In this section:
|What is gas hydrate?|
|Why are gas hydrates important?|
|Gas hydrates and climate change|
|Gas hydrates in the geological past|
|Gas hydrates as a geohazard|
|Determining past climatic conditions from sediment cores|
|References and further reading|
What is gas hydrate?
Gas hydrate is a clathrate - a chemical substance consisting of a lattice of one type of molecule trapping and containing a second type of molecule (pictured right). It is formed under very specific temperature and pressure conditions when molecules of methane become physically trapped within water ice. In the deep seabed, vast amounts of methane produced by the decay of organic matter and as a by-product of microbial activity. As the methane rises towards the seafloor, it reaches a zone known as the Gas Hydrate Stability Zone (GHSZ), where the pressures and temperatures are right for the methane to become trapped as gas hydrate. Molecules of water ice within the marine sediment trap methane and form a 'cage' by freezing around the methane molecule, thus forming gas hydrate.
The Gas Hydrate Stability Zone represents a fixed range of pressure (P) and temperature (T) conditions under which gas hydrate is stable. The depth below the seabed at which gas hydrate forms is completely controlled by the P/T conditions so in some areas this means that gas hydrate occurs at great depth below the seafloor, and in other places - where the temperatures are low enough - hydrate can occur at or just below the seabed. In all cases, gas hydrate will only form if there is a sufficient supply of methane.
In marine sediment, gas hydrate usually appears as an white, ice-like substance that often forms lumpy crystals or layers within the sediment. When gas hydrate is brought to the surface, the reduction in pressure and increase in temperature causes the clathrate structure to break down (dissociate), releasing the methane from its 'cage' and allowing it to escape as gas. It is this escaping gas that can be ignited to make the ice appear to be on fire.
Gas hydrates are known to exist (where the conditions are right) all over the world, both in the marine environment and in permafrost areas on land. Because they contain methane (CH4), the world's gas hydrate deposits represent a significant reservoir of carbon, not least because many gas hydrate deposits are underlain by natural gas (methane) deposits. Gas hydrates may represent a future source of fuel if an efficient and safe method of extracting them can be developed. However, gas hydrates are more of a concern to scientists in terms of their potential as a geological hazard, and how they would respond to changes in climate.
The physical conditions under which gas hydrate is stable are very restricted. Changes in pressure and/or temperature can cause the clathrate structure to break down, resulting in the release of methane. If global temperatures were to rise, and the water at the bottom of the sea were to get warmer, this might cause any gas hydrate in the region to become unstable. If the gas hydrate were to break down, it is possible that large quantities of methane could be released. At present, it is unknown how much of this methane would be released into the atmosphere, and how much of it would remain dissolved in the water column. Methane is a well-known and extremely potent greenhouse gas (25 times more powerful than CO2), so the release of large quantities of it into the atmosphere could accelerate global warming dramatically. Some scientists (Archer & Buffett 2005) have predicted that a 3ºC rise in global temperatures would release around 4000 Gt of carbon (mostly as methane) into the atmoshere from gas hydrate breakdown.
The Arctic is a region that is particularly sensitive to climate change. Most climate models predict a rapid increase in Arctic air and surface water temperature over the coming decades. We already know that over the past decade, the Arctic has been warming up twice as fast as the global average. This is caused by several factors, mainly 1) the lowering of the albedo effect (reflection of the sun's energy) as the ice mass at the poles is reduced through melting; 2) less trapped energy being consumed by evaporation; 3) thinning of the atmosphere, allowing more of the sun's energy to affect the surface; 4) the heat exchange between ocean and atmosphere will be faster due to less sea ice. As the sea surface waters warm up, this will in turn lead to warming of the oceanic bottom waters and affect the stability of gas hydrates where the physical conditions allow them to exist close to or at the seabed. In the Arctic, gas hydrates that could be affected by this scenario lie in around 350m water depth - shallow enough to be affected by increases in the surface water temperature alone.
Gaseous methane can be released from hydrate in sediment beneath the seabed, if a rise in temperature or a reduction in pressure causes the zone in which the hydrate exists to move outside the hydrate stability zone. The released gas will either migrate back into the stability zone and form hydrate again, if the stability zone still exists above the gas, or migrate upwards to the seabed if it is not trapped by geological structures along the way. The hydrate that is most likely to release gas into the ocean and atmosphere is that which exists in areas where the hydrate stability field is removed completely by the change in conditions (see cartoon below). Its escape to the seabed is likely to be through features that are expressed as pockmarks at the seabed, and by destabilisation of the seabed in form of slumps or slides at different scales - see the section below on geohazards for more detail. In deeper water, the change in conditions does not remove the hydrate stability field completely, but thins it, releasing gas from its base. This gas may migrate back into the hydrate stability zone and reform hydrate, but there is evidence from localities such as offshore Vancouver and the Storegga area that gas may escape through chimney-like structures that cut through the hydrate stability zone. Also, some gas may migrate upslope beneath the base of the stability zone to where it intersects the seabed.
Climate warming in the Arctic will have the immediate effect of increasing sea levels as the ice cap melts, resulting in increased pressure at the seafloor due to the extra mass of water. This will cause the base of the GHSZ (the deepest point in the seabed at which gas hydrate can form) to move downwards relative to the seafloor to a zone where the temperature conditions are warmer (due to the geothermal gradient) and gas hydrate is stable again under a regime of higher pressures. However, as climate warming continues the seafloor will also start to gradually warm up, allowing the base of the GHSZ to migrate upwards towards the surface again. Eventually, after several hundred years or more, the base of the GHSZ could reach the seafloor and gas hydrate dissociation could take place. As well as causing the GHSZ to move up and down relative to the seafloor, climate change may also cause the GHSZ to thin or thicken, depending on the magnitude and speed of the changes in physical conditions. The overall effect of these changes is that methane may be released from gas hydrate over a prolonged period of time due to the fluctuations in the position of the GHSZ, or that the major release of methane via gas hydrate dissociation occurs several hundred years after the onset of climatic warming.
One way to determine how Arctic gas hydrates might react to future global warming is to look at how they behaved at the end of the last ice age, when global warming produced widespread ice melt, sea level rise and elevated global temperatures. By taking samples of seafloor sediments (sediment cores), the climatic history of the region can be reconstructed. We already know that periods of time in between ice ages (the interglacials) experienced elevated levels of methane in the atmosphere - this has been determined by looking at the isotopic ratios of carbon in microfossils found in seafloor sediments of a known age. We can also use marine microfossils to work out the temperature of the seawater at given intervals in the past, how quickly the seawater temperatures and sea levels rose following the end of the ice age, and how closely this relates to elevated levels of atmospheric methane.
At present, the hydrate stability zone in the area of interest exists beneath the seabed in water depths greater than ~400 m. However, at the end of the last glacial the lower water temperature enabled the stability zone to exist in water depths shallower than ~300m. Simplistically, this would imply that all the hydrate beneath the seabed in this depth range has dissociated to methane gas and water. However, this is complicated by lower global sea level in the last glacial and by the effect of the load of the ice sheet on the land and continental shelf which depressed the seabed of the adjacent continental slope, because of the flexural rigidity of the lithosphere. The effect of change in water depth (increase in pressure) upon the hydrate stability zone is immediate, but a change in temperature at the seabed takes time to propagate down through the sediments and so lags behind the effects of a change in water depth. An increase in water depth, accompanying or preceding an increase in temperature, will further delay the effect of the temperature increase upon the hydrate stability zone, and, if the temperature change is not great enough, can suppress its effect entirely.
Gas hydrates as a geohazard
Since hydrates prevent sediment compaction, their in situ dissociation related to climatic change has been suggested as an important factor in creating weak sediment layers, along which sediment failure can be triggered (McIver, 1982), after which the methane released from the hydrate reservoir into the water column and eventually into the atmosphere could contribute to further climate change.
Gas hydrate binds together sediment particles in the seabed, making the sediment stronger - a bit like cement holding a brick wall together. If the hydrate breaks down it will weaken the sediment and may cause submarine landslides. This in turn could release methane into the atmosphere. Submarine landslides can cause tsunamis and catastrophic flooding of coastal areas.
A key part of this project involves looking past climatic changes to find out how this has affected the stability of gas hydrate. To do this, we need a way of determining the seawater temperature in the study area at given periods in the geological past. The seafloor is covered in fine sediment that accumulates over time, forming layers. Logic dictates that (assuming the pile of sediment is undisturbed), the layer of sediment on the seafloor surface is the youngest, with the sediment becoming progressively older as we move downwards. So as we move down through the pile of sediment, we are effectively moving backwards through time. This, combined with carbon dating techniques, can enable scientists to date - in both relative and absolute terms - a particular interval or layer within the sedimentary sequence.
Marine sediment comprises mineral grains, fragments of rock and organic matter and - most importantly - the remains of microscopic organisms such as foraminifera. Foraminifera are tiny creatures that live either in the water column (planktonic) or on the seabed (benthic). They secrete a calcareous shell for protection, which is preserved in the seafloor sediment long after the organism inside has died. These shells are composed of a mineral called calcite (CaCO3), which can also contain variable (but small) amounts of magnesium. The source for the elements that make up the shell are seawater - during its lifetime, the foraminifer extracts calcium, magnesium, carbon and oxygen from the seawater to make its calcareous shell.
The chemical make-up of the foraminifera shells is key in determining past climatic conditions. Oxygen and carbon exist in nature in various stable isotopic species. There are three stable isotopes of oxygen: 16O, 17O, and 18O, with relative natural abundances of 99.76%, 0.04% and 0.20%, respectively. Because of the higher abundances and the greater mass difference between 16O and 18O, research on oxygen isotopic ratios normally concerns 18O/16O ratios (denoted as ∂18O as a comparison to a known standard). Carbon occurs as two stable isotopes: 12C and 13C, with relative natural abundances of 98.89% and 1.11%, respectively.
In seawater, the behaviour of oxygen isotopes (and therefore the isotopic composition of oxygen in seawater) is determined by their mass: the lighter isotope 16O preferentially partitions into water vapour during evaporation whilst the heavier 18O isotope remains in seawater. During condensation and precipitation (i.e., rain or snowfall), the opposite occurs: heavier 18O oxygen condenses into the liquid water whilst 16O preferentially remains in vapour. As the hydrological cycle is intimately linked to climatic conditions, so the oxygen isotope composition in seawater (and therfore the calcareous shells of foraminifera) reflects the climatic conditions of the time.
During interglacial periods, the evaporation-precipitation cycle generally maintains an overall balance in oxygen isotope ratios in seawater. However, during glacial periods the oceans become increasingly depleted in 16O and enriched in 18O as precipitation (snow/rain) enriched in 16O falls on land and becomes trapped in ice sheets, preventing its return to the sea (see cartoons, left) and causing a global fall in sea level. As a result, you would expect foraminfera shells from animals that were alive during glacial periods will contain a higher proportion of 18O than the same animals living during an interglacial period. However, this picture is complicated by the way in which the different isotopes of oxygen are taken up during the formation of calcite during shell growth. Temperature plays a key role in this, with 18O becoming more preferentially partioned into calcite as temperatures decrease. Whilst this does not severely affect oxygen isotope records in benthic forams, where temperature fluctuations in the deep sea are a matter of only 1 or 2ºC, for planktonic foraminifera it can have huge implications - forams living in the upper, warmer parts of the water column can contain quite different ∂18O values to forams living in deeper, colder waters during the same period in time.
As glacial periods come to an end and the ice sheets melt, the meltwater that returns to the ocean brings an increase in 16O, thus eventually returning the seawater to a more 'balanced' isotopic composition.
The concepts outlined above are a very simplistic view of how oxygen isotopes are distributed in the ocean. In reality, the picture is further complicated by regional mixing of water masses from different sources, the proximity of ice sheets or rivers, regional climatic regimes (i.e., temperature and frequency of rainfall) and a host of more localised effects, many of which are still only partially understood. However, the use of oxygen isotope analyses in foraminifera have allowed palaeoceanographers to develop a global oxygen isotope stratigraphy that reflects the patterns of climate change and sea level variations (as a result of ice volume) in the geological past. Data from deep-sea benthic forams are particularly useful because they are insulated from large seasonal, latitudinal, and geographical variations in temperature and salinity and therefore are more representative of global change.
Archer, D. & Buffett, B.A. (2005) Time-dependent response of the global ocean clathrate reservoir to climatic and anthropogenic forcing. Geochemistry Geophysics Geosystems 6, Q03002, doi 10.1029/2004GC000854.
Chand, S. & Minshull, T.A. (2003) Seismic constraints on the effects of gas hydrate on sediment physical properties and fluid flow: a review. Geofluids 3, 275-289.
Lear, C.H., Elderfield, H. & Wilson, P.A. (2000) Cenozoic Deep-Sea Temperatures and Global Ice Volumes from Mg/Ca in Benthic Foraminiferal Calcite. Science 287, 269-272. DOI: 10.1126/science.287.5451.269
McIver, R.D. (1982) Role of naturally occurring gas hydrates in sediment transport. AAPG Bulletin 66 (6), 789-792.
Maslin, M., Owen, M., Day, S. & Long, D. (2004) Linking continental-slope failures and climate change: Testing the clathrate gun hypothesis. Geology 32(1), 5356.