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

石灰岩为世界25%的人口提供饮用水，并包含世界已知碳氢化合物储量的50%以上。石灰岩的高溶解度允许形成洞穴，控制地下的水和碳氢化合物的流动。因此，了解洞穴形成的过程对于改进水和碳氢化合物资源的特征是必要的。在形成石灰岩的碳酸盐台地环境中(如巴哈马、尤卡坦和佛罗里达)，厚度超过60米的不饱和岩带(包气带)被提议限制有机碳从土壤到地下水位的移动，否则氧化成二氧化碳(CO2)将导致石灰岩基岩的腐蚀。与这种解释相反的是，洞穴系统出现在低于现代海平面100多米的碳酸盐台地中。这些洞穴被认为是在过去海平面低于今天的时候与淡水地下水接触而形成的。因为当这些洞穴形成时，包气带的厚度已经远远超过60米，因此对它们形成的地球化学过程知之甚少。在这个项目中，通过包气带快速流动路线将溶解有机碳(DOC)和二氧化碳气体移动到地下水位的假设是为了提供一种侵蚀石灰岩的机制，并在厚厚的包气带下形成洞穴。这一假设将在关岛得到验证，那里的构造抬升造成了厚达180米的包气带。由深层包气带中的生物过程产生的二氧化碳形成的洞穴与碳酸盐台地中的洞穴是由不同化学成分的水混合而形成的范例背道而驰。因此，这一建议探讨的概念有可能改变对包气带地貌和生物地球化学的理解，挑战关于混合溶解是碳酸盐台地景观中溶解和洞穴形成的主要因素的经典观点。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，并为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项目制定实践活动。将通过收集关岛地下渗气、渗透和地下水位中的水来检验地下二氧化碳呼吸而不是混合在成生石灰岩中的溶解这一假设。无井监测井提供了取样包气气体和含水层的通道。充满空气的洞穴允许渗入的补给和气体在整个包气带收集。在一次大的降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，从而表明其氧化是否会导致溶解。通过包气带的二氧化碳剖面将被用来确定产生二氧化碳气体的深度。这里测试的最重要的假设预测了包气带和地下水位中DOC氧化产生的二氧化碳。二氧化碳和氧气浓度将一起用于确定1)包气带二氧化碳是由呼吸作用产生的，还是从补给水中脱气的；以及2)二氧化碳是否已通过扩散损失到大气中(土壤的典型特征，可能是土壤呼吸的示踪物)。将使用锶(锶)同位素追踪石灰岩的溶解情况；之所以选择关岛进行这项研究，是因为包气带和潜水带中石灰岩的年龄不同，可以利用锶同位素比率区分在潜水带中的溶解和在包气带中发生的溶解，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.