Methane clathrate, methane hydrate or methane ice - (n.)
In the presence of high concentrations of certain gases in the water, at low temperatures and high pressures, "clathrates" can form: open-structured water ice hosting gases such as methane, carbon dioxide or hydrogen sulfide. Methane clathrate is such an icy compound where the trapped gas is methane. One unit of ice, by volume, can contain as much as 164 units of methane gas, by volume. Methane ice is present in permafrost regions in Siberia and North America and is widespread on the seafloor in the vicinity of continents, below regions of high productivity, at depths of more than 300 m and temperatures near freezing. When methane is formed by bacterial decay within organic-rich sediments in, it rises to escape into the water. As it rises, it enters into a colder zone, since heating within the Earth produces a temperature increase downward within the sediment. When the stability zone (lying somewhat below the seafloor within the sediment) for methane clathrate is reached it forms, thereby trapping the gas underneath. This typical sequence — a layer of gas-rich sediment below a layer of ice-rich sediment — provides a strong reflector for sound. When sending sound waves from a ship and listening to the return, within the sediment (a technique called "acoustic profiling") this strong echo layer is readily recognized, as a "bottom-simulating reflector" or "BSR." In this fashion, methane ice distribution has been mapped worldwide. In places, methane ice has been recovered by dredging or drilling, and thus its properties are quite well known. Methane clathrate can also be made in the laboratory.
Methane ice may be involved in the fluctuations of atmospheric methane seen in polar ice cores. From this record, it is known that methane rose rapidly whenever climate changed from glacial to interglacial conditions (during "deglaciation"). Warming of water bathing the seafloor could have led to large-scale release of methane from the melting of methane ice. Evidence for such a process is seen on the floor of the Barents Sea, which is the shelf sea north of Norway and forms part of the Arctic. Fields of giant craters have been detected within that sea off the coast of Norway, in a region rich in methane clathrate deposits. The biggest of the craters was measured as 700 m wide and 30 m deep, indicating catastrophic explosions of methane. It is thought that these craters were formed during deglaciation. Direct measurements show that large amounts of methane can escape from the Sea of Okhotsk on occasion, where the seafloor is rich in organic matter and harbors methane ice. Pressure is increased on the seafloor during deglaciation (from the rise in sea level). Thus, if the seafloor is the source of the methane increase seen in ice cores, a marked rise in temperature must be responsible for release of methane. This would imply that intermediate waters must have been considerably colder than now, during glacial time. Also, the contribution of large amounts of methane would provide an additional source for the increase of carbon dioxide observed during deglaciation. Evidence for catastrophic release of methane has been found in the more distant geologic record as well, within deep-sea sediments, for a period at the end of the Paleocene about 55 million years ago. The event led to widespread extinctions in the fauna living on the deep-sea floor.
Some good references on methane clathrates: Flammable ice. by E. Suess, G. Bohrmann, J. Greinert and E. Lausch. Scientific American, 281 (5) 76-83, 1999; Probing Gas Hydrate Deposits. By Robert Kleinberg & Peter Brewer. American Scientist, Vol. 89 (No. 3) 2001; Gas hydrates: relevance to world margin stability and climate change. Edited by J.-P. Henriet and J. Mienert. Geological Society Special Publications, vol. 137, 1998; Potential effects of gas hydrate on human welfare. by K. A. Kvenvolden, in Proceedings of the National Academy of Sciences USA, Vol. 96, 3420-3426, 1999.