RAPID DEVELOPMENT OF SECONDARY POROSITY WITHIN FRESHWATER LENSES OF CARBONATE ISLANDS
Combining flank margin cave morphology and geochemical modeling should provide information on the rates of secondary porosity development within the phreatic zone of carbonate islands. Although Bahamian flank margin caves are believed to have formed during a 12 ky time window, Altar Cave, a flank margin cave on San Salvador Island, developed as syngenetic karst concurrently with the deposition of its host rock during the last interglacial highstand indicating it formed in less than 12 ky. The cave has an areal footprint of about 450 m2 and a minimum volume of 1200 m3. Water collected from lakes, cave pools, and wells across San Salvador Island has an average Ca2+ concentration of about 2 mM in excess of the value expected from seawater evaporation and dilution. Speciation-solubility modeling shows most samples are in equilibrium with respect to aragonite and supersaturated with respect to calcite, suggesting aragonite is the primary contributor of excess Ca2+ to San Salvador waters. Samples with salinity less than 10 psu have Ca2+/Cl- and Sr2+/Cl- molar ratios that are greater than seawater value, with the highest ratios indicating 48 times enrichment of Ca2+ and 122 times enrichment of Sr2+, respectively. These enrichments in low salinity samples suggest aragonite dissolution and aragonite-to-calcite transformation in fresh water may be responsible for most of the dissolution occurring on San Salvador Island. Mixing corrosion and sulfate reduction appear to have little influence on dissolution. The magnitude of excess Ca2+ combined with catchment area and recharge indicates an average of 31 m3/km2 of carbonate rock would be dissolved across the island and flushed to the ocean annually. At this rate, 138 m3, or only about 12% of the measured volume of the cave, would dissolve in 10 ky at the site where Altar Cave is located. Average island-wide dissolution rates underestimate the amount of secondary porosity that develops at the edge of a freshwater lens, which may result from solute transport out of the system due to higher specific discharge as the lens thins and cross-sectional area decreases.