The influence of soil age and regional climate on clay mineralogy and cation exchange capacity of moist tropical soils: A case study from Late Quaternary chronosequences in Costa Rica

Abstract

Tropical soils exhibit large differences across landscapes in many attributes, notably clay mineralogy (2:1 vs. 1:1 clays), bulk geochemistry (Ca, Al), pH, cation exchange capacity (CEC), organic matter and soil texture. In order to examine the factors controlling these variables, a series of three chronosequences (≤120ka) of tropical soils on uplifted terraces along the Pacific coast of Costa Rica were studied. The three study locations differ mainly as a function of soil moisture and leaching, with conditions ranging from 2700mm/yr mean annual precipitation (MAP) on the southern Nicoya Peninsula (4-month dry season), to 3200mm/yr in the Esterillos region (3-month dry season), and 4250mm/yr on the Osa Peninsula (no dry season). Analytical methods include X-ray diffraction (XRD), transmission-analytical electron microscopy (TEM-AEM), inductively coupled plasma optical emission and mass spectrometry (ICP-OES, ICP-MS), ammonium acetate extraction (1M NH4OAc, pH=7, for CEC) and C:N analysis.Soil weathering reactions and related decrease in CEC occur three-to-four times faster in a sequence of Inceptisol-to-Oxisol soils in the wettest climate (Osa) compared to the less-moist Esterillos area; in the even drier monsoon climate soils (Nicoya), this evolved, low-CEC (<10cmolc/kg) state does not occur, even after 120ka of soil formation, and rate of compositional alteration is approximately five to ten times slower than at Osa. The dominant exchangeable cation at all sites is Ca. Interlayer K and Al increase relative to Ca over time, resulting in interstratified K-S with smectite layers that become progressively more vermiculite-like and illite-like.The age-related evolution of tropical soils appears to be a predictable sequence in lowland tropical landscapes where periodic tectonism, erosion or volcanism produces unweathered parent material at the land surface. Empirical data from this project enables the extrapolation of simple equations applicable to tropical volcanic arc landscapes where presence of uplifted marine terrace soils facilitates determination of soil age. The two variables controlling soil composition can be combined into an effective age (ageeff) that takes into account soil age and weathering intensity (factoring in MAP and wet-dry months from climate data), and equations are of the form, e.g. ECEC=−18.1∗ln(ageeff)+77, with R2 values of 0.75 to 0.87. Greatest scatter occurs in the youngest soils. Given the apparent prevalence of this sequence and the systematic nature of its reaction progression, these results could be useful for modeling tropical soils.