In intensely weathered humid-tropical soils, soil solutions may be oversaturated with respect to kaolinite (the clay mineral Al2Si2O5(OH)4) at some depths and undersaturated at others, so that silica is dissolved and exported from some horizons while aluminium is residually enriched — the desilication–allitization pathway of tropical pedogenesis. Such a gradient (topsoil oversaturated, subsoil undersaturated with respect to kaolinite) has been attributed to a field study of Plinthic Acrisols in the upper Amazon basin (Fritsch et al. 2007), while the same group's later synthesis for nearby laterites describes the opposite sign, with kaolinite dissolution prevailing in the topsoil at lower Si activity. No speciation-corrected, field-validated account exists of the conditions under which the kaolinite saturation index changes sign with depth, of how sensitive that sign is to correction of aluminium speciation for organic complexation, or of how the saturation gradient maps quantitatively onto net silica export and aluminium enrichment. The underlying equilibrium thermodynamics is not itself the gap — the gibbsite–kaolinite–silica stability framework is classical — and the open core is a quantitative, field-tested linkage between saturation-state depth profiles and desilication rates.

Background

Desilication–allitization is the pedogenetic process by which humid-tropical soils lose silica (Si) while aluminium (Al) and iron accumulate in residually enriched form, transforming primary-mineral-derived clays into Al oxides. The key solid phases are kaolinite, a 1:1 layered aluminosilicate clay (Al2Si2O5(OH)4), and gibbsite (Al(OH)3), which appears when dissolved silica activity falls low enough that kaolinite itself becomes unstable. Whether a soil solution dissolves or precipitates kaolinite at a given depth is judged by the saturation index: the ratio of the ion-activity product (IAP) of the dissolution reaction, computed from the activities (effective thermodynamic concentrations) of Al³⁺, H₄SiO₄ (dissolved silicic acid) and H⁺, to the mineral's equilibrium solubility product K; oversaturation (IAP/K > 1) favours precipitation, undersaturation (IAP/K < 1) favours dissolution, and the crossover between them is at IAP/K = 1. Because the ratio itself is strictly positive, the sign of the saturation index is used below in its conventional logarithmic form log10(IAP/K), which is positive under oversaturation, negative under undersaturation and zero at the crossover. Ever since the classical laboratory equilibration studies of Kittrick (1967), the gibbsite–kaolinite–silica phase boundaries have been drawn on activity diagrams with axes pH + (1/3)log a(Al³⁺) versus log a(H₄SiO₄), and soil solutions are plotted on them to infer which Al-bearing phase is stable. Two complications are central in the field: (i) dissolved organic carbon (DOC) complexes Al strongly, so speciation (the distribution of a dissolved element among its free ionic and its organic or inorganic complexed forms) must be computed rather than assumed: the analytically measured dissolved Al can exceed the free Al³⁺ activity by an order of magnitude or more, shifting apparent saturation states, particularly in DOC-rich topsoils; and (ii) the real phase assemblage includes disordered kaolinites and hydroxy-interlayered vermiculite (HIV, a 2:1 clay whose interlayer hydroxy-Al filling gives it an ill-defined solubility), so the 'kaolinite' saturation index is computed against a nominal end-member. Against this framework, Fritsch, Herbillon, Do Nascimento, Grimaldi and Melfi (2007) studied Plinthic Acrisols (strongly weathered, iron-mottled tropical soils) on Tertiary sediments of the upper Amazon basin, combining detailed morphological and mineralogical investigation of a representative 25-ha site with hydro-geochemical data on iron and groundwater dynamics; an oversaturation of topsoil solutions and undersaturation of subsoil solutions with respect to kaolinite (or its HIV form), measured with zero-tension lysimeters (passive collectors of freely percolating soil water installed at depth) across rainy and dry seasons, has been attributed to that study but remains unconfirmed. Were such a gradient established, it would imply depth-dependent silica mobility that conditions where Si is exported and Al is residually enriched. Related field work shows the pattern is not a general law: the same group's integrated model of laterite and podzol genesis in the same region (Fritsch et al. 2011) describes allitization there as a topsoil phenomenon driven by lower Si activity in the topsoil, where kaolinite dissolution prevails; Lucas (2001) showed that plants pump Si from depth and return it to the topsoil in litter, raising topsoil and groundwater silica toward kaolinite saturation; Lucas et al. (2012), working on an Amazonian podzol–ferralsol system, found groundwaters in kaolinitic horizons distributed between two alternative kaolinite solubility lines, i.e. near equilibrium under a control by kaolinite precipitation/dissolution, and clearly oversaturated at the channel bottom where litter mineralization raises Si, but computed their stability diagram from average compositions without correcting for complexation by dissolved organic matter; Watanabe, Funakawa and Kosaki (2006) applied the same equilibrium framework across humid Asia and identified unquantified kinetics and unconstrained organic-ligand and colloidal Al as their main limitations; Ganor, Lu, Zheng and Zhu (2007) documented that natural waters are commonly persistently oversaturated with respect to Al-bearing secondary silicates, so equilibrium thresholds alone cannot predict weathering; and Kleber, Schwendenmann, Veldkamp, Rößner and Jahn (2007) showed that topsoil enrichment in Si-bearing phases can be decoupled from mineral weathering by biogenic Si cycling. Together these results define the unresolved question: what actually sets the sign and magnitude of the kaolinite saturation index as a function of depth in a given highly weathered tropical profile, and how does that saturation-state gradient govern net desilication?

Problem Statement

Provide a quantitative, speciation-corrected, field-validated account of the depth dependence of kaolinite saturation in highly weathered humid-tropical soils, comprising: (i) the conditions of pH, free Al³⁺ activity, H4SiO4 activity and solution composition (including dissolved organic carbon) under which the kaolinite saturation index changes sign with depth and in which direction, and the processes that set those variables with depth, taking into account Al–organic complexation and the actual phase assemblage (well-crystallized and disordered kaolinite, hydroxy-interlayered vermiculite, gibbsite) whose solubilities differ; (ii) an assessment of whether such saturation-state depth patterns are robust to correction of aluminium speciation for organic complexation, or artifacts of uncorrected speciation and of the uncertain solubility of the reacting phase; and (iii) a quantitative linkage showing how this saturation-state gradient governs net desilication — the balance between dissolved silica export and biologically recycled silica, and the associated residual aluminium enrichment (allitization) — validated against measured depth profiles of solution chemistry and mineralogy.

The account is to be a general, literature-based synthesis across highly weathered tropical profiles, validated against the measured depth profiles of solution chemistry and mineralogy published in the cited field studies, with site conditions (parent material and mineralogy, vegetation and biological Si supply, hydrology and water residence time, dissolved organic carbon and ligand composition) entering explicitly as the variables that set the sign and magnitude of the saturation index, so that the differences documented between profiles — including opposite signs of the depth gradient — are explained by identified controls rather than assumed away.

References

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  2. Y. Lucas (2001), 'The Role of Plants in Controlling Rates and Products of Weathering: Importance of Biological Pumping', Annual Review of Earth and Planetary Sciences 29:135–163, doi:10.1146/annurev.earth.29.1.135, https://doi.org/10.1146/annurev.earth.29.1.135
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