraulically relevant hydrofacies types (HF-types), derived by sedimentological analysis, helps to determine the hydraulic conductivity distribution and thus contribute to understanding the hydraulic dynamics in fluvioglacial aquifers. In particular, the HF-type “open framework gravel (OW)”, which occurs with the HF-type “bimodal gravel (BM)” in BM/OW couplings, has an intrinsically high hydraulic conductivity and significantly impacts hydrogeological challenges such as planning excavation-pit drainage or the prognosis of plumes. The present study investigates the properties and spatial occurrence of HF-types in fluvioglacial deposits at regional scale to derive spatial distribution trends of HF-types, by analyzing 12 gravel pits in the Munich gravel plain (southern Germany) as analogues for outwash plains. The results are compared to the reevaluation of 542 pumping tests. Analysis of the HF-types and the pumping test data shows similar small-scale heterogeneities of the hydraulic conductivity, superimposing large-scale trends. High-permeability BM/OW couples and their dependence on recognizable discharge types in the sedimentary deposits explain sharp-bounded small-scale heterogeneities in the hydraulic conductivity distribution from 9.1 × 10−3 to 2.2 × 10−4 m/s. It is also shown that high values of hydraulic conductivity can be interpolated on shorter distance compared to lower values. While the results of the HF-analysis can be transferred to other fluvioglacial settings (e.g. braided rivers), regional trends must be examined with respect to the surrounding topography.

The potential contribution of soil moisture to fog formation in the Namib Desert

Fog is considered as an important source of water in many drylands, and the knowledge of possible sources of its formation is essential to make future predictions. Prior studies have suggested the presence of locally generated fog in drylands; however, its formation mechanism remains unclear. There have been studies on the effects of fog on soil moisture dynamics. On the contrary, no research has yet been conducted to understand the potential contribution of soil moisture to fog formation. This study, therefore, for the first time intends to examine such possibility in a fog-dominated dryland ecosystem, the Namib Desert. The study was conducted at two sites representing two different land forms (sand dunes and gravel plains) in the Namib Desert. We first examined evidence of fog formation through water vapor movement using field observations, and then simulated water vapor transport using HYDRUS-1D model. In the first part of the study, soil moisture, soil temperature and air temperature data were analyzed, and the relationships between these variables were taken as one of the key indicators for the linkage between soil water and fog formation. The analysis showed that increase in soil moisture generally corresponded to similar increase in air or soil temperature near the soil surface, which implied that variation in soil moisture might be the result of water vapor movement (evaporated soil moisture) from lower depths to the soil surface. In the second part of the study, surface fluxes of water vapor were simulated using the HYDRUS-1D model to explore whether the available surface flux was sufficient to support fog formation. The surface flux and cumulative evaporation obtained from the model showed positive surface fluxes of water vapor. Based on the field observations and the HYDRUS-1D model results, it can be concluded that water vapor from soil layers is transported through the vadose zone to the surface and this water vapor likely contributes to the fog formation in fog-dominated drylands, like the Namib Desert.

Geostatistische Reliefmodellierung der quartären Grundwasserleiterbasis in der Münchener Schotterebene unter Verwendung von Massendaten

Geostatistische Reliefmodellierung der quartären Grundwasserleiterbasis in der Münchener Schotterebene unter Verwendung von Massendaten

Flora, Life-forms and Biological Spectrum of Muscat Governorate

The present study evaluated the floristic diversity and biological spectrum of Muscat Governorate, Sultanate of Oman. Heterogeneity of geology, topography and climatic conditions create a diverse array of habitats across the 3,500 km2 study area, supporting a range of vegetation types. Floristic studies were conducted across 42 sites spread across various habitats such as sand dunes, alluvial plains, gravel plains, wadis, saline dunes, coastal areas, farms and plantations. A total of 264 species belonging to 183 genera and 54 families of angiosperm were documented during sampling the vegetation from different wilayats of the Governorate. Maximum number of species belonged to family Fabaceae (14%), followed by Poaceae (12%), Asteraceae (9%), Apocyanaceae and Convolvulaceae (3.66% each), Boraginaceae, Euphorbiaceae, Zygophylaceae (3% each). The biological spectrum was represented almost equally by therophytes and chamaephytes (30% each), followed by phanerophytes (19%), hemicryptophytes (13%), geophytes (4%), liana (3%), and helophytes (1%). The dominance of therophytes and chamaephytes reveals a thero-chamaephytic phytoclimate for the area. Different habitats are characterized by specific plant assemblages, but some species such as Acacia tortilis, Prosopis cineraria, Tetraena quaterense, Dipterygium gluacum, Aerva javanica, due to their broad ecological tolerance and adaptations, inhabit more than one habitats, while some species are just confined to certain parts of a single habitat.

Chemical Characteristics and Sources of Groundwater Sulfate in the Kashgar Delta, Xinjiang

Poor water quality is one of the main reasons for a shortage of water resources in the Kashi Prefecture of Xinjiang. The area of the Kashgar Delta is located in the western Kashi Prefecture and covers an area of 13329 km2. Sulfate concentrations exceed the groundwater quality class Ⅲ standard in 73.2% of the unconfined groundwater area and 53.2% of the confined groundwater area. In addition, the TDS content and the hardness of the water are high. The chemical characteristics and sources of groundwater sulfate were analyzed using the isotope method of δD, δ18O-H2O, and δ34S-SO42-. The results showed that:① The dissolution of salt minerals such as calcareous siltstone, calcareous sandstone, and gypsum in denuded mountainous areas controlled the hydrochemical compositions in the basin, and formed surface water and groundwater dominated by SO4. The evolution trend of groundwater chemical types was HCO3·SO4→SO4→SO4·Cl. Alluvial-proluvial gravel plains on the piedmont slopes were the main dissolution-runoff zones and the fluvial plain was main runoff accumulation zone. The chemical composition of the groundwater tended towards salinization, from upstream to downstream; ② The groundwater was mainly derived from atmospheric precipitation, and was affected by evaporation. The distribution of δD and δ18O in the groundwaters of different hydrogeological units was clear; from upstream to downstream, isotope values varied from low to increasingly higher, and evaporation varied from weak to strong, respectively. The distribution of isotopes in the confined groundwater in the river alluvium plain was relatively discrete, which was affected by the mixing effect of unconfined groundwater; ③ The sources of sulfate in the unconfined groundwater in the southern and northern alluvial-proluvial gravel plains on the piedmont slopes were the dissolution of marine-terrestrial and terrestrial deposit evaporates, respectively. The sources of sulfate in the unconfined groundwater in the river alluvium plain were the dissolution of terrestrial deposit evaporates and pollution from fertilizer leaching. The confined groundwater was affected by the dissolution of evaporates and the mixing effect of the unconfined groundwater and bacterial sulfate reduction.

Ancient landscapes of the Namib Desert harbor high levels of genetic variability and deeply divergent lineages for Collembola.

AimTo assess spatial patterns of genetic and species‐level diversity for Namib Desert Collembola using mitochondrial DNA cytochrome c oxidase subunit I (COI) gene sequences.LocationNamib Desert gravel plains.TaxonCollembola (springtails).MethodsA total of 77 soil samples were collected along NE‐SW (60 km) and E‐W (160 km) transects from within a 4,000 km2 area of the Namib Desert gravel plains. We extracted 434 springtails from the 37 samples which contained Collembola and sequenced them at the COI gene locus. In the absence of specific taxonomic keys and previous genetic data for these taxa, we used Generalized Mixed Yule Coalescent (GMYC) analyses to provide putative species‐level designations.ResultsWe obtained 341 successful COI sequences, 175 of which were unique haplotypes. GMYC analyses identified 30 putative species, with up to 28% sequence divergence (uncorrected p‐distance). The distribution of genetic variants was disjunct, with 97% of haplotypes and 70% of “GMYC species” found only at single sites.Main conclusionsDispersal events, although rare, may be facilitated by environmental events such as prevailing onshore winds or occasional flow of rainwater to the coast. We conclude that the high genetic diversity we observed is the result of ancient springtail lineages, patchy distribution of suitable habitats, and limited dispersal (gene flow) among habitable locations.

How soil granulometry, temperature, and water predict genetic differentiation in Namibian spiders (Ariadna: Segestriidae) and explain their behavior.

The Namib Desert is a biodiversity hotspot for many invertebrates, including spiders. Tube‐dwelling spiders belonging to the Ariadna genus are widespread in gravel plains. These sit‐and‐wait predators share a particular behavior, as they spend their life in tunnels in the soil, surrounding the entrance of their burrow with stone rings. We investigated five spider populations taking into account environmental parameters, functional traits, and molecular data. We have chosen the temperature at the soil surface and at the bottom of the burrow, the air humidity, and the soil granulometry to define the environment. The chosen functional traits were the diameter and depth of the burrows, the ratio between weight and length, the thermal properties of their silks, and the number of ring elements. The molecular branch lengths and the evolutionary distance emerging from cytochrome oxidase I gene sequences summarized the molecular analysis. Our study highlights a strong coherence between the resulting evolutionary lineages and the respective geographical distribution. Multivariate analyses of both environmental and molecular data provide the same phylogenetic interpretation. Low intrapopulation sequence divergence and the high values between population sequence divergence (between 4.9% and 26.1%) might even suggest novel taxa which deserve further investigation. We conclude that both the Kimura distance and the branch lengths are strengthening the environmental clustering of these peculiar sites in Namibia.

Hydrochemistry and Environmental Isotope Characteristics of Groundwater in the Kashgar Delta Area, Xinjiang, China

The hydrochemistry and isotope characteristics of groundwater of different geomorphological types in the Kashgar Delta in Xinjiang, China were studied using isotopic analyses. Results showed that groundwater in the study area was mainly recharged by precipitation, and that groundwater ions were mainly affected by weathering-leaching of rock salt and evaporite as well as cation exchange and adsorption. The hydrochemical type of phreatic water in the alluvial-proluvial gravel plain on the piedmont slope was mainly SO4·Cl-Na·Ca type, with an average age of 28a. The phreatic water in the fluvial plain had shallow buried depth and intense evaporation. The hydrochemical type of phreatic water varied from SO4·Cl-Na·Mg type to Cl-Na type, with an average age of 99a. Along the groundwater flow path eastward, confined water in the fluvial plain experienced gradually more intense cation exchange and stronger adsorption. The major hydrochemical type of which was the SO4·Cl-Na type, with an average age of 30405a.