DEEP VERSUS SHALLOW CIRCULATION IN THE BAHAMAS BANKS: EVIDENCE FROM SR CONCENTRATIONS AND ISOTOPE RATIOS

Cross-bank pressure gradients and thermo-haline circulation can cause water to circulate deep within carbonate banks. On Bahamas Bank islands, some tidally influenced ponds, blue holes, and other surface water bodies located several kilometers from the coast occasionally have salinities close to seawater values, suggesting they may have direct connections to the ocean. If water dissolves carbonate minerals during flow from the ocean to inland pools, isotope ratios of dissolved Sr may reveal the depth of circulation on the basis of the well-known variations in seawater Sr isotopes through time. Samples from four sites on San Salvador Island and two sites on Long Island, Bahamas have salinity values ranging from 5 to 52, which are linearly correlated (r² = 0.99) to Ca concentrations. This correlation indicates variations in salinity and Ca concentrations are controlled primarily by evaporation and dilution from precipitation. In contrast, Sr concentrations show little correlation with salinity. When corrected for salinity variations, Sr/Cl ratios range from half seawater values for water with salinity of 5 to 2.5 times seawater values for water with salinity of 52. In the samples near seawater salinity (20-40), the Sr/Cl ratios are about 1.4 times seawater values. For all samples, the ⁸⁷Sr/⁸⁶Sr ratios are identical within error to modern seawater values of 0.70917 indicating little of the excess Sr is derived from dissolution of carbonate minerals that are older than about 750 k.y. Assuming an average subsidence rate of about 10 m/m.y., excess Sr cannot originate from dissolution reactions at burial depths greater than about 7.5 m. Although these isotope ratios could indicate the lack of deep circulation through the platform, an alternate explanation is that excess Sr originates from aragonite to calcite conversion near the surface following deep circulation.