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DIVISION S-5-PEDOLOGY

Induration of C Horizons by Allophane and Imogolite in Costa Rican Volcanic Soils

^a Dep. of Soil Science and Geology, Agricultural Univ., Wageningen, The Netherlands
^b Unité de Science du Sol, INRA, Versailles, France
^c REPOSA, Guapiles, Costa Rica

toine.jongmans{at}bodeco.beng.wau.nl

Little is known about the contribution of allophane and imogolite to induration of soil horizons in the perudic tropics. In this study we observed, to a depth of 10 m, indurated sandy C horizons that contained isotropic translucent fine-textured coatings in Holocene and Pleistocene alluvial andesitic deposits situated in the perudic moisture regime in Costa Rica. The horizons have a firm to extremely firm consistence and do not slake in water. Optical microscopy showed that the coatings cover and bridge primary minerals, forming a three-dimensional framework in the horizons, which indicates a post-depositional origin. Bulk chemical analyses show the presence of allophane in the indurated horizons. Scanning electron microscopy (SEM) of undisturbed mineral surfaces confirmed the existence of the coating framework. Chemical analyses, obtained by SEM energy-dispersive x-ray, indicate an alumino-silicate coating with Al/Si ratios ranging from 1.6 to 2.1. Coatings were studied by TEM, carried out on an ultra-thin section (50 nm) cut from a coating that was in situ sampled by microdrilling in an uncovered thin section. All clay material in the horizons consists of coatings of allophane and imogolite. Rapid weathering of the andesitic parent material released large amounts of Al and Si, which accumulated as amorphous gels due to oversaturation of the soil solution near the wetting front or in deeper water-conducting layers. We conclude that the mechanisms for soil horizon induration include changes in moisture conditions due to short seasonal drying, climatic changes, tectonic uplifting, and erosion, which lead to irreversible drying of allophane and imogolite.

Abbreviations: SEM, scanning electron microscopy • TEM, transmission electron microscopy

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