Paper No. 37
Presentation Time: 6:00 PM


RYAN, Peter C.1, PINCUS, Lauren2 and FALCONES, Kristoffer2, (1)Dept. of Geology, Middlebury College, Middlebury, VT 05753, (2)Geology Department, Middlebury College, 276 Bicentennial Way, Middlebury, VT 05753,

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 cmolc/kg (compared to 3 – 15 cmolc/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 cmolc/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).