There are two types of marginal seas: the lagoonal type with deep water outflow, and the estuarine type with deep water inflow. Lagoonal basins are characterized by waters that have low nutrient concentrations and high salinity and are well aerated; their sediments are rich in calcium carbonate. Estuarine basins contain waters with high nutrient concentrations, low salinity, and relatively low oxygen values; their sediments are rich in silica and organic matter and tend toward being anaerobic. The Atlantic Ocean is “lagoonal” in that it has a deep water outflow and collects lime on its floor; the Pacific Ocean is “estuarine” with a deep water inflow and silica-rich sediments.
The oceanic fractionation of silica from lime through basin-basin exchange of water at different depth levels is due to both supply and preservation patterns. Supply of biogenous precipitates to the ocean floor exceeds influx of solutes to the ocean, requiring dissolution on the ocean floor to maintain steady state. Supply is greatest in estuarine basins. Dissolution of lime proceeds fastest in estuarine basins, which have CO2-rich deep waters, whereas dissolution of silica is greatest in lagoonal basins, where the deep water is farthest from saturation because of low dissolved nutrient concentrations. Trace elements may conceivably be affected also by this fractionation mechanism. Some implications for the geological record are discussed.