MINERAL REACTION PATHWAYS AND RATES IN A TROPICAL SOIL CHRONOSEQUENCE, NICOYA PENINSULA, COSTA RICA

Chemical weathering of primary minerals and leaching of soluble components in moist tropical environments tend to produce kaolinite-rich soils that are depleted in base cations and characterized by low Si:Al ratios. These soils often reach steady state conditions within 100 – 200 ka; however, the rates and pathways to these steady state conditions can be varied and may consist of intermediate-stage minerals such as smectite and/or interstratified kaolin-smectite (K-S). Some studies have observed that, with increasing soil maturity, early-formed smectite undergoes transition to interstratified K-S phases which in turn give way to kaolinite or halloysite. The rates and pathways of such reactions are important for understanding nutrient cycling in tropical soils as well as to applying systematic changes in soil mineralogy and geochemistry to interpreting landscape evolution (e.g. uplift rates). The field area for this study, the southeastern tip of the Nicoya Peninsula in northwestern Costa Rica, provides an ideal environment for the study of soil chronosequences due to the terraced landscape produced by uplift associated with subduction at the Mid-America Trench. Accordingly, the purpose of this study is to examine the mineralogy and geochemistry of parent materials (beach deposits) as well as soils on uplifted beach terraces; a particular focus is to determine reaction rates and sequences of mineral reactions as well as details of clay mineral structures and compositions. Preliminary XRD results indicate a full weathering sequence from parent materials rich in rock and fossil fragments to 10 ka soils rich in pedogenic smectite with relict primary minerals (including calcite) to a kaolin-dominated assemblage in 80 to 125 ka soils. Intermediate age soils (e.g. ~30 – 50 ka) show evidence of interstratified kaolinite-smectite, and FTIR analysis data demonstrate a decreasing Si:Al ratio consistent with the breakdown and leaching of tetrahedral layers in the transition of parent materials to smectite to K-S to kaolin. Further collection of XRD data of randomly oriented clays and mineral composition from ICP-AES will be applied to quantification of bulk soil mineralogy and chemistry, and DTA-TG will help to quantify mineral types and structures.