DISSOLUTION IN BAHAMIAN BLUE HOLES FROM TIDAL PUMPING

摘要

Blue holes provide a pathway for organic matter to enter aquifers in modern carbonate platforms and can be used to infer hydrologic and diagenetic processes. Dampened tidal amplitudes in blue holes on San Salvador Island (Ink Well Blue Hole) and Rum Cay (Mermaid Pond), Bahamas, relative to the ocean reflect hydraulic conductivity indicative of conduit-dominated flow. Dampened tidal amplitudes in wells on San Salvador Island reflect hydraulic conductivity of the matrix porosity, which is approximately two orders of magnitude lower than estimates of the conduit-dominated hydraulic conductivity. The difference in hydraulic conductivity causes hydraulic head to increase faster in the blue holes than in the matrix and drives the exchange of water between the blue holes and the matrix during each tidal cycle. This exchange of water causes differences in the oxidation-reduction potential, pH, σ~(13)C_(DIC) values, and carbonate mineral saturation states between high and low tide. Both blue holes are rich in organic matter, which impacts oxidation potential of the water when trapped at the density interface between fresh and marine water. Oxidation of the organic matter decreases pH and saturation indices in the blue holes. The water is closer to saturation at low tide than at high tide because matrix water that has reacted with carbonate minerals discharges to the blue holes during ebbing tides. Redox reactions in the water column and tidally forced fluid exchange enhances dissolution within the matrix and may give blue holes their characteristic circular shape. Tidally driven water exchange, both within blue holes and along high conductivity flow paths, is an important process for the alteration of modern carbonate platforms.