Pedogenesis in a Pleistocene fluvial system of the Northern Pantanal — Brazil

Abstract

Sedimentology and hydrology seem to play a predominant role in soil formation in the Pantanal, since the fluvial systems of this wetland provided Quaternary sediments and shaped the landscape. However, pedogenic processes are active in this region mainly due to the floods that occur during the summer and fall season. The goal of this work was to study the pedogenesis in a representative transect on the São Lourenço fluvial fan, which has an area of about 16,000 km 2 and is one of the largest sedimentary compartments of the Northern Pantanal, West Central Brazil. A transect of 125 m was studied in the late Pleistocene sedimentary lobe of the fluvial fan, where four landform elements were identified (paleo-channel, paleo-levee, paleo-floodplain and mound) and described to show the soil horizon boundaries along the landform unit. Samples were collected from trenches, auger holes, and boreholes, and were analyzed for soil description and characterization. Soil analyses carried out were: macroand micromorphological, physical, chemical, mineralogical, C isotope fractionation, and OSL dating analyses. The set of evidence shows that the studied transect originated from Pleistocene sediments due to the activity of the São Lourenço fluvial fan. The following soil-landform element relationships developed: Quartzipsamments in the paleo-channel; Plinthaquults in the paleo-levee and paleo-floodplain and; Natrustalfs in the mound. Evidences of pedogenic processes in the transect are: a) clay eluviation and illuviation, resulting in formation of the Bt horizon in the soils at the lower part of the transect and mound; b) gleization and plinthization, mainly at the lower part of the transect, resulting in formation of mottles, reduced soil matrix, and nodules (plinthite and petroplinthite); c) bioturbation, responsible for addition of materials at the soil surface, assisting in the formation/maintenance of the mound; and d) the sodium concentration, observed in soils that have higher clay contents, which are highest in the mound has the highest levels.

Soil CO2 concentrations and efflux dynamics of a tree island in the Pantanal wetland

“…Rapid wetting and drying events at the tree island infer equally dynamic changes in soil CO 2 concentrations and diffusion in the top 0.30 m of soil. These differences can be expected at other sites in the Pantanal, which has a highly diverse landscape, soil, and soil water dynamics during annual flood pulses [ Nascimento et al ., ]. Moreover, large changes in environmental conditions might be accompanied by additional processes unaccounted for in the steady state model, namely, the physical transfer of CO 2 out of the soil during precipitation events when water can rapidly occupy soil pores, thereby pushing out CO 2 [ Kim et al ., ; Huxman et al ., ] or additional release of CO 2 dissolved in precipitation during rain events [ Lee et al ., ].…”

The Quaternary alluvial systems tract of the Pantanal Basin, Brazil

“…Such belts are apparently related to a past regional incision event that proceeded major changes in depositional dynamics, which resulted in a transition from Pleistocene-aged braided fluvial-channel planforms, observed in the inactive part of the megafans, to the observed meandering and anastomosing forms in the active depositional lobes (Assine 2005. This event resulted in intense pedogenesis, which is reflected in aerially-and laterally-extensive horizons of preserved paleosols presenting both calcrete and ferricrete (Nascimento et al 2015).…”

“…Debris-flow-related alluvial fans occur on the basin-boarder pediment slopes associated with the eastern tableland escarpments, characterized by coarse-grained sandstone and matrix-supported conglomerate; and calcretes and ferricretes duricrusts associated with some of these deposits (Boggiani & Coimbra 1995, Nascimento et al 2015. Smaller-scale alluvial plains are mapped in the basin-border, but will not be discussed in this paper.…”

Genesis and Classification of Sodic Soils in the Northern Pantanal

“…Some other regions in Brazil, with more humid climates, also have soils with high levels of Na + , such as the northern Pantanal region (Nascimento et al, 2015); the genesis of these soils has been little studied. Unlike soils in the semi-arid region of Brazil, the sodic soils of the flood plain of the São Lourenço River, in the Pantanal region of Mato Grosso, occur mainly in the highest parts of the landscape (paleolevees) (Nascimento et al, 2015), which are used by wild or domesticated animals (beef cattle) as a refuge during the flood period (Silva and Girard, 2004).…”

“…Unlike soils in the semi-arid region of Brazil, the sodic soils of the flood plain of the São Lourenço River, in the Pantanal region of Mato Grosso, occur mainly in the highest parts of the landscape (paleolevees) (Nascimento et al, 2015), which are used by wild or domesticated animals (beef cattle) as a refuge during the flood period (Silva and Girard, 2004). Thus, the importance of sodic soils in the Pantanal extrapolates their expression in terms of area, for they also play an important role in the ecology of the region, which may include intentional consumption by some wild animals (geophagy) to suppress nutritional deficiencies or to neutralize toxic compounds (Gilardi et al, 1999;Coelho, 2016).…”