Along the Caribbean coast of the Yucatan Peninsula, brackish ground water (mixed fresh water and sea water) is channeled through upper Pleistocene limestone via fracture-controlled caverns. In caves, cenotes, and caletas at Xcaret, Yalku, and Tancah, this open-flow coastal mixing zone comprises three major layers: (1) an upper dilute zone of gradually increasing salinity with depth (slow mixing), (2) a thin intermediate zone of rapidly increasing salinity with depth (rapid mixing), and (3) a lower saline zone of gradually increasing salinity with depth. The intermediate layer occurs at different absolute salinities at different localities, and it generally corresponds to the level of a notch (apparently related to dissolution) in the wall rock of the caverns.

Chemical composition of the mixed ground water is consistent with that predicted from theoretical conservative mixing of fresh-water and sea-water end members sampled in the Yucatan coastal region. Inorganic-C alkalinity, Mg:Ca and Sr:Ca, and δ18O and δ13C of the mixed water give no indication of dissolution or precipitation of CaC03 as the ground water flows rapidly seaward. Field and petrographic observations, however, show that aragonite is preferentially dissolving throughout the modern mixing zone, and in particular in the saline portion.

Comparison of saturation indices to calculated mixing curves shows that the coastal mixing zone of the eastern Yucatan Peninsula is undersaturated with respect to CaC03 over a wide range of salinity, including mixtures of as much as 95% sea water for aragonite and 90% sea water for calcite. Supersaturation with respect to stoichiometric dolomite is maintained over the whole range of salinities, but super-saturation with respect to calcian dolomite (Ca57Mg43) can be confined to high-salinity mixtures, depending on the thermodynamic data base used in calculations. These data suggest that (1) dissolution of coastal limestone can take place in the sea-water-dominated portion of the mixing zone and (2) the mixing-zone model for dolomitization can be extended to much higher salinities than is indicated by the widely accepted "Dorag" model.

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