RUI: Landscape-scale Implications of Mineral Reaction Rates and Mechanisms in Tropical Soils: Insights from Soil Chronosequences and Synthesis Experiments

热带土壤并非都富含高岭石和营养枯竭，本项目旨在研究随着热带土壤成熟而发生的矿物反应（以及相关的化学变化）的速率和机制。最终，碱阳离子和二氧化硅的明显浸出驱动潮湿的热带土壤溶液向营养贫乏，低pH值，富铝组成，有利于高岭石的结晶；然而，联合国粮食及农业组织的数据表明，65%的热带土壤不符合最终成员Oxisol状态。在许多热带土壤中，铁-蒙脱石(S)作为高岭石(K)的亚稳前体形成，最近由PI和合作者在哥斯达黎加对潮湿热带土壤序列的研究表明，S向K的转变发生在大约10万年的时间范围内，通过中间的高岭石-蒙脱石（K-S）阶段。反应发生在一层接一层的基础上，在K-S晶体中保存S层使Al被隔离在可扩展的S中间层中，从而限制了溶液中的植物毒性Al。此外，K- s的阳离子交换容量约为K的5倍，进一步强调了更好地约束这些反应的必要性。因此，本项目试图验证这样一个假设，即湿润的热带土壤，无论年平均降水量（MAP）如何，在最终产生K之前都经历了从S到K-S的逐步转变。该假设将通过检查时间和气候（MAP）对哥斯达黎加太平洋沿岸三个独立气候带土壤时间序列中S到K-S到K反应的速率和机制的作用来验证。初步数据表明，速率与MAP呈正相关。矿物反应机制也将通过进行合成反应，其中溶液组成和时间的控制进行检查。米德尔伯里学院是一所住宿文理学院，有着让学生参与动手体验式研究的悠久传统。这在地质系尤其如此，几十年来，地质系培养了批判性思维、问题导向的课程，其中包括一篇为期一年的毕业论文。地质系人口结构的积极变化为地质系的所有项目提供了更大的机会，使其扩展到代表性不足的学生。2008年至2013年的地质学专业毕业生中，美国有色人种学生占15%(8/53)，而2000年至2007年的地质学专业毕业生中有色人种学生占不到2%（1/65）。在过去的15年里，女学生占地质学专业学生的46%，而在2012年和2013年的班级中，19名学生中有13名是女性。米德尔伯里15%的地质学论文是与教师共同撰写的，50%的米德尔伯里地质学专业的学生就读研究生院，所以这些人口结构的变化为更广泛的地质社区提供了增加多样性的机会。因此，这个项目将针对不同的学生群体，在一个强大的项目中，在本科研究中取得了成功的记录。该项目还包括国际本科生，其中包括与哥斯达黎加大学（UCR）地质学家Guillermo Alvarado以及UCR和明德学院的本科生一起到奥萨半岛进行实地考察，以及在西班牙格拉纳达的安达卢兹科学研究所进行分析和实验研究，在那里PI和学生将合作对热带土壤进行详细的矿物学和地球化学分析。美国国家科学基金会国际科学与工程办公室为该项目的国际部分做出了贡献。

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.