Clathrates are ice-like solids composed of water molecules that trap other substances, such as methane, in a cage-like structure. They are found naturally in large deposits under the seabed and permafrost regions. While they could be a potential energy source, their extraction could be uneconomical and pose serious dangers, including landslides and the acceleration of global warming.
A clathrate is a type of hydrate, or water compound, in which molecules of another substance are trapped inside a cage-like structure made up of water molecules. The trapped molecule is usually a gas at normal pressure and temperature. Clathrates are ice-like solids that generally form at high pressures and low temperatures. Among the best known and most studied is a methane hydrate which is found naturally in large deposits under the seabed in many parts of the world. These can be a potential source of energy, but there is also concern that a sudden release of large amounts of methane clathrate, perhaps caused by global warming, could be disastrous.
The basic unit of most clathrates is a dodecahedron composed of 20 water molecules arranged to form 12 pentagonal faces, with a hollow center that can be occupied by a “guest molecule”. The structure is essentially held together by hydrogen bonds between the water molecules, but stabilized by the guest molecules. Since dodecahedrons cannot be packed together to fill all available space, other polyhedral shapes also occur, to form a lattice. Because of this variation in cage shapes and the fact that not all cages are necessarily occupied, the clathrates cannot be given precise chemical formulas. The guest molecules of the clathrate can be hydrocarbon gases, such as methane or ethane, oxygen, nitrogen and carbon dioxide.
Methane hydrate is the clathrate compound that has attracted the most interest. This compound is found in large quantities in various locations along the edges of all continents and in the permafrost regions of Siberia and Alaska. It has been estimated that these deposits constitute the largest hydrocarbon reserve on the planet, far exceeding known reserves of coal, oil and natural gas. They are thought to have formed from methane produced by microbial activity under anaerobic conditions in sediments just below the surface of the seafloor or on land where temperatures are low enough. Even in tropical regions, seafloor temperatures are low enough for clathrates to form, where pressure allows them to solidify a few degrees above freezing.
Given the large quantities of methane stored in these fields, they have been considered as a potential source of natural gas. However, there could be serious technical difficulties in its extraction that make it uneconomical. During the 1960s and 1970s, the Soviet Union made several unsuccessful attempts to extract gas from the clathrate deposits of the Siberian permafrost. There are also concerns that the methods used to release the trapped gas could destabilize the deposits, which could lead to subsidence and landslides.
While clathrate deposits could represent a huge untapped energy resource, they could also pose a serious danger. They are not stable outside the temperature and pressure conditions in which they occur and it is feared that global warming could make them unstable. This poses a double threat.
First, melting of clathrate ice mixed with sediments at continental margins could lead to massive landslides and subsequent tsunamis. There is evidence from the relatively recent geological past that this could have happened off the coast of Norway. Second, methane is a potent ‘greenhouse’ gas that traps even more heat in the atmosphere than carbon dioxide. The sudden release of huge amounts of this gas could accelerate global warming, which in turn could cause further destabilisation. Again, there is geological evidence that this may have occurred in the past through natural processes, and as of 2011, there is particular concern about methane hydrate in permafrost deposits.
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