The Earth's oceans are salty despite receiving a constant supply of fresh water from rivers and rain. Ocean salinity is caused by the presence of sodium chloride and other minerals. According to the U.S. Geological Survey, the world's oceans contain about 50 quadrillion tons of salt and have an average salinity of about three and a half percent, which is usually expressed by oceanographers as 35 parts per thousand (ppt). The level of salinity is affected by the balance between processes that add minerals to the oceans and processes that remove minerals, and a similar balance between the addition and removal of fresh water from the oceans.
The world's oceans receive a continual supply of fresh water, chiefly from river inflow and rainfall directly on the ocean surface. Fresh water also enters through groundwater discharge directly from the ocean floor. Although groundwater inputs are only a few percent of total fresh water flow, they can be important locally. More than 2,000 years ago the Roman geographer Strabo described how residents of Latakia, Syria rowed out to the Mediterranean Sea to collect fresh drinking water from a mid-ocean spring. Calving ice floes also introduce water into Arctic ocean areas.
Evaporation is the key water removal mechanism in the oceans. Local variations in the rate of water inflow and removal by evaporation lead to changes in sea level but on a global scale, evaporation largely keeps pace with freshwater input, so that sea levels only change slowly. The relative amount of fresh water inflow and removal from a body of ocean water is one of the key features affecting the overall salinity. Areas where evaporation is substantial and inflow is small tend to be the saltiest oceans, while high-inflow, low-evaporation areas generally result in less-salty water.
Fresh water, regardless of its source, contains small amounts of dissolved substances such as sodium, magnesium, calcium and chlorine. River discharge and land runoff also deliver large amounts of mineral-containing sediment to the oceans. Other processes, including underwater volcanic eruptions, thermal vent discharges on the ocean floor, atmospheric dust and pollution also introduce additional matter into the oceans. These elements combine to form sodium chloride and other salts that give the ocean its characteristic salinity.
Ocean salinity would constantly increase unless there was a means for removing some of the minerals that are continually delivered. Removal mechanisms include deposition to the ocean floor and uptake by biological organisms. Shellfish and coral reefs, for example, sequester enormous quantities of calcium that would otherwise dissolve in seawater.
Variation in Salinity
The overall balance between water input and removal and mineral input and removal determines the salinity of a given body of water. In cold latitudes near the Arctic and Antarctic, evaporation rates are low and fresh water input is substantial, so salinity is about 32 ppt -- lower than the global average for ocean water of 35 ppt. Conversely, the Red Sea and Persian Gulf, where evaporation rates are very high and fresh water input is low, have the highest salinity of open ocean water, about 40 ppt. In landlocked bodies of water, such as the Great Salt Lake in Utah or the Dead Sea in the Middle East, high evaporation combined with low rates of mineral removal lead to salinity levels 10 times greater than ordinary sea water.
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