Methane Degassing

Methane is a potent greenhouse gas, 30 times stronger per molecule than CO2, and it is produced during both the short-term and long-term carbon cycles. In the (prehuman) short-term cycle, it is produced mainly from wetlands and animal exhalation such as from bovids and termites. (These organisms have bacteria located in their digestive systems that break down carbohydrates to methane.) In wetlands and other water-logged, organicrich sediments, methane forms from a variety of microbial processes and chemical pathways, but the overall reaction can be simplified as

Because methane is readily oxidized by dissolved O2 (see table 4.2), it cannot accumulate in the presence of O2, and that is why appreciable methane is found only in water-logged sediments where all dissolved O2 has been previously removed by microbial respiration. Also, methane forms preferentially in fresh water sediments, as opposed to marine sediments, because methanogenic microorganisms are outcompeted for organic substrate by other microbes using dissolved sulfate as an energy source (Claypool and Kaplan, 1974), and marine sediments are rich in interstitial sulfate. As a result, biogenic methane formation occurs in marine sediments only after the removal of O2 and sulfate by other microbes (Claypool and Kaplan, 1974; Froelich et al., 1979). The order of the pathways by which organic matter decomposition takes place follows changes in free energy (table 4.2).

In the long-term carbon cycle most methane is produced at depth in sediments by both biogenic and abiogenic organic matter decomposition. At low temperatures biogenic decomposition occurs via the same reaction as (4.4). In buried freshwater organic-rich sediments, such as peat, biogenic methane produced at depth can readily move upward through the sediment and escape to the overlying water and, if not oxidized to CO2 by O2 in the water, eventually to the atmosphere. By contrast, in buried marine sediments upward-diffusing methane is oxidized by reaction with interstitial dissolved sulfate (Martens and Berner, 1977; Reeburgh and Heggie, 1977; Valentine et al., 2002), which is essentially absent in the interstitial waters of freshwater sediments. Because the sulfate concentration in seawater is so high (> 100x the average for fresh waters), methane flux out of marine sediments is limited. The free energy yield for CH4 reacting with sulfate under sedimentary conditions shows that this reaction is thermodynamically favored (table 4.2).

Table 4.2. Some major microbially mediated organic matter decomposition reactions with standard-state free-energy change, aG0, and free-energy change for typical activities of gases and dissolved species in organic-rich marine sediments during early diagenesis, aG*.

Reaction

aG° (kJ/mol)

aG*

CH2O + O2 ^ CO2 + H2O

-475

-491

2CH2O + SO4-2 ^ H2S + 2HCO3-

-77

-110

2CH2O ^ CO2 + ch4

-58

-44

CH4 + 2O2 ^ CO2 + 2H2O

-581

-594

CH4 + SO4-2 + CO2 ^ H2S + 2HCO3-

-14

-25

Activity for CH4 assumed to be saturation with gas at 25°C; CH2O represented by sucrose. Data partly from Berner (1971).

Activity for CH4 assumed to be saturation with gas at 25°C; CH2O represented by sucrose. Data partly from Berner (1971).

By comparison with biogenic methane, abiogenic methane forms at much greater depths due to the thermal decomposition of the organic matter surviving bacterial decomposition. This remaining organic matter is normally referred to as kerogen. As kerogen matures, it loses CH4, CO2, and H2O (Durand, 1980) and trends toward pure carbon (graphite) at the highest metamorphic temperatures (several hundreds of degrees Celsius). Thermal maturation of kerogen gives rise to the formation of coal, oil, and natural gas (CH4), depending on the temperature, time of burial, and the nature of the starting organic material. If the natural gas can move into fractures leading to the earth's surface, it may escape into the atmosphere.

Another minor source of abiogenic methane is at mid-ocean ridge (MOR) hydrothermal vents (e.g., Welhan and Craig, 1979). The methane apparently results from the reaction of CO2 with H2 formed by the reduction of water accompanying iron mineral oxidation. Although this reaction has been found to be associated with the overall process of the serpentinization of ultramafic rocks, additional methane can form from the hydrothermal alteration of MOR basalts (Welhan, 1988).

Even though methane is a potent greenhouse gas, it does not attain levels in the present atmosphere sufficiently high enough to overshadow warming due to CO2. This is also likely for most of the Phanerozoic. If the relative abundance of coal swamps over time can be assumed to represent the relative rate of input of methane to the atmosphere (Berner and Mackenzie, unpublished ms), and levels of atmospheric O2 have not dropped below half the present level (Chaloner, 1989), then past levels of atmospheric CH4 could not have been high enough to dominate greenhouse warming. This is because CH4 is rapidly oxidized in the atmosphere by O2 to CO2 (mean residence time of about 10 years). Thus, the idea that over many millions of years high levels of atmospheric CH4 brought about sustained global warming in the past is difficult to maintain.

Methane can arise from yet another source. This is via the breakdown of methane hydrates. At sufficiently low temperatures and high pressures, methane will react with water to form crystalline hydrate phases known as clathrates. Because they are thermodynamically unstable under earth surface conditions or at great depths when termperatures become too high, the clathrates are found only at intermediate depths in marine and nonmarine sediments. The stability of the hydrates in seawater is shown in figure 4.3. For clathrates occurring in the uppermost portions of sediments this means water depths of 200 m at 0°C and 1000 m at 12°C. However, most clathrates are found at considerable depth in the sediment column. The depth range depends on gas concentration and the available pore space within certain stability limits. Conditions neccessary to stabilize hydrates in marine sediments are pressures of 3-5 MPa at 3°C and 8-12 MPa at 11°C (0stergaard et al.,

CH4 + water

Pressure Mpa

Figure 4.3. Pressure-temperature stability of methane hydrate in seawater. (Data from Peltzer and Brewer, 2000.)

2002), with the range of pressures representing in each case the effects of fine particles in stabilizing the hydrates. Potentially, gas hydrates can occur between the seafloor and a locus of sub-bottom depths where geothermal gradients intersect gas-gas hydrate-pore water equilibrium curves (Dickens, 2001). Important controls are gas composition, water activity, bottom water temperature, geothermal gradient, and water depth. This means that the clathrates are found either near the sediment surface in cool and moderately deep-water marine sediments or within a limited depth range in the sediment column. Occurrences in marine sediments are worldwide, and they are also found buried in thick permafrost soils in polar regions (Kvenvolden, 1993, 2002).

Because methane reacts with both dissolved O2 and sulfate, both must be absent for CH4 to build up to saturation with clathrates in marine sediments. This is not normally a problem in organic-rich sediments. However, upward moving methane gas, released from hydrates, must travel through an overlying zone where it can be removed by bacterial sulfate reduction (Reeburgh, 1983; Valentine et al., 2001). Also, the methane must pass rapidly through overlying oxygenated waters to avoid further removal via oxidation before it can be transferred to the atmosphere (Dickens, 2000; Valentine et al., 2001). The best places for clathrates to form so that their potential decomposition can give rise to a flux of CH4 out of the sediment is at shallow sediment depths in unusually organic-rich sediments, where sulfate reduction is very intense and all interstitial sulfate is removed over a short depth range. However, the sediments still have to be sufficiently cold and under sufficiently deep water for clathrates to have formed in the first place.

What triggers methane hydrates to break down to methane gas which escapes to the atmosphere? Some possible causes are submarine landslides, sea level changes, and changes in ocean circulation (Bice and Marotzke, 2002). The sudden release of large quantities of methane from clathrates has been used to explain several short-lived events during Phanerozoic history. This includes the late Paleocene/Eocene thermal maximum (e.g., Dickens et al., 1997), Mesozoic oceanic anoxic events (e.g., Hesselbo et al., 1990; Beerling et al., 2002; Bralower et al., 2002), an early Cretaceous episode of atmospheric CO2 increase (Jahren et al.,

2001), the Permo-Triassic extinction (Krull and Retallack, 2000; McLeod et al., 2000), and the Triassic-Jurassic extinction (Beerling and Berner,

2002). The evidence in the sedimentary record for sudden methane release is a rapid decrease in the carbon isotopic composition of sedimentary CaCO3 and organic matter. This comes about because methane is unusually light; in other words, it is highly depleted in 13C (average 813C values are -60%o to -80%o). After a large mass of isotopically light methane is released to the atmosphere, it is oxidized rapidly to CO2 and then cycled by weathering and sedimentation to form unusually light carbonates and organic matter that are deposited in sediments. No other reasonably sized source of light carbon has yet been suggested to explain large, short-term negative isotope anomalies (see chapter 5, figure 5.15). Besides negative anomalies, some positive anomalies may be due to methane hydrates. It has been suggested that some positive excursions in the geologic record may have been caused by the formation and storage of methane hydrates rather than the burial of excess organic matter (Dickens, 2003).

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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