Salt-affected soils on elevated landforms of an alluvial megafan, northern Pantanal, Brazil

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 meters long and 2 meters deep, and 121 soil samples were collected in a 0.25 x 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.

Flooding effect on mineralogical and geochemical changes in alkaline‐sodic soil system of northern Pantanal wetlands, Brazil

“…Soil samples were collected from a lateral side of the trench along a regular 0.25 × 0.25 m grid, comprising 121 samples. The results for soil exchangeable complex characterization, pH, micromorphology and particle size distribution are reported by Oliveira Junior et al ().…”

“…On the other hand, these waters exhibit a positive RA calcite (Alk > Ca 2+ ) and follow an alkaline pathway in the case of evapoconcentration (Al‐Droubi et al, ; Barbiero et al, ). Figure shows the soil solutions along the trench are more concentrated, but consistent with the river chemistry, suggesting the São Lourenço River imposed its chemical alkaline pathway on the soils of the trench (Oliveira Junior et al, ). After successive floods and evaporation events, calcite saturation is reached, resulting in an increase of alkalinity with evapoconcentration (Barbiero et al, ).…”

“…Based on ECa values, the transition between paleolevee and floodplain was abrupt and matched the limit of influence of the seasonal floods (Figures b). The pH is higher in the horizon assigned as Btk (Figure ), in the highest elevation of the trench, reaching values close to 9.0, whereas the floodplain presented the lowest pH values, close to 4.5 (Oliveira Junior et al, ).…”

“…Two samples were selected from the B horizons in contrasting situations: those in the non‐flooded area with calcite precipitations (Btk) and those in the seasonally flooded area (Bt2). These two horizons were considered for mineralogical investigations because of their contrasting geochemical attributes, such as EC, pH and cation exchange capacity (CEC) (Oliveira Junior et al, ). The Btk horizon shows a basin shape, hampering ion leaching, whereas the Bt2 horizon is gently inclined towards the floodplain, thus favouring ion leaching.…”

Dissolved arsenic in the upper Paraguay River basin and Pantanal wetlands

“…In the short term, they mainly receive water from the seasonal rains, but over the long term, they are also supplied by the overflows of the Taquari River. Therefore, this river arising from sandstone area imposes its chemical characteristics, in particular the low arsenic contents (Table 1) and a positive calcite residual alkalinity (RAcalcite) (Oliveira Junior et al, 2019;Rezende Filho et al, 2012). During the wet season, while freshwater lakes are generally supplied by overflow during the flood pulse, alkaline saline lakes receive a reduced amount of freshwater through subsurface flows from the vazantes.…”