Brine rejection

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Brine rejection is a process that occurs when salty water freezes. The salts do not fit in the crystal structure of water ice, so the salt is expelled.

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Since the oceans are salty, this process is important in nature. Salt rejected by the forming sea ice drains into the surrounding seawater, creating saltier, denser brine. The denser brine sinks, influencing ocean circulation.

Formation

Average salinity of sea ice as a function of ice thickness for cold sea ice sampled during the growth season. The standard error of the estimate is 1.5%0 for thin ice and 0.6%0 for thick ice. Ice Thickness vs. Average Salinity.gif
Average salinity of sea ice as a function of ice thickness for cold sea ice sampled during the growth season. The standard error of the estimate is 1.5‰ for thin ice and 0.6‰ for thick ice.

As water reaches the temperature where it begins to crystallize and form ice, salt ions are rejected from the lattices within the ice and are either forced out into the surrounding water, or trapped among the ice crystals in pockets called brine cells. Generally, sea ice has a salinity ranging from 0 psu at the surface to 4 psu at the base. [1] The faster that this freezing process occurs, the more brine cells are left in the ice. Once the ice reaches a critical thickness, roughly 15 cm, the concentration of salt ions in the liquid around the ice begins to increase, as leftover brine is rejected from the cells. [1] This increase is associated with the appearance of strong convective plumes, which flow from channels and within the ice and carry a significant salt flux. The brine that drains from the newly-formed ice is replaced by a weak flow of relatively fresh water from the liquid region below it. The new water partially freezes within the pores of the ice, increasing the solidity of the ice.

As sea ice ages and thickens, the initial salinity of the ice decreases due to the rejection of brine over time. [1] While the sea ice ages, desalinization occurs to such a degree that some multiyear ice has a salinity of less than 1 PSU. [2] This occurs in three different ways:

Role in deep water formation and thermohaline circulation

Arctic and Antarctic sea ice concentration climatology from 1981 to 2010, at the approximate seasonal maximum and minimum levels based on passive microwave satellite data. Climatology 81-10 min-max conc tmb.png
Arctic and Antarctic sea ice concentration climatology from 1981 to 2010, at the approximate seasonal maximum and minimum levels based on passive microwave satellite data.

Brine rejection occurs in the sea ice packs around at the north and south poles of the Earth[ clarification needed ]. The Arctic Ocean has historically ranged from roughly 14-16 million square kilometers in late winter to roughly 7 million km2 each September. [6] The annual increase of ice plays a major role in the movement of ocean circulation and deep water formation. The density of the water below the newly-formed ice increases due to brine rejection. Saltier water can also become colder without freezing.

The dense water that forms in the Arctic are called North Atlantic Deep Water (NADW), while the Antarctic Bottom Water (AABW) forms in the southern hemisphere. These two areas of brine rejection play an important role in the thermohaline circulation of all of Earth's oceans.

Brinicles

As sea ice freezes, it rejects increasingly salty water, which drains through narrow brine channels that thread through the ice. The brine flowing through the brine channels and out of the bottom of the ice is very cold and salty, so it sinks in the warmer, fresher seawater under the ice, forming a plume. The plume is colder than the freezing point of sea water under the ice, so the seawater can freeze where it touches the plume. Ice freezing around the edges of the plume gradually builds a hollow icicle-like tube, called a brinicle. These frozen stalactite-like forms are fragile during early stages, but if brine drainage ceases, they may freeze solid. In calm waters, brinicles can reach the sea floor, freezing it fairly abruptly. [7]

Climate change

The deep ocean basins are stably stratified, so mixing of surface waters with the deep ocean waters occurs only very slowly. The dissolved CO2 of the surface waters of the ocean is roughly in equilibrium with the partial pressure of CO2 in the atmosphere. As atmospheric CO2 levels are rising, the oceans are absorbing some CO2 from the atmosphere. When surface waters sink, they carry considerable amounts of CO2 into the deep oceans, away from the atmosphere. Because these waters are able to contain a large amount of CO2, they have helped slow the rise in atmospheric CO2 concentrations, thus slowing some aspects of climate change.

Climate change could have different effects on ice melt and brine rejection. Previous studies have suggested that as ice cover thins, it will become a weaker insulator, resulting in larger ice production during the autumn and winter. [8] The consequent increase in winter brine rejection will drive ocean ventilation and strengthen the inflow of warm Atlantic waters. Studies of the last glacial maximum have indicated that a drastic reduction in the production of sea ice, and thus reduction of brine rejection, would result in the weakening of the stratification in the global deep oceans and in CO2 release into the shallow oceans and the atmosphere, triggering global deglaciation. [9]

Life in channels and surrounding waters

Life in sea ice is energetically demanding, and sets limits at any hierarchical, organizational, and organismic level, ranging from molecules to everything that an organism does.[ clarification needed ] [9] Despite this fact, the brine-containing interstices and pockets found in sea ice host a variety of organisms, including bacteria, autotrophic and heterotrophic protists, microalgae, and metazoa. [10]

Brine rejection and the North Pacific Intermediate Water

Brine rejection plays a crucial role in ocean circulation. In coastal polynyas, it is the key to the ventilation of multiple water masses across both the Arctic and Antarctic. A coastal polynya is an area of open water surrounded by ice. [11] The reason that coastal polynyas are the most active areas of brine rejection is that these waters often see offshore winds which give the water direct contact with cold air. [12] This leads to heat loss and ice production. One area that is commonly studied to look at these impacts is the coastal polynyas of the Okhotsk Sea. The Okhotsk Sea has wide, shallow shelves, severe wintertime conditions, high background salinity, and easy summertime access, making it an ideal study location. [12] There have been many studies done that look at the influence of brine rejection in the Okhotsk Sea.

In a paper done by Shcherbina et al. (2003), they analyze the influence of brine rejection well. [13] Within the Okhotsk Sea, circulation is driven by the brine rejection that occurs in the winter months. As is typical for the rejection of brine, sea ice develops that is 70-90% fresher than seawater. The water underneath becomes saltier and colder, leading to an increase in density. This parcel of water in the Okhotsk Sea is referred to as dense shelf water (DSW). The saltier and colder a water parcel is, the denser it becomes, causing it to sink below other parcels of water. For this reason, the DSW will begin to sink within the water column. The parcel then moves southward along the coast of Sakhalin. From here, the water moves over into the Pacific and ventilates the North Pacific Intermediate Water (NPIW). The NPIW is known to be the densest water in the North Pacific, and it is a key water mass in ocean circulation.

Brine rejection has been shown to ventilate the North Pacific Ocean to a depth of 300-1,000 meters. Some studies have even shown it reaching mixing depths of 2,000 meters. [14] The mixing and ventilation of the water column is key in helping to replenish the oxygen within intermediate waters. It can also lead to an upwelling of nutrients, which can influence productivity. An increase in primary production can lead to an increase in other organisms from krill to whales.

Related Research Articles

<span class="mw-page-title-main">Salinity</span> Proportion of salt dissolved in water

Salinity is the saltiness or amount of salt dissolved in a body of water, called saline water. It is usually measured in g/L or g/kg.

<span class="mw-page-title-main">Brine</span> Concentrated solution of salt in water

Brine is water with a high-concentration solution of salt. In diverse contexts, brine may refer to the salt solutions ranging from about 3.5% up to about 26%. Brine forms naturally due to evaporation of ground saline water but it is also generated in the mining of sodium chloride. Brine is used for food processing and cooking, for de-icing of roads and other structures, and in a number of technological processes. It is also a by-product of many industrial processes, such as desalination, so it requires wastewater treatment for proper disposal or further utilization.

<span class="mw-page-title-main">Seawater</span> Water from a sea or an ocean

Seawater, or sea water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5%. This means that every kilogram of seawater has approximately 35 grams (1.2 oz) of dissolved salts. The average density at the surface is 1.025 kg/L. Seawater is denser than both fresh water and pure water because the dissolved salts increase the mass by a larger proportion than the volume. The freezing point of seawater decreases as salt concentration increases. At typical salinity, it freezes at about −2 °C (28 °F). The coldest seawater still in the liquid state ever recorded was found in 2010, in a stream under an Antarctic glacier: the measured temperature was −2.6 °C (27.3 °F).

<span class="mw-page-title-main">Sea ice</span> Outcome of seawater as it freezes

Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface. Sea ice covers about 7% of the Earth's surface and about 12% of the world's oceans. Much of the world's sea ice is enclosed within the polar ice packs in the Earth's polar regions: the Arctic ice pack of the Arctic Ocean and the Antarctic ice pack of the Southern Ocean. Polar packs undergo a significant yearly cycling in surface extent, a natural process upon which depends the Arctic ecology, including the ocean's ecosystems. Due to the action of winds, currents and temperature fluctuations, sea ice is very dynamic, leading to a wide variety of ice types and features. Sea ice may be contrasted with icebergs, which are chunks of ice shelves or glaciers that calve into the ocean. Depending on location, sea ice expanses may also incorporate icebergs.

<span class="mw-page-title-main">Thermohaline circulation</span> Part of large-scale ocean circulation

Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water. Wind-driven surface currents travel polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes. This dense water then flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters upwell in the North Pacific. Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's oceans a global system. The water in these circuits transport both energy and mass around the globe. As such, the state of the circulation has a large impact on the climate of the Earth.

<span class="mw-page-title-main">Polynya</span> Area of unfrozen sea within an ice pack

A polynya is an area of open water surrounded by sea ice. It is now used as a geographical term for an area of unfrozen seawater within otherwise contiguous pack ice or fast ice. It is a loanword from the Russian полынья, which refers to a natural ice hole and was adopted in the 19th century by polar explorers to describe navigable portions of the sea.

In oceanography, a halocline is a cline, a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water. Because salinity affects the density of seawater, it can play a role in its vertical stratification. Increasing salinity by one kg/m3 results in an increase of seawater density of around 0.7 kg/m3.

<span class="mw-page-title-main">Antarctic bottom water</span> Cold, dense, water mass originating in the Southern Ocean surrounding Antarctica

The Antarctic bottom water (AABW) is a type of water mass in the Southern Ocean surrounding Antarctica with temperatures ranging from −0.8 to 2 °C (35 °F) and absolute salinities from 34.6 to 35.0 g/kg. As the densest water mass of the oceans, AABW is found to occupy the depth range below 4000 m of all ocean basins that have a connection to the Southern Ocean at that level. AABW forms the lower branch of the large-scale movement in the world's oceans through thermohaline circulation.

Bottom water is the lowermost water mass in a water body, by its bottom, with distinct characteristics, in terms of physics, chemistry, and ecology.

<span class="mw-page-title-main">Ocean</span> Salt water covering most of Earth

The ocean is the body of salt water that covers ~70.8% of the Earth. In English, the term ocean also refers to any of the large bodies of water into which the world ocean is conventionally divided. Distinct names are used to identify five different areas of the ocean: Pacific, Atlantic, Indian, Antarctic/Southern, and Arctic. The ocean contains 97% of Earth's water and is the primary component of the Earth's hydrosphere, thus the ocean is essential to life on Earth. The ocean influences climate and weather patterns, the carbon cycle, and the water cycle by acting as a huge heat reservoir.

<span class="mw-page-title-main">Blood Falls</span> Red-colored seep of saltwater flowing from Taylor Glacier in Antarctica

Blood Falls is an outflow of an iron oxide–tainted plume of saltwater, flowing from the tongue of Taylor Glacier onto the ice-covered surface of West Lake Bonney in the Taylor Valley of the McMurdo Dry Valleys in Victoria Land, East Antarctica.

Marine chemistry, also known as ocean chemistry or chemical oceanography, is the study of chemical content in marine environments as influenced by plate tectonics and seafloor spreading, turbidity, currents, sediments, pH levels, atmospheric constituents, metamorphic activity, and ecology. Marine life has adapted to the chemistries unique to Earth's oceans, and marine ecosystems are sensitive to changes in ocean chemistry.

<span class="mw-page-title-main">Frost flower (sea ice)</span> Ice crystal found growing on young sea ice

Frost flowers are ice crystals commonly found growing on young sea ice and thin lake ice in cold, calm conditions. The ice crystals are similar to hoar frost, and are commonly seen to grow in patches around 3–4 cm in diameter. Frost flowers growing on sea ice have extremely high salinities and concentrations of other sea water chemicals and, because of their high surface area, are efficient releasers of these chemicals into the atmosphere.

Paleosalinity is the salinity of the global ocean or of an ocean basin at a point in geological history.

<span class="mw-page-title-main">Sea ice growth processes</span>

Sea ice is a complex composite composed primarily of pure ice in various states of crystallization, but including air bubbles and pockets of brine. Understanding its growth processes is important for climate modellers and remote sensing specialists, since the composition and microstructural properties of the ice affect how it reflects or absorbs sunlight.

<span class="mw-page-title-main">Brinicle</span> Sea ice formation

A brinicle is a downward-growing hollow tube of ice enclosing a plume of descending brine that is formed beneath developing sea ice.

<span class="mw-page-title-main">Antarctic sea ice</span> Sea ice of the Southern Ocean

Antarctic sea ice is the sea ice of the Southern Ocean. It extends from the far north in the winter and retreats to almost the coastline every summer. Sea ice is frozen seawater that is usually less than a few meters thick. This is the opposite of ice shelves, which are formed by glaciers; they float in the sea, and are up to a kilometre thick. There are two subdivisions of sea ice: fast ice, which are attached to land; and ice floes, which are not.

<span class="mw-page-title-main">Sea ice microbial communities</span> Groups of microorganisms living within and at the interfaces of sea ice

Sea Ice Microbial Communities (SIMCO) refer to groups of microorganisms living within and at the interfaces of sea ice at the poles. The ice matrix they inhabit has strong vertical gradients of salinity, light, temperature and nutrients. Sea ice chemistry is most influenced by the salinity of the brine which affects the pH and the concentration of dissolved nutrients and gases. The brine formed during the melting sea ice creates pores and channels in the sea ice in which these microbes can live. As a result of these gradients and dynamic conditions, a higher abundance of microbes are found in the lower layer of the ice, although some are found in the middle and upper layers. Despite this extreme variability in environmental conditions, the taxonomical community composition tends to remain consistent throughout the year, until the ice melts.

<span class="mw-page-title-main">Southern Ocean overturning circulation</span> Southern half of the global ocean current system

Southern Ocean overturning circulation is the southern half of a global thermohaline circulation, which connects different water basins across the global ocean. Its better-known northern counterpart is the Atlantic meridional overturning circulation (AMOC). This circulation operates when certain currents send warm, oxygenated, nutrient-poor water into the deep ocean (downwelling), while the cold, oxygen-limited, nutrient-rich water travels upwards at specific points. Thermohaline circulation transports not only massive volumes of warm and cold water across the planet, but also dissolved oxygen, dissolved organic carbon and other nutrients such as iron. Thus, both halves of the circulation have a great effect on Earth's energy budget and oceanic carbon cycle, and so play an essential role in the Earth's climate system.

A sea ice brine pocket is an area of fluid sea water with a high salt concentration trapped in sea ice as it freezes. Due to the nature of their formation, brine pockets are most commonly found in areas below −2 °C (28 °F), where it is sufficiently cold for seawater to freeze and form sea ice. Though the high salinity and low light conditions of brine pockets create a challenging environment for marine mammals, brine pockets serve as a habitat to various microbes. Sampling and studying these pockets requires specialized equipment and alterations to methodologies to accommodate the hyper-saline conditions and subzero temperatures.

References

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  2. ↑ Talley, L.D.; Pickard, G.L.; Emery, W.J.; Swift, J.H. (2011). Descriptive Physical Oceanography (6 ed.). Elsevier. doi:10.1016/C2009-0-24322-4. ISBN   9780750645522.
  3. 1 2 Lake, R.A.; Lewis, E.L. (1970). "Salt rejection by sea ice during growth". J. Geophys. Res. 75 (3): 583–597. Bibcode:1970JGR....75..583L. doi:10.1029/jc075i003p00583.
  4. ↑ Wettlaufer, J. S.; Worster, M. Grae; Huppert, Herbert E. (1997). "Natural convection during solidification of an alloy from above with application to the evolution of sea ice". Journal of Fluid Mechanics. 344 (1): 291–316. Bibcode:1997JFM...344..291W. doi:10.1017/S0022112097006022. S2CID   46262541.
  5. ↑ "Arctic vs. Antarctic | National Snow and Ice Data Center". nsidc.org. Retrieved 20 April 2017.
  6. ↑ "All About Sea Ice | National Snow and Ice Data Center". nsidc.org. Retrieved 20 April 2017.
  7. ↑ Davies, Ella (23 November 2011). "'Brinicle' ice finger of death filmed in Antarctic". Archived from the original on 23 November 2011.
  8. ↑ Holland, Marika M.; Bitz, Cecilia M.; Tremblay, Bruno (2006). "Future abrupt reductions in the summer Arctic sea ice". Geophysical Research Letters. 33 (23). Bibcode:2006GeoRL..3323503H. doi:10.1029/2006GL028024. S2CID   14187034.
  9. 1 2 Thatje, S.; Hillenbrand, C.D.; Mackensen, A.; Larter, R. (2008). "Life hung by a thread: endurance of Antarctic fauna in glacial periods" (PDF). Ecology. 89 (3): 682–692. Bibcode:2008Ecol...89..682T. doi:10.1890/07-0498.1. PMID   18459332.
  10. ↑ Giannelli, Virginia; Thomas, David N.; Haas, Christian; Kattner, Gerhard; Kennedy, Hilary; Dieckmann, Gerhard S. (2001). "Behaviour of dissolved organic matter and inorganic nutrients during experimental sea-ice formation". Annals of Glaciology. 33: 317–321. Bibcode:2001AnGla..33..317G. doi: 10.3189/172756401781818572 . S2CID   18231952.
  11. ↑ Fukamachi, Yasushi; Shirasawa, Kunio; Polomoshnov, Anatoliy M.; Ohshima, Kay I.; Kalinin, Ervin; Nihashi, Sohey; Melling, Humfrey; Mizuta, Genta; Wakatsuchi, Masaaki (2009). "Direct observations of sea-ice thickness and brine rejection off Sakhalin in the Sea of Okhotsk". Continental Shelf Research. 29 (11–12): 1541–1548. Bibcode:2009CSR....29.1541F. doi:10.1016/j.csr.2009.04.005. hdl: 2115/38838 .
  12. 1 2 Shcherbina, Andrey Y.; Talley, Lynne D.; Rudnick, Daniel L. (2004). "Dense water formation on the northwestern shelf of the Okhotsk Sea: 1. Direct observations of brine rejection". Journal of Geophysical Research. 109 (C9). Bibcode:2004JGRC..109.9S08S. doi: 10.1029/2003jc002196 .
  13. ↑ Shcherbina, Andrey Y.; Talley, Lynne D.; Rudnick, Daniel L. (2003). "Direct Observations of North Pacific Ventilation: Brine Rejection in the Okhotsk Sea". Science. 302 (5652): 1952–1955. Bibcode:2003Sci...302.1952S. doi:10.1126/science.1088692. PMID   14671300. S2CID   10266768.
  14. ↑ Detlef, Henrieka; Sosdian, Sindia M.; Belt, Simon T.; Smik, Lukas; Lear, Caroline H.; Kender, Sev; Pearce, Christof; Hall, Ian R. (2020). "Late quaternary sea-ice and sedimentary redox conditions in the eastern Bering Sea – Implications for ventilation of the mid-depth North Pacific and an Atlantic-Pacific seesaw mechanism". Quaternary Science Reviews. 248: 106549. Bibcode:2020QSRv..24806549D. doi:10.1016/j.quascirev.2020.106549. hdl: 10871/122887 . S2CID   224913802.
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