HOW DOES FRESH AND SALT WATER MIXING INFLUENCE CALCITE SATURATION IN AN ISOTOPE SUBSTAGE 5E FLANK MARGIN CAVE? AN EXAMPLE FROM MAJORS CAVE, SAN SALVADOR ISLAND, BAHAMAS

Flank margin caves in Quaternary eolian calcarenites on San Salvador Island, Bahamas are believed to form by dissolution during mixing of the fresh water lens and seawater. An example of such a flank margin cave is Majors Cave, which formed during Oxygen Isotope Substage 5e (ca. 125 kybp). The cave contains perennial pools of mixed meteoric water and seawater that are density stratified. Water was collected from pools in the cave in January 2004, nominally the wet season and June 2004, nominally the dry season. In January, the water from a pool near the entrance of the cave had salinities ranging from 0.68 to 0.75 times seawater value at the surface and 1.5 m depth, respectively. In June, salinity of water from the same pool had increased to 0.83 times seawater value at the surface and 0.85 times seawater value at a depth of 1.5 meters, but water in a pool at the back of the cave showed a stronger density stratification with salinities ranging from 0.77 to 0.82 times seawater values at the surface and 1.5 m depth, respectively. Drip water collected at the time of sampling was 0.71 times seawater salinity. Samples collected in January have major element/Cl ratios identical to seawater ratios, except for Ca/Cl ratios, which are greater than seawater ratios by 9 to 15%. Analyses are on-going for the samples collected in June. Those solute ratios that are similar to seawater ratios suggest the water is a mixture of fresh meteoric water and seawater, while excess Ca concentrations shown by elevated Ca/Cl ratios may originate from dissolution of calcite as rainwater percolates through the epikarst and cave roof. Mixtures of fresh water and seawater would be expected to be undersaturated with respect to calcite, but calculations using the speciation program PHREEQC indicate the water is supersaturated (SI=+0.5 to +1.2) with respect to calcite. Supersaturation may occur from degassing of CO₂ during mixing, evaporative concentration of solutes after water enters the cave, and/or from excess Ca. This supersaturation suggests continued growth of flank margin caves from dissolution by mixing of modern fresh water and seawater may be limited. These two sampling times suggest that mixing of meteoric water and seawater may be seasonal, but additional and higher resolution sampling will be needed to assess this hypothesis.