Seawater

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

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 salts (mostly, but not entirely, sodium chloride) dissolved in it. 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]

[edit] Explanation

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.

[edit] 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.^[3][4] 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.^[5]

[edit] 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^[6] 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.

[edit] 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 ocean-going vessels create fresh water from seawater with reverse osmosis.

Otherwise, seawater should not be drunk because of its high dissolved mineral content. In the long run, more water must be expended to eliminate these minerals (through excretion in urine) than is gained from drinking the seawater itself.^{[citation needed]}

[edit] Temporary/emergency potability

In extreme emergencies at sea, one is better off drinking urine, even untreated urine, to conserve fresh water in situations where one may run out. Any detrimental effects of drinking urine are much longer term and some authorities claim it is actually beneficial.

However, with extreme precautions taken to avoid perspiration and water loss and to retain minerals temporarily in the system, experiments have shown that even exhausting physical activity can continue for extended periods while drinking just over a litre (a quart) of seawater per day (sipped in small amounts) as one's only drinking water supply. Ocean rowing adventurers report fairly reliably that they plan to do this or actually do this as routine practice, but that kidney damage will result from drinking more than that amount. Silver citrate and rain catch can be used to assist in the dilution and fixing of minerals to make it less dangerous, but in general, it requires significant medical expertise to determine the effects of ingestion.

In the only known experiment of ocean rowing across the Atlantic using only plain untreated seawater as the drinking water supply, the subject and experimenter was a British doctor. The experiment did however prove that many people had died of thirst at sea unnecessarily.

[edit] Seawater for flushing toilet

Hong Kong has an extensive use of seawater for flushing toilets territory-wide. More than 90% of toilets in the territory are flushed by seawater as a means of conserving fresh water resources. The development of this approach was started in the 1960s and 1970s when water shortages became a severe problem as the population of the then British colony grew. This is unusual because saline water cannot be treated (in a waste water treatment plant) by the usual methods. The principal problem with treating marine waste water is the high dissolved sulfate concentration. Within anoxic (oxygen free) environments found in some stages of conventional wastewater treatment sulfate is reduced to the very smelly and toxic gas hydrogen sulfide.

[edit] See also

• Fresh water
• Salinity
• Sea ice
• Water

[edit] References:

1. ^ http://duedall.fit.edu/ocn1010eng/jan27sp.htm
2. ^ http://www.waterencyclopedia.com/Mi-Oc/Ocean-Chemical-Processes.html Thomson Gale, "Ocean Chemical Processes". Retrieved 12/2/06.
3. ^ Gale.
4. ^ Paul R. Pinet, Invitation to Oceanography, (St. Paul: West Publishing Company, 1996), pp. 126, 134.
5. ^ Pinet, p. 135.
6. ^ Pinet, 133.