BACTERIAL ACTIVITY WITHIN THE VADOSE AND PHREATIC ZONES IN LIMESTONES: THE SOURCE OF CO2-DRIVEN DEVELOPMENT OF SECONDARY POROSITY

Where does the CO2 necessary for dissolution of limestones come from? The currently accepted model suggests that acidic rainwater picks up additional CO2 as it passes through the soil. In carbonate island settings, water descends through the vadose zone and merges with the fresh to brackish groundwater that floats on the underlying marine groundwater. The current hypothesis for cave development in carbonate islands relies on physico-chemical mixing of these water masses to dissolve rocks. We argue that this hypothesis cannot explain the dissolution we observe in the vadose and phreatic zone. Particularly in island settings, rainwater is too rapidly buffered for it to maintain sufficient dissolutional potential. Bacterial generation of CO2 within the vadose and phreatic zones is the only known renewable source of CO2 that can drive dissolution in such settings, and is almost certainly the dominant source of CO2. Our studies show that rainwater samples collected on San Salvador Island, Bahamas have an average pH of ~6. Rainwater captured off of a roof following a torrential rainstorm was buffered (pH ~8.3) by contact with the tiny amounts of carbonate dust on the roof. Our studies show that rainwater is slightly acidified as it passes through the carbonate-rich soils that are found in the Bahamas. So, soils that cap the limestone rock in the Bahamas, do contribute a small amount of CO2 to transient water, but the organic content of the soils is low, and the minor acidification is short lived. Therefore, descending vadose water is unlikely to have any dissolutional potential. We have documented concentrations of CO2 (410-770 ppm above the current atmospheric concentration of 380 ppm) in the pores of dry cave wall rock. In addition, counts of bacteria in sterile-caught rainwater (~103 cells/ml), in water collected from dripstones (>104-105 cells/ml), and in groundwater (>106 cells/ml), indicate that there are abundant populations of heterotrophic bacteria that generate CO2. Furthermore, we have documented the presence of abundant populations of bacteria in association with the mixing zone, and this acidic water (pH 6 – 4) is always associated with large bacterial populations. It must be these bacteria in the rock pores, and in the water that provide the CO2 necessary to drive carbonic-acid-driven dissolution.