Carbonate dissolution in a modern mixing zone

Abstract

The potential for dissolution of calcite and aragonite, and precipitation of dolomite, by the mixing of fresh and saline ground waters has been demonstrated theoretically1–3. Although geologists have used these processes in the explanation of the diagenesis of recent4–7 and ancient8–12 carbonates, there has been only limited investigation of the geochemical processes of modern mixing zones13,14. Here we report the results of chemical and rock profiling through a modern mixing zone on Andros Island in the Bahamas. Our results demonstrate undersaturation of the waters in the mixing zone with respect to aragonite and calcite, and corresponding pervasive dissolution of the carbonate rocks at this level. Undersaturation is significantly enhanced by bacterial processes, which are also responsible for precipitation of an iron-rich crust. Despite chemical supersaturation, precipitation of dolomite was not observed, thereby casting some doubt on the mixing-zone model of dolomitization.