[rsa4046]

The chemical composition of the world ocean reflects the balance of inputs from the continents (as described, from riverine input as well groundwater), atmospheric cycling, and outputs: extraction via evaporite minerals in marginal environments, weathering at the seafloor, exchange over a range of temperatures with mid-ocean ridge basalt, and precipitation of minerals (mostly in the form of biogenic carbonates such as CaCO3, biogenic silica, etc.), as well as their subsequent dissolution, and lastly the biological processes of CO2 fixation and respiration of organic carbon (including electron acceptors other than O2, such as iron, sulfate, etc.).

It is the solubility of sparingly soluble phases such as CaCO3 that controls much of the seawater composition: surface seawater is close to saturation with respect to CaCO3 (calcite, aragonite). Because halite (rock salt, NaCl) is highly soluble, seawater is, conversely, fairly concentrated with respect to these ions. Seawater must be extensively evaporated to remove the far more soluble (evaporite) minerals. Over geologic time, the composition of seawater has changed, reflecting the relative pace of the various processes listed above that deliver and remove components from solution.