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Deep sea Thermal vents under threat?

The Chinese government has just lodged the first application to mine for minerals under the seabed in international waters, in this case on a ridge in the Indian Ocean 1,700 metres (more than 5,000ft) below the surface.

The Chinese are hoping to recover valuable metals such as copper, nickel and cobalt – used in mobile phones, laptops and batteries – as well as gold and silver, in an area of currently inactive "hydrothermal vents", underwater geysers driven by volcanic activity.

Having explored the area using remotely operated underwater vehicles, the Chinese want to mine the sulphide deposits of a region of seabed in the south-west Indian Ocean for the rich mineral ores they contain. They have already applied to do so to the International Seabed Authority (ISA), the Jamaica-based body set up under the 1982 UN Convention on The Law of the Sea to deal with the liabilities relating to seabed exploitation and the environmental damage it may cause.

The environmental worries thrown up by the prospect of deep-sea mining are considerable, not least after the Gulf oil spill, which has become an intractable problem owing to the depth of the seabed where the well is sited. It has become clear that once something goes wrong at such a depth – in this case 1,500 metres, or nearly 5,000ft – putting it right is immeasurably more difficult than at the surface.

Although no one knows exactly what damage a deep-sea mine would do to the marine ecosystem, experts have no doubt that removing a considerable part of the sea floor would cause a major disturbance.

Not only that, but plumes of sediment – which may well be toxic – could have an impact over a much wider area, especially for filter-feeding marine organisms, which are common on the seabed. Such plumes might also block out light, hindering the development of plankton.

Interest in deep-sea mining began in the mid-1960s with the publication of a book by JL Mero entitled The Mineral Resources of The Sea, which suggested that there was a near-limitless supply of certain metals contained in manganese nodules, potato-sized lumps of compressed sediment on the sea floor at depths of 5,000 metres or more. Over the next 20 years, the US, as well as France and Germany, conducted research projects on nodule mining, but these were eventually abandoned after hundreds of millions of dollars had been spent.

However, over the past 10 years the demand for metals from the exploding economies of the main developing countries, led by China, has led to a resurgence of interest in sea-bed minerals; the focus for this has now switched from nodules to hydrothermal vents, discovered by American scientists in 1977 in the Galapagos rift in the Pacific seafloor. Now they have been found all around the world, and have astonished scientists with the teeming communities of specialised deep-sea animals they support in the darkness – often at temperatures of more than 400C.

 

 
 
 
 
 

Hydrothermal Vents


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Hydrothermal vents have been features of the seafloor since the oceans first formed. Today's vents are sites of lush, thriving faunal communities that have developed around chemical energy sources in the absence of light.

First discovered in 1977 at a spreading ridge in the pacific Ocean near the Galapagos Islands, hydrothermal venting has now been detected along all open-ocean spreading centres called mid-ocean ridges.

 

Mid Oceanic Ridges

A mid-ocean ridge is a term for an underwater mountain system that consists of various mountain ranges (chains), typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading centre, which is responsible for seafloor spreading.

The uplifted seafloor results from convection currents which rise in the mantle as magma at a weakness in the oceanic crust, and emerge as lava, creating new crust upon cooling.

A mid-ocean ridge marks the boundary between two tectonic plates, and consequently is termed a divergent plate boundary.

The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system that is part of every ocean, making the mid-oceanic ridge system the longest mountain range in the world.
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Mid oceanic ridges

Hydrothermal Vents

thermal vent chimney

A hydrothermal vent is a lot like an underwater geyser. Sea water seeps down into the cracks and fissures created by the spreading of the sea floor, sometimes as much as two or three miles into the earth’s crust. As the water comes into contact with the veins and channels of superheated, molten magma, the sea water is superheated.

Then the hotter sea water rises to the surface back through the fissures, carrying with it minerals leached from the crustal rock below. The superheated seawater then spews out of the holes in the crust, rising quickly above the colder, denser waters of the deep ocean. As the hot seawater and the cold seawater meet, the minerals suspended in the hot water precipitate out (clump together and drop out) right at the vent opening.

This causes an accumulation, or build up, of the minerals deposited by the mineral rich water into some fantastic and geologically unique formations that have come to be called chimneys. One giant smoker discovered in 1991 reached 15 stories high!

The most spectacular kind of hydrothermal vent are called "black smokers", where a steady stream of "smoke" gushes from a chimney-like structures. The "smoke" consists of tiny metallic sulfide particles that precipitate out of the hot vent fluid as it mixes with the cold seawater.

Plumes from such vents can be traced in the ocean for hundreds of meters upwards and hundreds of kilometers horizontally. The chimneys are made out of sulfide minerals that precipitate out of the vent fluid and can grow 10's of meters high. Many large ore deposits now found on land were formed at hydrothermal vents millions or even billions of years ago.

Black smokers are an example of focused vents, in which almost all the vent fluid comes out of one small pipe.

Black or White Smoker?

The composition of the vent fluids depends on a number of factors:
- heat source (magma, chemical reactions)
- types of substrates
- depths / temperatures involved
"Black smokers" tend to be warmer than "white smokers". They can be over 400°C. Black smokers contain particles with high levels of sulphides (sulphur-bearing minerals). White smokers contain more lighter-hued mineral particles (often containing barium, calcium, and silicon).

Black smoker hydrothermal vent
Black smoker from the mid-Atlantic Ridge

Sometimes the hot fluids rising from depth are mixed with cold seawater and spread out before they emerge back onto the seafloor. These are called diffuse vents and are usually only a few tens of degrees above the near freezing deep ocean water.

Diffuse vent areas have warm water exiting the seafloor over a large area and consequently do not build sulfide chimneys. However, they still contain high levels of hydrogen sulfide and other compounds that specialized microbes can use for energy. This is the basis for an ecosystem that is largely independent of the sun and gives rise to the specialized vent animals such as large tubeworms and clams. The relatively low temperature allows the animals to remain immersed in the nutrient rich water and allows the diffuse vent sites to develop into complex ecosystems. Often chimneys with focused, high-temperature venting are surrounded by areas of diffuse, low-temperature venting.

Initially it was thought that slow and ultra-slow ocean spreading centres were too removed from magma sources to have enough heat for extensive venting activity. Recent discoveries are showing that this is not the case, and in fact, venting at the slower centres may result in potentially larger deposits of seafloor minerals.

Relative to the majority of the deep sea, the areas around submarine hydrothermal vents are biologically more productive, often hosting complex communities fueled by the chemicals dissolved in the vent fluids. Chemosynthetic archaea form the base of the food chain, supporting diverse organisms, including giant tube worms, clams, limpets and shrimp. Life on Hydrothermal vents

Active hydrothermal vents are believed to exist on Jupiter's moon Europa, and ancient hydrothermal vents have been speculated to exist on Mars.

A British scientific expedition has discovered the world's deepest undersea volcanic vents, 3.1 miles (5000 metres) down in the Cayman Trough in the Caribbean. Using a deep-diving vehicle remotely controlled from the Royal Research Ship James Cook, the scientists found slender spires made of copper and iron ores on the seafloor, erupting water hot enough to melt lead, nearly half a mile deeper than anyone has seen before.

The scientists will be posting updates about their progress live from the ship at www.thesearethevoyages.net

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