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Seawater

alt=Photo of surf Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, or 599 mM). This means that every kilogram (roughly one litre by volume) of seawater has approximately of dissolved salts (predominantly sodium () and chloride () ions). Average density at the surface is 1.025 g/ml. Seawater is denser than both fresh water and pure water (density 1.0 g/ml @ ) because the dissolved salts add mass without contributing significantly to the volume. The freezing point of seawater decreases as salt concentration increases. At typical salinity it freezes at about .[1] The coldest seawater ever recorded (in a liquid state) was in 2010, in a stream under an Antarctic glacier, and measured .[2]

Contents


Geochemistry

The thermal conductivity of seawater is 0.6 W/mK at 25 degC and a salinity of 35 g/kg.[3] The thermal conductivity decreases with increasing salinity and increases with increasing temperature.[4]

Salinity

Annual mean sea surface salinity for the World Ocean. Data from the World Ocean Atlas[5]SalinityAlthough 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 unusually salty water. The salinity in isolated bodies of water (for example, the Dead Sea) can be considerably greater still.

The density of surface seawater ranges from about 1,020 to 1,029 kg m 3, depending on the temperature and salinity. Deep in the ocean, under high pressure, seawater can reach a density of 1,050 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 1,500 metres/second, and varies with water temperature, salinity, and pressure.

Seawater composition (by mass) (salinity = 35)
Element Percent Element Percent
Oxygen 85.84 Sulfur 0.091
Hydrogen 10.82 Calcium 0.04
Chloride 1.94 Potassium 0.04
Sodium 1.08 Bromine 0.0067
Magnesium 0.1292 Carbon 0.0028

Human impacts

Climate change, rising atmospheric carbon dioxide, excess nutrients, and pollution in many forms are altering global oceanic geochemistry. Rates of change for some aspects greatly exceed those in the historical and recent geological record. Major trends include an increasing acidity, reduced subsurface oxygen in both near-shore and pelagic waters, rising coastal nitrogen levels, and widespread increases in mercury and persistent organic pollutants. Most of these perturbations are tied either directly or indirectly to human fossil fuel combustion, fertilizer use, and industrial activity. Concentrations are projected to grow in coming decades, with negative impacts on ocean biota and other marine resources.[6]

Compositional differences from freshwater

Seawater contains more dissolved ions than all types of freshwater.[7] However, the ratios of solutes differ dramatically. For instance, although seawater contains about 2.8 times more 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 also constitute 48% of river water solutes but only 0.14% of all seawater ions.[7][8] 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 much more quickly.[8] The most abundant dissolved ions in seawater are sodium, chloride, magnesium, sulfate and calcium.[9]

Origin


              
Diagram showing concentrations of various salt ions in seawater: <!-- chem --> 55%, <!-- chem --> 30.6%, <!-- chem --> 7.7%, <!-- chem --> 3.7%, <!-- chem --> 1.2%, <!-- chem --> 1.1%, Other 0.7%. Note that the diagram is only correct in units of wt/wt, not wt/vol or vol/vol.
Diagram showing concentrations of various salt ions in seawater: 55%, 30.6%, 7.7%, 3.7%, 1.2%, 1.1%, Other 0.7%. Note that the diagram is only correct in units of wt/wt, not wt/vol or vol/vol.
  
Total Molar Composition of Seawater (Salinity = 35)[10]
Component Concentration (mol/kg)
53.6
0.546
0.469
0.0528
0.0282
0.0103
0.0102
CT 0.00206
0.000844
BT 0.000416
0.000091
0.000068

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 after rainfall washed it out of the ground. Upon reaching the ocean, these salts concentrated as more salt arrived over time (see Hydrologic cycle.) Halley noted that most lakes that don t have ocean outlets (such as the Dead Sea and the Caspian Sea, see endorheic basin), have high salt content. Halley termed this process "continental weathering".

Halley's theory was partly correct. In addition, sodium leached out of the ocean floor when the ocean formed. The presence of salt s other dominant ion, 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.[11] Following the ocean's formation, sodium no longer leached from the ocean floor, but instead was captured in sedimentary layers covering the ocean bed. Plate tectonics possibly forces salt under the continental land masses, where it slowly leaches again to the surface.

Human consumption

Accidentally consuming small quantities of clean seawater is not harmful, especially if the seawater is taken along with a larger quantity of fresh water. However, drinking seawater to maintain hydration is counterproductive; more water must be excreted to eliminate the salt (via urine) than the amount of water from the seawater itself.[12]

The renal system actively regulates sodium chloride in the blood within a very narrow range around 9 g/L (0.9% by weight).

In most open waters concentrations vary somewhat around typical values of about 3.5%, far higher than the body can tolerate and most beyond what the kidney can process. A point frequently overlooked in claims that the kidney can excrete NaCl in Baltic concentrations (2%) is that the gut cannot absorb water at such concentrations, so that there is no benefit in drinking such water. Drinking seawater temporarily increases blood s NaCl concentration. This signals the kidney to excrete sodium, but seawater s sodium concentration is above the kidney s maximum concentrating ability. Eventually the blood s sodium concentration rises to toxic levels, removing water from cells and interfering with nerve conduction, ultimately producing fatal seizure and heart arrhythmia.

Survival manuals consistently advise against drinking seawater.[13] A summary of 163 life raft voyages estimated the risk of death at 39% for those who drank seawater, compared to 3% for those who did not. The effect of seawater intake on rats confirmed the negative effects of drinking seawater when dehydrated.[14] However the regulation of the uptake of seawater salts may be possible through the colon. The mother of the Robertson family who were castaway for 38 days in 1972, proposed the feasibility of hydration through unpottable water enemas.

The temptation to drink seawater was greatest for sailors who had expended their supply of fresh water, and were unable to capture enough rainwater for drinking. This frustration was described famously by a line from Samuel Taylor Coleridge's The Rime of the Ancient Mariner:

"Water, water, everywhere,
And all the boards did shrink;
Water, water, everywhere,
Nor any drop to drink."

Although humans cannot survive on seawater, some people claim that up to two cups a day, mixed with fresh water in a 2:3 ratio, produces no ill effect. The French physician Alain Bombard survived an ocean crossing in a small Zodiak rubber boat using mainly raw fish meat, which contains about 40 percent water (like most living tissues), as well as small amounts of seawater and other provisions harvested from the ocean. His findings were challenged, but an alternative explanation was not given. In Kon-Tiki, Thor Heyerdahl reported drinking seawater mixed with fresh in a 40/60% ratio. A few years later another adventurer named William Willis claimed to have drunk two cups of seawater and one cup of fresh per day for 70 days without ill effect when he lost part of his water supply.[15]

Most modern ocean-going vessels desalinate potable water from seawater using processes such as vacuum distillation or multi-stage flash distillation in an evaporator, or more recently by reverse osmosis. These energy intensive processes were not usually available during the Age of Sail. Larger sailing warships with large crews, such as Nelson's were fitted with distilling apparatus in their galleys.[16]

Other land and marine animals such as fish, whales, sea turtles and penguins can adapt to a high saline habitat. For example, the kidney of the desert rat can concentrate sodium far more efficiently than the human kidney.

See also

Notes

External links

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