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

石灰岩为世界上25%的人口提供饮用水，并含有世界上50%以上的已知碳氢化合物储量。石灰石的高溶解度使得洞穴的形成控制了地下水和碳氢化合物的流动。因此，了解导致洞穴形成的过程对于改善水和碳氢化合物资源的表征是必要的。在石灰岩形成的碳酸盐台地环境中（如巴哈马群岛、尤卡坦半岛和佛罗里达），厚度超过60米的非饱和岩石带（气包带）被认为可以限制有机碳从土壤到地下水位的运动，否则，氧化成二氧化碳（CO2）会导致石灰岩基岩的腐蚀。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比现在低的时候与新鲜地下水接触而形成的。因为当这些洞穴形成时，水汽带的厚度要比60米厚得多，所以人们对它们形成的地球化学过程知之甚少。在这个项目中，溶解有机碳（DOC）和二氧化碳气体通过渗透带快速流动路线移动到地下水位，假设这为腐蚀石灰岩和在厚的渗透带下形成洞穴提供了一种机制。这一假设将在关岛得到验证，那里的构造隆起形成了厚度达180米的气包带。深层气包带生物过程产生的二氧化碳形成洞穴，与碳酸盐台地中的洞穴是由不同化学成分的水混合形成的范式相反。因此，通过挑战混合溶蚀作用是碳酸盐岩台地景观中溶蚀作用和溶洞形成的主要因素的权威观点，本提案所探索的概念有可能改变对渗透带地貌学和生物地球化学的理解。该项目通过培训两名博士生，为三名本科生提供研究机会，为K-12教师制定有关碳酸盐含水层的课程计划，并为大学主导的社区外展项目提供实践活动，支持STEM教育。关于二氧化碳的地下呼吸作用，而不是混合作用，支配着古成因石灰岩的溶解作用的假设，将通过收集关岛地下水位的挥发气体、渗透和水来检验。无套管监测井提供了对含气和含水层取样的途径。充满空气的洞穴允许渗透补给和气体在整个渗透区被收集。在大降雨事件前后取样将测试快速流动路线对溶解的影响。水样中的DOC将表明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。通过气包带的二氧化碳剖面将用于确定产生二氧化碳气体的深度。这里测试的总体假设预测了通过氧化DOC在渗透区和地下水位产生二氧化碳。二氧化碳和氧气浓度将一起用于确定：1)水汽区二氧化碳是由呼吸作用产生的，还是由回灌水脱气产生的；2)二氧化碳是否因扩散（土壤的典型特征和土壤呼吸作用的可能示踪剂）而流失到大气中。石灰石的溶解将使用锶（Sr）同位素进行追踪；关岛之所以被选为本研究的研究对象，是因为构成渗透带和潜水带的石灰岩年龄不同，因此可以使用Sr同位素比率来区分在潜水带发生的溶解和在渗透带发生的溶解，然后溶质被输送到含水层。

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.