MINERALOGICAL AND GEOCHEMICAL EVOLUTION OF TROPICAL SOILS IN A COASTAL TERRACE SEQUENCE

Soils formed on fluvial and marine fill terraces along the Pacific coast of Costa Rica contain a record of tropical soil evolution from smectite-rich young soils (Holocene, < 10 ka) to evolved, kaolinite-rich, lateritic Oxisols and Alfisols (Pleistocene terraces > 120 ka). Intermediate-age soils developed on latest Pleistocene (e.g. 30 – 50 ka) terraces are dominated by interstratified kaolin-smectites (K-S) with compositions and cation exchange capacities (CEC) intermediate to the smectite and kaolinite end-members. Changes to soil geochemistry occur in conjunction with mineralogical changes: Holocene soils are enriched in base cations (5 – 10 wt % oxides of bulk soil) whereas the most-evolved soils are depleted in base cations (< 1 %) and enriched in bulk and exchangeable Al. The K-S that occurs in intermediate-age soils appears to play an important role in cycling of Ca and Al; for example, natural K-S retains a cation exchange capacity of 10 – 30 cmol_c/kg (compared to 3 – 15 cmol_c/kg for many tropical soil kaolinites), as well as the capacity to sequester Al in smectite interlayers. The smectites that form in the first 5 ka of pedogenesis are Mg- and Fe-rich (e.g. 5 – 15 wt % Fe₂O₃). HRTEM indicates that the transformation of smectite to kaolinite layers occurs on a layer-by-layer basis within crystallites, a process that helps to preserve Mg and Fe in octahedral sheets to the extent that 120 ka kaolinites contain 3 – 10 wt % Fe. The Fe-kaolinites retain the crystal form of precursor smectite and K-S and possess CEC values of 5 – 20 cmol_c/kg. Current research is designed to analyze a large suite of tropical clays from this terrace sequence for CEC as well as composition of exchange sites, and also to extend research to sites with mean annual precipitation (MAP) ~ 4000 mm (MAP of sites studied thus far = 2000-3100 mm).