Abstract: Geochemical measurements (Ca 2+ , Sr 2+ , Mg 2+ , Cl − , SO 4 2- , and 87 Sr/ 86 Sr) have been made on fluids obtained from two deep boreholes (Unda and Clino) drilled on the Great Bahama Bank. As a result of the fact that these holes were drilled through cemented Limestones, it was not possible to squeeze sediments to obtain interstitial pore waters, and instead the fluids were retrieved using a combination of pumping from the surface and passive water samplers deployed during logging. Geochemical analyses show that it is possible using these techniques, combined with tritium, to define the extent of contamination of these pore fluids by surface waters. Consequently, with appropriate corrections for surface contamination, estimates of the interstitial concentrations of Ca 2+ , Sr 2+ , Mg 2+ , Cl − , and SO 4 2- and 87 Sr/ 86 Sr ratios can be obtained. In both holes, dissolution and precipitation of carbonate minerals such as aragonjte, calcite, and dolomite are taking place in slightly modified sea water, leading to significant Increases in the concentration of Ca 2+ and Sr 2+ and decreases in Mg 2+ . The absence of significant geochemical gradients in the upper zone of Unda suggests that there is substantial influx of modern seawater. Deeper in Unda and throughout Clino there are large increases in Sr 2+ and Ca 2+ suggesting a more closed diagenetic system. In both Clino and Unda the 87 Sr/ 86 Sr ratio of the pore fluids, when corrected for surface contamination, was either similar to or more radiogenic than the co-occurring sediment. This not only suggests that there is not a significant upward vertical flow through the platform, but also supports the notion that the 87 Sr/ 86 Sr ratios of carbonates in such settings can be used to give the oldest possible age of deposition and the youngest possible age of diagenesis.