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[1] A simple model of the CaCO 3 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 modelcalculated CaCO 3 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 CaCO 3 precipitation, (2) the ''Neritan Ocean'' of indefinite saturation dominated by biogenic shallow-water CaCO 3 precipitation, and (3) the ''Cretan Ocean'' of low saturation dominated by biogenic pelagic CaCO 3 precipitation. In the latter mode, the deep ocean [CO 3 2À ] is remarkably stable, provided that the biogenic production of CaCO 3 exceeds the riverine flux of Ca 2+ and CO 3 2À. 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.

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