This section is divided into 3 parts; Marine Sediments,
Gas Hydrates, Marine
Sediments II
>Tetrahydrofuran
(THF) gas hydrate crystal synthesized in the laboratory of FRP member Dr. J.
Carlos Santamarina - from Georgia Tech.
Petroleum refers to any naturally occurring hydrocarbon found beneath the Earth's surface. It forms from when organic carbon is trapped in sediments and buried. Humans for millennia have exploited hydrocarbons. For example the Babylonians used the solid form of hydrocarbon known as asphalt to pave roads and seal boats and the Egyptians used tar to embalm mummies. The Greeks used condensate, which is a clear volatile form of oil, as the main ingredient in their "Secret Weapon of Byzantium" also know as "Greek Fire" which they used to decimate Turkish navies for five centuries. Around 500 BC the Chinese used natural gas to light their imperial palaces.
The
Ancient Greek Navy decimated the Turks with a secret weapon known as Greek Fire.
The chief ingredient was oil condensate which burns on water. Later in the 14th
century, the Greeks had forgotten about their secret weapon and were conquered
by the Turkish Empire wielding their own form of burning hydrocarbon.
Modern petroleum exploration began in the 1859 when the Pennsylvania Rock Oil Company began drilling into "oily rock" that had been discovered by people digging for salt. Natural gas had been used to light streetlights since the 1820's, but fell out of favour when most cities turned to electric lights in the 1920. At that time houses were not connected to gas mains so natural gas became worthless and oil companies burnt it off as waste product. Improvements to welding and pipeline technology after the Second World War led to more homes being connected to gas mains and now half the energy used by households in the developed world is natural gas. Globally natural gas consumption is excepted to double by 2020 leading to an energy supply crisis. Don't be alarmed for there is hope as a new from of energy has come on to the horizon - gas hydrate - and there is lots of it. In fact there is more organic carbon locked up in gas hydrates than all other sources combined, which includes natural gas, oil, coal, peat, organic carbon dissolved in the oceans and contained in all living organisms.
> Relative proportions
of organic carbon on Earth. There is more organic carbon in gas hydrate that
all other sources combined. - USGS
Gas
hydrate is a crystalline solid that consists of a gas trapped in a ring of water
molecules (see figure). There is no chemical bond between the gas molecule and
the enclosing water molecules so the gas is effectively caged within an ice
lattice. In fact, another name for this class of compound is clathrates, which
is based on the Latin word meaning "to enclose with bars."
There
are many naturally forming gases that are an appropriate size to fit within
the ice lattice structure, such as carbon dioxide, hydrogen sulphide, and several
low-carbon-number hydrocarbons. However, easily the most common gas found in
marine gas hydrates is methane. The presence of methane hosted in the ice molecule
tends to stabilise the lattice structure of the crystal so that is doesn't "pressure
melt" - ice, which is less dense than water, melts with increasing pressure
(unusual in natural solids), and the temperature at which it melts decreases
with increasing pressure. Conversely, the increased stability of the lattice
with the introduction of a gas molecule means that the melting point of gas
hydrate increases with pressure (this (admittedly crap) image!). It is therefore
stable at low temperature and high pressure – so unlike water it can exist
as a solid on the sea floor and in sediments near the sea floor. The density
of methane in methane hydrate is also much higher that the density of free methane
in a gas trap because the lattice holds each methane molecule close together.
Gas
hydrates require cold temperatures at moderate pressures or warm temperature
but at higher pressure. They are stable at water depths below ~500m, where the
temperature and pressure of the ocean moves into the gas hydrate stability field.
Gas hydrates are known to occur at least to depths of 4400m within the ocean,
and theoretically could occur at even greater depths, but usually there is not
enough organic matter to generate the methane. The gas hydrate is generally
confined to the upper sediment layers, as at depth the geothermal heat from
below tends to melts the gas hydrate. The sediment host may be 100m thick in
shallow water and up to1000m thick in deep water
Because
the bacterial decay of organic particles in sediment is required to produce
the methane in gas hydrates (see below), they are most abundant on the continental
slopes The organic poor sediment cover on the abyssal plain restricts their
formation (see figure below). Gas hydrates are also hosted below tundra at high
latitudes where the frozen earth stops the gas hydrate from melting and releasing
the entrapped methane.
Where
gas hydrate is stable in an ocean basin. The temperature drops with depth in
the ocean, but rises again with depth in the crust due to heating by radioactive
decay from below. The depth (pressure) and temperature at which methane hydrate
melts and release methane and water is also shown. Methane hydrate is stable
when the geotherm is on the left of this line. - -Marmo
In nature organic carbon from the detrital remains of dead organisms is converted into methane in two ways, biogenically (bacteria) and thermogenically (heat). Bacteria that live in marine sediments survive by consuming organic carbon from the dead biota deposited with the sediment. This bacterial activity occurs in the top 10's of meters of the ocean floor. The main reactions used by different bacteria are (1) bacterial oxidation, (2 & 3) bacterial sulphate reduction, (4) bacterial fermentation and (5) bacterial carbonate reduction:
| CH2O + O2 -> CO2 + H2O | (1) |
| 2CH2O + SO42– -> 2CO2 + S2– + 2H2O | (2) |
| CH4 + SO42–; -> CO2 + S2– + 2H2O | (3) |
| 2CH2O -> CH4 + CO2 | (4) |
| CO2 + 8e– + 8H+ -> CH4 + 2H2O | (5) |
Methane is produced in reaction 4 by fermentation and in reaction 5 by carbonate reduction. The CO2 required in reaction 5 is produced in reactions (1) to (4). The methane produced freezes in the sea water to form methane hydrate and is deposited in the space between grains of ocean sediment as cement.
Thermogenic alteration occurs when sediments containing organic carbon are deeply buried within a sedimentary basin resulting in elevated temperatures. When the temperature reaches ~100°C, the organic carbon breaks down to form methane (CH4) and carbon-dioxide (CO2). Methane formed by thermogenic alteration percolates up through the sedimentary pile and combines with water to form the methane hydrate.
Methane can also exist as a gas phase (free methane) that is not trapped within gas hydrate. This free methane can form thermogenically, biogenically or it can come from melted gas hydrate. Free methane is generally found in the pore space within porous rocks such as sandstone. The positive buoyancy of free methane means that it tends to migrate up through the crust to the surface unless an impermeable layer of rock traps it. As gas hydrate tends to cement sediments together it can in some circumstances act as traps to gas reservoirs (Fig. Gas_traps).
Two simple
situations where layers of impermeable sediment containing gas hydrate act as
seals on natural gas reservoirs. - -Marmo
Underwater landslides and tsunamis
Underwater landslides can produce potentially fatal tsunamis. Around 7000 years ago a tsunami generated by an underwater landslide off the Norwegian coast travelled across the North Sea and swamped Iceland and the Shetland Islands. Its estimated that land as high as 20m above the sea level along the coast of Iceland was affected by the tsunami.
Sea floor slopes on continental margins are stable if the slope is less than 5°. However, many continental margins with shallow slopes have scars from underwater landslides. A potential trigger for shallow slope landslides is sudden gas release from the sediments. This can occur if the methane hydrate layer in the sediment becomes unstable. The hydrate layer can melt if the temperature rises or there is a drop in the confining pressure (below). Melting suddenly releases the methane trapped in the hydrate along with any natural gas trapped below the hydrate layer. Twenty thousand years ago an ice age resulted in the formation of large ice cap that covered much of northern Europe and Canada, and resulted in a 120m drop in sea level. The drop in sea-level reduced the pressure at the sea floor (due to the fact that there was less overlying water). Consequently the methane hydrate layer melted, causing many underwater landslides on the North American continental margin – the scares of which are still visible today and perhaps submarine slide scars recently mapped off Wollongong.
^ A drop in sea-level reduces the pressure at the
sea floor and causes the melting of methane hydrate. The sudden release
of gas results in landslides and slumps. It can also result in a plume of gas
rapidly rising to the ocean surface. Gas in the water reduces the density of
water leading to the loss of buoyancy of ocean going craft. Is this what causes
the mysterious sinking of ships in the Bermuda Triangle?
Archimedes and the Bermuda Triangle
When a large proportion of gas is released at the ocean floor it races to the surface as a massive plume of small bubbles. If we sample a cubic meter of the plume we find (unsurprisingly) that it weighs less than a cubic meter of sea water - the plume of bubbles and water is less dense than the surrounding water.
Archimedes Principal states that an object immersed in a fluid experiences a buoyant force that is equal in magnitude to the force of gravity on the displaced fluid. So a ship floats because it displaces a mass of water that exceeds its own mass. However, if the density of the water is reduced, then the mass of water displaced is also reduced. A ship that finds itself sitting on a gas plume suddenly loses buoyancy and disappears below the surface.
Many people have noted the landslide scars in the ocean below Bermuda Triangle that extends from Miami to Cuba and Puerto Rico. They proposed that methane escape due to the destabilisation of methane hydrate is the cause of abnormally high loss of shipping in the area. It is also thought that such an occurrence would result in the almost instantaneous sinking of ships – so rapid that distress calls could not be transmitted. Well it’s a great story, but an investigation by the insurance company Lloyds of London in 1992 found that the loss of shipping in the "Devils Triangle" was no greater than anywhere else. As for the scars from submarine landslides, well they were also produced 13 000 years before Flight 19 took off.
Gas Hydrates and the Greenhouse
Many chemical compounds found in the Earth’s atmosphere act as "greenhouse gases." These gases allow sunlight, which is radiation in the visible and ultraviolet spectra, to enter the atmosphere unimpeded. When it strikes the Earth’s surface, some of the sunlight is reflected as infrared radiation (heat). Greenhouse gases tend to absorb this infrared radiation as it is reflected back towards space, trapping the heat in the atmosphere. Methane is the third most important greenhouse gas behind carbon dioxide and water vapour. However methane absorbs three times more heat than CO2, so that an increase in the amount of methane in the atmosphere would have a very large effect on the planet’s climate.
Methane hydrate deposits in the ocean and tundra regions contain three times more methane within them than is currently found in the atmosphere. Methane hydrates are therefore a very important sink for greenhouse gas. When sea level dropped during the last ice age, the destabilisation of hydrate and the release of methane may have been sufficient to heat the atmosphere via greenhouse effects and turn back the ice age.
Scientists are currently arguing how an increase in global temperatures due to activities like the burning of fossil fuel might effect the stability of methane hydrates. An increase of mean temperatures at high latitudes would almost certainly result in the release of methane in tundra regions. This would result in a positive feedback, with further warming and further thawing of tundra. The effects in the ocean are less clear as a warming of the ocean may lead to the breakdown of hydrates in surficial sediments, but hydrates at depth are probably too well insulated to be affected. Chemically, the methane will oxidise rapidly in sea water before it reaches the atmosphere. In addition, a sea-level rise might accompany warming of the Earth's atmosphere, which would increase the stability of hydrate due to the increase in pressure from the overlying ocean.
At atmospheric pressure the concentration of methane in hydrate is over 600 times greater than in the free gas form. Methane hydrate is also significantly denser than liquid natural gas. Methane hydrate may provide a cost effective way of transporting and storing methane. To produce methane hydrate is relatively simple, it just involves freezing water and natural gas together. Researchers from Norway have shown that the processes of freezing natural gas into methane hydrate is 25% less expensive that compressing natural gas to form liquid natural gas, which is the current industrial procedure for transportation and storage. Once the methane is produced it remains stable between -5° and -15° at atmospheric pressures, so it is relatively easy to store. The other great asset for transporting and storing methane in the hydrate form is that it is much less explosive than compressed liquid natural gas. Methane hydrate has formed in the pipes of the natural gas processing plant in Victoria with disastrous consequences.
This exercise looks at some of the processes controlling sediment composition from the coast to the trench.
This exercise couples with the above exercise. Real examples of sediments have been used to allow you to interpret them in 'real-life' conditions.
Sediment deposition preserves more than just dirt! Conditions of deposition: temperature, depth, current, proximity to land, rate of deposition are all preserved in the sequence. With a little know-how this wealth of information can be gleaned from the rocks to give an informative 'snap shot' of million - hundreds of millions of years ago!
Vast amounts of sediment are shed off the continental slope, often settling 1000's of km from the continental rise. This exercise looks at some of the dynamics behind the process and the resulting geology.
Hung over? Procrastinating? This exercise will keep the mental cogs turning and let you take it easy for a while.
Yet to be added, this page will contain PDF's of 'real' data to allow you to nut out a few more similar topics
We realise that not everyone has had experience with the intrinsic concepts of Marine Science. This 'computer' has numerous examples which will help you understand these fundamental facets! Very helpful, very colourful!