Search

The occurrence of high arsenic concentrations (up to 3000 μg L−1) in water of soda lakes of the Pantanal wetland is a remarkable case of natural arsenic contamination in South America. However, little is known about arsenic speciation in this environment, particularly regarding speciation changes related to lake trophic status and seasonal variations. To fill this gap, arsenic speciation analysis was carried out in surface (SW) and subsurface (SSW) waters sampled in five soda lakes with different eutrophication status, in two dry and one wet season. As(V) was the dominant species in these waters, while As(III), DMA, MMA and likely complex organic species were present in lower amounts. The results allow to conclude that the arsenic speciation in SW and SSW varies seasonally according to the regional wet or dry periods and lake water levels. In eutrophic turbid and in oligotrophic vegetated soda lakes, arsenic speciation was also characterized by spatial differences between edge and center or between the SW and SSW. Cyanobacteria or macrophytes/algae are involved in arsenic biotransformation in soda lakes through its metabolic and detoxification processes. Significant variation in surface water arsenic speciation occurs as a result of seasonal primary production fluctuation or water arsenic concentration changes in the soda lakes, increasing organoarsenics in dry periods, whereas in flood periods, As(V) prevails. Spatial distribution of arsenic species is significantly impacted by biogeochemical conditions at the water/sediment interface in soda lakes. [Display omitted] • Arsenic species largely varied in surface and subsurface water of soda lakes. • Cyanobacteria or macrophytes/algae are involved in As biotransformation. • Arsenic speciation varies seasonally with lake water level and trophic condition. • Spatial distribution of As species varies with biogeochemical conditions. [ABSTRACT FROM AUTHOR]

Abstract Salt-affected soils play an important role in the Pantanal wetlands ecosystem, in Brazil, but their occurrence on higher landforms is not well understood. In order to investigate the major processes involved in salt-affected soil formation, a trench was opened in a representative transition between the floodplain and a paleochannel-paleolevee complex not currently reached by floodwater. The trench was 6 meter long and 2 meter deep, and 121 soil samples were collected in a 0.25 × 0.25 m regular grid and submitted for soil (particle-size, pH, exchangeable cations) and solution extract (major cation and anions) analyses. Analytical results were subjected to geostatistical and principal components analyses (PCA). For selected horizons, an undisturbed sample was collected to perform thin sections and micromorphology description. The presence of degraded carbonates in the soils indicated an ancient formation. The regional environmental settings of the alluvial fan and particle size variation along the soil profiles suggested that the sediments of the study site were deposited during the Late Pleistocene and Holocene, in a river channel-levee complex and an abandoned channel by avulsion processes. The depressed topography of the abandoned channel, accompanied by moderately drier climates during early Holocene, likely favored low leaching of salts and higher evapotranspiration, enabling the development of salt-affected soils. Later erosion of the region, which is expected in older lobes of alluvial fans, triggered a relief inversion in the area, preserving the more clayey areas as a slightly higher landform with remaining salt-affected soils. Currently, the seasonal flooding mainly reaches the footslope of this landform, triggering redoximorphic processes, expressed as redox concentrations and depletions, and solodization processes, expressed by lower values of pH, cation exchange capacity (CEC), base saturation and exchangeable sodium percentage (ESP) than at the summit. Thus, the occurrence of salt-affected soils on higher landforms is the reflection of a complex sedimentological history. In this way, the genesis of salt-affected soils on higher landforms, which seems to be poorly understood in many situations worldwide, should be carefully investigated in order to address formation mechanisms, especially in extremely dynamic environments of tropical wetlands. Highlights • Salt-affected soils were formed in more arid climates in the Early-Middle Holocene. • Salt accumulations were a result of water concentration in abandoned channels. • Erosion processes in the Late Holocene left these soils in topographical highs. • Current hydropedological and geomorphological framework points to soil solodization. [ABSTRACT FROM AUTHOR]

Abstract: The influence of the pedogenic and climatic contexts on the formation and preservation of pedogenic carbonates in a climosequence in the Western Ghats (Karnataka Plateau, South West India) has been studied. Along the climosequence, the current mean annual rainfall (MAR) varies within a 80km transect from 6000mm at the edge of the Plateau to 500mm inland. Pedogenic carbonates occur in the MAR range of 500–1200mm. In the semi-arid zone (MAR: 500–900mm), carbonates occur (i) as thick hardpan calcretes on pediment slopes and (ii) as nodular horizons in polygenic black soils (i.e. vertisols). In the sub-humid zone (MAR: 900–1500mm), pedogenic carbonates are disseminated in the black soil matrices either as loose, irregular and friable nodules of millimetric size or as indurated botryoidal nodules of centimetric to pluricentimetric size. They also occur at the top layers of the saprolite either as disseminated pluricentimetric indurated nodules or carbonate-cemented lumps of centimetric to decimetric size. Chemical and isotopic (87Sr/86Sr) compositions of the carbonate fraction were determined after leaching with 0.25N HCl. The corresponding residual fractions containing both primary minerals and authigenic clays were digested separately and analyzed. The trend defined by the 87Sr/86Sr signatures of both labile carbonate fractions and corresponding residual fractions indicates that a part of the labile carbonate fraction is genetically linked to the local soil composition. Considering the residual fraction of each sample as the most likely lithogenic source of Ca in carbonates, it is estimated that from 24% to 82% (55% on average) of Ca is derived from local bedrock weathering, leading to a consumption of an equivalent proportion of atmospheric CO2. These values indicate that climatic conditions were humid enough to allow silicate weathering: MAR at the time of carbonate formation likely ranged from 400 to 700mm, which is 2- to 3-fold less than the current MAR at these locations. The Sr, U and Mg contents and the (234U/238U) activity ratio in the labile carbonate fraction help to understand the conditions of carbonate formation. The relatively high concentrations of Sr, U and Mg in black soil carbonates may indicate fast growth and accumulation compared to carbonates in saprolite, possibly due to a better confinement of the pore waters which is supported by their high (234U/238U) signatures, and/or to higher content of dissolved carbonates in the pore waters. The occurrence of Ce, Mn and Fe oxides in the cracks of carbonate reflects the existence of relatively humid periods after carbonate formation. The carbonate ages determined by the U–Th method range from 1.33±0.84kyr to 7.5±2.7kyr and to a cluster of five ages around 20kyr, i.e. the Last Glacial Maximum period. The young occurrences are only located in the black soils, which therefore constitute sensitive environments for trapping and retaining atmospheric CO2 even on short time scales. The maximum age of carbonates depends on their location in the climatic gradient: from about 20kyr for centimetric nodules at Mule Hole (MAR=1100mm/yr) to 200kyr for the calcrete at Gundlupet (MAR=700mm/yr, ). The intensity of rainfall during wet periods would indeed control the lifetime of pedogenic carbonates and thus the duration of inorganic carbon storage in soils. [Copyright &y& Elsevier]

• Geochemical modelling made it possible quantifying rates of pedogenic processes. • Soil formation rates on the Pantanal interfluves range from 396 to 638 kg ha−1 y−1. • It takes at least 125 y to generate the Pantanal alkaline lakes. • Downslope soils act as a buffer that preserves lake alkalinity. • Deep pH 3.5 horizon due to mixing of alkaline and acid groundwater. Within a soil system, it is generally difficult to relate the observed secondary phases and their spatial distribution to past or current processes. Geochemical modelling can help to identify present-days processes and to quantify formation rates if the system is sufficiently constrained. Here we used the PHREEQC model for this purpose, applying it to soil alkaline to acidic soil systems in the Nhecolândia area. A large amount of data was available, but scientific questions remained opened. We carried out a complementary field study to clarify a key point necessary for modelling, confirming the disconnection between two types of groundwater, perched, alkaline and deep, acidic. Our simulations showed that the observed soil system corresponds to the current biogeochemistry and that paleoclimatic genetic hypotheses are unnecessary. We found that the ridge soil formation rate ranges from 396 to 638 kg ha−1 y−1, depending on hypotheses. We showed that the downslope soils act as buffers to store labile species during the dry season, allowing alkalinity to be maintained from one year to the next. At least 125 y were necessary to obtain downslope alkaline lakes, and this alkaline nature can quickly disappear after changing drainage conditions. We explained how hyperacidic horizons (pH < 4) can neighbor alkaline horizons (pH > 10) with a sharp contact. When correctly informed, geochemical modelling appeared as an effective tool to answer questions about soil processes. [ABSTRACT FROM AUTHOR]

In situ fractionation and redox speciation of As in three different saline-alkaline lakes (green, black and crystalline lakes) in the Pantanal of Nhecolândia (Brazil) were performed by using Diffusive Gradients in Thin films (DGT). The results indicated that As is present mainly in dissolved form. Total As concentration was similar when using different filter membranes, demonstrating that the species adsorbed by DGT devices were <10 kDa. Higher concentrations of labile total As were observed in the center of the lakes, indicating that the nature of the organic matter influences the formation of As complexes. Total As concentrations determined by using ZrO 2 DGT were consistent with As concentration in ultrafiltered water samples collected in the black lake. However, part of the data about As(III) obtained in grab samples contrasted with DGT results. The differences observed may indicate that alterations in the species occur during the storage period before analysis by ultrafiltration. As(III) concentrations measured by DGT in the black and crystalline lakes were 1-3 μg L-1 and 4-7 μg L-1, respectively, accounting for only 4%–8% of the total DGT inorganic As. In the green lake, As(III) concentrations were significantly higher at the center (217 μg L−1). Both the phytoplankton community and the dissolved organic carbon influence the As speciation and bioavailability in the lakes of Nhecolândia. The DGT approach used in the present work was able to perform As speciation and demonstrates that in situ sampling analytical techniques are essential in understanding As speciation and its behavior in complex natural aquatic systems. [Display omitted] • DGT was capable to quantify in situ As species in harsh conditions lakes. • Fractioning by using DGT showed presence of As as compounds >10 kDa and <0.22 μm. • In situ measurements using DGT shows As species changes during the sample storage. • Phytoplankton may influence As(III) levels in the Nhecolândia green lakes. [ABSTRACT FROM AUTHOR]

Several studies have revealed that the complexity in the distribution of drainage network patterns is not random and controlled by major parameters, variable in space but also throughout geological time. Drainage networks in the Eastern Anti-Atlas of Morocco consist of complex spatial arrangements with various types of patterns, such as trellis, angular, dendritic and parallel. The objective was to distinguish, quantify and rank the relationship that may exist between the different drainage networks patterns, geology and geomorphology. A total of 230 basins were extracted from the ASTER-GDEM Elevation Data (USGS), which were assigned 16 parameters reflecting their topography, morphometry, slope and geology. The statistical treatment of the dataset (16 variables x 230 observations) was carried out through principal component analysis (PCA), linear discriminant analysis (LDA) and agglomerative hierarchical clustering (AHC), in order to investigate the complexity of drainage network patterns and their distribution. The PCA showed that the topographical, slope and geological parameters, i.e. primarily the parameter associated with structural control, best explains the variation in the type of the drainage pattern. The LDA made it possible to distinguish between the four types of drainage patterns with a success rate of 90%, using 3 discriminant functions that were better correlated with geological and slope parameters. LDA and AHC statistical treatments show confusion between the parallel, trellis and angular patterns, on the one hand, due to similar factors responsible for their formation, and on the other because of transitions phenomenon from one drainage pattern to another over time or space. Such possible drainage network shifting may be explained by the geological events that have occurred in the Eastern Anti Atlas from Lower Mesozoic to the Quaternary. Image 1 • 230 basins were analyzed by 16 parameters reflecting topography, morphometry, and geology. • Structural control, lithological homogeneity and relief variation control the variation of drainage patterns. • Four types of drainage patterns were distinguished, with a success rate of 90%, using 3 discriminant functions. • Statistical treatments show confusion and similarity between parallel, trellis and angular patterns. [ABSTRACT FROM AUTHOR]