Methane hydrates - energy source, climate control and ice worms!

Methane hydrates are ice-like mixtures of water and methane, in which the methane molecules are trapped inside three-dimensional structures of water molecules. These types of structures are known as clathrates. An example is shown on the diagram to the left. A methane molecule is held within each dodecahedral space within a framework of water molecules.

Methane hydrates are stable at pressures which are higher than atmospheric pressure, and at temperatures which range up to around 20° C. The stability curve is shown as the dashed blue line on the diagram below (left). At 0° C methane hydrates will form where the pressure is equivalent to that of approximately 250 m of water. At 10° C hydrates will form where the total depth of water and sediment is around 1000 m. An example of the variation in water and sediment temperature with depth is shown as a red solid line on the diagram. From the diagram it may appear that methane hydrate is stable in deep ocean water, but in fact the level of methane in the water is too low for hydrates to form. Within the sediments the temperature increases quite significantly with depth, and at around 500 m below the sea floor the temperature becomes too warm (> 25° C) for hydrates to be stable - even at high pressure. As shown below (right) methane hydrate is stable within the upper few hundred metres of the sea-floor sediment in areas of the ocean where the water depth is greater than 300 m (greater depths where the water is very warm).

Methane hydrates are also stable a few tens of metres below surface in permafrost areas.

Within sediments the methane can either be produced in situ by bacterial consumption of organic material⁽¹⁾, or it can migrate upward from underlying gas-producing sediments and sedimentary rocks, in the same manner as conventional gas reserves. The methane hydrate acts like a cement, and such cemented layers can be several hundreds of metres thick. In some cases the upper surface is situated below the sea floor, but in other cases the hydrates exist right on the sea floor.

There is a lot of methane hydrate around. The US Geological Survey conservatively estimates that the methane in sea-floor and permafrost hydrates represents an amount of energy which is at least twice that of all coal, oil and conventional gas reserves put together. At present there is no suitable technology for recovering sea-bed methane hydrates, but government agencies (eg. USGS, Geological Survey of Canada and others) and energy corporations are scrambling to map out the distribution of methane-bearing areas and the magnitude of the resource. We can only hope that they don't rush it, because the ecological implications of getting it wrong could be huge - both in the areas where the extraction is taking place, and globally.

Although we have only known about the extent of methane hydrate deposits for a few years, it is likely that they have existed for as long as there have been oceans (ie. for most of geological time). It is also likely that they have played a significant role in affecting the earth's climate. When methane is released from the hydrate form some remains as methane, but most of it reacts with oxygen to form carbon dioxide. Both methane and carbon dioxide are greenhouse gases, and releases of methane can affect the earth's climate. There is strong evidence that this has happened in the past. The two papers listed below refer to a particularly strong input of carbon into the atmosphere at the end of the Paleocene Epoch (55 m.y. ago), and show evidence that much of this was a product of the breakdown of methane hydrates. The beginning of the Eocene is marked by the sudden diversity of mammalian organisms, including the appearance of primates, as well as by the extinction of some marine species.

Large amounts of methane from hydrates can be released to the atmosphere in several ways.

• Firstly, if there is a rise in ocean-water temperature (because of a general change in the climate or because of a change in ocean currents) the sea-floor sediments will also warm up and some of the hydrates will become unstable - releasing methane. (see the paper by Katz et al.)
  

• A global increase in temperature might also cause some permafrost hydrates to break down, both because of higher temperatures, and because higher water levels would casue flooding of some near-coastal permafrost regions.
  
• Global temperatures would be generally low during a period of glaciation, but the large amount of glacial ice would lead to a drop in ocean water levels. This could cause the breakdown of the sea-floor hydrate in areas where the water depth decreased to less than 500 m.
  

As noted above, there is a relationship between hydrates and methanogenic bacteria. It is also likely that certain bacteria as well as other microorganisms live within and consume methane hydrate deposits. It has been discovered only recently, however, (1997) that larger organisms, including polychaete worms thrive in methane hydrate deposits. It is likely that these "ice worms", which are 2 to 5 cm in length, live off the smaller organisms within the hydrate.

(image source: Penn.State University, see website link below)

References:

Norris, R. and Rohl, U., Carbon cycling and chronology of climate warming during the Paleocene/Eocene transition, Nature, V. 401, p. 775-778 (October 1999)

Katz, M., Pak, D., Dickens, G. and Miller, K., The source and fate of massive carbon input during the latest Paleocene thermal maximum, Science, V. 286, p. 1531-1533, (November 1999)

There is a lot of stuff about methane hydrates on the internet, including the following:

USGS: http://marine.usgs.gov/fact-sheets/gas-hydrates/title.html

G.S.C.: http://sts.gsc.nrcan.gc.ca/page1/hydrat/hydrates.html

Energy Research Clearing House: http://www.hydrate.org/

Ice worms: http://www.science.psu.edu/alert/iceworms.htm

1. A bacterium known as Methanogenium frigidum produces methane out of carbon and hydrogen under these conditions.

Steven Earle, 1999. Return to Earth Science News