Heterogeneous distributions of CO sub(2) may be more important for dissolution and karstification in coastal eogenetic limestone than mixing dissolution

Mixing dissolution, a process whereby mixtures of two waters with different chemical compositions drive undersaturation with respect to carbonate minerals, is commonly considered to form cavernous macroporosity (e.g. flank margin caves and banana holes) in eogenetic karst aquifers. On small islands, macroporosity commonly originates when focused dissolution forms globular chambers lacking entrances to the surface, suggesting that dissolution processes are decoupled from surface hydrology. Mixing dissolution has been thought to be the primary dissolution process because meteoric water would equilibrate rapidly with calcium carbonate as it infiltrates through matrix porosity and because pCO sub(2) was assumed to be homogeneously distributed within the phreatic zone. Here, we report data from two abandoned well fields in an eogenetic karst aquifer on San Salvador Island, Bahamas, that demonstrate pCO sub(2) in the phreatic zone is distributed heterogeneously. The pCO sub(2) varied from less than log -2.0 to more than log -1.0atm over distances of less than 30m, generating dissolution in the subsurface where water flows from regions of low to high pCO sub(2) and cementation where water flows from regions of high to low pCO sub(2.) Using simple geochemical models, we show dissolution caused by heterogeneously distributed pCO sub(2) can dissolve 2.5 to 10 times more calcite than the maximum amount possible by mixing of freshwater and seawater. Dissolution resulting from spatial variability in pCO sub(2) forms isolated, globular chambers lacking initial entrances to the surface, a morphology that is characteristic of flank margin caves and banana holes, both of which have entrances that form by erosion or collapse after cave formation. Our results indicate that heterogeneous pCO sub(2), rather than mixing dissolution, may be the dominant mechanism for observed spatial distribution of dissolution, cementation and macroporosity generation in eogenetic karst aquifers and for landscape development in these settings.