Book 2.indb - US Climate Change Science Program

Abrupt Climate ChangeBox 5.1. Chemistry, Physics, and Occurrence of Methane HydrateA clathrate is a substance in which a chemical lattice or cage of one type of molecule traps another type ofmolecule. Gas hydrates are substances in which gas molecules are trapped in a lattice of water molecules(Fig. 5.3). The potential importance of methane hydrate to abrupt climate change results from the fact that largeamounts of methane can be stored in a relatively small volume of solid hydrate. For example, 1 cubic meter(m 3 ) of methane hydrate is equivalent to 164 m 3 of free gas (and 0.8 m 3 of water) at standard temperature andpressure (Kvenvolden, 1993). Naturally occurring gas hydrate on Earth is primarily methane hydrate and formsunder high pressure–low temperature conditions in the presence of sufficient methane. These conditions aremost often found in relatively shallow marine sediments on continental margins, but also in some high-latitudeterrestrial sediments (Fig. 5.4). Although the amount of methane stored as hydrate in geological reservoirs isnot well quantified, it is very likely that very large amounts are sequestered in comparison to the present totalatmospheric methane burden.The right combination of pressure and temperature conditions forms what is known as the hydrate stability zone,shown schematically in Figure 5.5. In marine sediments, pressure and temperature both increase with depth,creating a relatively narrow region where methane hydrate is stable. Whether or not methane hydrate formsdepends not only on temperature and pressure but also on the amount of methane present. The latter constraintlimits methane hydrate formation to locations of significant biogenic or thermogenic methane (Kvenvolden, 1993).When ocean bottom water temperatures are near 0 ºC, hydrates can form at shallow depths, below ~200 mwater depth, if sufficient methane is present. The upper limit of the hydrate stability zone can therefore be atthe sediment surface, or deeper in the sediment, depending on pressure and temperature. The thickness ofthe stability zone increases with water depth in typical ocean sediments. It is important to note, however, thatmost marine methane hydrates are found in shallow water near continental margins, in areas where the organiccarbon content of the sediment is sufficient to fuel methanogenesis. In terrestrial sediments, hydrate can format depths of ~200 m and deeper, in regions where surface temperatures are cold enough that temperatures at200 m are within the hydrate stability zone.Figure 5.3. Photographs of methane hydrate as nodules, veins, and laminae in sediment.Courtesy of USGS (http://geology.usgs.gov/connections/mms/joint_projects/methane.htm).169