Every ton of seawater contains about 30 kg (some 60 pounds) of solutes, with the vast majority being sodium, chloride, sulphate, magnesium and calcium from dissolved halite, epsom salt, and gypsum. These sea-salts and their major ions have profound effects on the solubility of gases and minerals in seawater, such as atmospheric CO₂ and calcium carbonate hard-parts of marine organisms. The high salinity of seawater, specific ion-ion interactions and complex formation all affect the acid/base balance and thus the pH of seawater. The chemistry of modern seawater is fairly well established, but how the ocean’s sea-salt content changed through geologic time and which effects these major ion changes had on the planetary carbon budget, atmospheric CO₂, ocean acidity and climate is still a research frontier.