predavanje 7. junija 2007, Atrij (!!!) ZRC SAZU, Novi trg 2  ob 19:00!!! John E. Mylroie & Joan R. Mylroie  Department of Geosciences, Mississippi State University, ZDA Bahamian Geology and Hydrology: A Tropical Carbonate Laboratory  The Bahamian Archipelago, a solely carbonate province, lies in the southwestern North Atlantic Ocean, extending for 1400 km from Florida in the northwest, to Cuba and Hispaniola in the southeast. To the northwest, islands large and small are found primarily on large carbonate banks in a positive water-budget climate. To the southeast, islands are smaller, as are the carbonate banks, in a negative water-budget climate. The entire group of banks and islands is tectonically stable, isostatically subsiding at 1-2 m/100ka since the mid-Mesozoic. The surface lithologies consist of Quaternary carbonate sediments with interspersed paleosols. Glacioeustatic sea-level highstand positions of ~10 ka duration are represented as eolian and subtidal suites of limestones, recording a depositional record of platform flooding and carbonate sediment production. Terra rossa paleosols, covering a well-developed epikarst, represent integrated aerosol dust deposition of ~100 ka-long platform exposure events during glaciations. Absolute dating of late Pleistocene fossil corals by U/Th methods has produced an excellent record of the last interglacial (MIS substage 5e). Holocene eolianites have been dated by 14C to reveal platform flooding at ~5000 ybp, and sea-level stabilization at ~3000 ybp. Pleistocene eolianites have been dated by AAR, but subdivisons of the eolianites into separate MIS 5a, 5c and 5e deposits are controversial. Terra rossa paleosols have been differentiated by paleomagnetic secular variation measurements.  The hydrology of young carbonate islands is controlled by karst processes, but in a manner very different from that found in old, dense carbonates of continents. The Carbonate Island Karst Model (CIKM) explains this unique karst as a result of: 1) sea water/fresh water mixing, 2) glacioeustatic sea-level change, 3) local tectonics, 4) degree of diagenetic maturity, and 5) carbonate/non-carbonate rock interactions. Carbonate island porosity decreases with time, but permeability increases as preferred flow paths develop. Voids form at the millimeter to meter scale. The largest voids, flank margin caves, form in the distal margin of the fresh-water lens, under the flank of the enclosing landmass, as a result of superposition of the vadose/phreatic mixing zone and the sea water/fresh-water mixing zone, enhanced by organic collection and decay. Flank margin caves are sea-level indicators, and their size is controlled by the length of time sea level stays at a given horizon. The morphology of flank margin caves yields information about the configuration and discharge of the fresh-water lens when the caves were forming. Erosive breaching of flank margin caves provides a measure of surficial denudation rates. As sea level falls during glaciations, carbonate banks increase in size, and conduit flow becomes competitive. Carbonate deposition and dissolution cycles caused by glacioeustasy create a mass flux of CO2, ~1 x 1018 grams carbon, sufficient to explain the atmospheric CO2 excursions of the Quaternary.