Playa Sediments Research Articles

Persistent microbial communities in hyperarid subsurface habitats of the Atacama Desert: Insights from intracellular DNA analysis.

Desert environments constitute one of the largest and yet most fragile ecosystems on Earth. Under the absence of regular precipitation, microorganisms are the main ecological component mediating nutrient fluxes by using soil components, like minerals and salts, and atmospheric gases as a source for energy and water. While most of the previous studies on microbial ecology of desert environments have focused on surface environments, little is known about microbial life in deeper sediment layers. Our study is extending the limited knowledge about microbial communities within the deeper subsurface of the hyperarid core of the Atacama Desert. By employing intracellular DNA extraction and subsequent 16S rRNA sequencing of samples collected from a soil pit in the Yungay region of the Atacama Desert, we unveiled a potentially viable microbial subsurface community residing at depths down to 4.20 m. In the upper 80 cm of the playa sediments, microbial communities were dominated by Firmicutes taxa showing a depth-related decrease in biomass correlating with increasing amounts of soluble salts. High salt concentrations are possibly causing microbial colonization to cease in the lower part of the playa sediments between 80 and 200 cm depth. In the underlying alluvial fan deposits, microbial communities reemerge, possibly due to gypsum providing an alternative water source. The discovery of this deeper subsurface community is reshaping our understanding of desert soils, emphasizing the need to consider subsurface environments in future explorations of arid ecosystems.

GEOCHRONOLOGY OF PHREATOPHYTIC MOUNDS ON THE ATMUR EL KIBIESH, EGYPT: WITH DESCRIPTIONS OF PLANTS COLLECTED DURING THE EXPEDITION TO THE EASTERN SAHARA, EGYPT, AND SUDAN (APPENDIX I)

ABSTRACTIn the hyperarid eastern Sahara, west of the Nile River in Egypt, areas with vegetated eolian mounds have attracted people and animals because of shallow groundwater that at times of high water tables may be reached by hand digging shallow wells. An eolian phreatophytic mound with a living arak bush (Silvadora persica L.) on top, one of three known from this region of SW Egypt, provided a stratigraphic record of its growth. The geochronology of the mounds aggradation and that of a nearby tarfa mound (Tamarix nilatica Bunge) was determined by radiocarbon dating plant macrofossils within the stratigraphic succession. Eolian aggradation of the mound postdates deflation that eroded playa sediments of the Neolithic pluvial that ended ca. 5000 BP and appears to be due to a resurgence of the shallow aquifer. Subsequent deflation of the mounds is apparently due to post-1500 BP aridity. Regional vegetation is described in the Appendix I.

Identification of Playa Lakes and tracking their evolution pathways using geochemical models in the Great Indian Thar desert

Playa Lakes of the great Indian Thar desert are unique and intricate systems with pronounced scientific and ecological significance. In this study, the combined use of geospatial and field data assisted in depicting and understanding the changes within these natural systems. The purpose of this study is to provide a comprehensive dataset of Playa Lakes, which can help with an overall understanding of playas from a geochemical, ecological, and economic perspective. The 1163 surface depressions were accounted for as possible playas considering the threshold area of >5 km2. A total of 42 representative surface water samples were collected from the selected playas (Sambhar, Didwana, Pachpadra, and Pokhran) and hydrochemical analysis was carried out to identify the geochemical evolution of these playas. The major water types were Na-Cl and Na-Cl-SO4 type. Hardie and Eugster's model was used to explain the possible sequence of evolutionary pathways and brine shifts in the selected playas. Simulated evaporation modeling suggests precipitation of major evaporites (chlorides, carbonates, and sulphates) during progressive evaporation. Surface sediment analysis (X-ray diffraction) confirmed the presence of calcite, halite, and dolomite minerals in the playa sediments. The greatest economic value of these playas may be mineral production and providing food and habitat for migratory birds, though conservation and policy intervention is required for their sustainable utilization.

Deciphering Depositional Environment of Playa Lakes Using Grain Size Parameters in the Arid and Semi-Arid Region of Rajasthan, India

This study encompasses the grain size distribution of the playa lakes (Pachpadra, Pokhran, and Didwana) of the Thar Desert in Rajasthan, India. The grain size of sediment particles is the most fundamental feature, giving essential information regarding their origin, transport history, and depositional conditions. The aeolian and fluvial transport processes were evaluated through environmentally sensitive grain size subpopulations to identify the differential sedimentary sources and dynamics in the playas. End-member modelling further determined the sediment grain size distribution through statistical analysis. The playa sediments mainly consist of very fine sand (46–54%) and very coarse silt (22–37%). The results show that the average fine fraction of Pachpadra, Pokhran, and Didwana playa was 46.29%, 66.11%, and 66.28%, respectively. In contrast, the average coarser fraction deposition in Pachpadra, Pokhran, and Didwana corresponds to 53.71%, 33.89%, and 33.72%, respectively. This suggests that the playas mostly contain aeolian sediment rather than fluvial sediment transported by dust/sand storms. Additionally, the textural pattern and depositional distribution of the sediments determined through the Passega CM diagram and bivariate plots indicate that 82% of the samples were poorly sorted, and 18% were very poorly sorted. Furthermore, an environmentally sensitive grain size component (ESGSC) was also assessed to identify the spatial variability and transport processes of sediment between these playas. Three ESGSCs in Pokhran (250 µ, 31 µ, and 2 µ) and Pachpadra (125 µ, 31 µ, and 4 µ), while two ESGSCs in Didwana playa (125 µ and 16 µ) were identified, indicating sediment deposition with moderate velocity in a low energy environment with a mixed sediment population transported by aeolian and fluvial activities.

Applying Wind Erosion and Air Dispersion Models to Characterize Dust Hazard to Highway Safety at Lordsburg Playa, New Mexico, USA

Lordsburg Playa, a dry lakebed in the Chihuahuan Desert of southwestern New Mexico (USA), is crossed by Interstate Highway 10 (I-10). Dust from the playa threatens highway safety and has caused dozens of fatal accidents. Two numerical models—the U.S. Department of Agriculture’s Single-Event Wind Erosion Evaluation Program (SWEEP) and the American Meteorological Society and U.S. Environmental Protection Agency Regulatory Model (AERMOD)—were used to simulate and predict the generation and dispersion of windblown soil, dust, and PM10 from playa hotspots and estimate PM10 concentrations downwind. SWEEP simulates soil loss and particulate matter emissions from the playa surface, and AERMOD predicts the concentration of transported dust. The modeling was informed by field and laboratory data on Lordsburg Playa’s properties, soil and land use/land cover databases, and weather data from meteorological stations. The integrated models predicted that dust plumes originating on the playa—including a large, highly emissive area away from the highway and a smaller, less emissive site directly upwind of the interstate—can lead to hourly average PM10 concentrations of tens, to hundreds of thousands, of micrograms per cubic meter. Modeling results were consistent with observations from webcam photos and visibility records from the meteorological sites. Lordsburg Playa sediment contains metals, as will its dust, but human exposures will be short-term and infrequent. This study was the first to successfully combine the SWEEP wind erosion model and the AERMOD air dispersion model to evaluate PM10 dispersion by wind erosion in a playa environment. With this information, land managers will be able to understand the potential levels of dust and PM10 exposure along the highway, and better manage human health and safety during conditions of blowing dust and sand at Lordsburg Playa.

Commentary and Review of Modern Environmental Problems Linked to Historic Flow Capacity in Arid Groundwater Basins

Environmental problems may develop in groundwater basins when water levels change due to long-term wetter or drier climate or land development. A term related to water-level elevation is flow capacity, which develops in aquifers when the water table is at or very close to land surface. Non-capacity develops in systems where the water table is too deep for capillary water to reach the land surface. Flow capacity is the maximum amount of water that an aquifer can transmit. Sufficient moisture is not available for flow capacity to be established in most aquifers in arid zones and these aquifers are at non-capacity, but many aquifers in today’s deserts were at flow capacity when paleoclimates were cooler and moister during the late Pleistocene. Climate change and anthropogenic activities can cause aquifers to move toward flow capacity but in the last 15,000 years, almost always toward non-capacity. This paper reviews environmental and geotechnical problems associated with the transition of groundwater basins from flow capacity to non-capacity, and vice versa. Five relevant topics are discussed and evaluated: (1) The effects of flow capacity and non-capacity on groundwater basins targeted for waste repositories; (2) The salt contamination of groundwater where flow capacity was present in the Late Pleistocene and is no longer present; (3) Trace element enrichment in salt crusts in playa sediments and environmental risks to groundwater when the flow systems transition from flow capacity to non-capacity; (4) The development and retention of environmental tracers in arid groundwater flow systems at flow capacity that cannot be explained under conditions of non-capacity; and (5) The relationship of flow capacity to fossil hydraulic gradients and non-equilibrium conditions where there is little groundwater extraction. A case example is provided with each of these topics to demonstrate relevance and to provide an understanding of topics as they relate to land management.

The role of Mineral Salts Company in pollution of Mighan playa sediments with heavy metals by contamination indices and multivariate analysis methods, Arak, Iran

Heavy metal concentrations were investigated for 30 sediments collected from different regions of Mighan playa/lake. The means of Cr, Cu, Ni, Pb, Zn and Cd in sediments of playa/lake were much lower than the soil guideline values of Iran and background values of region (BVR). However, the maximum concentrations of Cr, Cu, Ni, Pb, Zn and Cd were higher than BVR. Only 7% of Cr, Cu, Zn, 27% of Ni, 14% of Pb and 38% of Cd concentration exceeded the BVR. The heavy metal Cr, Zn, Ni and Cd are the most important metals in different land use. About 11% of the samples in the lake land use contain Cr, Ni, Pb and Zn has concentration higher than BVR. The concentration of Cu and Pb in 33% and 67% samples is exceeded the BVR in Tail of Mineral Salts Company. The spatial distribution patterns of Cr, Cu, Ni, Pb, Zn and Cd were generally similar and increase from Mighan playa/lake to the Arak city and mainly affected by anthropogenic sources. Among the six types of land use, the concentrations of Cr, Ni, Zn and Cd in the range land and wastewater sludge were significantly higher than those in the other land use (p <0.05). In factor analysis, Cr, Ni, Zn in factor1, Cd in factor2, Pb, Zn in factor3 were originated from the municipal sewage, industrial plants activities and Arak urban traffic.

Stratigraphic relationship between Rimrock Lake and Hay Lake playa sediments, Harney Basin, eastern Oregon, USA

Stratigraphic relationship between Rimrock Lake and Hay Lake playa sediments, Harney Basin, eastern Oregon, USA

The Role of Hydrates, Competing Chemical Constituents, and Surface Composition on ClNO2 Formation.

Atomic chlorine (Cl•) affects air quality and atmospheric oxidizing capacity. Nitryl chloride (ClNO2) - a common Cl• source-forms when chloride-containing aerosols react with dinitrogen pentoxide (N2O5). A recent study showed that saline lakebed (playa) dust is an inland source of particulate chloride (Cl-) that generates high ClNO2. However, the underlying physiochemical factors responsible for observed yields are poorly understood. To elucidate these controlling factors, we utilized single particle and bulk techniques to determine the chemical composition and mineralogy of playa sediment and dust samples from the southwest United States. Single particle analysis shows trace highly hygroscopic magnesium and calcium Cl-containing minerals are present and likely facilitate ClNO2 formation at low humidity. Single particle and mineralogical analysis detected playa sediment organic matter that hinders N2O5 uptake as well as 10 Å-clay minerals (e.g., Illite) that compete with water and chloride for N2O5. Finally, we show that the composition of the aerosol surface, rather than the bulk, is critical in ClNO2 formation. These findings underscore the importance of mixing state, competing reactions, and surface chemistry on N2O5 uptake and ClNO2 yield for playa dusts and, likely, other aerosol systems. Therefore, consideration of particle surface composition is necessary to improve ClNO2 and air quality modeling.

Dust emission source characterization for visibility hazard assessment on Lordsburg Playa in Southwestern New Mexico, USA

In drylands around the world, ephemeral lakes (playas) are common. Dry, wind-erodible playa sediments are potent local and regional sources of dust and PM10 (airborne particles with diameters less than 10 μm). Dust clouds often cause sudden and/or prolonged loss of visibility to travelers on downwind roadways. Lordsburg Playa, in southwestern New Mexico, USA is bisected by Interstate Highway 10. Dust storms emanating from the playa have been responsible for numerous visibility-related road closures (including 39 road closures between 2012 and 2019) causing major economic losses, in addition to well over a hundred dust-related vehicle crashes causing at least 41 lost lives in the last 53 years. In order to improve understanding of the surfaces responsible for the dust emissions, we investigated the critical wind friction velocity thresholds and the dust emissivities of surfaces representing areas typical of Lordsburg Playa’s stream deltas, shorelines, and ephemerally flooded lakebed using a Portable In-Situ Wind ERosion Laboratory (PI-SWERL). Mean threshold friction velocities for PM10 entrainment ranged from less than 0.30 m s− 1 for areas in the delta and shoreline to greater than 0.55 m s− 1 for ephemerally flooded areas of the lakebed. Similarly, we quantified mean PM10 vertical flux rates ranging from less than 500 μg m− 2 s− 1 for ephemerally flooded areas of lakebed to nearly 25,000 μg m− 2 s− 1 for disturbed delta surfaces. The unlimited PM10 supply of the relatively coarse sediments along the western shoreline is problematic and indicates that this may be the source area for longer-term visibility reducing dust events and should be a focus area for dust mitigation efforts.

A closer look at mineral aerosol emissions from the Makgadikgadi Pans, Botswana, using automated SEM-EDS (QEMSCAN®)

ABSTRACT This paper demonstrates the use of Quantitative Evaluation of Minerals by SCANning electron microscopy (QEMSCAN®), an automated scanning microscopy technique, which combines scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), providing ultra-fast analysis of particle grains at a micron-scale resolution. We evaluate its application in aerosol studies by comparing surface and airborne samples from the Makgadikgadi Pan in Botswana. The playa is a major global dust emitter and its aerosols have a widespread effect on atmospheric, biological and terrestrial processes. Sampling was conducted at a carefully selected surface location and associated BSNE dust trap stack at 0.25, 0.5, 0.85 1.65 meters. The dominant minerals identified here are quartz, halite, thernadite, mica, calcite and feldspar. Surface sample results from QEMSCAN are in line with other forms of elemental and mineralogical analyses. When comparing surface samples with elevated trap samples, we noted a fining and fractionation during grain entrainment, resulting in a compositional shift with height. We also observed some ultra-fine fraction losses from the BSNE traps. Overall, the single location here establishes the link between fluvial playa basin inputs, sediment storage, evaporation products and aeolian losses and outputs from a dry lake surface, not unlike semi-arid evaporative dust sources elsewhere.

Mega‐monsoon variability during the late Triassic: Re‐assessing the role of orbital forcing in the deposition of playa sediments in the Germanic Basin

AbstractThe formation of the supercontinent Pangaea during the Permo–Triassic gave rise to an extreme monsoonal climate (often termed ‘mega‐monsoon’) that has been documented by numerous palaeo‐records. However, considerable debate exists about the role of orbital forcing in causing humid intervals in an otherwise arid climate. To shed new light on the forcing of monsoonal variability in subtropical Pangaea, this study focuses on sediment facies and colour variability of playa and alluvial fan deposits in an outcrop from the late Carnian (ca225 Ma) in the southern Germanic Basin, south‐western Germany. The sediments were deposited against a background of increasingly arid conditions following the humid Carnian Pluvial Event (ca234 to 232 Ma). Theca2·4 Myr long sedimentary succession studied shows a tripartite long‐term evolution, starting with a distal mud‐flat facies deposited under arid conditions. This phase was followed by a highly variable playa‐lake environment that documents more humid conditions and finally a regression of the playa‐lake due to a return of arid conditions. The red–green (a*) and lightness (L*) records show that this long‐term variability was overprinted by alternating wet/dry cycles driven by orbital precession andca405 kyr eccentricity, without significant influence of obliquity. The absence of obliquity in this record indicates that high‐latitude forcing played only a minor role in the southern Germanic Basin during the late Carnian. This is different from the subsequent Norian when high‐latitude signals became more pronounced, potentially related to the northward drift of the Germanic Basin. The recurring pattern of pluvial events during the late Triassic demonstrates that orbital forcing, in particular eccentricity, stimulated the occurrence and intensity of wet phases. It also highlights the possibility that the Carnian Pluvial Event, although most likely triggered by enhanced volcanic activity, may also have been modified by an orbital stimulus.

Electrical resistivity tomography and forward modelling in hyper-arid environments – A case study from Erg Chebbi, Southeast Morocco

Electrical Resistivity Tomography (ERT), a non-destructive geophysical method, which aims at detecting sediment conductivity and its distribution including e.g. stratification, has been used in manifold research topics, for example in sedimentology, hydrology, and geoarchaeology. However, ERT is mostly used in humid to semi-arid environments, where at least some underground moisture guarantees reliable conductivity. In this paper, for the first time, the results of ERT measurements are presented in combination with geomorphological and sedimentological investigations in a dune-interdune system in a hyper-arid environment. Erg Chebbi in South-East Morocco and its environment is characterized by very scarce precipitation and high evaporation rates, which presents significant methodological challenges. Reliable results were generated, displaying a distinct border between a dune body and underlying playa sediments. In the latter, a thin calcareous crust, found by drillings, was not displayed by ERT data due to their spacial depth resolution. To account for this, robust resistivity forward modelling was used to explain contradicting ERT data and corresponding sedimentological results. Major advantages and drawbacks concerning measurement set ups, modelling and interpretation arc finally discussed. In conclusion, the use of ERT in a hyper-arid environment proved to be a successful and reliable tool if mentioned precautions (in field validation, suitable array layout) are taken into account.

Sedimentology, mineralogy, and geochemistry of the Late Quaternary Meyghan Playa sediments, NE Arak, Iran: palaeoclimate implications

Playas are shallow ephemeral lakes that form in arid and semi-arid regions. Iran has a large number of playas such as Meyghan Playa, which is located in the northeast of Arak city that borders the central Iran and Sanandaj-Sirjan zones. This study aims to investigate the mineralogical, sedimentological, and geochemical characteristics of the playa sediments. In order to determine the palaeoenvironment, we carried out X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM) studies. Meyghan Playa sediments consist of very fine-grained sediments and contain both evaporite and clastic minerals. The evaporite minerals include calcite, gypsum, halite, glauberite, and thenardite, whereas clastic minerals are quartz and clay. The calcite abundance decreases from the margin to the central portion of the playa but gypsum and halite abundances show an increasing trend from the margin to the center. This observation is consistent with the general zonation of other playas. Variations of calcite and gypsum concentration profiles present increasing and decreasing trends with depth, which could be ascribed to the changes in climatic factors. These factors include brine chemical modifications owing to changes in evaporation and precipitation rates and variations in relative abundance of anions-cations or in the rate of clastic and evaporite minerals due to variations in the freshwater influx (climatic changes) with time. A decrease in calcite and increase in sulfate minerals (especially gypsum) with depth is probably due to the higher water level and rainfall, a more humid climate, and salinity variations.

Pedogenic evidence of Urmia Lake's maximum expansion in the late Quaternary

The Urmia Lake recession, and the resulting increase in vast barren area around it, has been rapid in recent decades and thus has attracted attention of environmentalists and geoscientists alike. However, evidence for its different strandlines and the largest extension of the lake during the late Quaternary has not been reported yet. The principal goals of the current research were to figure out pedological evidence of its maximum expansion during the late Quaternary and mapping out the different strandlines along the western shores of Urmia Lake over time. Thirty-two soil pedons at different landscape positions, geomorphic surfaces and distances from Urmia Lake were studied during autumn in 2013 and 2014. Micromorphological and mineralogical studies were performed for some diagnostic horizons. Results showed along the transect under study three different entities diagnostic of the lake's history (1) lithological discontinuities, (2) saline lacustrine sediments, (3) buried soils. However, almost 2 km from the closest pedon to the lake, no evidence of saline playa sediments were detected. Based on previous chronology results from Urmia Lake, which indicated high lake levels with very low salinity since about 180 ky BP, the sediments of this part of the study area were considered to have been deposited at the same period of time, indicating a maximum expansion of Urmia Lake during the late Quaternary. The current results demonstrate that soil pedons which were situated at various distances from Urmia Lake denote different stages of the lacustrine evolution. Along with the distance from the lake the development of soils increases. Due to a longer and more intensive impact of pedogenic processes on the topstratum the overall soil system becomes more “mature”. The study of buried soils and their comparison to upper non-buried soils revealed that in some cases, the buried soils were more evolved than their upper soils or vice versa. Through these observations, the periods of exposure to soil-forming processes after each sedimentation period was detected and schematic diagrams illustrating periods of highstands and lowstands (recessions) of Urmia Lake were reconstructed. This study demonstrates that even though multiple episodes of lake recession were apparent, the level of Urmia Lake increased during hydroclimatic events conducive to lake expansion, which affected the soil evolution due to several fluctuations during the late Quaternary.

Investigating Anthropogenic and Geogenic Sources of Groundwater Contamination in a Semi-Arid Alluvial Basin, Goshen Valley, UT, USA

Groundwater resources can be impacted by contamination from geogenic and anthropogenic inputs but it can be difficult to disentangle contaminant sources. In this study, we investigated the sources and distribution of NO3 and As in Goshen Valley, UT, a semi-arid alluvial basin in the western USA that contains geothermal waters, playa soils, agriculture, and legacy mining. Surface water, springs, and wells were analyzed for As and NO3 concentrations in relation to major ions, trace elements, and stable isotopes in water (δ18O and δD), and other isotopic tracers. Major ion concentrations showed high spatial variability ranging from freshwater to brackish water, with the highest salinity found in geothermal springs and springs discharging from playa sediments (Playa Springs). Radiogenic 87Sr/86Sr ratios in the Playa Springs suggest that Sr is sourced from crystalline basement rocks. The highest NO3 concentrations were found in groundwater beneath agricultural areas, particularly dairy farms, with isotopic values indicating manure, not fertilizers, as the major source. Many of the NO3-contaminated wells contained old groundwater (based on 14C and 3H), suggesting that reinfiltration of pumped groundwater may be a source of NO3 pollution. The Playa Springs also had the highest As concentrations, with moderate As concentrations found in other geothermal springs. Wells containing moderate As concentrations were found in areas where the groundwater interacts with alluvial sediments or carbonate rocks. Surprisingly, nearby mining and mineral processing seems to have minimal effect on As contamination in the alluvial aquifer. This study has implications for understanding water quality in regions that are impacted by multiple potential contaminant sources.

Evaluating Micrometeorological Estimates of Groundwater Discharge from Great Basin Desert Playas.

Groundwater availability studies in the arid southwestern United States traditionally have assumed that groundwater discharge by evapotranspiration (ETg ) from desert playas is a significant component of the groundwater budget. However, desert playa ETg rates are poorly constrained by Bowen ratio energy budget (BREB) and eddy-covariance (EC) micrometeorological measurement approaches. Best attempts by previous studies to constrain ETg from desert playas have resulted in ETg rates that are within the measurement error of micrometeorological approaches. This study uses numerical models to further constrain desert playa ETg rates that are within the measurement error of BREB and EC approaches, and to evaluate the effect of hydraulic properties and salinity-based groundwater density contrasts on desert playa ETg rates. Numerical models simulated ETg rates from desert playas in Death Valley, California and Dixie Valley, Nevada. Results indicate that actual ETg rates from desert playas are significantly below the uncertainty thresholds of BREB- and EC-based micrometeorological measurements. Discharge from desert playas likely contributes less than 2% of total groundwater discharge from Dixie and Death Valleys, which suggests discharge from desert playas also is negligible in other basins. Simulation results also show that ETg from desert playas primarily is limited by differences in hydraulic properties between alluvial fan and playa sediments and, to a lesser extent, by salinity-based groundwater density contrasts.

Evidence for regional aeolian transport of freshwater micrometazoans in arid regions.

While separated by large expanses of dry terrain unsuitable for aquatic biota, aridland waters possess high biodiversity. How aquatic micrometazoans disperse to, and colonize, these isolated ephemeral habitats are not well understood. We used a multi-faceted approach including wind tunnel and rehydration experiments, and next-generation sequencing to assess potential movement of diapausing propagules of aquatic invertebrates by anemochory across regional scales (102-105 km). Wind tunnel experiments using dry playa sediments with added micrometazoan propagules demonstrated that after entrainment by saltation and downwind transport were subsequently recoverable as viable animals when rehydrated. Further, rehydration of fallen natural dust yielded micrometazoans, including rotifers, gastrotrichs, microcrustaceans, and nematodes. Using conserved DNA primers, we identified >3,300 eukaryotic Operational Taxonomic Units (excluding fungi) in the dust including some taxa found in rehydration experiments. Thus, we provide strong evidence that anemochory can disperse micrometazoans among isolated, ephemeral ecosystems in North American deserts and likely elsewhere.

Ractopamine in particulate matter emitted from beef cattle feedyards and playa wetlands in the Central Plains.

Beef cattle in the United States are routinely administered ractopamine, a β-adrenergic receptor agonist, to enhance growth. The present study is the first to quantify ractopamine in feedyard-emitted particulate matter and playa wetlands near feedyards. Ractopamine was present in 92% of particulate matter samples, 16% of playa sediment samples, and 3% of playa water samples, at maximum concentrations of 4.7 μg/g, 5.2 ng/g (dry wt), and 271 ng/L, respectively. These data suggest that aerial transmission and deposition of particulate matter is a transport mechanism for ractopamine between feedyards and aquatic systems in the region. Environ Toxicol Chem 2018;37:970-974. © 2017 SETAC.

Sand-grain micromorphology used as a sediment-source indicator for Kharga Depression dunes (Western Desert, S Egypt)

Abstract Roundness and surface-feature characteristics of sand grains collected from two dune ridges in Kharga Depression (southern Egypt) were tested for potential use as source-to-sink indicators of dunes movement. Grain examination was accommodated through Scanning Electron Microscope (SEM) analysis. Five grain types were distinguished: A) fresh; B) sheet precipitated with “raindrop” structures; C) platy precipitated; D) broken; and E) with chemically etched surfaces—each type diagnostic of a specific geomorphic inheritance. Regarding the level of sphericity, these grains were subdivided into nine roundness classes (0.1–0.9), where angular grains are marked by 0.1 and very well-rounded grains by 0.9. Significant roundness and grain-type surface variations are observed both along dune ridges and between them. Poorly and medium-rounded grain populations dominate, along with sheet-precipitated grains. The contribution of well- and very well-rounded grains is low. The northern part of both eastern and western dune ridges is characterized by grains that represent high-energy aqueous environments with well-rounded grains, whereas platy precipitated grains with a lower level of roundness are concentrated in the middle part of the dune ridges. The southern part of the Kharga Depression is again characterized by sheet-precipitated grains. Our results indicate that the northern part of dune ridges in the Kharga Depression is mainly built of sands that originate from beyond the depression ( e.g., Ghard Abu-Maharik) and the weathered deposits of the Nubian and Moghra Sandstones. The dunes in central and southern part of the Kharga Depression also derive sand from a local depression bottom comprised of playa and fluvial deposits. The growing importance of the local sand source may be explained by the lowering of the local groundwater table, which resulted in playa drying. This groundwater loss resulted in the degradation of the vegetation cover, facilitating an increase in wind entrainment of playa sediments.