Brine evolution in two inland evaporative environments: influence of microbial mats in mineral precipitation

This paper gives new insight into the precipitation sequences in six playa basins that host microbial mats. The study basins are distributed across two evaporitic endorheic drainage systems located in the Central part of Spain with markedly different hydrochemistry and mineralogy. One group, in the north, consists of highly alkaline, brackish to saline lakes containing a high concentration of chloride with dominant carbonate over sulphates. A second group of lakes are mesosaline to hypersaline, with sulphate the dominant anion over chloride. Mineral assemblages identified in both contain several phases that provide evidence for mixed carbonate-sulphate precipitation pathways, in the north, and sulphate-dominated pathways in the south. Regardless of their ionic composition, saline lakes support thin veneers of microbial mats which, by integrating several lines of evidence (hydrochemical and physical analyses, statistical analyses of ions, mineralogical assemblages, textural relationships among mineral phases and microbial mats) are shown to modify the chemical behavior of the evaporitic sediment and promote the formation of carbonates and sulphates from Ca-poor waters with high Mg/Ca ratios. Geochemical changes induced in the environment surrounding the microorganism favor the nucleation of hydrated Mg-carbonates (hydromagnesite and nesquehonite), calcite and dolomite. Simultaneously, the microbial mats provide nucleation sites for gypsum crystals, where they are subjected to episodic stages of growth and dissolution due to saturation indices close to zero. In addition, the bubbles produced by the metabolic activities of microorganisms are shown to promote the precipitation of hydrated Mg-sulphates, despite permanent subsaturation levels. Although common in the studied playa basins, this effect has not been previously reported and is key to understanding sulphate behavior and distribution. Modern and natural evaporitic microbial environments are important analogs for understanding brine evolution and mineral precipitation pathways in shallow water settings that have existed since the Archean on Earth and perhaps on Mars.     

Mapping subsurface karst features with GPR: results and limitations

Ground penetrating radar (GPR) has been applied, with relative success, to locate paleo-collapses and cavities and to detect and characterise karst. One of its main advantages is that, while the penetration depth is limited to several tens of meters or even just several meters, the obtained resolution can be in the scale of centimeters. In this paper, we illustrate the applicability of GPR prospecting to the study of alluvial karst and the structures associated with subsidence areas. GPR radargrams obtained with two central frequency antennas (50 and 100 MHz) are balanced against direct observation of geological features of collapse structures in vertical exposures of gravel quarries. GPR-surveys offer the possibility of obtaining nearly continuous vertical cross-sections of the subsoil, and integration of data within a 3D frame. However, the study of the internal structure of the subsoil by means of the GPR-profiles has been usually neglected. In this work, we show that some hints about the evolution of individual dolines can be established from the study of the geometry of the sedimentary filling by means of GPR. The obtained results indicate that GPR allows to characterise the structures associated with karst features and can therefore be useful evaluating hazard susceptibility in doline fields, because: (1) when no surface evidences exists, it permits the detection of karst hazards in the subsoil, and (2) when surficial evidences of karst activity are present, it permits the characterisation of processes associated with subsidence.     

Seasonal Variability of Mineral Formation in Microbial Mats Subjected to Drying and Wetting Cycles in Alkaline and Hypersaline Sedimentary Environments

Interactions of the microbial mat community with the sedimentary environment were evaluated in two shallow, ephemeral lakes with markedly different hydrochemistry and mineralogy. The characterization of growing and decaying microbial mats by light microscopy observations and fluorescence in situ hybridization was complemented with biogeochemical and mineralogical measurements. The lakes studied were Eras and Altillo Chica, both located in Central Spain and representing poly-extreme environments. Lake Eras is a highly alkaline, brackish to saline lake containing a high concentration of chloride, and in which the carbonate concentration exceeds the sulfate concentration. The presence of magnesium is crucial for the precipitation of hydromagnesite in microbialites of this lake. Altillo Chica is a mesosaline to hypersaline playa lake with high concentrations of sulfate and chloride, favoring the formation of gypsum microbialites. Differences in the microbial community composition and mineralogy of the microbialites between the two lakes were primarily controlled by alkalinity and salinity. Lake Eras was dominated by the cyanobacterial genus Oscillatoria, as well as Alphaproteobacteria, Gammaproteobacteria and Firmicutes. When the mat decayed, Alphaproteobacteria and Deltaproteobacteria increased and became the dominant heterotrophs, as opposed to Firmicutes. In contrast, Deltaproteobacteria was the most abundant group in Lake Altillo Chica, where desiccation led to mats decay during evaporite formation. In addition to Deltaproteobacteria, Cyanobacteria, Actinobacteria, Alphaproteobacteria and Gammaproteobacteria were found in Altillo Chica, mostly during microbial mats growth. At both sites, microbial mats favored the precipitation of sulfate and carbonate minerals. The precipitation of carbonate is higher in the soda lake due to a stronger alkalinity engine and probably a higher degradation rate of exopolymeric substances. Our findings clarify the distribution patterns of microbial community composition in ephemeral lakes at the levels of whole communities, which were subjected to environmental conditions similar to those that may have existed during early Earth.     

Sedimentary effects of flood‐producing windstorms in playa lakes and their role in the movement of large rocks

This paper sheds light on the hydrodynamic conditions of transport and sedimentary effects of wind‐induced water currents produced during strong windstorms in low gradient systems. Repeated field surveys were conducted in a playa lake in central Spain to determine the impacts of major winter storms on the bed form morphology in real time. The succession of storms that passed through the area from mid‐December 2013 to early February 2014 left behind a variety of sedimentary structures: mainly ripple marks showing complex patterns and erosional structures. The latter include obstacle scours, grooves and other tool marks. In situ observations revealed that strong storm events in almost flat, extremely shallow lakes (less than 5 cm) have enough hydraulic energy to erode and remove high volumes of sediments and may also lead to large stones sliding across the bed, thus creating long grooves. Sole marks found in ancient continental successions have been typically attributed to fluvial conditions. We suggest that shallow lake basins should not be discounted when storm‐generated structures are preserved in ancient rocks. The identification of such sedimentary structures provides valuable information for reconstructing hydrodynamic conditions and paleoclimatic conditions in semi‐arid environments. Copyright © 2014 John Wiley & Sons, Ltd.     

Magnetometry and ground-penetrating radar surveys applied to tracing potential collectors of mining-derived pollutants in coastal sediments (Piscinas Bay,Montevecchio mining area,SW Sardinia)

Dispersion of pollutants from mining areas can result in risks to human health. The dynamic interaction of geological processes can generate complex situations that favor enrichment in toxic elements by sedimentary and diagenetic mechanisms. In order to explore the distribution of iron-rich minerals in coastal and river sediments, a geophysical campaign was performed along the outlet of the Piscinas and Naracauli Rivers that drain two abandoned mining areas in SW Sardinia. Both rivers flow into the Mediterranean Sea, a bay where fluvial, marine, and eolian processes interact. The geophysical campaign comprised magnetometry and ground-penetrating radar surveys. Magnetic properties were controlled by means of magnetic susceptibility measurements along the beach and dune areas, through sampling of surface sediments, natural outcrops and trenches, and considering different grain size intervals. Results indicate enrichment in ferromagnetic minerals at the leeside of dunes and berms. Sedimentological interpretation is supported by means of ground-penetrating radar. Due to both sedimentary and diagenetic processes, the 100- to 300-μm fraction of fine sands exhibits high susceptibility. Implied tenors of toxic elements in the magnetite may represent a serious hazard to environmental and health security. The results inferred from magnetic anomalies support the applicability of the geophysical approach in order to locate high concentrations of iron-rich particles both at surface and below ground and the usefulness of joint evaluation of magnetic susceptibility and ground-penetrating radar in order to characterize the sedimentary and geomorphology-controlled magnetite distribution.