BIOGENIC EXPLANATION FOR THE DEVELOPMENT OF SECONDARY POROSITY IN LIMESTONES

Today, the explanation for secondary porosity development, including caves, in carbonate islands and in carbonate coastal areas relies primarily on physico-chemical mixing-zone corrosion. We argue that this explanation does not correctly address the dissolutional potential of meteoric water, water passing through the vadose zone, and the groundwater. Analyses of rainwater indicate that it is buffered to pH values of 8.5 immediately after it contacts the ground in the Bahamas, and thus is not likely to accomplish much dissolution. Study of possible mixing corrosion (Moore and Martin, 2006) via mixing of waters occurring in the Bahamas indicates that it is incapable of explaining significant porosity development. Our studies document abundant microbially-generated CO₂ (410-770 ppm above the current atmospheric concentration of 380 ppm) in the pores of cave wall rock; and that primarily heterotrophic bacteria are abundant in rainwater (~10³ cells/ml), in water collected from dripstones (>10⁴-10⁵ cells/ml), and in groundwater (>10⁶ cells/ml). These data indicate that bacterial generation of CO₂ within the vadose and phreatic zones is the probable driving force in classical carbonic acid limestone dissolution. Our observations also indicate that porosity and permeability may be enhanced even during periods when the rock is not saturated with water. Bacteria living in rock pores within the vadose zone create conditions that lead to secondary porosity from the microscopic scale to moderate macroscopic voids depending on availability of water and organics. Bacteria in rock pores in the phreatic zone create conditions that enlarge pores within the rock and dissolve it from the inside out; and bacteria living in the water in flooded voids create conditions that lead to enlargement of the void by dissolving inward from the void walls.