The ocean depths

The hidden face of the Earth

While recent exploration of the ocean depths has unveiled many secrets, it has raised just as many questions. Do the ocean depths conceal new exploitable resources? Are pollution and climate change damaging their unique ecosystems? How do we preserve them? It is these and similar questions to which scientists like those in the Hermes(1) project are starting to find answers.

Gorgonocepahlus caput  medusae, lives 290  metres down in the Skagerak region off the Norwegian coast. © T.Lundalv, TMBL Gorgonocepahlus caput medusae, lives 290 metres down in the Skagerak region off the Norwegian coast. © T.Lundalv, TMBL
Studied by  researchers from the Hermes project, the Nazaré oceanic canyon reveals the complex relationship that  this underwater gorge has with its environment. © NOCS Studied by researchers from the Hermes project, the Nazaré oceanic canyon reveals the complex relationship that this underwater gorge has with its environment. © NOCS
Lophelia pertusa colony living on the Sacken Reef, in Norway. © T.Lundalv, TMBL Lophelia pertusa colony living on the Sacken Reef, in Norway. © T.Lundalv, TMBL
The subject of much  research, most of the continental  margins’ potential is concentrated in  its depths. Source Planète-Terre / Pierre-André Bourque The subject of much research, most of the continental margins’ potential is concentrated in its depths. Source Planète-Terre / Pierre-André Bourque

Mountains, volcanoes, canyons, water springs… the picture of the seabed conjured up in most people’s minds and in traditional oceanic literature scarcely does justice to its diversity and complexity. With a rich variety of unique structures, and governed by complex systems, the ecosystems of this benthic world (2) are also interdependent, which dramatically complicates the task of scientists. Right now, research is grappling with the specificities of each separate part of the submarine terrain. While the seas and oceans cover 70 % of the globe, we know scarcely 1% of the beings that inhabit them.

Life on the continental slope

Oceanographers are very interested in the continental margins which extend the continental shelves up to the abyssal plains. These continental margins, where the continental and ocean plates meet, are littered with vents through which many different gases – in particular methane – escape. It is also there that terrestrial sediments transported by rivers and marine currents pile up before falling into the abysses. Unlike the abyssal plains, where life is rare, numerous organisms colonise the continental slope, feeding mainly off the organic matter contained in the sediments covering it. Here we find a whole range of unique ecosystems, highly fragile because specifically adapted to the extreme conditions of the great depths where luminosity is weak, the temperature low and oxygen rare.

Any sudden variation of this biotope could potentially wipe a myriad of unknown organisms off the face of the Earth. “The oil industry and the fishing sector are looking more and more covetously at the sea depths, as resources available in more accessible places become scarce. It is therefore urgent to better understand the benthic environment in order to assess its fragility and, in the longer term, to propose sustainable forms of exploitation to politicians,” says Philip Weaver, a researcher at the National Oceanography Centre, Sout - hampton – NOCS (UK) and coordinator of the Hermes project, a multi-disciplinary scientific platform set up to study the edges of the European continental shelves. All this represents a major challenge for the European Union, with its three million square kilometres of continental shelf. Given the immensity of the task, Hermes researchers are focusing on seven strategic zones, viewed as ecological ‘hot spots’.

Portugal’s Grand Canyon

These zones include the Portuguese canyons. Nazaré, a gigantic canyon off the Portuguese coast, stretches for some 250 km, almost comparable in length with its American cousin in Colorado, reaching depths of 5 000 m in places. Nazaré constitutes one of the final stages for terrestrial sediment transported towards the abyssal plains at the foot of the continental slope. The organisms living in the abysses are highly dependent on this sediment, which carries large quantities of organic matter.

The mechanism of this phenomenon is simple but effective: the sediments accumulate in the canyon, creating large slopes over time. With successive deposits of sediment, the slopes become increasingly unstable. When they collapse following a geological event like an earthquake, or simply lose their balance, they create avalanches of sediment called turbidity currents. Pushed by gravity, large columns of sediment-filled water pass along the bottom of the canyon, ending up at the canyon mouth, forming a bathyal cone or sediment fan. In this way, sediments travel long distances at speeds of up to several tens of kilometres per hour. These currents, the frequency of which varies from one canyon to the next (approximately every 400 years in the case of Nazaré), are strong enough to break underwater telecommunications cables or uproot observation stations anchored by scientists into the walls of the canyon. “Turbidity currents hollow out the canyons and ravage everything in their path. We are seeking to determine the ability of ecosystems to recover from such events. This will enable us to evaluate the impact of the various human activities that can potentially be situated around the canyons,” Philip Weaver adds.

Mud volcanoes

The accumulation of sediment at the foot of the continental slopes explains another geological phenomenon which is characteristic of the edges of the continental shelves. Large amounts of hydrocarbons escape from the sea depths, through simple gas vents, pockmarks(3) and mud volcanoes. Scientists call these exhausts cold seeps, to distinguish them from the very hot hydrothermal springs found close to the ocean ridges where volcanic activity is intense. “These emanations, consisting mainly of methane, result from the decomposition of the organic matter held in the sediments, or may derive from deeper-lying oil reserves. Methane, as well as the water contained in the sediments, is trapped under the sediment layer. Under the effect of pressure these escape through fissures in the seabed,” explains Jean- Paul Foucher, coordinator of the cold seeps project at Hermes and a researcher at Ifremer (4) Marine Geosciences department. “Right now, we are trying to map and better understand the mud volcanoes, the abundant discharges of gas and water at times, and the particular ecosystem that characterises them.” Populating these unusual structures is a multitude of organisms, the list of which is being constantly added to with the progress of submarine cartography. A mission was recently completed in the Gulf of Cadiz on the James Cook, an oceanographic vessel belonging to the NOCS. But several other expeditions of the Hermes programme are targeting the submarine mud volcanoes, one of the most impressive of which, Häkon Mosby, lies 1 100 m down off the Norwegian coast. “For a little over a decade, we have observed exceptional degassing activity at the surface of Häkon Mosby. This mud volcano has been repeatedly examined in order to better understand its ups and downs and the eco - system behind it. On the Häkon Mosby, as on other mud volcanos or pockmarks, part of the methane discharged is trapped in the form of hydrate crystals, a solid mixture of gas and water, which could constitute an enormous future source of energy, but also represent a potential danger owing to the heating of the oceans (5).”

Inhabitants of the extremes

How do mud volcanoes work? At what depth does the source of this irregular eruption of water, methane and other gases lie? As Jean- Paul Foucher points out, “each volcano is different, which presents scientists with an enormous task.

But this type of structure is the habitat of extremophiles, whose main source of energy is not photosynthesis but chiliosynthesis, the production of energy from the chemical components of the cold fluids, mainly methane. This unusual fauna and the intense microbial activity consume a large portion of the methane given off by the cold seeps. The rest is digested inside the water column. But researchers are afraid of the impacts of climate change on the biotope specific to cold seeps, because if the extremophiles were to disappear, the massive liberation of unconsumed gas into the atmosphere could have disastrous consequences: methane has a global heating power (GHP) 23 times higher than CO2, making it a powerful greenhouse gas.

Study of this biotope is adding to our still very partial knowledge of the interactions between the oceans and the atmosphere, and in parti - cular confirms their vital influence on climate. Anticipating climate change necessarily involves a more precise study of marine phenomena, in particular those governing this “dark side” of the planet which the ocean depths represent.

Julie Van Rossom

  1. Hotspot Ecosystem Research on the Margins of European Seas.
  2. The term benthic refers to the life specific to the seabed.
  3. A pockmark is a trace left on the surface of the sediments by the percolation of fluids through the sedimentary column.
  4. Institut Français de Recherche pour l’Exploitation de la Mer
  5. See here The Strange World of Oceanic Methane, in RTD Info, no 48, February 2006, p. 9.


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Cold water corals

The word “coral” immediately conjures up an image of superb tropical beaches. But there is also another type of coral that grows in the dark depths of the seas, less brightly coloured than its well-known southern cousins but spread over a wider zone, and one which has been recently brought to light with improvements in deep sea exploration technologies. These coral reefs grow at depths of 40 – 6 500m in all Europe’s seas, from the  Norwegian fjords to the temperate Mediterranean waters, as in the rest of the world. Whilst Lophelia Pertusa, a  white coral, is the most illustrious representative of this category, no less than 1 300 different species have been recorded so far in the north-east Atlantic alone. These cold water coral reefs, stretching at times over several kilometres, lie at the centre of a rich ecosystem which provides protection, home and food to a number of marine organisms, including a multitude of fish of commercial value. However, quite apart from the impacts of climate  change and offshore oil and gas drilling, whole blocks of cold water coral are torn away by deep sea trawl nets,   which drag the sea bottoms looking for fish. These corals grow ten times more slowly than tropical corals, which means that a hundred, if not a thousand years’ growth can be reduced to nothing in an instant. In recent years, a handful of countries, including Norway, Ireland and the United Kingdom, have finally responded to repeated calls from the scientific community, which is fascinated by this biotope from the shadowy depths, and have imposed measures to protect these poorly known specimens.


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