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

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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.