Methane clathrate

From Wikipedia, the free encyclopedia

"Burning ice". Methane, released by heating, burns; water drips.
Inset: clathrate structure (University of Göttingen, GZG. Abt. Kristallographie).
Source: United States Geological Survey.

Methane clathrate (CH₄·5.75H₂O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice.^[1] Originally thought to occur only in the outer regions of the Solar System, where temperatures are low and water ice is common, significant deposits of methane clathrate have been found under sediments on the ocean floors of the Earth.^[2]

Methane clathrates are common constituents of the shallow marine geosphere and they occur in deep sedimentary structures and form outcrops on the ocean floor. Methane hydrates are believed to form by migration of gas from deep along geological faults, followed by precipitation or crystallization, on contact of the rising gas stream with cold sea water. In 2008 research on Antarctic Vostok and EPICA Dome C ice cores revealed that methane clathrates were also present in deep Antarctic ice cores and record a history of atmospheric methane concentrations, dating to 800,000 years ago.^[3] The ice-core methane clathrate record is a primary source of data for global warming research, along with oxygen and carbon dioxide.

Structure and composition

The nominal methane clathrate hydrate composition is (CH₄)₄(H₂O)₂₃, or 1 mole of methane for every 5.75 moles of water, corresponding to 13.4% methane by weight, although the actual composition is dependent on how many methane molecules fit into the various cage structures of the water lattice. The observed density is around 0.9 g/cm³, which means that methane hydrate will float to the surface of the sea or of a lake unless it is bound in place by being formed in or anchored to sediment.^[4] One litre of fully saturated methane clathrate solid would therefore contain about 120 grams of methane (or around 169 litres of methane gas at 0°C and 1 atm).^{[nb 1]}

Methane forms a structure I hydrate with two dodecahedral (12 vertices, thus 12 water molecules) and six tetradecahedral (14 water molecules) water cages per unit cell. (Because of sharing of water molecules between cages, there are only 46 water molecules per unit cell.) This compares with a hydration number of 20 for methane in aqueous solution.^[5] A methane clathrate MAS NMR spectrum recorded at 275 K and 3.1 MPa shows a peak for each cage type and a separate peak for gas phase methane.^{[citation needed]} In 2003, a clay-methane hydrate intercalate was synthesized in which a methane hydrate complex was introduced at the interlayer of a sodium-rich montmorillonite clay. The upper temperature stability of this phase is similar to that of structure I hydrate.^[6]

Methane hydrate phase diagram. The horizontal axis shows temperature from -15 to 33 Celsius, the vertical axis shows pressure from 0 to 120,000 kilopascals (0 to 1,184 atmospheres). For example, at 4 Celsius hydrate forms above a pressure of about 50 atmospheres.

Natural deposits

Worldwide distribution of confirmed or inferred offshore gas hydrate-bearing sediments, 1996.
Source: USGS

Gas hydrate-bearing sediment, from the subduction zone off Oregon

Specific structure of a gas hydrate piece, from the subduction zone off Oregon

Methane clathrates are restricted to the shallow lithosphere (i.e. < 2,000 m depth). Furthermore, necessary conditions are found only in either continental sedimentary rocks in polar regions where average surface temperatures are less than 0 °C; or in oceanic sediment at water depths greater than 300 m where the bottom water temperature is around 2 °C. In addition, deep fresh water lakes may host gas hydrates as well, e.g. the fresh water Lake Baikal, Siberia.^[7] Continental deposits have been located in Siberia and Alaska in sandstone and siltstone beds at less than 800 m depth. Oceanic deposits seem to be widespread in the continental shelf (see Fig.) and can occur within the sediments at depth or close to the sediment-water interface. They may cap even larger deposits of gaseous methane.^[8]

Oceanic

There are two distinct types of oceanic deposit. The most common is dominated (> 99%) by methane contained in a structure I clathrate and generally found at depth in the sediment. Here, the methane is isotopically light (δ¹³C < -60‰) which indicates that it is derived from the microbial reduction of CO₂. The clathrates in these deep deposits are thought to have formed in situ from the microbially produced methane, since the δ¹³C values of clathrate and surrounding dissolved methane are similar.^[8] However, it is also thought that fresh water used in the pressurization of oil and gas wells in permafrost and along the continental shelves world wide, combine with natural methane to form clathrate at depth and pressure, since methane hydrates are more stable in fresh water than in salt water. Local variations may be very common, since the act of forming hydrate, which extracts pure water from saline formation waters, can often lead to local, and potentially significant increases in formation water salinity. Hydrates normally exclude the salt in the pore fluid from which it forms, thus they comprise high electric resistivity just like ice, and sediments containing hydrates have a higher resistivity compared to sediments without gas hydrates (Judge [67])^{[citation needed]}.^[9]:9

These deposits are located within a mid-depth zone around 300–500 m thick in the sediments (the gas hydrate stability zone, or GHSZ) where they coexist with methane dissolved in the fresh, not salt, pore-waters. Above this zone methane is only present in its dissolved form at concentrations that decrease towards the sediment surface. Below it, methane is gaseous. At Blake Ridge on the Atlantic continental rise, the GHSZ started at 190 m depth and continued to 450 m, where it reached equilibrium with the gaseous phase. Measurements indicated that methane occupied 0-9% by volume in the GHSZ, and ~12% in the gaseous zone.^[10][11]

In the less common second type found near the sediment surface some samples have a higher proportion of longer-chain hydrocarbons (< 99% methane) contained in a structure II clathrate. Carbon from this type of clathrate is isotopically heavier (δ¹³C is -29 to -57 ‰) and is thought to have migrated upwards from deep sediments, where methane was formed by thermal decomposition of organic matter. Examples of this type of deposit have been found in the Gulf of Mexico and the Caspian Sea.^[8]

Some deposits have characteristics intermediate between the microbially and thermally sourced types and are considered to be formed from a mixture of the two.

The methane in gas hydrates is dominantly generated by microbial consortia degrading organic matter in low oxygen environments, with the methane itself produced by methanogenic archaea. Organic matter in the uppermost few centimetres of sediments is first attacked by aerobic bacteria, generating CO₂, which escapes from the sediments into the water column. Below this region of aerobic activity, anaerobic processes take over, including, successively with depth, the microbial reduction of nitrite/nitrate, metal oxides, and then sulfates are reduced to sulfides. Finally, once sulfate is used up, methanogenesis becomes a dominant pathway for organic carbon remineralization.

If the sedimentation rate is low (about 1 cm/yr), the organic carbon content is low (about 1% ), and oxygen is abundant, aerobic bacteria can use up all the organic matter in the sediments faster than oxygen is depleted, so lower-energy electron acceptors are not used. But where sedimentation rates and the organic carbon content are high, which is typically the case on continental shelves and beneath western boundary current upwelling zones, the pore water in the sediments becomes anoxic at depths of only a few centimeters or less. In such organic-rich marine sediments, sulfate then becomes the most important terminal electron acceptor due to its high concentration in seawater, although it too is depleted by a depth of centimeters to meters. Below this, methane is produced. This production of methane is a rather complicated process, requiring a highly reducing environment (Eh -350 to -450 mV) and a pH between 6 and 8, as well as a complex syntrophic consortia of different varieties of archaea and bacteria, although it is only archaea that actually emit methane.

In some regions (e.g., Gulf of Mexico) methane in clathrates may be at least partially derived from thermal degradation of organic matter, dominantly in petroleum.^{[12][citation needed]} The methane in clathrates typically has a biogenic isotopic signature and highly variable δ¹³C (-40 to -100‰), with an approximate average of about -65‰ .^{[13][citation needed][14][citation needed][15]} Below the zone of solid clathrates, large volumes of methane may form bubbles of free gas in the sediments.^[10][16][17]

The presence of clathrates at a given site can often be determined by observation of a "bottom simulating reflector" (BSR), which is a seismic reflection at the sediment to clathrate stability zone interface caused by the unequal densities of normal sediments and those laced with clathrates.

Reservoir size

The size of the oceanic methane clathrate reservoir is poorly known, and estimates of its size decreased by roughly an order of magnitude per decade since it was first recognized that clathrates could exist in the oceans during the 1960s and '70s.^[18] The highest estimates (e.g. 3×10¹⁸ m³)^[19] were based on the assumption that fully dense clathrates could litter the entire floor of the deep ocean. Improvements in our understanding of clathrate chemistry and sedimentology have revealed that hydrates form in only a narrow range of depths (continental shelves), at only some locations in the range of depths where they could occur (10-30% of the GHSZ), and typically are found at low concentrations (0.9-1.5% by volume) at sites where they do occur. Recent estimates constrained by direct sampling suggest the global inventory occupies between 1×10¹⁵and 5×10¹⁵ m³ (0.24 to 1.2 million cubic miles).^[18] This estimate, corresponding to 500-2500 gigatonnes carbon (Gt C), is smaller than the 5000 Gt C estimated for all other geo-organic fuel reserves but substantially larger than the ~230 Gt C estimated for other natural gas sources.^[18][20] The permafrost reservoir has been estimated at about 400 Gt C in the Arctic,^{[21][citation needed]} but no estimates have been made of possible Antarctic reservoirs. These are large amounts; for comparison the total carbon in the atmosphere is around 800 gigatons (see Carbon: Occurrence).

These modern estimates are notably smaller than the 10,000 to 11,000 Gt C (2×10¹⁶ m³) proposed^[22] by previous workers a reason to consider clathrates to be a geo-organic fuel resource (MacDonald 1990, Kvenvolden 1998). Lower abundances of clathrates do not rule out their economic potential, but a lower total volume and apparently low concentration at most sites^[18] does suggest that only a limited percentage of clathrates deposits may provide an economically viable resource.

Continental

Methane clathrates in continental rocks are trapped in beds of sandstone or siltstone at depths of less than 800 m. Sampling indicates they are formed from a mix of thermally and microbially derived gas from which the heavier hydrocarbons were later selectively removed. These occur in Alaska, Siberia, and Northern Canada.

In 2008, Canadian and Japanese researchers extracted a constant stream of natural gas from a test project at the Mallik gas hydrate site in the Mackenzie River delta. This was the second such drilling at Mallik: the first took place in 2002 and used heat to release methane. In the 2008 experiment, researchers were able to extract gas by lowering the pressure, without heating, requiring significantly less energy.^[23] The Mallik gas hydrate field was first discovered by Imperial Oil in 1971-1972.^[24]

Commercial use

The sedimentary methane hydrate reservoir probably contains 2–10 times the currently known reserves of conventional natural gas, as of 2013^[update].^[25] This represents a potentially important future source of hydrocarbon fuel. However, in the majority of sites deposits are thought to be too dispersed for economic extraction.^[18] Other problems facing commercial exploitation are detection of viable reserves and development of the technology for extracting methane gas from the hydrate deposits.

A research and development project in Japan is aiming for commercial-scale extraction near Aichi Prefecture by 2016.^[26][27] In August 2006, China announced plans to spend 800 million yuan (US$100 million) over the next 10 years to study natural gas hydrates.^[28] A potentially economic reserve in the Gulf of Mexico may contain approximately 100 billion cubic metres (3.5×10^¹² cu ft) of gas.^[18] Bjørn Kvamme and Arne Graue at the Institute for Physics and technology at the University of Bergen have developed a method for injecting CO
2 into hydrates and reversing the process; thereby extracting CH₄ by direct exchange.^[29] The University of Bergen's method is being field tested by ConocoPhillips and state-owned Japan Oil, Gas and Metals National Corporation (JOGMEC), and partially funded by the U.S. Department of Energy. The project has already reached injection phase and was analyzing resulting data by March 12, 2012.^[30]

On March 12, 2013, JOGMEC researchers announced that they had successfully extracted natural gas from frozen methane hydrate.^[31] In order to extract the gas, specialized equipment was used to drill into and depressurize the hydrate deposits, causing the methane to separate from the ice. The gas was then collected and piped to surface where it was ignited to prove its presence.^[32] According to an industry spokesperson, "It [was] the world's first offshore experiment producing gas from methane hydrate".^[31] Previously, gas had been extracted from onshore deposits, but never from offshore deposits which are much more common.^[32] The hydrate field from which the gas was extracted is located 50 kilometres (31 mi) from central Japan in the Nankai Trough, 300 metres (980 ft) under the sea.^[31][32] A spokesperson for JOGMEC remarked "Japan could finally have an energy source to call its own".^[32] The experiment will continue for two weeks before it is determined how efficient the gas extraction process has been.^[32] Marine geologist Mikio Satoh remarked "Now we know that extraction is possible. The next step is to see how far Japan can get costs down to make the technology economically viable."^[32] Japan estimates that there are at least 1.1 trillion cubic meters of methane trapped in the Nankai Trough, enough to meet the country's needs for more than ten years.^[32]

Hydrates in natural gas processing

Routine operations

Methane clathrates (hydrates) are also commonly formed during natural gas production operations, when liquid water is condensed in the presence of methane at high pressure. It is known that larger hydrocarbon molecules like ethane and propane can also form hydrates, although longer molecules (butanes, pentanes) cannot fit into the water cage structure and tend to destabilise the formation of hydrates.

Once formed, hydrates can block pipeline and processing equipment. They are generally then removed by reducing the pressure, heating them, or dissolving them by chemical means (methanol is commonly used). Care must be taken to ensure that the removal of the hydrates is carefully controlled, because of the potential for the hydrate to undergo a phase transition from the solid hydrate to release water and gaseous methane at a high rate when the pressure is reduced. The rapid release of methane gas in a closed system can result in a rapid increase in pressure.^[4]

It is generally preferable to prevent hydrates from forming or blocking equipment. This is commonly achieved by removing water, or by the addition of ethylene glycol (MEG) or methanol, which act to depress the temperature at which hydrates will form (i.e. common antifreeze). In recent years, development of other forms of hydrate inhibitors have been developed, like Kinetic Hydrate Inhibitors (which by far slow the rate of hydrate formation) and anti-agglomerates, which do not prevent hydrates forming, but do prevent them sticking together to block equipment.

Effect of hydrate phase transition during deep water drilling

When drilling in oil- and gas-bearing formations submerged in deep water, the reservoir gas may flow into the well bore and form gas hydrates owing to the low temperatures and high pressures found during deep water drilling. The gas hydrates may then flow upward with drilling mud or other discharged fluids. When the hydrates rise, the pressure in the annulus decreases and the hydrates dissociate into gas and water. The rapid gas expansion ejects fluid from the well, reducing the pressure further, which leads to more hydrate dissociation and further fluid ejection. The resulting violent expulsion of fluid from the annulus is one potential cause or contributor to the "kick".^[33] (Kicks, which can cause blowouts, typically do not involve hydrates: see Blowout: formation kick).
Measures which reduce the risk of hydrate formation include:

• High flow-rates, which limit the time for hydrate formation in a volume of fluid, thereby reducing the kick potential.^[33]
• Careful measuring of line flow to detect incipient hydrate plugging.^[33]
• Additional care in measuring when gas production rates are low and the possibility of hydrate formation is higher than at relatively high gas flow rates.^[33]
• Monitoring of well casing after it is "shut in" (isolated) may indicate hydrate formation. Following "shut in", the pressure rises while gas diffuses through the reservoir to the bore hole; the rate of pressure rise exhibit a reduced rate of increase while hydrates are forming.^[33]
• Additions of energy (e.g., the energy released by setting cement used in well completion) can raise the temperature and convert hydrates to gas, producing a "kick".

Blowout recovery

Concept diagram of oil containment domes, forming upsidedown funnels in order to pipe oil to surface ships. The sunken oil rig is nearby.

At sufficient depths, methane complexes directly with water to form methane hydrates, as was observed during the Deepwater Horizon oil spill in 2010. BP engineers developed and deployed a subsea oil recovery system over oil spilling from a deepwater oil well 5,000 feet (1,500 m) below sea level to capture escaping oil. This involved placing a 125-tonne (276,000 lb) dome over the largest of the well leaks and piping it to a storage vessel on the surface.^[34] This option had the potential to collect some 85% of the leaking oil but was previously untested at such depths.^[34] BP deployed the system on May 7–8, but it failed due to buildup of methane clathrate inside the dome; with its low density of approximately 0.9 g/cm³ the methane hydrates accumulated in the dome, adding buoyancy and obstructing flow.^[35]

Methane clathrates and climate change

Methane is a powerful greenhouse gas. Despite its short atmospheric half life of 7 years, methane has a global warming potential of 62 over 20 years and 21 over 100 years (IPCC, 1996; Berner and Berner, 1996; vanLoon and Duffy, 2000). The sudden release of large amounts of natural gas from methane clathrate deposits has been hypothesized as a cause of past and possibly future climate changes. Events possibly linked in this way are the Permian-Triassic extinction event and the Paleocene-Eocene Thermal Maximum.
Climate scientists like James E. Hansen predict that methane clathrates in the permafrost regions will be released consequent to global warming, unleashing powerful feedback forces which may cause runaway climate change that cannot be controlled.

Recent research carried out in 2008 in the Siberian Arctic has shown millions of tonnes of methane being released^{[36][37][38][39][40]} with concentrations in some regions reaching up to 100 times above normal.^[41]

In their Correspondence in the September 2013 Nature Geoscience journal, Vonk and Gustafsson cautioned that the most probable mechanism to strengthen global warming is large-scale thawing of Arctic permafrost which will release methane clathrate into the atmosphere.^[42] While performing research in July in plumes in the East Siberian Arctic Ocean, Gustafsson and Vonk were surprised by the high concentration of methane.^[43]

In 2014 based on their research on the northern United States Atlantic marine continental margins from Cape Hatteras to Georges Bank, a group of scientists from the US Geological Survey, the Department of Geosciences, Mississippi State University, Department of Geological Sciences, Brown University and Earth Resources Technology, claimed there was widespread leakage of methane.^{[44] [45]}

Natural gas hydrates versus liquified natural gas in transportation

Since methane clathrates are stable at a higher temperature than liquefied natural gas (LNG) (−20 vs −162 °C), there is some interest in converting natural gas into clathrates rather than liquifying it when transporting it by seagoing vessels. A significant advantage would be that the production of natural gas hydrate (NGH) from natural gas at the terminal would require a smaller refrigeration plant and less energy than LNG would. Offsetting this, for 100 tonnes of methane transported, 750 tonnes of methane hydrate would have to be transported; since this would require a ship of 7.5 times greater displacement, or require more ships, it is unlikely to prove economic.^{[citation needed]}

Nanoporous methane storage – an impossible target?

18 February 2015 | Emma Stephen

The structure of one of the best predicted MOFs © Cory Simon

Is it possible to design a material to fulfil current methane storage goals? This is the question that a multi-disciplinary research team set out to answer by rapidly screening hundreds of thousands of possible methane storage materials in a computational study. Methane could reduce global dependence on oil so the search is on for nanoporous materials to act as fuel tanks for this tricky-to-store gas; but things are not looking promising.

‘Natural gas storage in porous materials provides the key advantage of being able to store significant natural gas at low pressures than compressed gas at the same conditions,’ explains engineer Mike Veenstra of Ford Motor Company, US, who was not involved in the research. ‘The advantage of low pressure is the benefit it provides both on-board the vehicle and off-board at the station. On the vehicle, low pressure reduces the tank attributes along with the other components. At the station, low pressure reduces the compressor stages along with the attributes of other components.’

Randall Snurr at Northwestern University, US, Berend Smit at the University of California, Berkeley, US, and their colleagues around the globe are part of the Materials Genome Initiative that intends to revolutionise materials chemistry in the same way the Human Genome Project did for the biological sciences. They have developed a computer algorithm that simulates model structures, and identifies their pore sizes in relation to deliverable methane storage capacity, based on the current idea of what it is possible to make. The simulations are particularly useful for metal–organic frameworks (MOFs), and considering the vast combinations of inorganic and organic molecular building blocks available, few have been synthesised and tested. ‘Experimentalists have synthesised somewhere between 5000 and 10,000 MOFs and less than 250 zeolite structures over several decades. In this computational study, we examined over 650,000 such structures in a much shorter time,’ explains Snurr.

A huge number of materials can be generated by combining metals and organic linkers

The best experimentally measured capacity of any material falls short of the current US Advanced Research Project Agency-Energy (ARPA-E) methane storage target. Smit says the conventional approach is to rely on experiments to answer questions as to whether the target is feasible or not: ‘Here we make a bold statement: the current approach is not going to get us there.’ Instead, he says that using calculations to rapidly determine relevant methane storage candidates saves valuable time and resources.

‘Here we make a bold statement: the current approach is not going to get us there’

Calculations reveal an optimal pore diameter of around 11Å for methane storage; any larger and the van der Waals interactions between the gas molecules and the pore walls cannot be felt. The ideal material also has a high density of adsorption sites to optimise methane interactions. Comparing the calculated structures against current goals, the researchers quickly established that it may be impossible to reach the desired target using such materials. With a computational infrastructure in place, the effect of different temperatures and pressures on the structures can be readily recalculated, showing how these factors impact on the deliverable methane capacity. However, even by adopting more favourable parameters, no materials were able to reach the ARPA-E target. From the large number of structures examined computationally, the highest predicted deliverable capacity echoes those observed for the current top sorbent materials, meaning that these materials may already be at their limit.

Joe Zhou, an expert in materials at Texas A&M University, US says ‘research in materials science depends on theoretical simulation to set the boundary, guide the synthesis, and eventually confirm the experimental results.’ He says that for methane storage, understanding what is achievable in industry is critical. ‘Generally speaking, theoretical work focusing on enumerating possible materials is common, but to explore the performance limits is a definite step toward guiding the invention of novel materials.’

MOFs and related materials are not only useful for methane storage. The huge amount of computational data from this study can now be data mined to identify their performance in alternative applications. Similar materials genome-style screening studies could be carried out to identify what is realistically possible to achieve in other areas, including lithium-ion batteries and photovoltaics. Smit says, ‘I am fascinated by the insights these screening studies give; normally one can say something about a few materials – now we have insights on the performance of an entire class of materials. This has completely changed my view on how to do materials research.’

References

This article is free to access until 26 March 2015. Download it here:
C M Simon et al, Energy Environ. Sci., 2015, DOI: 10.1039/c4ee03515a

Arctic Sea Ice Extent, January 17, 2015, DMI, Danish Meteorological Institute

Total sea ice extent on the northern hemisphere during the past years, including climate mean; plus/minus 1 standard deviation. The ice extent values are calculated from the ice type data from the Ocean and Sea Ice, Satellite Application Facility (OSISAF), where areas with ice concentration higher than 15% are classified as ice.

The total area of sea ice is the sum of First Year Ice (FYI), Multi Year Ice (MYI) and the area of ambiguous ice types, from the OSISAF ice type product. The total sea ice extent can differ slightly from other sea ice extent estimates. Possible differences between this sea ice extent estimate and others are most likely caused by differences in algorithms and definitions. Some time in 2013 sea ice climatology and anomaly data will become available here.


                       Sea ice extent in recent years for the northern hemisphere.
                       The grey shaded area corresponds to the climate mean
                       plus/minus 1 standard deviation.

The plot above replaces an earlier sea ice extent plot, that was based on data with the coastal zones masked out. This coastal mask implied that the previous sea ice extent estimates were underestimated. The new plot displays absolute sea ice extent estimates. The old plot can still be viewed here for a while.

Arctic Ocean

From Wikipedia, the free encyclopedia

A bathymetric/topographic of the Arctic Ocean and the surrounding lands.

The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions.^[1] The International Hydrographic Organization (IHO) recognizes it as an ocean, although some oceanographers call it the Arctic Mediterranean Sea or simply the Arctic Sea, classifying it a mediterranean sea or an estuary of the Atlantic Ocean.^[2][3] Alternatively, the Arctic Ocean can be seen as the northernmost part of the all-encompassing World Ocean.

Almost completely surrounded by Eurasia and North America, the Arctic Ocean is partly covered by sea ice throughout the year^[4] (and almost completely in winter). The Arctic Ocean's surface temperature and salinity vary seasonally as the ice cover melts and freezes;^[5] its salinity is the lowest on average of the five major oceans, due to low evaporation, heavy fresh water inflow from rivers and streams, and limited connection and outflow to surrounding oceanic waters with higher salinities. The summer shrinking of the ice has been quoted at 50%.^[1] The US National Snow and Ice Data Center (NSIDC) uses satellite data to provide a daily record of Arctic sea ice cover and the rate of melting compared to an average period and specific past years.

History

For much of European history, the north polar regions remained largely unexplored and their geography conjectural. Pytheas of Massilia recorded an account of a journey northward in 325 BC, to a land he called "Eschate Thule," where the Sun only set for three hours each day and the water was replaced by a congealed substance "on which one can neither walk nor sail." He was probably describing loose sea ice known today as "growlers" or "bergy bits;" his "Thule" was probably Norway, though the Faroe Islands or Shetland have also been suggested.^[6]

Emanuel Bowen's 1780s map of the Arctic features a "Northern Ocean".

Early cartographers were unsure whether to draw the region around the North Pole as land (as in Johannes Ruysch's map of 1507, or Gerardus Mercator's map of 1595) or water (as with Martin Waldseemüller's world map of 1507). The fervent desire of European merchants for a northern passage, the Northern Sea Route or the Northwest Passage, to "Cathay" (China) caused water to win out, and by 1723 mapmakers such as Johann Homann featured an extensive "Oceanus Septentrionalis" at the northern edge of their charts.

The few expeditions to penetrate much beyond the Arctic Circle in this era added only small islands, such as Novaya Zemlya (11th century) and Spitsbergen (1596), though since these were often surrounded by pack-ice, their northern limits were not so clear. The makers of navigational charts, more conservative than some of the more fanciful cartographers, tended to leave the region blank, with only fragments of known coastline sketched in.

This lack of knowledge of what lay north of the shifting barrier of ice gave rise to a number of conjectures. In England and other European nations, the myth of an "Open Polar Sea" was persistent. John Barrow, longtime Second Secretary of the British Admiralty, promoted exploration of the region from 1818 to 1845 in search of this.

In the United States in the 1850s and 1860s, the explorers Elisha Kane and Isaac Israel Hayes both claimed to have seen part of this elusive body of water. Even quite late in the century, the eminent authority Matthew Fontaine Maury included a description of the Open Polar Sea in his textbook The Physical Geography of the Sea (1883). Nevertheless, as all the explorers who travelled closer and closer to the pole reported, the polar ice cap is quite thick, and persists year-round.

Fridtjof Nansen was the first to make a nautical crossing of the Arctic Ocean, in 1896. The first surface crossing of the ocean was led by Wally Herbert in 1969, in a dog sled expedition from Alaska to Svalbard, with air support.^[7] The first nautical transit of the north pole was made in 1958 by the submarine USS Nautilus, and the first surface nautical transit occurred in 1977 by the icebreaker NS Arktika.

Since 1937, Soviet and Russian manned drifting ice stations have extensively monitored the Arctic Ocean. Scientific settlements were established on the drift ice and carried thousands of kilometres by ice floes.^[8]

In World War II the European region of the Arctic Ocean was heavily contested : the Allied commitment to resupply the Soviet Union via its northern ports was opposed by German naval and air forces.

Geography

The Arctic region; of note, the region's southerly border on this map is depicted by a red isotherm, whereby all territory to the north averages temperatures of less than 10 °C (50 °F) in July.

The Arctic Ocean occupies a roughly circular basin and covers an area of about 14,056,000 km² (5,427,000 sq mi), almost the size of Russia.^[9][10] The coastline is 45,390 km (28,200 mi) long.^[9][11] It is surrounded by the land masses of Eurasia, North America, Greenland, and by several islands.

It is generally taken to include Baffin Bay, Barents Sea, Beaufort Sea, Chukchi Sea, East Siberian Sea, Greenland Sea, Hudson Bay, Hudson Strait, Kara Sea, Laptev Sea, White Sea and other tributary bodies of water. It is connected to the Pacific Ocean by the Bering Strait and to the Atlantic Ocean through the Greenland Sea and Labrador Sea.^[1]

Countries bordering the Arctic Ocean are: Russia, Norway, Iceland, Greenland, Canada and the United States.

Extent and major ports

There are several ports and harbors around the Arctic Ocean^[12]

United States

In Alaska, the main ports are Barrow (

 WikiMiniAtlas

71°17′44″N 156°45′59″W / 71.29556°N 156.76639°W / 71.29556; -156.76639 (Barrow)) and Prudhoe Bay (

 WikiMiniAtlas

70°19′32″N 148°42′41″W / 70.32556°N 148.71139°W / 70.32556; -148.71139 (Prudhoe)).

Canada

In Canada, ships may anchor at Churchill (Port of Churchill) (

 WikiMiniAtlas

58°46′28″N 094°11′37″W / 58.77444°N 94.19361°W / 58.77444; -94.19361 (Port of Churchill)) in Manitoba, Nanisivik (Nanisivik Naval Facility) (

 WikiMiniAtlas

73°04′08″N 084°32′57″W / 73.06889°N 84.54917°W / 73.06889; -84.54917 (Nanisivik Naval Facility)) in Nunavut,^[13] Tuktoyaktuk (

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69°26′34″N 133°01′52″W / 69.44278°N 133.03111°W / 69.44278; -133.03111 (Tuktoyaktuk)) or Inuvik (

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68°21′42″N 133°43′50″W / 68.36167°N 133.73056°W / 68.36167; -133.73056 (Inuvik)) in the Northwest territories.

Greenland

In Greenland, the main port is at Nuuk (Nuuk Port and Harbour) (

 WikiMiniAtlas

64°10′15″N 051°43′15″W / 64.17083°N 51.72083°W / 64.17083; -51.72083 (Nuuk Port and Harbour)).

Norway

In Norway, Kirkenes (

 WikiMiniAtlas

69°43′37″N 030°02′44″E / 69.72694°N 30.04556°E / 69.72694; 30.04556 (Kirkenes)) and Vardø (

 WikiMiniAtlas

70°22′14″N 031°06′27″E / 70.37056°N 31.10750°E / 70.37056; 31.10750 (Vardø)) are ports on the mainland. Also, there is Longyearbyen (

 WikiMiniAtlas

78°13′12″N 15°39′00″E / 78.22000°N 15.65000°E / 78.22000; 15.65000 (Longyearbyen)) on the island of Svalbard next to Fram Strait.

Russia

In Russia, major ports sorted by the different sea areas are:

• Murmansk (68°58′N 033°05′E / 68.967°N 33.083°E / 68.967; 33.083 (Murmansk)) in the Barents Sea
• Arkhangelsk (64°32′N 040°32′E / 64.533°N 40.533°E / 64.533; 40.533 (Arkhangelsk)) in the White Sea
• Labytnangi (66°39′26″N 066°25′06″E / 66.65722°N 66.41833°E / 66.65722; 66.41833 (Labytnangi)) Salekhard (66°32′N 066°36′E / 66.533°N 66.600°E / 66.533; 66.600 (Salekhard)), Dudinka (69°24′N 086°11′E / 69.400°N 86.183°E / 69.400; 86.183 (Dudinka)), Igarka (67°28′N 86°35′E / 67.467°N 86.583°E / 67.467; 86.583 (Igarka)) and Dikson (73°30′N 080°31′E / 73.500°N 80.517°E / 73.500; 80.517 (Dikson)) in the Kara Sea
• Tiksi (71°38′N 128°52′E / 71.633°N 128.867°E / 71.633; 128.867 (Tiksi, city of the Polar Heros)) in the Laptev Sea
• Pevek (69°42′N 170°17′E / 69.700°N 170.283°E / 69.700; 170.283 (Pevek)) in the East Siberian Sea

Arctic shelves

The ocean's Arctic shelf comprises a number of continental shelves, including the Canadian Arctic shelf, underlying the Canadian Arctic Archipelago, and the Russian continental shelf, which is sometimes simply called the "Arctic Shelf" because it is greater in extent. The Russian continental shelf consists of three separate, smaller shelves, the Barents Shelf, Chukchi Sea Shelf and Siberian Shelf. Of these three, the Siberian Shelf is the largest such shelf in the world. The Siberian Shelf holds large oil and gas reserves, and the Chukchi shelf forms the border between Russian and the United States as stated in the USSR–USA Maritime Boundary Agreement. The whole area is subject to international territorial claims.

Underwater features

An underwater ridge, the Lomonosov Ridge, divides the deep sea North Polar Basin into two oceanic basins: the Eurasian Basin, which is between 4,000 and 4,500 m (13,100 and 14,800 ft) deep, and the Amerasian Basin (sometimes called the North American, or Hyperborean Basin), which is about 4,000 m (13,000 ft) deep. The bathymetry of the ocean bottom is marked by fault block ridges, abyssal plains, ocean deeps, and basins. The average depth of the Arctic Ocean is 1,038 m (3,406 ft).^[14] The deepest point is Litke Deep in the Eurasian Basin, at 5,450 m (17,880 ft).

The two major basins are further subdivided by ridges into the Canada Basin (between Alaska/Canada and the Alpha Ridge), Makarov Basin (between the Alpha and Lomonosov Ridges), Nansen Basin (between Lomonosov and Gakkel ridges), and Nansen Basin (Amundsen Basin) (between the Gakkel Ridge and the continental shelf that includes the Franz Josef Land).

Oceanography

Water flow

Distribution of the major water mass in the Arctic Ocean. The section sketches the different water masses along a vertical section from Bering Strait over the geographic North Pole to Fram Strait. As the stratification is stable, deeper water masses are more dense than the layers above.

Density structure of the upper 1,200 m (3,900 ft) in the Arctic Ocean. Profiles of temperature and salinity for the Amundsen Basin, the Canadian Basin and the Greenland Sea are sketched in this cartoon.

In large parts of the Arctic Ocean, the top layer (about 50 m (160 ft)) is of lower salinity and lower temperature than the rest. It remains relatively stable, because the salinity effect on density is bigger than the temperature effect. It is fed by the freshwater input of the big Siberian and Canadian streams (Ob, Yenisei, Lena, Mackenzie), the water of which quasi floats on the saltier, denser, deeper ocean water. Between this lower salinity layer and the bulk of the ocean lies the so-called halocline, in which both salinity and temperature are rising with increasing depth.

Due to its relative isolation from other oceans, the Arctic Ocean has a uniquely complex system of water flow. It is classified as a mediterranean sea, which as “a part of the world ocean which has only limited communication with the major ocean basins (these being the Pacific, Atlantic, and Indian Oceans) and where the circulation is dominated by thermohaline forcing”.^[15] The Arctic Ocean has a total volume of 18.07×10⁶ km³, equal to about 1.3% of the World Ocean. Mean surface circulation is predominately cyclonic on the Eurasian side and anticyclonic in the Canadian Basin.^[16]

Water enters from both the Pacific and Atlantic Oceans and can be divided into three unique water masses. The deepest water mass is called Arctic Bottom Water and begins around 900 meters depth.^[15] It is composed of the densest water in the World Ocean and has two main sources: Arctic shelf water and Greenland Sea Deep Water. Water in the shelf region that begins as inflow from the Pacific passes through the narrow Bering Strait at an average rate of 0.8 Sverdrups and reaches the Chukchi Sea.^[17] During the winter, cold Alaskan winds blow over the Chukchi Sea, freezing the surface water and pushing this newly formed ice out to the Pacific. The speed of the ice drift is roughly 1–4 cm/s.^[16] This process leaves dense, salty waters in the sea that sink over the continental shelf into the western Arctic Ocean and create a halocline.^[18]

This water is met by Greenland Sea Deep Water, which forms during the passage of winter storms. As temperatures cool dramatically in the winter, ice forms and intense vertical convection allows the water to become dense enough to sink below the warm saline water below.^[15] Arctic Bottom Water is critically important because of its outflow, which contributes to the formation of Atlantic Deep Water. The overturning of this water plays a key role in global circulation and the moderation of climate.

In the depth range of 150–900 meters is a water mass referred to as Atlantic Water. Inflow from the North Atlantic Current enters through the Fram Strait, cooling and sinking to form the deepest layer of the halocline, where it circles the Arctic Basin counter-clockwise. This is the highest volumetric inflow to the Arctic Ocean, equaling about 10 times that of the Pacific inflow, and it creates the Arctic Ocean Boundary Current.^[17] It flows slowly, at about 0.02 m/s.^[15] Atlantic Water has the same salinity as Arctic Bottom Water but is much warmer (up to 3 °C). In fact, this water mass is actually warmer than the surface water, and remains submerged only due the role of salinity in density.^[15] When water reaches the basin it is pushed by strong winds into a large circular current called the Beaufort Gyre. Water in the Beaufort Gyre is far less saline than that of the Chukchi Sea due to inflow from large Canadian and Siberian rivers.^[18]

The final defined water mass in the Arctic Ocean is called Arctic Surface Water and is found from 150–200 meters. The most important feature of this water mass is a section referred to as the sub-surface layer. It is a product of Atlantic water that enters through canyons and is subjected to intense mixing on the Siberian Shelf.^[15] As it is entrained, it cools and acts a heat shield for the surface layer. This insulation keeps the warm Atlantic Water from melting the surface ice. Additionally, this water forms the swiftest currents of the Arctic, with speed of around 0.3-0.6 m/s.^[15] Complementing the water from the canyons, some Pacific water that does not sink to the shelf region after passing through the Bering Strait also contributes to this water mass.

Waters originating in the Pacific and Atlantic both exit through the Fram Strait between Greenland and Svalbard Island, which is about 2700 meters deep and 350 kilometers wide. This outflow is about 9 Sv.^[17] The width of the Fram Strait is what allows for both inflow and outflow on the Atlantic side of the Arctic Ocean. Because of this, it is influenced by the Coriolis force, which concentrates outflow to the East Greenland Current on the western side and inflow to the Norwegian Current on the eastern side.^[15] Pacific water also exits along the west coast of Greenland and the Hudson Strait (1-2 Sv), providing nutrients to the Canadian Archipelago.^[17]

As noted, the process of ice formation and movement is a key driver in Arctic Ocean circulation and the formation of water masses. With this dependence, the Arctic Ocean experiences variations due to seasonal changes in sea ice cover. Sea ice movement is the result of wind forcing, which is related to a number of meteorological conditions that the Arctic experiences throughout the year. For example, the Beaufort High—an extension of the Siberian High system—is a pressure system that drives the anticyclonic motion of the Beaufort Gyre.^[16] During the summer, this area of high pressure is pushed out closer to its Siberian and Canadian sides. In addition, there is a sea level pressure (SLP) ridge over Greenland that drives strong northerly winds through the Fram Strait, facilitating ice export. In the summer, the SLP contrast is smaller, producing weaker winds. A final example of seasonal pressure system movement is the low pressure system that exists over the Nordic and Barents Seas. It is an extension of the Icelandic Low, which creates cyclonic ocean circulation in this area. The low shifts to center over the North Pole in the summer. These variations in the Arctic all contribute to ice drift reaching its weakest point during the summer months. There is also evidence that the drift is associated with the phase of the Arctic Oscillation and Atlantic Multidecadal Oscillation.^[16]

Sea ice

Sea cover in the Arctic Ocean, showing the median, 2005 and 2007 coverage ^[19]

Much of the Arctic Ocean is covered by sea ice which varies in extent and thickness seasonally. The mean extent of the ice is decreasing since 1980 from the average winter value of 15,600,000 km² (6,023,200 sq mi) at a rate of 3% per decade. The seasonal variations are about 7,000,000 km² (2,702,700 sq mi) with the maximum in April and minimum in September. The sea ice is affected by wind and ocean currents which can move and rotate very large areas of ice. Zones of compression also arise, where the ice piles up to form pack ice.^[20][21]

Icebergs occasionally break away from northern Ellesmere Island, and icebergs are formed from glaciers in western Greenland and extreme northeastern Canada. These icebergs pose a hazard to ships, of which the Titanic is one of the most famous. Permafrost is found on most islands. The ocean is virtually icelocked from October to June, and the superstructure of ships are subject to icing from October to May.^[12] Before the advent of modern icebreakers, ships sailing the Arctic Ocean risked being trapped or crushed by sea ice (although the Baychimo drifted through the Arctic Ocean untended for decades despite these hazards).

Climate

Changes in ice between 1990–1999

Under the influence of the Quaternary glaciation, the Arctic Ocean is contained in a polar climate characterized by persistent cold and relatively narrow annual temperature ranges. Winters are characterized by the polar night, cold and stable weather conditions, and clear skies; summers are characterized by continuous daylight (midnight sun), damp and foggy weather, and weak cyclones with rain or snow^{[citation needed]}

The temperature of the surface of the Arctic Ocean is fairly constant, near the freezing point of seawater. Because the Arctic Ocean consists of saltwater, the temperature must reach −1.8 °C (28.8 °F) before freezing occurs.

The density of sea water, in contrast to fresh water, increases as it nears the freezing point and thus it tends to sink. It is generally necessary that the upper 100–150 m (330–490 ft) of ocean water cools to the freezing point for sea ice to form.^[22] In the winter the relatively warm ocean water exerts a moderating influence, even when covered by ice. This is one reason why the Arctic does not experience the extreme temperatures seen on the Antarctic continent.

There is considerable seasonal variation in how much pack ice of the Arctic ice pack covers the Arctic Ocean. Much of the Arctic ice pack is also covered in snow for about 10 months of the year. The maximum snow cover is in March or April — about 20 to 50 cm (7.9 to 19.7 in) over the frozen ocean.

The climate of the Arctic region has varied significantly in the past. As recently as 55 million years ago, during the Paleocene–Eocene Thermal Maximum, the region reached an average annual temperature of 10–20 °C (50–68 °F).^[23] The surface waters of the northernmost^[24] Arctic ocean warmed, seasonally at least, enough to support tropical lifeforms^[25] requiring surface temperatures of over 22 °C (72 °F).^[26]

Animal and plant life

Three polar bears approach USS Honolulu near the North Pole.

Endangered marine species in the Arctic Ocean include walruses and whales. The area has a fragile ecosystem which is slow to change and slow to recover from disruptions or damage.^[12] Lion's mane jellyfish are abundant in the waters of the Arctic, and the banded gunnel is the only species of gunnel that lives in the ocean.

The Arctic Ocean has relatively little plant life except for phytoplankton.^[27] Phytoplankton are a crucial part of the ocean and there are massive amounts of them in the Arctic, where they feed on nutrients from rivers and the currents of the Atlantic and Pacific oceans.^[28] During summer, the sun is out day and night, thus enabling the phytoplankton to photosynthesize for long periods of time and reproduce quickly. However, the reverse is true in winter when they struggle to get enough light to survive.^[28]

Natural resources

Petroleum and natural gas fields, placer deposits, polymetallic nodules, sand and gravel aggregates, fish, seals and whales can all be found in abundance in the region.^[12]
The political dead zone near the center of the sea is also the focus of a mounting dispute between the United States, Russia, Canada, Norway, and Denmark.^[29] It is significant for the global energy market because it may hold 25% or more of the world's undiscovered oil and gas resources.^[30]

Environmental concerns

The Arctic ice pack is thinning, and in many years there is also a seasonal hole in the ozone layer.^[31] Reduction of the area of Arctic sea ice reduces the planet's average albedo, possibly resulting in global warming in a positive feedback mechanism.^[32] Research shows that the Arctic may become ice free for the first time in human history by 2040.^[33]
Warming temperatures in the Arctic may cause large amounts of fresh meltwater to enter the north Atlantic, possibly disrupting global ocean current patterns. Potentially severe changes in the Earth's climate might then ensue.^[32]

As the extent of sea ice diminishes and sea level rises, the effect of storms such as the Great Arctic Cyclone of 2012 on open water increases, as does possible salt-water damage to vegetation on shore at locations such as the Mackenzie's river delta as stronger storm surges become more likely.^[34]

Other environmental concerns relate to the radioactive contamination of the Arctic Ocean from, for example, Russian radioactive waste dump sites in the Kara Sea^[35] and Cold War nuclear test sites such as Novaya Zemlya.^[36] In addition, Shell planned to drill exploratory wells in the Chukchi and Beaufort seas during the summer of 2012, which environmental groups filed a lawsuit about in an attempt to protect native communities, endangered wildlife, and the Arctic Ocean in the event of a major oil spill.^[37]