Geology and Hydrogeology of Carbonate Islands

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Book Chapter

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About a hundred years ago, Alexander Agassiz, after making a fortune from Michigan copper and becoming the world authority on sea urchins [Revision of the Echini (1873)],undertook to investigate coral reefs and limestone islands. Agassiz's coral reef expeditions, which he financed largely himself, lasted about a decade (1893-1902) and took him to the Bahamas, Bermuda, the Florida Keys, the Great Barrier Reef, the Fijis, Tongatapu, the Society islands, the Cook islands, the Carolines, the Marshalls, Guam, and Niue to name only carbonate islands that are examined in this book. Intellectually, the driving force behind those studies was Darwin' s theory of coral reefs [Structure and Distribution of Coral Reefs (1842)]. Now, studies of carbonate island geology are energized by concepts and data of plate tectonics; deep-sea and on-island drilling; isotope geochemistry and geochronology; facies models and diagenetic pathways; sea-level curves and Milankovitch cycles. At roughly the same time, W. Badon Ghyben in the Netherlands (1888) and A. Herzberg in Germany (1901) independently published analyses of the hydrostatics whereby fresh groundwater floats on ocean-derived saline groundwater in coastal settings. Now, in addition to the Ghyben-Herzberg principle and Ghyben-Herzberg lenses of island settings, we have brackish-water mixing zones, dual-aquifer conceptualizations, hydrologic budgets, and variable density flow and transport modeling. We now know of the temperature-driven flow of Kohout convection and endoupwelling at greater depths, beneath the meteoric realm. There have been feedback studies relating the rocks to the flows, and the flows to the rocks, and these studies shed light on old questions such as dolomitization. According to one of our chapters, the deep flows explain Darwin's paradox - how the oligotrophic reefs of carbonate islands can exist in the first place, in such vast nutrient deserts. The purpose of this book is to sample the geological and hydrogeological knowledge of particular islands now, some hundred years after Agassiz and Ghyben and Herzberg.

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Geology and Hydrogeology of Carbonate Islands, v. 54, p. v-vii

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