FROZEN HEAT | Volume 1

here on continental margin settings, which combine the appropriate pressure and temperature conditions for meth- ane hydrate formation with regions of high sedimentation rates and elevated “primary productivity,” which is the rate at which organic carbon is produced in surface waters (Reeburgh 2007). As discussed in Volume 1, Chapter 1, the rapid burial of organic carbon that “rains” to the sea floor can promote microbial breakdown of that organic mate- rial, with methane as a key by-product. At the appropriate pressure and temperature conditions, this methane can be

incorporated into methane hydrates within the sediments, usually at water depths greater than ~500 m. Because the combination of high primary productivity and high organic- carbon burial rate is mostly confined to continental margins (Hedges and Keil 1995; Buffett and Archer 2004), continen- tal margins host most of the world’s gas hydrate while sedi- ments of deep ocean basins are relatively free of methane hydrate, even though the deep-ocean pressure and tempera- ture conditions are suitable for gas-hydrate formation (See Volume 1 Chapter 1).

Methane consumption in the environment et a e c s ti i t e e vir e t

Zone 4

Zone 4

Atmosphere Atmosphere

Methane chemically reacts to form many compounds, including carbon dioxide Aerobic methane oxidation Methane chemically reacts to form many compounds, including carbon dioxide Aerobic methane oxidation

Zone 3

Zone 3

Where methane rises from the sea oor in plumes of bubbles, much of the methane dissolves before reaching the surface Where methane rises from the sea or in plumes of bu bles, much of the methane di solves before reaching the surface

Water column Water column

Aerobic methane oxidation Aerobic methane oxidation

Methane and oxygen are biologically and chemically converted to carbon dioxide Methane and oxygen are biologically and chemically converted to carbon dioxide

Zone 2

Zone 2

Oxygenated sediments Oxygenated sediments

Aerobic methane oxidation Aerobic methane oxidation

Methane and oxygen are biologically and chemically converted to carbon dioxide near the sea oor Methane and oxygen are biologically and chemically converted to carbon dioxide near the sea or

Zone 1

Zone 1

Anoxic sediments Anoxic sediments

Anaerobic oxidation of methane Anaerobic oxidation of methane

Methane dissolved in pore water and sulphate are biologically converted to bicarbonate, hydrogen sulphide and water Methane di solved in pore water and sulphate are biologically converted to bicarbonate, hydrogen sulphide and water

Gaseous methane can bypass the sediment- based bio lter by migrating along permeable paths, such as faults Gaseous methane can bypa s the sediment- based bio lter by migrating along permeable paths, such as faults

Figure 2.3: Methane consumption in the environment. Near sea-floor methane hydrate is being continuously broken down, releasing methane dissolved in pore water. As methane moves through sediment into the water column and atmosphere, it is consumed in a variety of chemically and microbially controlled reactions. As listed on the left, dissolved-phase methane can then be consumed by microbes as part of an extended chemosynthetic food chain (see also Fig. 2.7) or consumed chemically. As shown on the right, gaseous methane can bypass the microbially controlled reactions in the sediment because microorganisms can access only dissolved methane (Treude et al. 2005b; Treude and Ziebis 2010). Methane in bubbles entering the water column tends to dissolve into the water, where it can then be consumed by aerobic microbes. The methane “biofilter” removes much of the methane that would otherwise be transported into the atmosphere. Figure is not drawn to scale. For hydrates in the marine environment, the water depth (Zone 3) would generally be 300-500 metres or more, Zone 2 would be on the order of 1 centimetre thick, and Zone 1 would be on the order of 10 metres thick.

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