Salt Lake System Research Articles

Boron and lithium isotopic constraints on their origin, evolution, and enrichment processes in a river–groundwater–salt lake system in the Qaidam Basin, northeastern Tibetan Plateau

The Nalenggele River Catchment (NRC) is the largest river sourced from the northern slope of the Eastern Kunlun Mountains (EKM). The NRC terminates in the Qaidam Basin and feeds tail salt lakes. The river–groundwater system (freshwater) and the terminal salt lakes display high concentrations of B and Li, ranking the Qaidam Basin as a world–class mineral system. However, the source, pathway, and enrichment mechanism of these elements remain unclear. In this study, the B and Li isotopes from rivers and groundwater from mountain to basin were used to trace their origin and evolution processes. B and Li are found in high concentrations even in river and groundwater with low total dissolved solids (TDS), with average concentrations tens to hundreds times higher than those of natural freshwater systems. The observed B and Li isotopic footprints are significantly depleted in heavy isotopes in the NRC if compared to other geological systems dominated by natural weathering processes. Following the confluence of one of the main tributaries of the Hongshui River (HSR) sourced from the EKM, the B and Li concentrations increase sharply, while the δ11B and δ7Li values gradually decrease. This study indicates that the mechanism accounting for the B and Li endowment of the river–groundwater–salt lake system is mainly related to the Li–B fertility of the sources (deep fluids and country rocks), favorable tectonic conduits for water circulation, and high evaporation rates. Specifically, the deep fluids upwelling is responsible for the sourcing of B and Li. These fluids are then discharged to the river and groundwater through geothermal springs developing along deep faults or within volcanic craters. In this frame, the river and groundwaters act as carriers, and input B and Li into the lake basin. Ultimately, B and Li undergo an evaporation–mediated fractionation ending with the formation of brines and eventually accounting for the formation of the world–class Li–B salt lake deposits.

A strategy of synergistically using ether oxygen and phenolic hydroxyl groups for Ultra-High selective and fast Cs+ isolation

Hypercrosslinked polymers (HCPs) with synergistic effect of ether oxygen and phenolic hydroxyl groups was designed and fabricated for Cs+ isolation in the salt lake system. The separation factors (S.F.)(Cs+/K+) of the adsorbents were significantly increased through regulating the content of the ether oxygen group in the frameworks. The S.F. of PO-HCPs-6 was increased by nearly 5 times to an ultra-high value (S.F. = 57) compared with the Cs+ adsorbents reported in the literature. Moreover, the mild and sufficient polymerization process endowed PO-HCPs-6 with an extremely fast kinetic adsorption rate. PO-HCPs-6 could quickly reach 90% of the equilibrium adsorption capacity in just 1 min. Finally, after 5 times adsorption–desorption experiments, the remarkable elution rate (>92%) and reusability adsorption capacity (>91%) of PO-HCPs-6 proved it has inspiring application potential for the Cs+ isolation in the salt lake brine system.

Effects of salinity and particle size on radium desorption from river sediments in the Qinghai-Tibet Plateau

Natural radium isotopes have been widely used to study groundwater discharge in different systems. Therefore, it is of great significance to understand the desorption behavior of radium isotopes on sediments to trace water-land exchange processes. However, there is very limited studies observing the desorption Ra isotopes to lake water of the brine lake. 224Ra desorption experiments with different salinities and particle sizes were carried out by collecting samples of brackish water from Qinghai Lake, brine from Dabuxun Lake and river sediments entering the lakes. The results show that the desorption activity of 224Ra from the river sediments to lake water of Qinghai Lake is 0.2 dpm/g when the salinity is 10.07‰. The maximum desorption activity of 224Ra from river sediments to lake water of Dabuxun Lake is 0.195 dpm/g at a salinity of 40.81‰. A salinity of 41.81‰ and particle size of 16.28 μm are the threshold points affecting the desorption behavior of Ra. When the salinity is less than 40.81‰, the desorption activity of Ra increases linearly with increasing salinity. When the salinity is greater than 40.81‰, the desorption activity of Ra decreases nonlinearly with increasing salinity and tends toward a stable low value. When the particle size is larger than 16.28 μm, the small particle size promotes desorption. The smaller the particle size is, the greater the desorption activity is. When the particle size is less than 16.28 μm, the small particle size inhibits desorption. The smaller the particle size is, the smaller the desorption activity. The co-precipitation of Ra2+ with supersaturated Ca2+, SO42− and other ions may be the main reason for the threshold point of salinity and particle size in Ra desorption process in salt lake system.

Ancient Microorganisms and Carotenoids Preserved in Fluid Inclusions in Halite from Chaka Salt Lake, Western China: Evidence from Micro‐observation and in situ Raman Spectroscopy

AbstractTrapped ancient microorganisms in halite fluid inclusions are of special interest to the understanding of biology and ecology in salt lake systems. With the integration of petrologic, microthermometric, and Raman spectroscopic analyses, this study utilizes fluid inclusions from Chaka Salt Lake, eastern Qaidam Basin, NW China, to assess the possibility of microorganism‐trapping by fluid inclusions. Here, we report that the solid phase of some primary fluid inclusions contains carotenoids, which is interpreted as evidence of Dunaliella algae, and that the coexisting liquid phase comprises SO42‐. The homogenization temperatures of single‐phase primary fluid inclusions indicate that the precipitation temperature of the Holocene halite in Chaka Salt Lake ranges from 13.5°C to 36.4°C. This suggests that fluid inclusions in halite are a good medium for trapping and preserving ancient microorganisms and organic matter in salt lakes, and that Raman spectroscopy has good potential to identify halophilic archaea.

Evaluating historical trends and influences of meteorological and seasonal climate conditions on lake chlorophyll a using remote sensing

Hansen CH, Burian SJ, Dennison PE, Williams GP. 2019. Evaluating historical trends and influences of meteoorological and seasonal climate conditions on lake chlorophyll a using remote sensing. Lake Reserv Manage. 36:45–63.Evaluations of long-term water quality trends and patterns in lakes and reservoirs are often inhibited by irregular historical records. This study uses historical Landsat satellite imagery to construct a more complete historical record of algal biomass (measured via chlorophyll a [Chl-a]) and presents a framework for developing seasonal algal estimation models using open source tools for processing and model development. This approach is both physically based (using observed patterns of variability and algal succession in the lake) and data driven (relying on statistical methods for model development). We use a generalized linear regression modeling technique to develop lake-specific, seasonal models for each lake in the multilake Great Salt Lake system in Utah. The 32-yr constructed history of estimated Chl-a enables analysis of long-term trends within the lake system as well as evaluations of local climate influences on Chl-a concentrations. The estimated historical record exhibits a shift in seasonality (i.e., maximum Chl-a occurs earlier in the growing season), as well as increasing trends of extreme Chl-a concentrations. We also evaluated relationships between meteorological conditions and Chl-a using the enhanced historical record and found localized sensitivity to short-term weather events such as high wind, high temperatures, or precipitation events. Seasonal climate conditions including high winter precipitation, summer temperatures, and early spring snow water equivalent are consistent with higher Chl-a extremes in the historical record. Improved understanding of the trends and climate influences provides useful context and guidance for future monitoring efforts and management strategies.

Geological controls and the impact of human society on the composition of peloids of present-day salt lakes (coastal zones of the Black, Azov, and Dead Seas)

Modern salt lakes belong to a growing class of natural systems undergoing compositional change due to increased human activity along their shores. To understand the full consequence of anthropogenic-driven change, detailed geological and geochemical study of such salt lake systems is required. Notably further study is required of the changes recorded in the peloids (lake muds) commonly found in these salt lake systems. Differences in regional geology respectively determine the dominant sulphate mineralsalt type and chemical composition of Crimean salt lake peloids, and the chloridecarbonate mineral-salt type and chlorine-calcium composition of the Dead Sea peloids. Local geological factors furthermore influence the peloids’ geochemical and grain size characteristics: Cu-Rb-Zn-Mo-Ca-Cl association is found in Dead Sea peloids and Fe-V-Mn-Co-Y-Pb association is found in Crimean lake muds. Finally, the beddingstructure of the peloid layer affects the granulometric characteristics of the peloids. The impact of human society (e.g., the anthropogenic factor) influences the salt, granulometric and microelemental compositions of peloids. A complete lack of halite in the Lake Kuchuk-Adzhigol mud is most likely related to intensive anthropogenic desalination, as well as to the input of drainage and waste water. The coarsest muds were found in Lake Saky, which reflects changes in the hydrochemical regime of the lake. Ferrous sediments of the Cimmerian stage are widespread in the East of Crimea. Until recently, these deposits were exploited through open pit mining (e.g., the iron ore mine Kamish-Burun), which could amplify Fe-Ti-Cr-K-V-Pb-Y-Mn-As-Co association in the Lake Tobechik muds.

Distribution of glycerol dialkyl glycerol tetraether (GDGT) lipids in a hypersaline lake system

Isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) of archaeal origin and branched (br)GDGTs of bacterial origin occur in a diverse range of lacustrine sedimentary environments. They have attracted attention as potential temperature proxies, providing high resolution (palaeo)environmental reconstruction from continental interiors. For this study, the distribution of GDGTs in, and application of GDGT-based proxies to, surface samples from Chaka Salt Lake (China), as well as soils and in-flow river sediments, were investigated to assess whether GDGT-based proxies were applicable to this hypersaline lake system. We show that iso- and brGDGTs were present in all sediments and soils from the Chaka Salt Lake system. GDGT-0 and crenarchaeol were generally the two most abundant isoGDGTs, suggesting Thaumarchaeota as a major biological source of isoGDGTs. The low ratio of crenarchaeol/crenarchaeol regioisomer suggests that Thaumarchaeota of the lake sediments is likely Thaumarchaeota group I.1b derived from the surrounding alkaline soils, arguing against the use of the TEX86 proxy in this system. Because alkaline soils generally have high isoGDGT concentrations, it is likely that a large allochthonous input of isoGDGTs would be a pervasive challenge to palaeoclimate applications in such settings. On the other hand, the brGDGT distributions in the lake and river sediments differed markedly from those in the surrounding soils, suggesting that, instead of deriving from the surrounding soils, at least part of the brGDGTs is synthesized in situ or delivered from more distal upland soils. Taken together, our results indicate that the mixed sources of GDGTs in Chaka Salt Lake complicate the application of GDGT-based proxies, and it will be challenging to use such proxies in this system.

Geochemical Examination of Surficial Soil Overlying Uranium Deposit in Manyoni, Central Tanzania

Geochemical compositions of surface to shallow soil samples of Manyoni uranium deposit in Tanzania were determined by X-ray fluorescence analysis (XRF). Uranium, calcium and chlorine are abundant at the depth of 40 cm to 140 cm. Strong correlations between U, CaO and chlorine suggest uranium and carbonates deposition in a Salt Lake system. Strontium content is higher (370 to 480 ppm) than the average upper continental crust UCC (320 ppm) at the depth of 60 cm to 120 cm suggesting Sr accumulation in carbonates. TiO2 and Fe2O3 abundances appear along the profile but have normal concentrations below the average UCC content. Vanadium is positively correlated to Fe2O3 and Ni (mafic sources) concentrations and negatively correlated to U. Uranium precipitation is suggestive of abrupt evaporation during dry season at Manyoni depositional basin. Chlorine (Cl) with high concentration in the vertical profile may be yielded in the sediments after evaporation of the lake water.

A visual assessment of the recovery of 3D seismic lines in the Cooper Basin, South Australia

This report examines the recovery rates of 3D seismic lines in the Cooper Basin, SA, 1992–2015. There are now more than 70,103 km of 3D seismic line in the Cooper Basin. There are a variety of impacts caused by seismic exploration, with the visual impact being the prime concern raised by stakeholders. The recovery rates of 3D seismic lines vary between differing land systems. Some land systems, such as dune and floodplain, are dynamic, while others, such as gibber plain, are less so. A historical database consisting of 2,018 videos and photographs was used to score the visibility of seismic lines at various locations and times. A new methodology of visually scoring this data was created, using a scale ranging from 1 (the survey dominates the landscape) to 5 (the survey is no longer visible). Data analysis showed that seismic lines in dune land systems are likely to recover within seven years, seismic lines in floodplain land systems are likely to recover within eight years, and seismic lines in gibber plain land systems are likely to take 10–20 years to recover to a visibility score of 4 or 5. To arrive at a more accurate determination of seismic line recovery in salt lake, wetland and tableland land systems, more extensive data collection is recommended.

Validity and slopes of the linear equation of state for natural brines in salt lake systems

Many density-dependent groundwater flow simulations rely on a linear equation of state that relates the fluid density to the total dissolved solute content (TDS). This approach ignores non-linear volume of mixing effects, as well as the impact of any chemical reactions. These effects can be considered by using geochemical codes that implement algorithms that calculate the density of a fluid based on the concentration of individual solute species. While in principle such algorithms could be used in-lieu of a linear equation of state in a groundwater model, the computational overhead is such that the use of a more simplified equation of state is preferred. This requires that the assumption of linearity as well as the appropriate value of the linear slope have to be determined. Here, published density measurements of 7 chemically-distinct salt lake brines are compared with densities calculated by PHREEQC-3, confirming the applicability of PHREEQC’s algorithm to salt lake brines, as well as to seawater brines and artificial brines from laboratory experiments. Further, calculations with PHREEQC-3 are used to assess the impact of mineral precipitation reactions during evaporative concentration. Results show that the density–TDS relationship is essentially linear over a wide concentration range, and that slopes range between 0.64 and 0.75, with the upper end of the range applying to Na–CO3–Cl brines and the lower end to Na–Cl brines. Mineral precipitation of highly-soluble evaporate minerals such as halite and trona limit TDS, and may lead to considerable errors in coupled flow simulations based on a linear equation of state at high concentrations. Misrepresentation of the slope may lead to an error of up to 20% in the calculated length of the brine nose bordering a salt lake, or of the Rayleigh number, which indicates if a density stratification is stable or not.

Drought tolerances of three stem-succulent halophyte species of an inland semiarid salt lake system.

Succulent halophytes of the genus Tecticornia are dominant in salt marshes of inland lakes of Australia. We assessed the drought responses of a C4 species, Tecticornia indica subsp. bidens (Nees) K.A.Sheph. & Paul G.Wilson, and two C3 species, Tecticornia auriculata Paul G.Wilson (K.A.Sheph. & Paul G.Wilson) and Tecticornia medusa (K.A.Sheph. & S.J.van Leeuwen) that occur in the Fortescue Marsh, north-west Australia. In a glasshouse experiment, the three species were grown individually and in different combinations, with varying number of plants per pot to achieve comparable dry-down rates among pots. Prior to the imposition of drought (by withholding water) the three species showed differences in dry mass and physiological variables. As the soil dried out, the three species showed similar reductions of transpiration, osmotic potential and photochemical efficiency. Shoot growth was depressed more than root growth. Tissue water loss from portions of the succulent shoots accounted for ~30% of transpiration during severe drought stress. There was no osmotic adjustment. Shoot tissue concentrations of Na+ and Cl- tended to increase during drought, and those of K+ decreased; however, these changes were not always statistically significant. Chlorophyll concentration decreased but betacyanin concentration increased. Despite occupying distinct positions in a water and salinity gradient, the three Tecticornia species had remarkably similar responses to soil water deficit.

Physicochemical Conditions and Microbial Diversity Associated with the Evaporite Deposits in the Laguna de la Piedra (Salar de Atacama, Chile)

The Salar de Atacama, located in Northern Chile, is a wide salt flat that is characterized by several salt lakes, which are locally called lagunas. The Laguna de la Piedra is one of the salt lake systems that is located in the northernmost sector of the Salar de Atacama. The present paper examines some physicochemical properties of the Laguna de la Piedra as well as the microbial diversity of the evaporite deposits. Under extreme desiccation and ambient UV flux, the evaporite deposits can create favorable endolithic microniches for the development of microorganisms. In the Laguna de la Piedra these deposits host a variety of halophilic microorganisms, which were investigated by using an optical and environmental scanning electron microscope (ESEM) as well as molecular diversity studies based on the small subunit ribosomal (SSU) rRNA of Bacteria, Archaea and Eucarya. We detected a single phylotype of halophilic archaea and a oxytrichid ciliate. Within the bacteria, a variety of Cyanobacteria, Bacteroidetes, Alpha-, Beta- and Deltaproteobacteria, as well as members of the candidate division TM6, were identified.

Vegetation and landscape connectivity control wildfire intervals in unmanaged semi-arid shrublands and woodlands in Australia

Aim The aim of this study was to determine how spatial variation in vegetation type and landscape connectivity influence fire intervals in a semi-arid landscape with low relief and complex mosaics of woodland and shrubland vegetation. Location Our study focused on a 15,500-km2 area of relatively undisturbed and unmanaged land in south-western Australia, referred to as the Lake Johnston region. Methods We modelled fire-interval data from a 67-year (1940–2006) digital fire history database using a two-parameter Weibull function, and tested for the effects of vegetation type and landscape connectivity on estimates of the length of fire intervals (Weibull parameter b) and the dependence of fire intervals on fuel age (Weibull parameter c). Results Vegetation type and landscape connectivity significantly influenced fire interval probability distributions. Fire intervals in shrublands (dense low shrub assemblage) were typically shorter (b = 46 years) and more dependent on fuel age (c = 2.33) than most other vegetation types, while fire intervals in open eucalypt woodlands were much longer (b = 405 years) and were less dependent on fuel age (c = 1.36) than in shrub-dominated vegetation types. Areas adjacent to or surrounded by salt lakes burnt less frequently (b = 319 years) and fire intervals were less dependent on fuel age (c = 1.48) compared with more exposed areas (e.g. b < 101 years, c > 1.68). Fire intervals in thickets (dense tall shrub assemblage) were longer (b = 101 years) than would be expected from fuel loads, most likely because they were protected from fire by surrounding fuel-limited woodlands. Main conclusions Fire intervals in south-western Australia are strongly influenced by spatial variation in vegetation (fuel structure) and landscape connectivity. The importance of fuel structure as a control of fire intervals in south-western Australia contrasts with other landscapes, where topographical gradients or climatic influences may override the effects of underlying vegetation. We found that, regardless of low relief, topographical features such as large salt lake systems limited the connectivity and spread of fire among landscape units in an analogous manner to lakes or mountainous features elsewhere.

Response of fringing vegetation to flooding and discharge of hypersaline water at Lake Austin, Western Australia

Patterns and dynamics of the salt marsh vegetation that surrounds many of the salt lake systems of arid/semi-arid Australia are poorly known. Lake Austin is a very large salt lake with extensive areas of fringing salt marsh; it is located in the arid Yilgarn Region of Western Australia. In this study, the changes in this vegetation over a 4-year period (1998–2002), during which both a major flooding event and addition of hypersaline groundwater from a nearby mining operation occurred, are reported. The monitoring program, based on Before-After-Control-Impact (BACI) principles, was designed to detect impacts of discharging hypersaline water into the lake; however, the flooding event, the result of above average rainfall in early 2000, complicated the results. The rains of 2000 and subsequent inundation of the vegetation immediately fringing the lake bed and major inlet channels resulted in dramatic changes to the species composition of annual and short-lived species and growth of perennial species. Flooding resulted in substantial death and damage to perennial shrubs (particularly Halosarcia fimbriata) due most likely to a combination of several weeks/months of inundation and smothering by macroalgae and Ruppia, with smaller plants and those closer to the lake bed impacted upon to a greater degree. Seed germination and recruitment of new Halosarcia plants was substantial as floodwaters receded with the majority of these seedlings surviving some 2 years after flooding despite the severe drought that followed the flood. Growth rates of seedlings differed substantially and were linked to subtle differences in micro-topography. Recruitment following flooding was also demonstrated in in vitro experiments involving inundated soil cores, provided water was relatively non-saline (conductivity <30 mS cm−1). A conceptual model is proposed to explain changes in fringing vegetation in response to frequency, depth, period and salinity of flooding in relation to micro-topography. Despite the profound effect of flooding, impacts of discharge were identified, with changes in topsoil pH and salinity greater in areas closer to the discharge than those further away. Impacts on vegetation characteristics were not detected.

Thermodynamics of salt lake system: Representation, experiments, and visualization

AbstractA coherent, systematic approach for the determination of solid–liquid phase behavior of a multi‐component salt lake system for use in the synthesis of crystallization process is presented. It centers on the thermodynamics of such a salt lake system and integrates three interrelated activities—representation of the system phase behavior as a phase diagram/thermodynamic model, experimental determination of the necessary data and visualization of the relevant crystallization regions. To illustrate this approach, the thermodynamics of a simplified salt lake system Li+, Na+, K+, Mg2+//Cl−, SO42− ‐H2O at 25°C and 1 atm was determined. The identification of process alternatives using the resulting phase diagram for recovering Li2SO4·H2O was also illustrated. © 2008 American Institute of Chemical Engineers AIChE J, 2008

On the evolution of salt lakes: Episodic convection beneath an evaporating salt lake

The transient evolution of salt lake brines and the accompanying underlying groundwater salinity distribution with depth is discussed. A convective‐diffusive equation with a novel surface source term modeling the evaporative process is introduced, and an approximate analytical solution of this equation is obtained using a perturbation approach. A linear stability analysis of the resulting solution is carried out. For typical field‐scale parameters, the model predicts that instability may initially occur within timescales on the order of weeks or decades and that a near‐surface layer of salty fluid whose thickness is on the order of decimeters is responsible for the initial onset of convection. Furthermore, the analysis indicates that when convection starts, the decimeter layer mixes, and this results in the loss of the destabilizing force. In this period, the salty fluid diffuses downward. This results in a return to a hydrologic system, where the salt profile is then well mixed throughout, but now with a background concentration with a higher average salinity. The process can then repeat for an extensive but finite period as the surface layer and subsurface groundwater concentration eventually become hypersaline and where salt precipitation occurs. We have called this phenomenon episodic convection. Variable‐density groundwater flow and solute transport modeling experiments of a field‐based salt lake system confirm the existence of episodic convection and semiquantitatively validate the findings of the analytical approach.

Salt lakes of the northern agricultural region, Western Australia

A survey of 11 sites covering three large (>10 km long, 6 sites) playa lakes and four pans (<1 km2, 5 sites) of the Yarra Yarra salt lake system in the Northern Agricultural Region of Western Australia commenced in 2001. These salt lakes are shallow and ephemeral, with inundation being more regular following winter rainfall, but summer inundation also occurred in 2001. Salinity was generally higher in playas (156–368 g l−1) than pans (30–284 g l−1), but salinity responded noticeably to heavy rainfall events, especially in pans. pH values in the playa lakes (6.68–7.82) were less variable than in the pans (6.81–8.08). The range of dissolved oxygen concentrations was greater in pans (3.7–14.4 mg l−1) than in playas (3.9–8.2 mg l−1). Cationic concentrations generally followed the pattern of sea water cation dominance. Benthic microbial communities comprised either cohesive to loosely mucilaginous mats, or thin films of diatoms. Five genera of diatom and two species of filamentous cyanobacteria were recorded.

A conspectus of the Western Australian Bossiaea species (Bossiaeeae: Fabaceae)

An overview is provided of the We stern Australian species of Bossiaea. Thirty-eight species are recognised of which the following are described here as new: B. arcuata, B. atrata, B. aurantiaca, B. barbarae, B. barrettiorum, B. calcicola, B. celata, B. eremaea, B. flexuosa, B. inundata, B. laxa, B. saxosa, B. simulata and B. smithiorum. Descriptions, distribution maps, and a key to identification are provided and most species are illustrated. Attention is drawn to a number of entities whose taxonomic status awaits clarification. The genus Bossiaea, as pre sently understood, is represented in Western Australia by thirty-eight species, more species than occur in the remainder of Australia. Of the species found in Western Australia, only B. bossiaeoides (A.Cunn. ex Benth.) Court and B. walkeri F.Muell. occur in an adjacent state or territory or beyond. The majority of Western Australian species occur in the south-west where they occupy a diversity of habitats. Many species are conspicuous, and sometimes locally dominant, members of heathland and eucalypt-dominated woodland and forest, but some occupy fairly hostile environments. Several species are associated only with salt lake systems where they typically occur on t he perimeter of lakes above the samphire zone, or on islands or spits of land that protrude into the salt-affected areas. In years of severe drought some of these species associated with hostile environments produce no flowers, and the outer stems die back from the perimeter of the plant towards the centre. When favourable conditions return, new stems are produced from the centre of the plant. Bossiaea Ve nt. is the largest genus in the endemic papilionoid tribe Bossiaeeae (Benth.) Hutch. which comprises also Platylobium Sm., Goodia Salisb., Ptychosema Benth., Muelleranthus Hutch. and Aenictophyton A.T.Lee (Ross & Crisp, 2005). Bossiaea and Platylobium, which until now have been maintained as separate genera, are differentiated from other members of the tribe Bossiaeeae by having the flowers 1- several from the axils or occasionally pseudoracemose and subtended by papery or scarious bracts, and the leaves simple, unifoliolate (i.e. with 2 pulvini, one at the base of the petiole and one at the top below the leaflet) or reduced to exstipulate scales. Bossiaea and Platylobium both have distichous phyllotaxis. Platylobium has been

Interpretation of a Sub-Audio Magnetic Survey over the lena Shear near Cue, Western Australia

The Lena Shear is a gold-anomalous structure that sits in an Archaean greenstone belt and extends for more than 7 km. It is partially obscured by regolith and an extensive salt lake system over its length. Identification of the shear under cover is hampered by the poor magnetic susceptibility contrasts of the host lithologies. A Sub-Audio Magnetic (SAM) survey over an exposed section of the Lena Shear recorded anomalous equivalent magnetometric resistivity (EQMMR) values proximal to the shear, but not directly associated with the geologically mapped location of the shear as expected. An interpretation of the Sub-Audio Magnetic survey highlights current channelling in structural and geological elements that were previously unknown. The relationship between the shear zone and the anomalous EQMMR response is unresolved because of the stronger response of an adjacent ultramafic unit. It is likely that deep weathering of rock units, contacts and shears has combined to produce the observed anomalies.

Formation of analcime and moganite at Lake Lewis, central Australia: significance of groundwater evolution in diagenesis

Abstract Lake Lewis basin in the Northern Territory is a hydrologically closed intermontane basin in which clay-rich palaeolacustrine sediments are infused with highly evolved brines. Acquisition of dissolved silica and sodium by groundwaters from the Proterozoic Arunta granite-gneiss terrain, and from arkosic alluvium and weathering products sourced from these bedrocks, is important to diagenesis at the salt lake. Supersaturation of groundwaters with respect to SiO 2 and the availability of catalytic ions, coupled with high rates of evaporation, favour neoformation of opal and chalcedonic silica from solution near the groundwater discharge zone. Moganite, a distinctive silica polymorph that is commonly associated with evaporites, has been identified amongst precipitated silica species. Down-gradient from the zone of silica precipitation, beneath the playa, brines are SiO 2 -deficient. Long periods of weathering and diagenesis of lacustrine clays immersed in semi-stagnant brines has transformed detrital minerals into substantial proportions of amorphous or gelatinous aluminosilicate material. Aluminosilicate phases have become metastable in the presence of evolved SiO 2 -deficient interstitial brine. Analcime, a sodium-bearing zeolite group mineral, is stable in the presence of the Na-rich brine and is crystallizing authigenically below the watertable from the amorphous aluminosilicate material in the lacustrine sediment. No volcanic rocks or pyroclastic sediments are involved in this diagenetic occurrence of analcime. The appearance of analcime in the evolution of the salt lake system probably commenced within the last 100 000 years and is occurring in the present-day hydrologic and climatic regime of this arid zone area.