This section is divided into 2 parts; Hydrothermal Systems and Oceanic volcanism

Hydrothermal Systems;
Vents

Hydrothermal vents are found at modern mid-ocean ridges and less frequently in back-arc basins (eg Manus Basin rift zone). and on seamounts. At these locations seawater seeps down through the cracks in oceanic crust, heats up, then rises, erupting to the surface through vents. The heated water dissolves many elements including sulphur and metals, such as copper, zinc and gold, carrying them to the sea floor. When the hot plume encounters the freezing water, the metals precipitate as sulphide and sulphate minerals. The build up of minerals can form elaborate chimney structures, often referred to as "black smokers". The "smoke" that you see is actually the precipitating minerals. Even though the water escaping from the vents can reach temperatures up to 400 C, the high pressure keeps the water from boiling. The intense heat is limited to a very small area within about 15 cm of the vent opening. The surrounding water is only 2 C. Hydrothermal vents play an important role in the geochemical cycles and heat balance of he oceans.

Life at Vents

In 1977 scientists in the submersible Alvin discovered a bizarre collection of organisms living at a vent off the Galapagos Islands. Similar communities have since been found at several hundred hydrothermal sites around the world. Organisms are attracted to the vents by the chemical cocktail that exists there. No sunlight penetrates to the deep ocean so there is no chance of photosynthesis. Instead a similar form of energy production called chemosynthesis is occurring. Many different species of bacteria and archaea (sometimes called extremophiles, the most primative organisms yet discovered) make up the primary producers. They utilize the sulphur, hydrogen, methane and other compounds that are produced when seawater reacts with magma in the crust. Many of these microbes can survive at temperatures above 100 C. The most abundant chemical dissolved in vent water is hydrogen sulfide (which at low concentrations smells like rotten eggs). Microbes utilize the energy released from the oxidation of hydrogen sulphide to produce food. These sulfur microbes form mat-like coatings on the surrounding rocks or live within the tissues of other animals.

Worms, clams, crabs, shrimps and octopuses also live around the vents. The most spectacular organisms so far discovered are probably the giant tube worms. These worms can be over 3 m tall, have fabulous red tops and grow in groups that sway in the current. The have no mouth or gut, instead relying on symbiotic microbes for their food. The red tip contains haemoglobin, which binds oxygen and hydrogen sulphide for the microbes. Giant clams have similar relationships with microbes.
During a recent research cruise to the Manus Basin on the RV Franklin (May 2000), Australian CSIRO scientists recovered a huge black smoker chimney. It was 2.7 metres long, 80 cm in diameter at the base, and weighed more than 800 kg. It is thought to have been an actively venting chimney. The scientists recovered large numbers of microbes from the surface of the chimney. A major goal of the expedition was to identify particular microbes that can be used to process minerals on dry land, and so develop more efficient and cleaner ways to extract metals.

Figure Vent fauna photo (from Lutz and Haymon)
Microbes have also been found in the subsurface rocks of hydrothermal systems. A recent ODP cruise to the Manus Basin (January 2001) recovered microbes from depths of up to 130 m under a chimney field. Cultivation experiments on board the ship proved that these deep biota flourish under anaerobic conditions in seawater at temperatures as high as 90°C, (experiments at higher temperatures ate currently being conducted).

Hydrothermal mineral deposits

Ancient vent sites are often exploited for the sulphide minerals. In Australia they are known as "sedex deposits" which stands for fine-grained sediment–hosted Pb-Zn-Ag deposits of mid-Proterozoic age. Examples include the Broken Hill, Mount Lyell and Mount Morgan deposits.

Mining the ocean

Mineral deposits on the bottom of the ocean first attracted attention in the 1960’s. when people started to fantasize about mining zinc from sites 2 kilometres deep in the Red Sea. Other deposits have been investigated over the years, including nickel and copper nodules. The high cost of recovery of these fairly low-grade deposits has so far made deep sea mining uneconomical. This may all be about to change.
Massive sulphides were discovered in the Bismarck Sea in 1985. More recently a group of Papuan, Australian and Canadian scientists have visited the area. They discovered that the eastern Manus Basin contains three active hydrothermal zones, which they named PACMANUS, SuSu Knolls and DESMOS. ODP Leg 193 (January 2001), explored the subsurface parts of these active, mineralized hydrothermal system (see http://www-odp.tamu.edu/publications/prelim/193_prel for more information on the ODP discoveries). Unlike the deep sea deposits previously considered for mining, these massive sulphide deposits are high grade and occur at relatively shallow depths (less than 2 km). The New Guinea Government has granted two underwater exploration and development licences to Nautilus Mineral Corporation PL.

Dr Ray Binns, from CSIRO Exploration and Mining, was the cruise leader on the 2001 ODP expedition. He has stated that the average composition of research samples from PACMANUS and SuSu include 10% and 15% copper, and 26% and 3% zinc respectively. PACMANUS has 15 grams of gold and 200 grams of silver to the tonne, while the figures at SuSu are 21g/t and130g/t. He feels that these metal levels could change the economics of sea-floor mining and cause it to be taken seriously..
The Eastern Manus Basin hydrothermal vents are located in a 80-100km wide area of thin oceanic crust. This is a pull-apart rift zone between two transform faults, which releases felsic lavas featuring copper, zinc, lead, silver and gold.

How to find your very own hydrothermal vent

Vent fluids can be detected because they have different physical properties and chemical composition from the surrounding seawater. For example they differ in temperature (hotter), pH (more acidic), turbidity (higher), O₂ content (generally lower), Mg and SO₄ (lower) and He, Fe, H₂S, Mn (higher). Some of these differences are still discernible 100’s of km (or in the case of Helium, 1000’s of km) from the vent field. The NOAA-VENTS Plume Studies Group (http://www.pmel.noaa.gov/vents/) has developed methods for measuring and mapping hydrothermal plumes based on the detection of temperature and particle anomalies.
Scientists can use a CTD (an instrument that measures conductivity, temperature and density) to locate hydrothermal vents and track the dispersing plume. The CTD is lowered via a cable and towed behind the ship. Data is collected in real time and relayed to the ship. Additional sensors can be added to measure water chemistry and turbidity (similar instruments can be used to track other types of plumes, for example AGSO have mapped sewage plumes emitted from Sydney’s deep ocean outfalls).
Hydrothermal fluids expelled from the vent are rapidly mixed and diluted with seawater. As we would expect the major differences in temperature and suspended solid concentration (ssc) occur close to the vent opening. However the emerging plume is still less dense than the surrounding water so rises up (for tens of hundreds of meters) and will continue to rise until it reaches a point where it’s density is the same as the surrounding water (ie it is neutrally buoyant). When the plume reaches this point it ceases to rise and will be laterally dispersed by ocean currents.
Baker and Lupton published a paper in Nature in 1990 (346, 556-558) documenting the results of a study they undertook on a megaplume field on the Juan de Fuca Ridge. They mapped the steady state plume annually between 1986 and 1988 using CTDT (conductivity, temperature, density and transmissometer) casts and tows. In addition they collected samples for chemical analysis. They found that in each year the plume was ~200 m thick, centred ~150 m above the floor of the axial valley and covered an area of 100 km 3

Now for the question ‘why would we want to locate a hydrothermal vent?’

As we have already mentioned, hydrothermal vents can be sources of valuable mineral deposits, so that’s one reason you might like to locate them. In addition finding vents and studying plume tracers can help us understand ocean circulation and mixing. NOAA has also indicated that hydrothermal plumes are likely to be very important for the transport and distribution of the marine microbes that live under the seafloor. Also concentrations of gases (e.g. He, H₂S and CH₄) and trace metals (e.g. Fe and Mn) are high enough in vent plumes sto significantly effect their oceanic geochemical budgets. Finally, hydrothermal vents and plumes may also give us an insight into tectonic processes. It seems that vents are more common in areas with fast spreading rates.
(ps tube worms are reputed to make great pets).

Active hydrothermal sites (stars) at convergent margins of the western Pacific Ocean. It is probable that many more hydrothermal vents exist in this area and that continued surveying of the sea floor will bring them to light.

Oceanic volcanism