Dissolved strontium and calcium concentrations in fresh-water lenses (FWL) and associated mixing zones (MZ) on two small, Holocene ooid-sand islands in the Schooner Cays, Bahamas, were monitored during a 1-yr period to quantitatively analyze the transformation of aragonite to calcite. The observed characteristics of this mass transfer are functions of climate and hydrology.
Aragonite-to-calcite transformation in all hydrologic zones is primarily associated with meteoric recharge. The transformation occurs throughout the FWL and in the MZ to relative salinities of 19% and 36% sea water on the two islands. Rates of transformation are rapid in all zones and are greatest in the FWL. A limestone composed of 100% calcite should form from an aragonite precursor within 4,700 to 15,600 yr in the FWL, and within 8,700 to 60,000 yr in the upper MZ.
Efficiencies of transformation can vary between hydrologic zones due to PCO2 effects; yet, the efficiency of the entire system (FWL + MZ) is high (87%). This indicates that most CaCO3 derived from aragonite dissolution is reprecipitated as calcite somewhere in the fresh-water system or upper mixing zone.
CO2 effects, fresh-water-sea-water mixing, and the differing solubilities between aragonite and calcite all drive the mass transfer. The latter is the most significant, accounting for up to nine times more mass transfer than CO2 effects and at least ten times more mass transfer than fresh-water-sea-water mixing. Differing solubilities should also cause mass transfer to occur throughout the hydrologic cycle, but it apparently becomes ineffective after the rainy season, possibly due to the inhibition of calcite precipitation.