Tropical soils are not uniformly kaolinite-rich and nutrient-depleted, and this project aims to examine rates and mechanisms of mineral reactions (and associated chemical changes) that occur with maturation of tropical soils. Ultimately, pronounced leaching of base cations and silica drives humid tropical soil solutions towards a nutrient-poor, low pH, Al-rich composition that favors crystallization of kaolinite; however, data of the Food and Agricultural Organization of the United Nations indicate that 65 % of tropical soils do not fit this end-member Oxisol state. Pedogenic Fe-smectite (S) forms in many tropical soils as a metastable precursor to kaolinite (K), and a recent study of a humid tropical soil sequence by the PI and collaborator in Costa Rica indicates that the transformation of S to K occurs over time spans on the order of about 100,000 years via intermediate kaolinite-smectite (K-S) phases. The reaction occurs on a layer-by-layer basis, and the preservation of S layers within K-S crystals enables Al to be sequestered in expandable S interlayers, thus limiting plant-toxic Al in solution. Furthermore, the cation exchange capacity of K-S is about 5 times greater than K, further underscoring the need to better constrain these reactions. Accordingly, this project seeks to test the hypothesis that humid tropical soils, regardless of mean annual precipitation (MAP), undergo step-wise transformation from S to K-S before ultimately producing K. This hypothesis will be tested by examining the roles of time and climate (as MAP) on the rates and mechanisms of the S to K-S to K reaction in soil chronosequences from three separate climate zones along the Pacific coast of Costa Rica. Preliminary data indicate that rate is positively correlated to MAP. Mineral reaction mechanisms will also be examined by carrying out synthesis reactions where solution composition and time are controlled. Middlebury College is a residential liberal arts college with a long tradition of involving students in hands-on, experiential research. This is particularly true in the Geology Department, which over many decades has cultivated a critical-thinking, problem-oriented curriculum that includes a required year-long senior thesis. Positive demographic changes in the Geology Dept provide a much greater opportunity for this and all projects in the department to extend to typically underrepresented students. Graduating geology majors from the classes of 2008 to 2013 are comprised of 15 % (8/53) American students of color, whereas the geology majors from 2000 to 2007 consisted of less than 2 % American students of color (1/65). Female students comprise 46 % of geology majors over the past 15 years, and in the classes of 2012 and 2013 combined, 13/19 students are female. 15% of Middlebury geology theses result in co-authored publications with faculty, and 50 % of Middlebury geology majors attend graduate school, so these demographic shifts present the opportunity to add diversity to the broader geologic community. Thus, this project will target a diverse group of students in a strong program with a proven record of success in undergraduate research. The project also has an international undergraduate scope, which includes a field excursion to the Osa Peninsula to do field work with University of Costa Rica (UCR) geologist Guillermo Alvarado and undergraduate students from UCR and Middlebury College, as well as analytical and experimental research at the Instituto Andaluz de Ciencias de la Tierra in Granada Spain, where the PI and students will collaborate on detailed mineralogical and geochemical analysis of tropical soils. The NSF Office of International Science and Engineering contributed to the international component of this project.
