seawater

Annual mean sea surface salinity for the World Ocean. Data from the World Ocean Atlas 2001.

Sea water in the Straits of Malacca. Taken from onboard the SuperStar Virgo

Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of ~3.5%, or 35 parts per thousand. This means that every 1 kg of seawater has approximately 35 grams of dissolved salts (mostly, but not entirely, the ions of sodium chloride: Na⁺, Cl^-). The average density of seawater at the surface of the ocean is 1.025 g/mL; seawater is denser than fresh water (which reaches a maximum density of 1.000 g/mL at a temperature of 4°C) because of the added weight of the salts and electrostriction.^[1]

Salinity

Main article: Salinity

Although the vast majority of seawater has a salinity of between 3.1% and 3.8%, seawater is not uniformly saline throughout the world. Where mixing occurs with fresh water runoff from river mouths or near melting glaciers, seawater can be substantially less saline. The most saline open sea is the Red Sea, where high rates of evaporation, low precipitation and river inflow, and confined circulation result in the formation of unusually salty seawater. The salinity in isolated seas and salt-water lakes (for example, the Dead Sea) can be considerably greater.

The density of surface seawater ranges from about 1020 to 1029 kg·m^-3, depending on the temperature and salinity. Deep in the ocean, under high pressure, seawater can reach a density of 1050 kg·m^-3 or higher. Seawater pH is limited to the range 7.5 to 8.4. The speed of sound in seawater is about 1500 m·s^-1, and varies with water temperature and pressure.

Compositional differences from fresh water

Seawater is more enriched in dissolved ions of all types than fresh water.^[2] However, the ratios of various solutes differ dramatically. For instance, although seawater is ~2.8 times more enriched with bicarbonate than river water based on molarity, the percentage of bicarbonate in seawater as a ratio of all dissolved ions is far lower than in river water; bicarbonate ions constitute 48% of river water solutes, but only 0.41% of all seawater ions.^[2][3] Differences like these are due to the varying residence times of seawater solutes; sodium and chlorine have very long residence times, while calcium (vital for carbonate formation) tends to precipitate out much more quickly.^[3]

Geochemical explanations

Chemical composition of sea salt

Scientific theories behind the origins of sea salt started with Sir Edmond Halley in 1715, who proposed that salt and other minerals were carried into the sea by rivers, having been leached out of the ground by rainfall runoff. Upon reaching the ocean, these salts would be retained and concentrated as the process of evaporation (see Hydrologic cycle) removed the water. Halley noted that of the small number of lakes in the world without ocean outlets (such as the Dead Sea and the Caspian Sea, see endorheic basin), most have high salt content. Halley termed this process "continental weathering".

Halley's theory is partly correct. In addition, sodium was leached out of the ocean floor when the oceans first formed. The presence of the other dominant ion of salt, chloride, results from "outgassing" of chloride (as hydrochloric acid) with other gases from Earth's interior via volcanos and hydrothermal vents. The sodium and chloride ions subsequently became the most abundant constituents of sea salt.

Ocean salinity has been stable for billions of years, most likely as a consequence of a chemical/tectonic system which removes as much salt as is deposited; for instance, sodium and chloride sinks include evaporite deposits, pore water burial, and reactions with seafloor basalts^[5] Since the ocean's creation, sodium is no longer leached out of the ocean floor, but instead is captured in sedimentary layers covering the bed of the ocean. One theory is that plate tectonics result in salt being forced under the continental land masses, where it is again slowly leached to the surface.

Potability

Even on a ship or island in the middle of the ocean, there can be a "shortage of water" meaning, of course, a shortage of fresh water. This is described famously by a line from Samuel Taylor Coleridge's The Rime of the Ancient Mariner:

"Water, water, every where

Nor any drop to drink."

Seawater can be turned into drinkable (potable) water by one of a number of desalination processes, or by diluting it with fresh water to reduce the salinity. Almost all large ocean-going vessels create fresh water from seawater using vacuum evaporators, flash evaporators or by the use of reverse osmosis.

Otherwise, seawater should not be drunk because of its high salt content. In the long run, more water must be expended to eliminate the salt (through excretion in urine) than the amount of water that is gained from drinking the seawater itself. ^[6]

The effect of seawater intake has been studied in laboratory setting in rats. (Etzion and Yagil; Metabolic effects in rats drinking increasing concentrations of sea-water. Comp Biochem Physiol A. 1987;86(1):49-55.) [1]

Drinking seawater

Accidentally consuming small quantities of seawater is not harmful. However some people cling to a persistent and incorrect belief that humans can survive at sea by drinking only seawater. This misconception probably originated from questionable reports claiming that the French physician Alain Bombard survived an ocean crossing using only seawater and other provisions harvested from the ocean.

The amount of sodium chloride in human blood and in urine is always kept within a very narrow range of 9 g per L (0.9% by weight). Drinking seawater (which contains about 3.5% ions of dissolved sodium chloride) temporarily increases the concentration of sodium chloride in the blood, so the only way to excrete the excess sodium chloride in the urine is by sacrificing internal water from cells. The cells eventually give so much water to try to dilute the salt that they die from dehydration, quickly followed by organs and eventually the organism.

Survival manuals consistently advise against drinking seawater. For example, the book "Medical Aspects of Harsh Environments" (Chapter 29 - Shipboard Medicine) [2] presents a summary of 163 life raft voyages. The risk of death was 39% for those who drank seawater, compared to only 3% for those who did not drink seawater.

See also

• Thermohaline circulation
• Fresh water
• Salinity
• Sea ice
• Seawater pH
• Water

References

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