The Tidal Capillary Fringe as a Hot Spot for Dissolution in Eogenetic Carbonate Platforms
Classical models maintain mixing of fresh and saline groundwaters is the primary driver of cavernous porosity in eogenetic carbonate rocks. A growing body of evidence indicates, however, that subsoil respiration of CO2 may play an underappreciated role in dissolution. Here we present data from San Salvador Island, Bahamas, which suggests organic matter oxidation and karstification may be enhanced at water tables that are influenced by ocean tides. Similar to many eogenetic carbonate islands, San Salvador’s thin vadose zones facilitate rapid transport of surficial organic matter to freshwater lenses, where subsequent oxidation consumes dissolved oxygen (DO) and organic matter to produce CO2. High organic matter fluxes on San Salvador and other islands have led to dysaerobic to anaerobic conditions in freshwater lenses. By measuring changes in the concentrations and relative amounts of CO2, O2 and Ar in subsurface gasses collected directly above the water table from uncased monitoring wells, we show that falling water table elevations pull oxygenated air into newly exposed, wetted pore spaces in eogenetic carbonate bedrock (the tidal capillary fringe) during the transition from higher to lower tides. Oxygenation fuels aerobic oxidation of organic matter to CO2 which, in turn, dissolves carbonate bedrock. Depth profiles of dissolved oxygen concentrations in wells suggest tidal oscillation also oxygenates the freshwater lens top, with rising water tables entraining oxygen trapped in air bubbles within the tidal capillary fringe. DO concentrations at the water table were at least 5% saturated but declined to near 0% saturation about 1 meter below the water table. Timeseries measurements of Specific Conductance demonstrate small-scale oscillations, on the order of 5-10 µS/cm, over each tidal cycle. SpC maxima typically occur while water table elevations are increasing, resulting in hysteresis in water table-SpC relationships. We interpret this hysteresis as evidence of tidal pumping of water from less connected to more connected pores, either within the freshwater lens or the tidal capillary fringe. The combination of dissolution and transport within the tidal capillary fringe may establish water tables as hotspots of dissolution, creating laterally continuous, but vertically restricted, regions of enhanced porosity and permeability.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019