Ocean water is a complex, mineral-rich solution characterized by its salinity, the measure of all dissolved solids by mass. The average salinity of the global ocean is approximately 35 parts per thousand (ppt), meaning that every kilogram of seawater contains about 35 grams of dissolved salts. These dissolved components are charged particles called ions, the fundamental building blocks of sea salt.
The Dominant Duo: Chloride and Sodium
The vast majority of the ocean’s dissolved salt content is composed of just two ions: the negatively charged chloride ion (Cl-) and the positively charged sodium ion (Na+). These two ions are the components of common table salt, sodium chloride, which is why seawater tastes distinctly salty. Chloride is the single most abundant ion, accounting for approximately 55% of the total dissolved solids by mass.
Sodium is the second most abundant ion, making up about 30.6% of the total dissolved mass. Together, chloride and sodium ions constitute over 85% of all the dissolved ions in seawater. In a sample of average seawater with a salinity of 35 ppt, the concentration of chloride is around 19.3 grams per kilogram, while sodium is present at about 10.7 grams per kilogram.
The Supporting Cast: Other Major Ions
Beyond the dominant duo, four other ions are consistently present in significant quantities and are classified as major ions. These include:
- Sulfate (SO4 2-).
- Magnesium (Mg2+).
- Calcium (Ca2+).
- Potassium (K+).
Sulfate and magnesium are the third and fourth most abundant, respectively, and together these four supporting ions account for nearly all of the remaining dissolved salt mass.
The principle of constant proportions, also known as Marcet’s Principle, governs ocean chemistry. This principle dictates that while the total concentration of salt may vary slightly across the globe, the relative ratios of these six major ions to one another remain nearly constant in the open ocean. For instance, the ratio of calcium to potassium will be the same whether the water is from the frigid Arctic or the tropical Pacific. This consistency allows oceanographers to determine the total salinity of a water sample by accurately measuring the concentration of just one of the major ions, traditionally chloride.
The Source of Salinity: Geological Origins
The dissolved ions in the ocean originate primarily from geological processes, involving a constant cycle of input and removal. One main input mechanism is the weathering and erosion of continental rock, where rainwater, made slightly acidic by dissolved atmospheric carbon dioxide, chemically breaks down minerals. This process releases ions like calcium and magnesium, which are subsequently carried to the sea by rivers.
Another source of ions is activity occurring on the ocean floor, particularly at hydrothermal vents. Seawater seeps into cracks in the ocean crust where it is heated by magma, undergoing chemical reactions that alter its composition. The superheated fluid then vents back into the ocean, having gained ions like chloride and metals from the surrounding rock while losing others like magnesium and sulfate. This continuous input is balanced by removal processes, such as the deposition of minerals onto the seafloor, the formation of evaporite deposits, and the uptake of ions by marine organisms for shells and skeletons.
The Role of Dissolved Ions in Ocean Systems
Dissolved ions influence the physical properties of seawater, altering how it behaves compared to fresh water. One of the most immediate consequences is an increase in density; the added mass of the ions makes seawater denser than fresh water, contributing to the stratification of the ocean into layers. Density differences drive global ocean currents, often referred to as the ocean conveyor belt, which distribute heat around the planet.
Another physical property affected by salinity is the freezing point of water, which is lowered by the presence of dissolved ions. Average seawater does not freeze at 0°C like fresh water, but rather at about -1.8°C. This property is especially important in polar regions, where it prevents large bodies of water from freezing solid and allows marine life to survive in the water column. Ions like calcium are necessary, as they are used by organisms such as corals and mollusks to construct their protective calcium carbonate shells and skeletons.