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

Abstract

In the northern Pantanal wetlands of Brazil, alkaline sodic soils play a significant role in the ecological functioning of the region, but little is known about their mineralogical equilibria in current flooding dynamics, which is necessary for good management of these areas. This study focused on a transition between sodic and non-sodic soils, identified by low frequency electromagnetic induction survey. Samples of B horizons from alkaline-sodic and non-sodic soils were analyzed via soil solution from saturated paste, decomposition of X-ray diffraction (XRD) patterns and transmission electron microscopy with X-ray dispersive energy spectroscopy (TEM-EDS) analysis. The transition between alkaline-sodic and non-sodic soils matches with the transition between flooded and non-flooded areas. The soil solution shows a concentration factor of about 40 along this transition. The formation of sodic soils results from an accumulation by evaporation of solutions with a positive calcite residual alkalinity (RA calcite ), probably imposed by the chemistry of the São Lourenço River. An increasing Na:Ca ratio caused solonization of the exchange complex. These alkaline-sodic soils, however, can be interpreted as declining soils in the current context with an evacuation of soluble salts during flood cycles. The detailed investigation of the fine clay fraction (< 0.2 μm) using full profile modelling and decomposition of XRD patterns and TEM-EDS analysis demonstrates gradual transformations of trioctahedral to dioctahedral smectite by Mg -Al substitutions on octahedral sites, leading to the prevalence of kaolinitesmectite interstratified minerals when the conditions become more acidic via the solodization process.

Soils Surrounding Saline-Alkaline Lakes of Nhecolândia, Pantanal, Brazil: Toposequences, Mineralogy and Chemistry

“…This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4422092 P r e p r i n t n o t p e e r r e v i e w e d minerals were likely transported with little modification and remained restricted to the sand and silt fractions (Assine et al, 2015;Oliveira Júnior et al, 2020).…”

“…According to published studies on similar systems (Barbiero et al, 2008(Barbiero et al, , 2016Furquim et al, 2008Furquim et al, , 2010aFurquim et al, , 2010bGuerreiro et al, 2019;Andrade et al, 2020) Calcite in the other green horizons could however have been ignored due to the method of separation of the clay fraction during which soluble minerals can be dissolved ( §2.4).…”

“…The soils surrounding these lakes become alkaline, which results in values of soil electrical conductivity (ECs), pH and dissolved organic carbon (DOC) in soil water reaching 80 dS.m -1 , 11 and 750 mgC.L -1 , respectively (Mariot et al, 2007;Barbiero et al, 2008). They exhibit a number of characteristic features, in particular, the presence of deep, low-permeability green horizons, which are the sites of neoformation of specific secondary clay minerals, stevensite or Fe-illite (Barbiero et al, 2008;Furquim et al, 2008Furquim et al, , 2010aFurquim et al, , 2010bFurian et al, 2013;Andrade et al, 2020;Vidoca et al, 2020;Souza Oliveira et al, 2021). The 3-D morphology of the green horizons may govern part of the lake groundwater dynamics (Sakamoto et al, 2004; This preprint research paper has not been peer reviewed.…”