MYSTERIES OF THE OCEAN The strange world of oceanic methane
The seabed was long thought to be virtually devoid of life below the zone to which light is able to penetrate. Over the past three decades, however, revelations about the extremely active anaerobic biosphere of the deep-sea abysses have revolutionised perceptions of the oceanic carbon cycle, in which methane plays the star role. At the same time, this has raised questions about the impact of this powerful greenhouse gas on climate change.
Consisting of one carbon atom and four hydrogen atoms, the methane molecule – CH4 – is familiar under another name: natural gas. This fossil fuel extracted from the entrails of the Earth is today a vital source of energy, being relatively clean burning and an efficient heat producer, releasing significantly less CO2 while it is burning than other hydrocarbons.
Natural gas is, however, just one of the specific forms – resulting from a thermochemical transformation of geologically buried organic matter – adopted by the methane present in the global ecosystem. The latter’s production in nature, where it is one of the fundamental building blocks of the carbon cycle, is both abundant and widespread. This is due to the continuous process of biological degradation of living matter by anaerobic micro-organisms (i.e. which only live in an oxygen-free environment). Throughout the biosphere, these ‘anoxic’ sites where methane is generated are characterised by humid conditions that favour this bacterial process, such as the mud of marshes, the sediment at the bottom of seas and lakes, peat bogs and permafrost. The stomach of ruminants is another source.
Upsurge of interest In these times of global warming, there has been an upsurge of interest in methane. Independently of its interest as a source of energy, it is also a powerful greenhouse gas, all the more menacing as its warming effect is 20 times that of carbon dioxide. Today, it is considered to be responsible for 20% of the present greenhouse effect. This role is not attributed – at least not for now – to nature itself, but to emissions of anthropogenic origin. The quantity of methane in the atmosphere has more than doubled over the past two centuries due to human activities, compared with a 30% increase in carbon dioxide.
Although science’s growing interest in methane is largely due to increased awareness of its impact on climate, considerable questions are also being raised regarding its role on the seabed. Whereas it was always believed that the ocean depths were largely devoid of life below the level to which light is able to penetrate, the recent discovery – over no more than the last 30 years – of intense underwater microbiological activity has overturned accepted ideas. “One of the most marked consequences of this change in perspective is that the seabed is now known to act as a giant anaerobic bioreactor producing vast quantities of methane,” explains Professor Bo Barker Jørgensen, of the Max Planck Institute of Marine Biology in Bremen, working with the European project DeepBug.
Formidable biological barrier The key to the global mechanism at work in this methane production lies in the presence, in oceanic sediments, of billions of anaerobic bacteria, among which are found methanogenic microbes which give out methane rather like oxygen-breathing species give out CO2. These cohorts feed off living matter that originates on the surface. The marine plankton, which are very abundant in the higher luminous waters of the sea, generate a kind of organic rain which is supplemented by the decomposed remains of waste, dead plants and animals, excrement, etc. This residue very gradually carpets the ocean bed where it mixes with mineral matter. At the same time, rivers flow down into the sea, carrying with them particles of continental origin charged with organic nutriments. In this way, a sometimes very thick – up to several hundred metres – substrate builds up over thousands of years, which is the habitat of these bacteria and where methane is produced.
"But we have discovered that 90% of oceanic methane is degraded immediately it is produced by microbiological processes which use the large concentration of sulphate carried by marine waters to the seabed,” continues Bo Jørgensen. “This natural methane barrier plays a vital role in climate control at global level.”
It was just six years ago that a young German microbiologist, Antje Boetius, first began to unravel the mysteries of this process. “He established that it was the work of extraordinary microbial colonies, combining bacteria and archeobacteria. A phenomenon that holds even deeper mysteries, when one considers that, in evolutionary terms, the gap that separates these micro-organisms and plants is comparable to that between the plant and animal worlds.”
Methane cages While this mechanism of the methane barrier of microbiological origin can be understood as the key element in the oceanic carbon cycle – without which the climate of the planetary geosphere would be thrown into total disarray – major questions remain about the remaining 10% of gas that is continuously produced and that escapes this process. It has been discovered that non-degraded methane is retained at high pressure and low temperature in strange compounds known as ‘methane hydrates’. The latter have a crystal structure, with a structure very like ice, in which methane molecules are trapped in a kind of ‘cage’ consisting of water molecules. These trapping structures are commonly known as clathrates.
The oceans of the world are believed to contain vast stocks of these methane hydrates, mainly on the edges of continents, that is, on the continental slopes running down from the continents to the seabed. Although much uncertainty surrounds the figures, there is thought to be around 1 200 billion tonnes of them, mixed to varying degrees with the sediment in which they form veins and inclusions.
Security concerns The ‘security’ of these methane cages is a current subject of debate – and the cause of certain fears. First of all, this is because they act as genuine methane ‘concentrators’: one volume of clathrate releases about 170 volumes of natural gas when it dissociates. Secondly, because such dissociation is physically conceivable once the temperature and pressure conditions necessary for the stable existence of these compounds cease to be met.
Two risk hypotheses are therefore taken very seriously, one ‘climatic’ (causing a rise in temperature of the ocean floor) and the other ‘mechanical’ (a sudden disturbance of the pressure conditions). In both cases the danger lies in a massive methane release by the clathrates.
The mechanical hypothesis relates essentially to the appearance of instabilities in the layers of sediment deposited on the underwater continental slopes, which could result in the slippage of vast quantities of matter. In such an event, the clathrates present could dissociate under the impact, provoking the release of the methane they store.
It is not out of the question that present ocean warming could produce such movements. Two European programmes (Costa and Hydratech) have looked at this possibility, focusing in particular on the septentrional continental margins of Norway and the Barents Sea, and on the technology for the remote detection of methane hydrates. The data collected to date would seem to suggest that these hydrate concentrations are found at a considerable depth, making them relatively safe from slope-instability phenomena.
To a lesser degree, this ‘mechanical’ risk linked to clathrates can also be induced by human activities. The offshore drilling for hydrocarbons, which heats the seabed, could have a dangerous destabilising effect and thus trigger a methane release. Oil companies are aware of this risk, which is why they are very interested in these projects, sometimes participating in them.
The clathrate gun hypothesis As to the risk of an increase in temperatures on the seabed causing the dissociation of clathrates, this is more of a long-term problem. Oceans react slowly to atmospheric warming. Nevertheless, climate records show a marked historical concordance between periods of global warming and methane quantities in the atmosphere. The continental causes of increases in methane emissions must also be taken into account, such as the expansion of marshy areas and the melting of the permafrost in high latitudes.
Another theory has also been put forward to explain the link: the clathrate gun hypothesis, formulated in 2002 by the American James Kennet, Professor at Santa Barbara University. He believes that methane hydrates build up during ice ages (loading of the gun) and are then dissociated at the first signs of warming, releasing large quantities of methane (discharge of the gun) which in turn, in a feedback movement, speeds up the temperature rise. The author himself admits that this is no more than a line of enquiry, especially as it is still not known at what speed clathrates build up and how many of them there are. But this hypothesis nevertheless underscores the fact that, on a geological scale, oceanic methane is very probably a key component of the carbon cycle and, as such, is of undoubted climatic importance. There is therefore an urgent need to find out more about the conditions determining the formation, disassociation and global stock of clathrates.
Launched in January 2005, 45 partners (including nine SMEs) from 15 countries are working on the important Hermes (Hotspot Ecosystem Research on the Margins of the European Seas) project. Their aim is to explore the marine ecosystems present in more than 15 000 kilometres ...
Quantifying and mapping the flows
The Metrol (Methane Flux Control on Ocean Margin Sediments) project, coordinated by Christian Borowski of the Max Planck Institute in Bremen (DE), is interested in quantifying the oceanic methane flows that result from the diverse biological processes. The researchers have studied three specific ...
Launched in January 2005, 45 partners (including nine SMEs) from 15 countries are working on the important Hermes (Hotspot Ecosystem Research on the Margins of the European Seas) project. Their aim is to explore the marine ecosystems present in more than 15 000 kilometres of Europe’s deep ocean margins, many of which have implications for the methane question. They will analyse the vast anoxic expanses of the Black Sea and Baltic, the underwater mud volcanoes which are found in abundance in Greece and Norway, and the chimneys and faults that emit gas (often methane) and around which extraordinary biological communities congregate that are particularly rich in worms of the polychete family. “The aim of this research on anoxic microbial ecosystems, which are often associated with flows of fluids and gas hydrates, is to identify and describe the biodiversity of the key microbes responsible for the carbon sources and sinks, in order to arrive at an understanding of their energy budgets and structure of their ecosystem,” the principal project partners wrote recently in an issue of the magazine Oceanography. They will have access to the latest available technologies in the marine exploration sector and a budget of €15 million spread over four years. The multidisciplinary information collected will be the result of a large number of oceanographic research campaigns, including those using the Remotely Operated Vehicles (ROVs) from the Southampton Oceanography Centre, (project coordinator), the French research institute Ifremer and the Bremen Institute in Germany.
The Metrol (Methane Flux Control on Ocean Margin Sediments) project, coordinated by Christian Borowski of the Max Planck Institute in Bremen (DE), is interested in quantifying the oceanic methane flows that result from the diverse biological processes. The researchers have studied three specific zones of the European continental margins. The Black Sea, for example, is the world’s largest anoxic expanse with no oxygen at all below a depth of 200 m – and a seabed dropping to 2 200 m in places. This makes it the ideal site for studying the behaviour of methane, a gas that escapes from a multitude of sources. The Baltic Sea (which has large anoxic areas) and the North Sea are also the subject of research. The latter, which is very important economically, is well known to Statoil, the Norwegian oil partner in the project.
In all these regions where methane production is very abundant, due to the high organic content of the sediment, innovative techniques for flow quantification have been developed. Researchers combine seismic and acoustic measurements (giving information about the seabed structure) with analyses of the gas bubbles obtained from the sedimentary cores obtained in situ. The end result is a genuine regional mapping of methane production and a first step towards a reliable global inventory of marine methane emissions.
The strange world of oceanic methane
