Seawater
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Water salinity |
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Salinity levels |
Fresh water (< 0.05%) Brackish water (0.05–3%) Saline water (3–5%) Brine (> 5% up to 26%–28% max) |
Bodies of water |
Seawater, or sea water, is
Seawater pH is typically limited to a range between 7.5 and 8.4.[3] However, there is no universally accepted reference pH-scale for seawater and the difference between measurements based on different reference scales may be up to 0.14 units.[4]
Properties
Salinity
Although the vast majority of seawater has a salinity of between 31 and 38 g/kg, that is 3.1–3.8%, seawater is not uniformly saline throughout the world. Where mixing occurs with freshwater runoff from river mouths, near melting glaciers or vast amounts of precipitation (e.g.
Density
The density of surface seawater ranges from about 1020 to 1029 kg/m3, depending on the temperature and salinity. At a temperature of 25 °C, the salinity of 35 g/kg and 1 atm pressure, the density of seawater is 1023.6 kg/m3.[7][8] Deep in the ocean, under high pressure, seawater can reach a density of 1050 kg/m3 or higher. The density of seawater also changes with salinity. Brines generated by seawater desalination plants can have salinities up to 120 g/kg. The density of typical seawater brine of 120 g/kg salinity at 25 °C and atmospheric pressure is 1088 kg/m3.[7][8]
pH value
The
The pH value of seawater is naturally as low as 7.8 in deep ocean waters as a result of degradation of organic matter in these waters.
Measurement of pH is complicated by the
Chemical composition
Seawater contains more dissolved
Small amounts of other substances are found, including

55%, Na+
30.6%, SO2−
4 7.7%, Mg2+
3.7%, Ca2+
1.2%, K+
1.1%, Other 0.7%. Note that the diagram is only correct when in units of wt/wt, not wt/vol or vol/vol.
Element | Percent by mass |
---|---|
Oxygen | 85.84 |
Hydrogen | 10.82 |
Chlorine | 1.94 |
Sodium | 1.08 |
Magnesium | 0.1292 |
Sulfur | 0.091 |
Calcium | 0.04 |
Potassium | 0.04 |
Bromine | 0.0067 |
Carbon | 0.0028 |
Component | Concentration (mol/kg) |
---|---|
H 2O |
53.6 |
Cl− |
0.546 |
Na+ |
0.469 |
Mg2+ |
0.0528 |
SO2− 4 |
0.0282 |
Ca2+ |
0.0103 |
K+ |
0.0102 |
CT | 0.00206 |
Br− |
0.000844 |
BT | 0.000416 |
Sr2+ |
0.000091 |
F− |
0.000068 |
Microbial components
Research in 1957 by the
Bacteria are found at all depths in the
Some bacteria interact with
Bacteria-like
Pandoravirus salinus, a species of very large virus, with a genome much larger than that of any other virus species, was discovered in 2013. Like the other very large viruses Mimivirus and Megavirus, Pandoravirus infects amoebas, but its genome, containing 1.9 to 2.5 megabases of DNA, is twice as large as that of Megavirus, and it differs greatly from the other large viruses in appearance and in genome structure.
In 2013 researchers from
The ocean has a long history of human waste disposal on the assumption that its vast size makes it capable of absorbing and diluting all noxious material.[26] While this may be true on a small scale, the large amounts of sewage routinely dumped has damaged many coastal ecosystems, and rendered them life-threatening. Pathogenic viruses and bacteria occur in such waters, such as Escherichia coli, Vibrio cholerae the cause of cholera, hepatitis A, hepatitis E and polio, along with protozoans causing giardiasis and cryptosporidiosis. These pathogens are routinely present in the ballast water of large vessels, and are widely spread when the ballast is discharged.[27]
Other parameters
The
Origin and history
The water in the sea was thought to come from the Earth's volcanoes, starting 4 billion years ago, released by degassing from molten rock.[30]: 24–25 More recent work suggests much of the Earth's water may come from comets.[31]
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 do not 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.[15]: 133
Human impacts
Climate change, rising levels of carbon dioxide in Earth's atmosphere, 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, and industrial activity. Concentrations are projected to grow in coming decades, with negative impacts on ocean biota and other marine resources.[32]
One of the most striking features of this is
).Seawater is a means of transportation throughout the world. Every day plenty of ships cross the ocean to deliver goods to various locations around the world. Seawater is a tool for countries to efficiently participate in international commercial trade and transportation, but each ship exhausts emissions that can harm marine life, air quality of coastal areas. Seawater transportation is one of the fastest growing human generated greenhouse gas emissions.[34] The emissions released from ships pose significant risks to human health in nearing areas as the oil and gas released from the operation of merchant ships decreases the air quality and causes more pollution both in the seawater and surrounding areas.[35]
Another human use of seawater that has been considered is the use of seawater for agricultural purposes. In areas with higher regions of sand dunes, such as Israel, the use of seawater for irrigation of plants would eliminate substantial costs associated with fresh water when it is not easily accessible.[36] Although it is not typical to use salt water as a means to grow plants as the salt gathers and ruins the surrounding soil, it has been proven to be successful in sand and gravel soils.[36] Large-scale desalination of seawater is another factor that would contribute to the success of agriculture farming in dry, desert environments.[36] One of the most successful plants in salt water agriculture is the halophyte. The halophyte is a salt tolerant plant whose cells are resistant to the typically detrimental effects of salt in soil.[37] The endodermis forces a higher level of salt filtration throughout the plant as it allows for the circulation of more water through the cells.[37] The cultivation of halophytes irrigated with salt water were used to grow animal feed for livestock; however, the animals that were fed these plants consumed more water than those that did not.[37] Although agriculture from use of saltwater is still not recognized and used on a large scale, initial research has shown that there could be an opportunity to provide more crops in regions where agricultural farming is not usually feasible.
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 obtained from the seawater itself.[38] In normal circumstances, it would be considered ill-advised to consume large amounts of unfiltered seawater.
The renal system actively regulates the levels of sodium and chloride in the blood within a very narrow range around 9 g/L (0.9% by mass).
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
Survival manuals consistently advise against drinking seawater.[39] 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.[40]
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 in place of normal drinking water, 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% 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 could not be given. In his 1948 book The Kon-Tiki Expedition, Thor Heyerdahl reported drinking seawater mixed with fresh in a 2:3 ratio during the 1947 expedition.[41] A few years later, another adventurer, 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.[42]
During the 18th century, Richard Russell advocated the medical use of this practice in the UK,[43] and René Quinton expanded the advocation of this practice to other countries, notably France, in the 20th century. Currently, it is widely practiced in Nicaragua and other countries, supposedly taking advantage of the latest medical discoveries.[44][45][verification needed]
Purification
Like any other type of raw or contaminated water, seawater can be evaporated or filtered to eliminate salt, germs, and other contaminants that would otherwise prevent it from being considered potable. Most oceangoing vessels desalinate potable water from seawater using processes such as vacuum distillation or multi-stage flash distillation in an evaporator, or, more recently, 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 HMS Victory, were fitted with distilling apparatus in their galleys.[46] The natural sea salt obtained by evaporating seawater can also be collected and sold as
A number of regional cuisines across the world traditionally incorporate seawater directly as an ingredient, cooking other ingredients in a diluted solution of filtered seawater as a substitute for conventional dry seasonings. Proponents include world-renowned chefs Ferran Adrià and Quique Dacosta, whose home country of Spain has six different companies sourcing filtered seawater for culinary use.[47] The water is marketed as la sal perfecta, "the perfect salt", containing less sodium with what is considered a superior taste. A restaurant run by Joaquín Baeza sources as much as 60,000 litres a month from supplier Mediterranea[47]
Animals such as fish, whales,
Mineral extraction
Minerals have been extracted from seawater since ancient times. Currently the four most concentrated metals –
The future of mineral and element extractions
In order for seawater mineral and element extractions to take place while taking close consideration of sustainable practices, it is necessary for monitored management systems to be put in place. This requires management of ocean areas and their conditions, environmental planning, structured guidelines to ensure that extractions are controlled, regular assessments of the condition of the sea post-extraction, and constant monitoring.[55] The use of technology, such as underwater drones, can facilitate sustainable extractions.[56] The use of low-carbon infrastructure would also allow for more sustainable extraction processes while reducing the carbon footprint from mineral extractions.[56]

Another practice that is being considered closely is the process of desalination in order to achieve a more sustainable water supply from seawater. Although desalination also comes with environmental concerns, such as costs and resources, researchers are working closely to determine more sustainable practices, such as creating more productive water plants that can deal with larger water supplies in areas where these plans weren't always available.[57] Although seawater extractions can benefit society greatly, it is crucial to consider the environmental impact and to ensure that all extractions are conducted in a way that acknowledges and considers the associated risks to the sustainability of seawater ecosystems.
Standard
ASTM International has an international standard for artificial seawater: ASTM D1141-98 (Original Standard ASTM D1141-52). It is used in many research testing labs as a reproducible solution for seawater such as tests on corrosion, oil contamination, and detergency evaluation.[58]
Ecosystems
The minerals found in seawater can also play an important role in the ocean and its ecosystem's food cycle. For example, the Southern Ocean contributes greatly to the environmental carbon cycle. Given that this body of water does not contain high levels of iron, the deficiency impacts the marine life living in its waters. As a result, this ocean is not able to produce as much phytoplankton which hinders the first source of the marine food chain.[59] One of the main types of phytoplankton are diatoms which is the primary food source of Antarctic krill. As the cycle continues, various larger sea animals feed off of Antarctic krill, but since there is a shortage of iron from the initial phytoplankton/diatoms, then these larger species also lack iron. The larger sea animals include Baleen Whales such as the Blue Whale and Fin Whale.[59] These whales not only rely on iron for a balance of minerals within their diet, but it also impacts the amount of iron that is regenerated back into the ocean. The whale's excretions also contain the absorbed iron which would allow iron to be reinserted into the ocean’s ecosystem. Overall, one mineral deficiency such as iron in the Southern Ocean can spark a significant chain of disturbances within the marine ecosystems which demonstrates the important role that seawater plays in the food chain.
Upon further analysis of the dynamic relationship between diatoms, krill, and baleen whales, fecal samples of baleen whales were examined in Antarctic seawater.[59] The findings included that iron concentrations were 10 million times higher than those found in Antarctic seawater, and krill was found consistently throughout their feces which is an indicator that krill is in whale diets.[59] Antarctic krill had an average iron level of 174.3mg/kg dry weight, but the iron in the krill varied from 12 to 174 mg/kg dry weight.[59] The average iron concentration of the muscular tissue of blue whales and fin whales was 173 mg/kg dry weight, which demonstrates that the large marine mammals are important to marine ecosystems such as they are to the Southern Ocean.[59] In fact, to have more whales in the ocean could heighten the amount of iron in seawater through their excretions which would promote a better ecosystem.
Krill and baleen whales act as large iron reservoirs in seawater in the Southern Ocean. Krill can retain up to 24% of iron found on surface waters within its range.[59]The process of krill feeding on diatoms releases iron into seawater, highlighting them as an important part of the ocean's iron cycle. The advantageous relationship between krill and baleen whales increases the amount of iron that can be recycled and stored in seawater.[59] A positive feedback loop is created, increasing the overall productivity of marine life in the Southern Ocean.
Organisms of all sizes play a significant role in the balance of marine ecosystems with both the largest and smallest inhabitants contributing equally to recycling nutrients in seawater. Prioritizing the recovery of whale populations because they boost the overall productivity in marine ecosystems as well as increasing iron levels in seawater would allow for a balanced and productive system for the ocean. However, a more in depth study is required to understand the benefits of whale feces as a fertilizer and to provide further insight in iron recycling in the Southern Ocean.[59] Projects on the management of ecosystems and conservation are vital for advancing knowledge of marine ecology.
Environmental impact and sustainability
Like any mineral extraction practices, there are environmental advantages and disadvantages.

There are also risks that come with extracting from the seafloor. Many biodiverse species have long lifespans on the seafloor, which means that their reproduction takes more time.[55] Similarly to fish harvesting from the seafloor, the extraction of minerals in large amounts, too quickly, without proper protocols, can result in a disruption of the underwater ecosystems.[55] Contrarily, this would have the opposite effect and prevent mineral extractions from being a long-term sustainable practice, and would result in a shortage of required metals. Any seawater mineral extractions also risk disrupting the habitat of the underwater life that is dependent on the uninterrupted ecosystem within their environment as disturbances can have significant disturbances on animal communities.[55]
See also
- Brine – Concentrated solution of salt in water
- Brine mining – Extracting materials from saltwater
- Brackish water – Water with salinity between freshwater and seawater
- Fresh water – Naturally occurring water with low amounts of dissolved salts
- Ocean color – Explanation of the color of oceans and ocean color remote sensing
- Saline water – Water that contains a high concentration of dissolved salts
- Sea ice – Outcome of seawater as it freezes
- Seawater pH – Measure of the level of acidity or basicity of an aqueous solution
- Surface tension of seawater – Tendency of a liquid surface to shrink to reduce surface area
- Thalassotherapy – Form of therapy using seawater
- Thermohaline circulation – Part of large-scale ocean circulation
- CORA dataset – Oceanographic temperature and salinity dataset global ocean salinity
References
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External links
Tables
- Tables and software for thermophysical properties of seawater, MIT
- G. W. C Kaye, T. H. Laby (1995). "Physical properties of sea water". Tables of physical and chemical constants (16th ed.). Archived from the original on 8 May 2019.