Collaborative Research: What hydrogeochemical processes control weathering in the deep critical zone of unburied karst landscapes?

石灰石为世界上25%的人口提供饮用水，并含有世界上已知的碳氢化合物储量的50%以上。石灰石的高溶解度允许形成洞穴，控制地下水和碳氢化合物的流动。因此，了解有助于洞穴形成的过程对于改善水和碳氢化合物资源的特性是必要的。在石灰石形成的碳酸盐台地环境中（例如巴哈马、尤卡坦和佛罗里达），建议厚度超过60 m的非饱和岩石区（渗流区）限制有机碳从土壤向地下水位的移动，否则氧化成二氧化碳（CO2）会导致石灰石基岩的腐蚀。与这种解释相反，洞穴系统出现在现代海平面以下100多米深处的碳酸盐台地中。这些洞穴被认为是在过去海平面低于今天的时候与新鲜地下水接触形成的。由于这些洞穴形成时，渗流带的厚度远大于60米，因此对其形成的地球化学过程知之甚少。在这个项目中，溶解的有机碳（DOC）和CO2气体通过渗流区快速流动路线的地下水位的运动被假设为提供一种机制，腐蚀石灰岩，并在厚的渗流区下创建洞穴。这一假设将在关岛得到检验，那里的构造隆起造成了厚度达180米的渗流带。洞穴的形成是由深层包气带中生物过程产生的二氧化碳造成的，这与碳酸盐台地中洞穴是由不同化学成分的沃茨混合而形成的范例相反。因此，这个建议探索的概念有可能改变的地貌和包气带的地球化学的理解，挑战典型的观点，混合溶解是溶解和洞穴形成的主要代理人在碳酸盐岩台地景观。该项目通过培训两名博士生来支持STEM教育，为三名本科生提供研究机会，为K-12教师制定关于碳酸盐含水层的课程计划，并为大学领导的社区外展计划开展实践活动。通过收集渗流气体，在关岛的地下水位上，水的渗透和水的问题。无套管监测威尔斯井提供了对渗流气体和含水层进行采样的通道。充满空气的洞穴允许渗透补给和气体被收集在整个包气带。在大雨事件之前和之后的采样将测试快速流动路线对溶解的影响。水样中的DOC将指示DOC是否被输送到地下水位，从而其氧化是否会导致溶解。通过渗流带的CO2剖面将用于确定CO2气体产生的深度。在这里测试的总体假设预测生产的CO2在包气带和在地下水位氧化DOC。CO2和氧气浓度将一起用于确定1）包气带CO2是否由呼吸作用产生或从补给水中脱气，以及2）CO2是否通过扩散（典型的土壤和土壤呼吸的可能示踪剂）损失到大气中。将使用锶（Sr）同位素追踪石灰石的溶解情况;选择关岛进行这项研究，是因为构成渗流区和潜水区的石灰石年龄不同，可以使用Sr同位素比率区分潜水区的溶解和渗流区的溶解，然后将溶质输送到含水层。

Limestone provides 25 percent of the world's population with drinking water and contains more than 50 percent of the world's known hydrocarbon reserves. Limestone's high solubility allows for the formation of caves that control the flow of water and hydrocarbons below-ground. Understanding the processes that contribute to the formation of caves is thus necessary for improved characterization of water and hydrocarbon resources. In carbonate platform environments where limestones form (e.g. Bahamas, Yucatan and Florida), zones of unsaturated rock (vadose zone) that exceed 60 m in thickness have been proposed to limit movement of organic carbon from soil to the water table, where oxidation to carbon dioxide (CO2) would otherwise drive corrosion of limestone bedrock. In contrast to this interpretation, cave systems occur in carbonate platforms at depths of more than 100m below modern sea level. These caves are thought to have formed in contact with fresh groundwater at times in the past when sea level was lower than it is today. Because vadose zones would have been much thicker than 60 m when these caves formed, the geochemical processes responsible for their formation are poorly understood. In this project, the movement of dissolved organic carbon (DOC) and CO2 gas to the water table via vadose zone fast flow routes is hypothesized to provide a mechanism for corroding limestone and create caves beneath thick vadose zones. This hypothesis will be tested on the island of Guam, where tectonic uplift has created vadose zones that are up to 180 m in thickness. Cave formation by CO2 that is produced by biological processes in the deep vadose zone runs contrary to the paradigm that caves in carbonate platforms form as a result of mixing waters of different chemical composition. Concepts explored by this proposal thus have potential to transform understanding of the geomorphology and biogeochemistry of the vadose zone by challenging canonical views that mixing dissolution is the principal agent of dissolution and cave formation in carbonate platform landscapes. This project supports STEM education via the training of two PhD students, providing research opportunities for three undergraduate students as well as developing lesson plans about carbonate aquifers for K-12 teachers and hands-on activities for university-led community outreach programs.The hypothesis that subsoil respiration of CO2, rather than mixing, dominates dissolution in eogenetic limestone will be tested by collecting vadose gases, infiltration and water at water tables on the island of Guam. Uncased monitoring wells provide access for sampling vadose gases and the aquifer. Air-filled caves allow infiltrating recharge and gases to be collected throughout the vadose zone. Sampling before and after a large rain event will test the influence of fast-flow routes on dissolution. DOC in water samples will indicate whether DOC is transported to the water table and thus whether its oxidation could result in dissolution. CO2 profiles through the vadose zone will be used to determine the depths at which CO2 gas is produced. The overarching hypothesis tested here predicts production of CO2 in the vadose zone and at the water table by oxidation of DOC. CO2 and oxygen concentrations will be used together to determine 1) if vadose zone CO2 is produced by respiration or is degassed from recharging water and 2) if CO2 has been lost to the atmosphere by diffusion (typical of soils and a possible tracer of soil respiration). Dissolution of limestone will be traced using Strontium (Sr) isotopes; Guam was selected for this study because differences in the age of limestone that comprises the vadose and phreatic zone allow use of Sr isotope ratios to discriminate between dissolution in the phreatic zone from dissolution that occurs in the vadose zone followed by transport of solutes into the aquifer.