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Abstract

In open marine carbonate platform systems there is a potential for dolomitiza- tion by circulation of saline ground waters because of the high magnesium content of sea water and the long residence time of the carbonate sediments in the shallow subsurface. Saline ground water circulation is driven by hydraulic head, which may be generated either by differences in sea-surface elevation caused by tides, winds or ocean currents, or by differences in water density related to salinity and/or temperature. Buoyant circulation occurs where saline ground waters are entrained by the fresh ground water circulation, while the increase in salinity of bank surface waters due to evaporation induces reflux. Finally, thermal contrasts between cold ocean waters and ground waters warmed by the geothermal heat flux gives rise to convective circulation. The scale of any platform, the steepness of its margins, and relative position of sea level are major controls on the type of circulation that may occur. In addition, climate, the distribution of permeability in the carbonate sediments, and geographic position in relation to other land masses and the atmospheric and oceanic circulations are significant.

Active circulation of saline ground water occurs beneath North Andros Island on the Great Bahama Bank in the form of major outflows (0.3-3.9xl05 m3day<sup>-1</sup>) from karstic blue holes along the margin of the Tongue of the Ocean. The elevated salinity of these waters indicates that they derive by reflux from the Great Bahama Bank, and flow eastward under North Andros Island. Ground water temperatures are, however, low, suggesting that cold normal salinity sea waters from depth within the adjacent oceans are also involved in the circulation. It is concluded that this circulation results from the combined effects of differences in temperature, salinity, and elevation potential. Numerical modeling of saline ground water circulation is needed in order to understand the sensitivity of the circulation to the differing controlling parameters, and may enable prediction of the three-dimensional distribution of resultant diagenetic alteration.

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