A simple model of the CaCO3 saturation state of the ocean is presented. It can be solved analytically and is intended to identify the fundamental controls on ocean carbonate ion concentration. It should also attract researchers unfamiliar with complex biogeochemical models. Despite its limitations, the model-calculated CaCO3 saturation state of today's ocean agrees well with observations. In general, the model reveals three distinctly different modes of operation: The Strangelove Ocean of high supersaturation which is dominated by inorganic CaCO3 precipitation, (2) the Neritan Ocean of indefinite saturation dominated by biogenic shallow-water CaCO3 precipitation, and (3) the Cretan Ocean of low saturation dominated by biogenic pelagic CaCO3 precipitation. In the latter mode, the deep ocean 32-> is remarkably stable, provided that the biogenic production of CaCO3 exceeds the riverine flux of Ca2+ and CO32-. This explains the overall constancy of the saturation state of the ocean documented over the last 100 Ma. The model is then used to address diverse questions. One important result is that the recovery of the oceanic carbonate chemistry from fossil fuel neutralization in the future will be accelerated due to expected reduced biogenic calcification.