Salinity is the measure of dissolved salt content in a body of water, averaging about 35 parts per thousand (3.5% salt by weight) in the ocean. This dissolved material originates primarily from the erosion of continental rocks and geological processes beneath the seafloor. Understanding this process requires examining the sources that supply these ions and the mechanisms that remove them.
The Primary Source of Ocean Salt
The primary reason for the ocean’s saltiness is continental weathering and runoff. Rainwater absorbs carbon dioxide from the atmosphere, forming a weak carbonic acid that falls onto land. This slightly acidic rain chemically breaks down rocks and minerals, dissolving ions from the surrounding soil. Rivers and groundwater then carry these dissolved mineral ions to the ocean basin. Over geologic timescales, this continuous flow of fresh water carrying dissolved solids has supplied the majority of the salt found in the global ocean.
Secondary Geological Contributors
Another significant source of dissolved ions comes from geologic activity occurring on the ocean floor itself. Submarine volcanism, including underwater eruptions, directly releases minerals and gases from Earth’s interior into the water. Hydrothermal vents, often found along mid-ocean ridges, also add salt to the deep sea. Seawater seeps into crust cracks, is heated by magma, and reacts with oceanic rock, dissolving large amounts of minerals. This superheated water is then vented back into the ocean basin, providing a substantial source of elements like chloride and metals.
Maintaining the Balance: Salt Removal
The ocean’s salt concentration is not increasing indefinitely because natural processes act as constant “sinks” to remove ions, creating a dynamic equilibrium. One removal process is the precipitation of salt, where ions combine to form solid minerals that form sediment layers, often resulting in massive evaporite deposits in enclosed basins. Biological uptake also plays a significant role, as marine organisms like mollusks and corals use dissolved ions to construct their hard shells and skeletons. When these organisms die, their remains settle on the seafloor, incorporating the ions into deep-sea sediments. Finally, reverse weathering involves the reaction of ions, such as potassium and magnesium, with clay minerals to form new mineral structures, permanently locking these elements away.
The Chemical Makeup of Seawater
The salinity of the ocean is a complex mixture of many different dissolved ions, not just common table salt. The two most abundant ions are chloride (Cl⁻), which makes up approximately 55% of the total dissolved salts, and sodium (Na⁺), accounting for about 30.6%. Together, these two ions form sodium chloride, or common table salt, which constitutes roughly 85% of the sea’s total salt content. Other major constituents include sulfate (SO₄²⁻) at about 7.7%, magnesium (Mg²⁺) at 3.7%, and smaller amounts of calcium (Ca²⁺) and potassium (K⁺).