DISSOLUTION IN BAHAMIAN BLUE HOLES

Blue holes provide points of entry for organic carbon (OC) to aquifers on carbonate platforms. The OC can be oxidized to CO₂ when trapped at the top of the pycnocline, driving localized dissolution at the contact of the pycnocline and the blue hole walls. More widely distributed dissolution requires the CO₂-rich water to exchange with aquifer water, such as occurs during tidal pumping at semi-diurnal frequencies in the Bahamas. CO₂ concentrations may also vary at diel frequencies from primary production and plant respiration in the blue holes. At longer time scales, water composition may vary from seasonal variations in rainfall and evapotranspiration. To assess variations in water compositions at tidal, diel, and seasonal frequencies, we have measured profiles of the chemical composition of the water column in Ink Well Blue Hole on San Salvador Island a total of six times under different conditions, including high and low tide and in the morning and afternoon, in October 2010 following the rainy season and in May 2012 following the dry season. Specific conductivity (SpC) at the surface was about 10 mS/cm higher in October than in May, suggesting elevated evapotranspiration during the summer months may increase salinity. SpC at the bottom of the blue hole was similar in both seasons, although values were about 10 mS/cm greater at high than low tide reflecting vertical movement of the pycnocline. Regardless of tidal stage, time of day, or season, carbon isotope values of the dissolved inorganic carbon pass through a minimum at intermediate values of SpC reflecting localized mineralization of OC at the pycnocline. Saturation indices of the water did not have a similar minimum and instead had an approximately inverse linear relationship with SpC, with values decreasing from around +0.4 to -0.2 in water with low to high SpC, respectively. Similarly, Ca concentrations in excess of those expected from simple mixing between water with low and high SpC are elevated by 2 to 3 mM in the low SpC water. Excess Ca was about 1 mM higher in October than May, when SpC was also elevated. These results indicate that sufficient mixing occurs within the blue hole water column and into the aquifer to disperse chemical variations at tidal and diel frequencies, as well as distribute the potential for dissolution throughout the aquifer matrix.