Methane hydrates are ice-like formations consisting of water and extremely high concentrations of methane (CH4). They exist around the world, buried in ocean sediments at continental margins or beneath permafrost in the Arctic (Milkov 2004).
Methane hydrates have recently attracted attention because of the enormous energy reserve they represent – one cubic meter of methane hydrate is estimated to contain 164 times the energy of the same volume of methane gas (Kvenvolden 2003). At a time when the utilization of methane from shale formations in the United States has drastically reduced energy costs, there is great potential for methane hydrates as a future source of methane for human use.
At the same time, methane hydrates have drawn concern from environmental scientists as a “tipping point” in the global carbon balance (Archer, Buffett, and Brovkin 2009). Globally, methane hydrates are estimated to contain 500-2500 gigatons of carbon – twice as much as all other methane reservoirs and more carbon than is currently in the atmosphere (Milkov 2004).
Although these interests and concerns may compete in the future, at the present they have together driven significant scientific interest in methane hydrates.
There are three essential requirements for methane hydrate formation: high pressure, low temperature, and high organic carbon deposition.
For methane to form and remain as solid hydrate, the ambient pressure around the methane must be extremely high and the ambient temperature near freezing. Pressure increases with depth beneath the ocean floor as a result of the weight of the overlying sediment and water. However, temperature also increases with depth according to the geothermal gradient (Turcotte and Schubert 2002). The result of these opposing pressure and temperature gradients with depth is that methane hydrate is stable only within a specific depth band, known as the gas hydrate stability zone (GHSZ) (K. A. Kvenvolden 1998).
For methane hydrate to form even within the GHSZ, methane gas must be supersaturated in the sediment pore water (the water filling the space between sediment grains). For high concentrations of methane gas to be present below the sea floor, there must first be large deposits of organic carbon within the sediment. Organic carbon is, in effect, dead phytoplankton and zooplankton. A small fraction of this “detritus” sinks to the ocean floor, and over time is buried by sediments transported from the continents to the ocean floor by rivers (Falkowski 2012). This buried organic carbon is utilized as an energy source by microorganisms known as methanogens, which produce methane gas in the process of methanogenesis. If the source of organic carbon is large enough, as is the case around many continental margins (where phytoplankton productivity is highest), methanogenesis can produce enough methane gas for hydrate formation to occur (Wellsbury and Parkes 2003).
Anaerobic Oxidation of Methane
In sediments shallower than the GHSZ or in sediment pore water under-saturated with methane, hydrate cannot form. Yet methane is still present, as a gas dissolved in the pore water – similar to how carbon dioxide is dissolved in soda. At very shallow depths below the seafloor, however, the dissolved methane disappears. At the same depth, the concentration of dissolved sulfate (SO42-), which is naturally high in seawater at the seafloor, drops to zero. This depth below the seafloor, above which sulfate concentrations are high and below which methane concentrations are high, is known as the sulfate-methane transition zone (SMTZ) (Knittel and Boetius 2009). The depth of the SMTZ below the seafloor can vary from centimeters to tens of meters, depending on the depth at which methanogenesis occurs, the rate at which pore water containing dissolved methane rises towards the seafloor, and the rate at which seawater containing dissolved sulfate sinks into the sediments (Knittel and Boetius 2009).
The disappearance of dissolved methane and sulfate, and thus the formation of the SMTZ, is the direct result of a process known as anaerobic oxidation of methane (AOM). AOM is carried out by a consortium of anaerobic methanotrophs (ANME) and sulfate-reducing bacteria (SRB) that together metabolize methane and sulfate to produce energy (Knittel and Boetius 2009). These microorganisms are the focus of much of the current microbiological research surrounding methane hydrates.
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Falkowski, Paul. 2012. “Ocean Science: The Power of Plankton.” Nature 483 (7387).
Knittel, Katrin, and Antje Boetius. 2009. “Anaerobic Oxidation of Methane: Progress with an Unknown Process.” Annual Review of Microbiology 63 (1): 311–334.
Kvenvolden, K. A. 1998. “A Primer on the Geological Occurrence of Gas Hydrate.” Geological Society, London, Special Publications 137 (1): 9–30.
Kvenvolden, K. A. 2003. “Natural Gas Hydrate: Background and History of Discovery.” In Natural Gas Hydrate, edited by Michael D. Max, 9–16.
Milkov, Alexei V. 2004. “Global Estimates of Hydrate-Bound Gas in Marine Sediments: How Much Is Really out There?” Earth-Science Reviews 66 (3–4): 183–197.
Turcotte, David L. and G. Schubert. 2002. Geodynamics. Cambridge; New York: Cambridge University Press.
Wellsbury, Peter, and R. John Parkes. 2003. “Deep Biosphere: Source of Methane for Oceanic Hydrate.” In Natural Gas Hydrate, edited by Michael D. Max, 91–104.