Playa Environments Research Articles

Melanoides tuberculata and Zootecus insularis gastropod shells provide a snapshot into past hydroclimatic conditions of arid environments: New perspectives from Oman

Shells of the aquatic gastropod Melanoides tuberculata and the terrestrial gastropod Zootecus insularis were analysed using high-resolution isotope sampling (up to 274 samples per shell) to assess their potential use as a proxy for hydroclimatic and palaeoenvironmental reconstruction in drylands. A total of 169 snails (fossil and modern) were collected from 37 sites in Northern Oman and Dhofar, with each site selected for its specific geomorphological, archaeological or ecological context. This included fluvial terraces, playa environments, modern oasis gardens, irrigation channels and Neolithic (6000–3200 BCE) and Early Bronze Age (3200–2000 BCE) archaeological sites. The δ18O data obtained from these gastropods could be classified into eight different patterns, three for the aquatic snails (Type 1 A–C) and five for the terrestrial snails (Type 2 A–E), which were linked to the environmental context of their habitat. Furthermore, the use of the aquatic snails enabled us to distinguish between groundwater and surface water signals, whereas the terrestrial snails were employed to reconstruct changes in rainfall origin, humidity, evaporation, regular wet-dry cycles, and/or the presence of meteorological events such as cyclones. According to the results, gastropods can be used to elucidate the long-term, local evolution of rain-fed floodplain ecosystems in drylands and to identify the hydrological resources present in the vicinity of archaeological sites, particularly with regard to type (e.g., surface water vs groundwater).

Lithostratigraphic and Paleoenvironmental Characterization of the Senonian Series of the Errachidia-Boudenib-Erfoud Basin, South-East Morocco

Six lithostratigraphic sections distributed in different parts of the Errachidia-Boudenib-Erfoud basin (SE Morocco) were surveyed bed by bed and sampled in order to study stratigraphy, analyzed the facies, and characterized the depositional environment of the Senonian series. This study shows that this series is mainly constituted of three lithostratigraphic units: the first one is mixed, attesting to an alluvial plain environment disturbed by marine incursions at the base. The second is dominated by "kerkoub" sandstone bars intercalated by silt-clay intervals, reflecting a floodplain environment. The third is dominated by gypsum versicolored facies, marking an evaporitic playa environment. The correlation of the studied sections shows that the Senonian sedimentary prism opens progressively to the East. The series ends with Tertiary deposition: western marine carbonates and eastern continental conglomerates. The non-deposition of these faces in the center of the basin could be the result of the presence of a paleorelief at the Tinejdad-Goulmima transition.

Structural influences on groundwater circulation in the Makgadikgadi salt pans of Botswana? Implications for martian playa environments

Across the surface of Mars, evidence of past lacustrine and evaporitic environments has been found within basins and craters, where often layered sedimentary deposits and hydrated minerals are observed. However, the intensity, duration, and precise phases of aqueous processes during their deposition remain unresolved mostly for our inability to model subsurface structures. Although several geological processes and locations on Earth have been previously proposed as examples to describe these deposits on Mars, we lack a strong visualization of what water activity might have looked like during evaporitic stages within basins and craters. Here we propose to investigate the shallow subsurface of the Makgadikgadi salt pans of Botswana as a potential analog for understanding groundwater upwelling on Mars. The pans are found within the Makgadikgadi Basin, a depression located at the southwestern end of a northeast-southwest set of graben linked with the East African Rift. The Makgadikgadi Pans are evaporitic environment rich in hydrated minerals and groundwater activity. The purpose of this work is to identify buried faults and areas of relative water saturation within the lacustrine sediment of the Makgadikgadi Basin by means of electrical resistivity surveys. This work represents the first electrical resistivity survey of the basin floor which provides a precursory investigation of the relationship between groundwater, faults, basement depth, and the lacustrine sediments. We present four electrical survey lines from different locations in the pans which reveal distinct sedimentary units. Several faults are inferred from the vertical displacement of these units and accompanying low resistivity where displacement is observed. These results provide a framework for visualizing the sedimentary sequences of infilled basins and craters on Mars, which can broaden the ongoing discussion of hydrogeological processes that were active in the planet’s past. We propose Meridiani Planum, as well as Oyama and Becquerel crater of Arabia Terra as locations to establish this framework. Since such processes are still ongoing in the Makgadikgadi Basin, imaging the subsurface of the pans helps explain the formation of layered and salty deposits on the surface of Mars, how they may have interacted with flowing water, and whether they might have hosted life.

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.

Micromorphological and mineralogical features of saline playa surface sediments from two large Trans-Uralian lakes

The proposed paper is devoted to the features of mineralogical composition and microstructure of saline playa surface sediments from two large drainless lakes in the south west of Western Siberia. The material composing the surface sediments of both playas is an unlithified mixture of clay and sand, with a significant admixture of organic matter. Coarse material is represented mainly by quartz, with an insignificant admixture of feldspar grains and micaceous fragments. In general, terrigenous component is characterized by a comparatively low degree of sorting. Clay material is scarce and composed, presumably of chlorite-hydromica material with a significant admixture of undecomposed organic matter, and traces of ferruginization. Carbonates and evaporates are the most common authigenic minerals. In both cases carbonates occur as microconcretions that correspond to the zone enriched with cysts, plant detritus and other degrading organic matter. Evaporates occur both as the efflorescence on the surfaces of the crusts. The study results have shown that surface crusts contain zones enriched with Artemia salina cysts, which are a significant component of sediments. Degrading crusts promote secondary mineral formation, especially formation of carbonates. Surface crusts of two studied playa environments differ in proportion of terrigenous material, clay minerals, as well as the composition of evaporates and carbonates.

Reservoir quality and diagenesis of Triassic sandstones and siltstones from arid fluvial and playa margin environments: A study of one of the UK's earliest producing oilfields

The Triassic stratigraphy of the UK contains many important reservoir rock units, present in all major offshore and onshore basins, that are rich in oil, gas and water resources. The Ormskirk Sandstone Formation (Sherwood Sandstone Group) and Tarporley Siltstone Formation (Mercia Mudstone Group) represent the main reservoirs found in one of the UK's earliest producing oilfields, at an unusually shallow depth of 30–90 m. In this study, environments of deposition, maximum burial depth, and reservoir quality have been evaluated from well Formby-7, drilled in 2012, using modern analytical techniques. Wireline data and core analysis data were made available by Aurora Energy Resources for the purposes of this study. Formby-7 was continuously cored so that the whole section was logged for sedimentary structures and grain size. Optical analysis of thin sections was conducted, along with SEM-EDS, using thin sections from core plugs. The Ormskirk Sandstone Formation was deposited in a proximal setting by a braided, dryland river with aeolian influence; the Tarporley Siltstone Formation was deposited in a more distal setting on the margins of a playa environment. The study found that the reservoir quality is excellent in the deeper Ormskirk sediments, but relatively poor in the overlying Tarporley sediments. Good reservoir quality present in the Ormskirk Sandstone Formation is linked to early depositional textures, grain size (medium sand) and general lack of pore-filling cements. In the Tarporley, reservoir quality is poor due to the grain size (very fine sand) and the variable presence of pore-filling gypsum associated with playa deposits. Minor feldspar diagenesis (both dissolution and precipitation) occurred in both the Sherwood and Mercia sediments. The small degree of mechanical compaction and lack of any signs of mesodiagenesis suggest that the clastic sediments in the Formby-7 well have not been buried deeper than about 1000 m. The findings reported here can be used to help the understanding of UK Triassic sedimentology and reservoir quality for oil and gas, geothermal energy, CCS (carbon capture storage) and water supply.

Evolution of sedimentary environments and geography of the Gavkhouni Playa during the Late Quaternary

Gavkhouni Playa, as lowland of the Zayandehrud River catchment area, is located in an inter-mountain basin in Central Iran. A total of 16 sedimentary cores were taken from the inner and outer parts of the basin, and surrounding areas to investigate depositional systems of the Gavkhouni Playa during the late Quaternary. Sedimentary facies were identified based on sedimentary texture, type of evaporite minerals, presence of organic materials, color changes, and other microscopic characteristics, and sampling was performed based on changes in types of sediment and sedimentary facies. A total of 80 sediment samples were selected for granulation analysis by two methods of wet sieving for coarse-grained particles and Laser Particle Sizer Analysette for smaller than sand (silt and clay) particles. Sedimentology parameters were measured by the Sedilizer software, and the results were statistically analyzed by the SPSS software. The major types of sediments in the Gavkhouni Playa sub-surface, mud and sedimentary environments included aeolian, fluvial, deltaic, lacustrine, and playa environments. According to 14C-AMS isotope dating analysis, the sedimentation rate for 40 ka period, is about 0.4 and 0.25 mm in western and eastern parts per year, respectively. Paleogeography of the Gavkhouni Playa and its extent have been reconstructed for the last 40 ka BP. The results showed that the Gavkhouni Basin extended to the western part of sand dunes during the late Pleistocene. During the early-middle Holocene, coastline regression and the Zayandehrud Delta progression toward the playa occurred in the northern part of the area. During this time, aeolian sands spread to the Gavkhouni Playa. In the Late Holocene, arid conditions caused a decrease in water level and coastline regression along with the domination of the playa environment in the Gavkhouni margin. But, there were still ephemeral lake conditions in the center of the Playa. Gavkhouni Playa experienced an increase in the level and progression of the coastline towards land about 3000 years ago. The presence of arid conditions about 1000 years ago (cal. BP) changed the Gavkhouni lands into a playa environment, and this situation was dominant in the area almost to the present day.

A new type of polygonal terrain formed by sulfate weathering in arid regions

Polygonal terrain (or simply, polygons) is a common geomorphological feature in arid regions. Polygons with diverse shapes and different sizes ranging from several meters to more than one hundred meters are widespread in playas. Here, we report a new geometric type of polygons in the playas in the western Qaidam Basin on the northern Tibetan Plateau. These polygons are characterized by raised rims that are interlocked with each other, forming irregular boundaries resembling jigsaw puzzle piece-shapes. The average size of these polygons is around 80 m, which is larger than most of the reported polygons in playas elsewhere. While most polygonal terrain in playa environments is bounded by cracks generated by desiccation or thermal contraction, these unusual features are delimited by raised rims likely caused by a chemical weathering process. Thenardite, a dense anhydrous sodium sulfate mineral originally formed in a saline playa, has been weathered and hydrated to form widespread gypsum. As gypsum leads to a larger volume than thenardite during precipitation, we propose that the volume expansion develops the raised rims of polygons as pressure ridges.

New type of sand wedge polygons in the salt cemented soils of the hyper-arid Atacama Desert

Polygonal grounds are landscape features commonly associated with periglacial environments originating from freeze-thawing cycles or frost-related processes. However, such a genesis is unlikely for polygonal grounds on alluvial surfaces in the warm and hyper-arid Atacama Desert due to the lack of enduring sub-zero temperatures and limited water availability, whereas a cracking mechanism based on thermal contraction and/or desiccation is more plausible. To differentiate between those mechanisms, we performed a quantitative morphometric terrain characterization in combination with a geochemical and sedimentological analysis on three polygonal networks located in the Yungay area of the Atacama Desert, Chile. Our data show that these sand wedge polygons differ from other polygonal features in the Yungay area such as salt polygons and mud crack polygons from playa environments in regard to composition, morphometry and topographical setting. The investigated polygonal soils are composed of siliciclastic sediment that is mainly cemented by sulfates (gypsum & anhydrite) in the shallow ground (~0–50 cm) and by nitrates and chlorides in the deeper ground (~50–100 cm) while being separated by about 1 m deep, salt-poor and V-shaped sand wedges. The low clay content (~2 wt%) makes an exclusive desiccation origin less relevant whereas the high salt content (63 wt%) and the high surface temperature variations make thermal contraction origin more likely. Morphometric data indicate a link between topography and polygon geometry, as the flat-centered polygons (mean size 3.9 to 4.7 m) are aligned either in slope direction or perpendicular to it, while being more elongated on steeper slopes, which is common to thermal contraction polygons. Although we cannot exclude that desiccation cracking plays a minor role for the formation of the here described polygons, we conclude that their genesis is dominated by thermal contraction.

Subsurface Microbial Ecology at Sediment-Groundwater Interface in Sulfate-Rich Playa; White Sands National Monument, New Mexico.

The hypersaline sediment and groundwater of playa lake, Lake Lucero, at the White Sands National Monument in New Mexico were examined for microbial community composition, geochemical gradients, and mineralogy during the dry season along a meter and a half depth profile of the sediment vs. the groundwater interface. Lake Lucero is a highly dynamic environment, strongly characterized by the capillary action of the groundwater, the extreme seasonality of the climate, and the hypersalinity. Sediments are predominantly composed of gypsum with minor quartz, thenardite, halite, quartz, epsomite, celestine, and clays. Geochemical analysis has revealed the predominance of nitrates over ammonium in all of the analyzed samples, indicating oxygenated conditions throughout the sediment column and in groundwater. Conversely, the microbial communities are primarily aerobic, gram-negative, and are largely characterized by their survival adaptations. Halophiles and oligotrophs are ubiquitous for all the samples. The very diverse communities contain methanogens, phototrophs, heterotrophs, saprophytes, ammonia-oxidizers, sulfur-oxidizers, sulfate-reducers, iron-reducers, and nitrifiers. The microbial diversity varied significantly between groundwater and sediment samples as their temperature adaptation inferences that revealed potential psychrophiles inhabiting the groundwater and thermophiles and mesophiles being present in the sediment. The dynamism of this environment manifests in the relatively even character of the sediment hosted microbial communities, where significant taxonomic distinctions were observed. Therefore, sediment and groundwater substrates are considered as separate ecological entities. We hope that the variety of the discussed playa environments and the microorganisms may be considered a useful terrestrial analog providing valuable information to aid future astrobiological explorations.

3-D radar imaging unlocks the untapped behavioral and biomechanical archive of Pleistocene ghost tracks

Footprint evidence of human-megafauna interactions remains extremely rare in the archaeological and palaeontological records. Recent work suggests ancient playa environments may hold such evidence, though the prints may not be visible. These so-called “ghost tracks” comprise a rich archive of biomechanical and behavioral data that remains mostly unexplored. Here we present evidence for the successful detection and 3-D imaging of such footprints via ground-penetrating radar (GPR), including co-associated mammoth and human prints. Using GPR we have found that track density and faunal diversity may be much greater than realized by the unaided human eye. Our data further suggests that detectable subsurface consolidation below mammoth tracks correlates with typical plantar pressure patterns from extant elephants. This opens future potential for more sophisticated biomechanical studies on the footprints of other extinct land vertebrates. Our approach allows rapid detection and documentation of footprints while enhancing the data available from these fossil archives.

Sulfur Cycling and Mass Balance at Meridiani, Mars

AbstractThe Opportunity rover was sent to Meridiani Planum, Mars, to investigate aqueous processes as inferred from orbiter observations of hematite. While the rover found the source of the hematite signal, it also identified abundant bedrock enriched in SO3 on average by >20 wt% throughout its >40‐km traverse. Geological mapping and orbiter data show that the layered sulfate deposits occur over ~4 × 105 km2, an area larger than Germany. Multiple scenarios for formation of the sediment and sulfur sources in the Meridiani sulfate‐rich bedrock have been proposed, including groundwater upwelling in a playa environment, volcanic ash, ice deposits, and fluvial input. Mass balance calculations show that none of the existing scenarios as currently portrayed can account for the total sediment volume and sulfur mass in the Meridiani region. Thus, after 14 Earth years of rover observations, there are still fundamental questions that remain regarding formation mechanisms of the sulfate‐rich bedrock.

Rock magnetic signature of a Miocene playa cycle in Central Asia and environmental implications

The increased aridification of Central Asia during the Miocene coincides in time with lake formations and the evolution of playa environments in the region. However, Miocene continental climate dynamics and the forcing of aridification are still not well constrained. Neogene lacustrine mudflat deposits in the Ili Basin in southeast Kazakhstan provide a well-exposed paleoclimate archive. Here, we present a detailed rock magnetic study of a middle Miocene playa cycle deposited in a closed basin. We use high-resolution rock magnetic parameters, lithological studies and geochemistry to reconstruct the playa environment and the depositional conditions. The rock magnetic mineralogy of the playa cycle is controlled by hematite and two fine-grained magnetite phases. Increased magnetic concentrations occur during dry mudflat conditions, with a lower groundwater table and increased aridity. The underlying processes controlling the observed variation in magnetic concentrations are a complex interplay of diagenetic processes during and after deposition. The data support an authigenic origin of both magnetite phases, one formed before and the other after sediment consolidation. Early diagenetic formation of fine-grained magnetite by microbial activity is followed by post-depositional formation of a secondary fine-grained magnetite phase. The rock magnetic results such as magnetic concentration-dependent parameters, ARM/SIRM and s-ratio indicate a sensitive record of (ground) water availability and aridity changes in the Ili Basin. We suggest that they can serve as an effective proxy for detailed paleo-environment reconstruction of playa evolution, not only in the middle Miocene Ili Basin but also in comparable floodplain/playa lake settings.

Geomorphologic Characteristics of Polygonal Features on Chloride-Bearing Deposits on Mars: Implications for Martian Hydrology and Astrobiology

Over 600 chloride-bearing deposits (chlorides) have been identified on the southern highlands of Mars. These chlorides have critical implications for hydrology and astrobiology: they are indicators of an evaporating super saturated solution, and they could have provided habitat environments for halophilic microorganisms and preserved organic matter. One of the prominent geomorphology characteristics of these chloride-bearing regions is the polygonal features within them. The origin of these polygonal features is still in debate. In this study, we have surveyed 153 locations of chlorides using 441 high resolution imaging science experiment (HiRISE) images to characterize the geomorphology of polygonal features. We identified 3 types of polygonal features of distinct geomorphologic characteristics: fractures, raised ridges, and transitional polygons between fractures and raised ridges. We evaluate previously proposed hypotheses of the formation of the polygonal features, and suggest that the 3 types of polygonal features are indicators of different stages of salt crust formation. Salt crust is usually formed through multiple groundwater activities, and it often occurs in playa environment on Earth. The unique hydrological and astrobiological implications of the chlorides with polygonal features make these deposits of high priority for future landed on and/or sample return exploration missions of Mars.

Magnesite formation in playa environments near Atlin, British Columbia, Canada

The hydromagnesite-magnesite playas near Atlin, British Columba, Canada are unique Mg-carbonate depositional environments that have formed at Earth’s surface since the end of the last deglaciation. This study elucidates the mechanisms, pathways, and rates of magnesite (MgCO3) formation in these near-surface environments, which are challenging to study in short-duration laboratory experiments because magnesite precipitation is extremely slow at low temperature. The Atlin playas, having formed over millennia, contain abundant magnesite as well as a suite of other Mg- and Ca-carbonate minerals. Mineralogical and textural evidence demonstrate that hydromagnesite [Mg5(CO3)4(OH)2·4H2O] forms at least in part through transformation of more hydrated phases, e.g., lansfordite (MgCO3·5H2O). Deposition of these hydrated Mg-carbonate minerals is limited by the evaporative flux, and thus, is effectively transport-controlled at the scale of the playas. Magnesite is a spatially distinct phase from hydromagnesite and its crystal morphology varies with depth indicating variable crystal growth mechanisms and precipitation rates. Particle size distributions and mineral abundance data indicate that magnesite formation is nucleation-limited. Furthermore, mineralogical data as well as stable and radiogenic isotope data support magnesite formation starting after the majority of hydromagnesite had been deposited likely resulting from long induction times and slow precipitation rates. Hydrated Mg-carbonate minerals precipitate relatively rapidly and control pore water chemistry while magnesite remains highly supersaturated, and thus, is reaction-controlled. This difference in controlling regime allows for magnesite abundance to increase over time without the loss of hydromagnesite such as through its transformation, which the data also does not support. We estimate rates of magnesite formation (nucleation + crystal growth) in the range of 10-17 to 10-16 mol/cm2/s over approximately 8000 years. This study helps to elucidate the geochemical conditions needed to form Mg-carbonate minerals in ancient and modern sedimentary environments and provides insights into facilitating long-term storage of anthropogenic CO2 within Mg-carbonate minerals.

Biosignature Preservation Potential in Playa Evaporites: Impacts of Diagenesis and Implications for Mars Exploration.

Assessing biosignature preservation potential (BPP) in ancient habitable environments on Mars is a top NASA priority. We address this goal through the study of Miocene-Pliocene evaporites of the Verde Formation (central Arizona). We assessed the effects of diagenesis on BPP, integrating outcrop-scale observations with six lab analyses: thin-section petrography, X-ray diffraction, Raman spectroscopy, total organic carbon (TOC), electron probe microanalysis (EPMA), and visible to near-infrared (VNIR) reflectance spectroscopy. We recognized five facies and their diagenetic pathways. Two facies included mudstones which contain clusters of displacive growth gypsum (DGG). Early DGG was altered during diagenesis by dissolution forming crystal cavities and later underwent recrystallization, cation substitution, and sulfate dehydration. Another facies was identified by lenticular beds dominated by halite and late diagenetic thenardite (Na2SO4). These pods are overlain by a sequence of interbedded gray and red mudstones which record cyclic oxidation and Fe-oxide cementation. During the Pleistocene, a lacustrine environment developed, accompanied by magnesite cementation of playa mudstones. TOC analyses were used as a proxy for inferring the BPP in each facies. The highest BPP was associated with both red and gray mudstone facies. This study provides a taphonomic framework for playa environments on Earth that record the impacts of diagenesis on BPP, with potential applications to Mars sample return (MSR) missions.

Elevated olivine weathering rates and sulfate formation at cryogenic temperatures on Mars

Large Hesperian-aged (~3.7 Ga) layered deposits of sulfate-rich sediments in the equatorial regions of Mars have been suggested to be evidence for ephemeral playa environments. But early Mars may not have been warm enough to support conditions similar to what occurs in arid environments on Earth. Instead cold, icy environments may have been widespread. Under cryogenic conditions sulfate formation might be blocked, since kinetics of silicate weathering are typically strongly retarded at temperatures well below 0 °C. But cryo-concentration of acidic solutions may counteract the slow kinetics. Here we show that cryo-concentrated acidic brines rapidly chemically weather olivine minerals and form sulfate minerals at temperatures as low as −60 °C. These experimental results demonstrate the viability of sulfate formation under current Martian conditions, even in the polar regions. An ice-hosted sedimentation and weathering model may provide a compelling description of the origin of large Hesperian-aged layered sulfate deposits on Mars.

Temporal dynamics of flooding, evaporation, and desiccation cycles and observations of salt crust area change at the Bonneville Salt Flats, Utah

The Bonneville Salt Flats (BSF) in Utah is a dynamic saline playa environment responding to natural and anthropogenic forces. Over the last century, the saline groundwater from below BSF has been harvested to produce potash via evaporative mining, mostly used as agricultural fertilizers, while the surface halite crust has provided a significant recreational site for land speed racing. Perceptions of changes in the salt crust through time have spurred debates about land use and management; however, little is known about the timescales of natural change as the salt crust responds to climatic parameters that drive flooding, evaporation, and desiccation (FED) cycles that control surface salt growth and dissolution. Climate data over the last ~30years are examined to identify annual patterns in surface water balance at BSF to identify annual and seasonal climate constraints on FED cycles. Landsat satellite data from 1986 to the present are used to map the areal extent of the surface halite salt crust at BSF at the end of the desiccation season (between August 15 and October 30) annually. Overall, the observed area of the desiccation-stage BSF halite crust has varied from a maximum of 156km2 in 1993 to a minimum of 72km2 in 2014 with an overall trend of declining area of halite observed over the ~30years of analysis. Climatic variables that influence FED cycles and seasonal salt dissolution and precipitation have also varied through this time period; however, the relationship between surface water fluxes and salt crust area do not clearly correlate, suggesting that other processes are influencing the extent of the salt. Intra-annual analyses of salt area and weather illustrate the importance of ponded surface water, wind events, and microtopography in shaping a laterally extensive but thin and ephemeral halite crust. Examination of annual to decadal changes in salt crust extent and environmental parameters at BSF provides insights into the processes driving change and the sustainability of land use in this dynamic environment.

Sediment characteristics and mineralogy of salt mounds linked to underground spring activity in the Lop Nor playa, Western China

Salt mounds are commonly distributed along playa margins and typically comprise alternating layers of loose fine sand and slightly hard halite-rich sediments as a result of long-term underground spring activity. A model of salt mound development was constructed for this study. It suggests that wind-blown sand supply and upward recharge of underground springs are two important factors in salt mound construction. Furthermore, it proposes that salt mound height is mainly controlled by the vertical transport range of underground springs and the thickness of the capillary fringe. A 1.5m representative profile dug from the center of salt mound LP1 in the Lop Nor playa revealed a fairly complicated mineral assemblage including halite, gypsum, anhydrite, glauberite, epsomite, anhydrite, calcite, bischofite, polyhalite, schoenite, kieserite and carnallite. This matches closely with the assemblage predicted by the EQL/EVP model. The groundwater in the area is highly concentrated brine rich in Cl− and Na+ and poor in Ca2+, displaying low alkalinity, and containing considerable amounts of SO42−, Mg2+ and K+. Chemical analysis of groundwater revealed considerable variation in the salinity and chemical composition of groundwater over time. The Cs-137 technique was used to measure the accumulated ages of the salt mounds. This method may prove useful in the research of relatively young playa environments where carbon dating techniques are unworkable because of an absence of carbon-rich materials in recent saline sediments.

Evaluation of relative atmospheric correction methods on ASTER VNIR–SWIR data in playa environment

Thermal emission and reflection radiometer (ASTER) L1B visible-near infrared and short wave-infrared (VNIR–SWIR) data were used to evaluate various relative calibration methods in playa environments. Internal average relative reflectance (IARR), log residual (LR), flat field (FF), and quick atmospheric correction (QUAC) methods were performed to convert at-sensor radiance data to apparent reflectance over several playas in Birjand, East of Iran. Results were evaluated comparing the calibrated image spectra of known locations and the laboratory spectra of field samples. The comparisons were done using spectral angle mapper (SAM) method and qualitative analysis of the spectral profiles. The SAM results and the qualitative analysis of the spectral profiles showed that spectra provided by applying FF method produced more similarities with laboratory spectra.