Methane clathrate, also called methane hydrate or methane ice, is a solid form of water that contains a large amount of methane within its crystal structure (a clathrate hydrate). Originally thought to occur only in the outer regions of the Solar System where temperatures are low and water ice is common, extremely large deposits of methane clathrate have been found under sediments on the ocean floors of Earth. 
Methane clathrates are common constituents of the shallow marine geosphere, and they occur both in deep sedimentary structures, and as outcrops on the ocean floor. Methane hydrates are believed to form by migration of gas from depth along geological faults, followed by precipitation, or crystallization, on contact of the rising gas stream with cold sea water.
At higher pressures Methane clathrates remain stable at temperatures up to 18 °C. The average methane clathrate hydrate composition is 1 mole of methane for every 5.75 moles of water, though this 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. One liter of methane clathrate solid would therefore contain, on average, 168 liters of methane gas (at STP).
Methane forms a structure I hydrate with two dodecahedral (20 vertices thus 20 water molecules) and six tetradecahedral (24 water molecules) water cages per unit cell. The hydratation value of 20 can be determined experimentally by MAS NMR. A methane clathrate spectrum recorded at 275 K and 3.1 MPa shows a peak for each cage type and a separate peak for gas phase methane. Recently, a clay-methane hydrate intercalate was synthesized in which a methane hydrate complex was introduced at the interlayer of a Na-rich montmorillonite clay. The upper temperature stability of this phase is similar to that of structure I hydrate.
Methane clathrates are restricted to the shallow lithosphere (i.e. < 2000 m depth). Furthermore, necessary conditions are found only either in polar continental sedimentary rocks where 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. 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.
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, as the δ13C values of clathrate and surrounding dissolved methane are similar.
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 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.
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. Methane 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.
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 bacterial degradation of organic matter in low oxygen environments. 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. In this region of aerobic bacterial activity sulfates are reduced to sulfides. If the sedimentation rate is low (<1 cm/kyr), the organic carbon content is low (<1% ), and oxygen is abundant, aerobic bacteria use up all the organic matter in the sediments. But where sedimentation rates and the organic carbon content are high, the pore waters in the sediments are anoxic at depths of only a few cm, and methane is produced by anaerobic bacteria. This production of methane is a rather complicated process, requires the activity of several varieties of bacteria, a reducing environment (Eh -350 to -450 mV), and a pH between 6 and 8. 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. The methane in clathrates typically has a bacterial isotopic signature and highly variable δ13C (-40 to -100‰), with an approximate average of about -65 ‰ .Kvenvolden, 1993; Dickens et al., 1995;  Below the zone of solid clathrates, large volumes of methane may occur as bubbles of free gas in the sediments.
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.
The size of the oceanic methane clathrate reservoir is poorly known, and estimates of its size have 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. The highest estimates (e.g. 3×1018 m³) were based on the assumption that fully dense clathrates could litter the entire floor of the deep ocean. However, improvements in our understanding of clathrate chemistry and sedimentology have revealed that hydrates only form in a narrow range of depths (continental shelves), only at 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 lies between 1×1015 and 5×1015 m³ (1 quadrillion to 5 quadrillion). This estimate, corresponding to 500-2500 gigatonnes carbon (Gt C), is smaller than the 5000 Gt C estimated for all other fossil fuel reserves but substantially larger than the ~230 Gt C estimated for other natural gas sources. The permafrost reservoir has been estimated at about 400 Gt C in the Arctic, but no estimates have been made of possible Antarctic reservoirs. These are large amounts. For comparison the total carbon in the Atmosphere is around 700 gigatons.
These modern estimates are notably smaller than the 10,000 to 11,000 Gt C (2×1016 m³) proposed by previous workers as a motivation considering clathrates as a fossil 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 does suggests that only a limited percentage of clathrates deposits may provide an economically viable resource.
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 and Siberia.
The sedimentary methane hydrate reservoir probably contains 2-10x the currently known reserves of conventional natural gas. This represents a potentially important future source of fossil fuel. However, in the majority of sites deposits are likely to be too dispersed for economic extraction. 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 by 2016. In August of 2006, China announced plans to spend 800 million yuan (US$100 million) over the next 10 years to study natural gas hydrates. A potentially economic reserve in the Gulf of Mexico may contain ~1010 m3 of gas. Bjørn Kvamme at the Institute for Physics and technology at the University in Bergen has developed a method for injecting CO2 into hydrates and reversing the process; thereby extracting CH4 by direct exchange .
Methane clathrates and climate change
Methane is a powerful greenhouse gas which, despite its atmospheric lifetime of around 12 years, nonetheless 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, the Paleocene-Eocene Thermal Maximum.
Natural gas hydrates (NGH) vs. liquified natural gas (LNG) in transportation
Since methane clathrates are stable at a higher temperature (−20 vs −162 °C) than LNG, there is some interest in converting natural gas into clathrates rather than liquifying it when transporting it by seagoing vessels. Accordingly, the production of NGH from NG at the terminal would require a smaller plant than LNG would.
Methane clathrates in popular fiction
The book Mother of Storms by John Barnes offers a fictional example of catastrophic climate change caused by methane clathrate release.
Clive Cussler's Fire Ice also mentions methane hydrate. It tells of how a Russian mining industrialist wants to detonate a bomb into three pockets off of the American coast creating large tsunamis. The intent was to swamp Boston, Charleston, and Miami.
In the anime Ergo Proxy, a string of explosions in the methane hydrate reserves wipes out 85% of human life on Earth.
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