Large scale inward and upward density-driven convective circulations occur in the porous structure of many marine carbonate platforms. Their geochemical implications are of prime importance for a variety of problems where a substantial transport of chemical species is required. In the present study we show, using simple thermal and hydraulic arguments, that there is a negative feedback between convective flow and heat transfer. This results in an upper limit on geothermally-driven vertical fluxes, irrespective of the hydraulic conductivity of the medium. This concept is potentially applicable to a variety of problems. Taking the example of two on-going debated questions in the reef scientific community, i.e., diagenesis and nutrient cycling, we show unambiguously that whereas convective circulations in coral reefs are compatible with dolomitization models, they are too low by at least one to two orders of magnitude to be a significant process in the nutrient budget of coral reefs. ¿ 2000 American Geophysical Union