Sea Level Rise

METHANE RELEASED TO ATMOSPHERE

METHANE RELEASED TO ATMOSPHERE

Figure 8: Methane release caused by relatively warm sea water that covers cold, hydrate-bearing coastal plain deposits in the Arctic.

Evidence for dissociation of oceanic gas hydrate by pressure reduction seems clearer at present than for the effect of warming. When sea level drops, the pressure at the seafloor and down into the sediments decreases instantaneously. Pressure is dependent on the weight of a column of water and sediment above a spot, and if that changes there is no delay in changing pressure at all depths, as there is in changing temperature. Thus a lowering of sea level, as occurred during buildup of continental icecaps that removed water from the ocean, will immediately reduce the pressure at the base of the GHSZ and cause gas hydrate to dissociate.

SEA LEVEL t

SEA LEVEL 2

BASE HYDRATE

BASE GAS HYDRATE 2

SEA LEVEL t

SEA LEVEL 2

LEVEL

FALL

BASE HYDRATE

BASE GAS HYDRATE 2

ZONE OF GAS HYDRATE WITH FREE GAS

METHANE

METHANE

Figure 9: A drop in sea level can reduce pressure on the gas hydrate, which will cause dissociation. That can introduce water and gas into sediment pore space previously occupied by solid gas hydrate and generate overpressures. The presence of fluids and excess pressures will weaken the sediments and cause sediment slides.

Figure 9: A drop in sea level can reduce pressure on the gas hydrate, which will cause dissociation. That can introduce water and gas into sediment pore space previously occupied by solid gas hydrate and generate overpressures. The presence of fluids and excess pressures will weaken the sediments and cause sediment slides.

The most recent glacial sea level lowering of about 120 m ended about 15,000 years ago, and must have caused significant dissociation of gas hydrate. The process would release of gas and water into a sediment that previously contained solid gas hydrate, which to some extent may have acted as a cement. The products of gas hydrate breakdown, gas + water, commonly occupy greater volume than the hydrate they were derived from, and thus the dissociation will increase the internal pressure in the pore space. Such pressures are called "overpressures", pressures greater than the column of water plus sediment above the spot.

These changes, conversion of solid hydrate to gas + water and generation of overpressures, weaken the sediment significantly and are likely to initiate sediment slides on continental slopes and rises (Figure 9). When the slide takes place, the removal of sediment reduces the load on the sediment that was below it, and thus creates another pressure reduction that may cause further gas hydrate dissociation, resulting in cascading slides. The methane released at the base of the GHSZ can escape to the ocean/atmosphere system when the capping sediment is slid away. Even on relatively flat slopes where slides are not triggered, evidence for buildup of pressure, mobilization of gas + water-rich sediments, and escape of methane, water, and sediment has been interpreted in apparent blowout structures in the Gulf of Mexico and U.S. Atlantic margin.

A seismic profile that shows the structure of such an inferred blowout off South Carolina is shown in Figure 10. Notice that the only strata disturbed are those within or just below the base of the GHSZ; its base is shown by the BSR. The subsidence of the seafloor is significant - more than 100 m over a large area - and the loss of volume has been calculated to exceed 13 cubic kilometers. Such a volume released might have increased the methane content of the atmosphere by 4% on the basis of gas concentrations sampled by drilling in the area. This is only a single event of course. During a sea level drop, we might expect comparable events all over the world, most of which would have occurred on continental slopes and rises where the event would have triggered a slide. Further evidence that gas hydrate processes and landslides are related is provided by a map of the shallow limit of gas hydrate stability compared to the tops of known landslide scars on the U.S. Atlantic margin that is shown in Figure 11.

Landslide Seismic Profile
Figure 10: Seismic profile and interpretation across a blowout and collapse caused by gas hydrate processes at the Blake Ridge, a sedimentary accumulation off South Carolina.

In the long term, methane from the gas hydrate reservoir might have had a stabilizing influence on global climate. When the Earth cools at the beginning of an ice age, expansion of continental glaciers binds ocean water in vast continental ice sheets and thus causes sea level to drop. Lowering of sea level would reduce pressure on sea-floor hydrates, which would cause hydrate dissociation and gas release, possibly in association with seafloor slides and collapse. Such release of methane could increase the greenhouse effect, and cause global warming. Through such a mechanism, global cooling might cause release of a greenhouse gas and result in warming. Thus we speculate that gas

Images Gas Hydrate Global Maps

Figure 11: Map showing locations of the tops of slide scars on the U.S. Atlantic continental margin compared to the depth of the theoretical upper (shallow) limit of gas hydrate stability (Booth et al. 1994). Clustering of slope failures within the zone of gas hydrate stability, just below its upper limit, is considered to be circumstantial evidence for a relation between slumps and gas hydrate processes comparable to those diagramed in Figure 9.

hydrate might be part of a great negative feedback mechanism leading to stabilization of Earth temperatures.

Figure 11: Map showing locations of the tops of slide scars on the U.S. Atlantic continental margin compared to the depth of the theoretical upper (shallow) limit of gas hydrate stability (Booth et al. 1994). Clustering of slope failures within the zone of gas hydrate stability, just below its upper limit, is considered to be circumstantial evidence for a relation between slumps and gas hydrate processes comparable to those diagramed in Figure 9.

At present we are just beginning to analyze the potential effects of this huge reservoir of methane on the global environment and much study is needed to understand the processes and determine which hypotheses are correct.

6. CONCLUSIONS

Gas hydrate is stable at ocean water depths exceeding about 500 m (shallower in the Arctic). Its presence is controlled by temperature, pressure and availability of appropriate gases. Almost all gas hydrate in the world is formed from bacterially generated methane, although other gases can and do form hydrate when they are available. The methane is most easily formed and sequestered as hydrate in the organic-rich, rapidly depositing sediments of the continental slopes and rises. Hydrate has also been identified in abyssal rise sediments well away from terrestrial sediment sources (Auzende et al., 2000).

Gas hydrate may form and be stable in sediments just beneath the sea floor, but temperature invariably increases downward in the sediments, so conditions will change out of the gas hydrate stability range at some depth below the sea floor. Because increased pressure makes gas hydrate more stable, the gas hydrate stability range tends to extend to greater depths below the sea floor at progressively increased water depths. Often the depth to the base of the gas hydrate stability zone (GHSZ) in a restricted region will approximately parallel the sea floor, but that relationship may break down as a result of variability in temperature, chemistry, and pressure, parameters that control gas hydrate stability. Temperature anomalies may result from such phenomena as circulation of warm fluids upward through faults, presence of salt domes (salt has a high thermal conductivity), or landslides that remove shallow, cooler sediments. Chemical changes that effect hydrate stability include variations in gas composition and salinity changes in pore water. Pressure changes can result from sea-level movement, sediment slides, or tectonic forces that cause vertical movements of the sea floor.

High concentrations of gas hydrate (possible places where methane might be extracted for energy) seem to occur where gas becomes trapped beneath a seal formed at the base of the GHSZ. We infer that the trapped gas nourishes the region above and creates zones of rich gas hydrate concentrations.

Methane is a potent greenhouse gas and methane from the gas hydrate reservoir may have escaped to the atmosphere and affected climate in the past. Some evidence suggests that the methane probably escaped to the atmosphere as a result of seafloor collapses and sediment slides. These mass sediment movements most likely were generated by dissociation of gas hydrate that weakened the sediment by generating overpressures and introduction of additional water and gas where a solid gas hydrate previously existed.

The potential significance of gas hydrate in ocean sediments to energy resources, climate, and seafloor stability are compelling reasons to continue our study of gas hydrate in the marine setting.

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