TIDAL EFFECTS ON THE CHEMICAL COMPOSITION OF WATER WITHIN A BLUE HOLE: A POSSIBLE CONTROL ON THEIR MORPHOLOGIC EVOLUTION

Tidal variations occur inland of the coast in karstic carbonate islands because of elevated matrix permeability. These tidal variations may alter the thickness of haloclines within conical blue holes. Haloclines decrease in thickness at high tide as the fresh water lens spreads across the wider upper portion of conical blue holes. The halocline traps organic carbon and can become undersaturated with respect to calcite. As the halocline spreads with a rising tide, more wall rock may come into contract with the undersaturated water leading to dissolution. To test this hypothesis we collected water samples from three sites in Ink Well Blue Hole, San Salvador Island, Bahamas. Buoys were located at the three sites 1 m from the edge of the blue hole, 7 m from the shore in the center, and 4 m halfway between the other two with PVC tubing suspended from the buoys to depths 2 m below the water surface. The water samples collected were measured for pH and specific conductivity (SpC) as well as Ca and H₂S concentrations. We suspended a pressure logger at a depth fixed relative to the land surface to record tidal fluctuations within the blue hole. The SpC values varied with tidal signals over time reflecting variations in the thickness of the halocline. The H₂S concentrations varied linearly with SpC, but at low tide the near shore site had H₂S concentrations up to 3x greater than the center site. The Ca concentrations increased with SpC and had the greatest range near shore. In contrast, pH decreased when SpC increased and had the smallest range near shore. The greatest H₂S and Ca concentration ranges and lowest pH values occurred at high tide as the halocline thinned. The elevated H₂S concentrations near shore may originate from the sediments. Increased H₂S concentrations, when oxidized to SO₄^2- close to the surface should decrease pH and dissolve calcite at the edge of the blue hole. Dissolution is recognized in the large Ca concentration range through the tidal cycles and likely reflects dissolution processes that would buffer changes in pH. Concentrated dissolution within the halocline would expand the blue hole creating a feedback that enhances the shallow edges of blue holes surrounding the deeper center that likely formed by collapse into a subsurface void.