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Shallow Hydrothermal Vents: The Bubbling Seas of the Greek Archipelago
From: The Natural History Museum
| By:
Eva Valsami-Jones |
EDITOR'S INTRODUCTION |
Submarine hydrothermal vents--areas where hot sea water gushes out of fissures in the underlying rocks of the sea bed--are usually located at great depths in the ocean. The vents at the Greek island of Milos are extremely unusual because they occur in shallow waters. Eva Valsami-Jones of The Natural History Museum has been studying the Milos vents since 1996. Here, she reveals how these remarkable natural phenomena provide unique insights into what goes on under the Earth's surface. |
 ne of the key factors in the creation of hydrothermal activity is the operation of plate tectonics. |
Plate-tectonics theory
Earth's crust is made of plates which are not fixed but 'float' on a sea of hotter, more mobile rock--a process known as plate tectonics. As they move close together, the plates collide and form subduction zones where the oceanic crust of one plate is sunk and lost. In the middle of oceans, the crust is split apart as plates move away from each other. Magma--molten rock from the lower layers--rises upwards to create new crust. The magmatic heat gives rise to hydrothermal activity. In subduction zone environments, hydrothermal activity is associated with new crust formation as a result of pulling forces at back-arc or fore-arc basins (large depressions that are major submarine topographic features). Typical examples of such environments can be found in the Pacific Ocean. |
 The discovery of hydrothermal vents in the late 1970s revolutionised the study of geology and plate tectonics. Scientists studying the heat released from Earth's surface discovered that the crust was cooling more rapidly than they expected, indicating that some other mechanism of heat release had to be involved. They predicted that water circulating through the crust, cooling it by convection, was the likely cause, and expected to find hydrothermal vents long before they were actually discovered. The first hydrothermal vents to be found and examined are close to the Galapagos Islands. The vents here are not the typical black smokers which are very hot, with temperatures of 350° Celsius or above--these vents are cooler, although equally important and spectacular. Following this discovery, more hydrothermal vents were located along the East Pacific Rise, the mid-Atlantic ridge, the Lau Basin and other settings both mid-oceanic and back-arc. Hydrothermal activity in the Aegean arc (Greece), including Milos, was discovered in the mid-1980s. |
Hydrothermal vent operation
Hydrothermal vents are formed when water percolates down into the crust, through rock-pore space and cracks. It sinks deeper and deeper and gradually heats up as the surrounding rocks get hotter. Eventually it reaches a critical temperature, becomes buoyant and rises again, usually along major discontinuities such as faults. Having reacted with the rock at high temperatures, the fluid is no longer simple seawater. It exhibits a different chemical make-up and is extremely acidic (it can have a pH as low as 3). When the hot hydrothermal water comes into contact with the cold seawater, rapid cooling forces the metals carried by the hydrothermal water to precipitate as metal sulphides which can no longer be transported. This creates a 'black smoke' made of minute particles of metal sulphides, iron sulphides and other metals. Despite the heat and acidity, which suggest an environment inhospitable to life, hydrothermal vents are unique environments of biological activity. |
The Milos hydrothermal system
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| Palaeohori Bay, Milos Island. | |
The island of Milos is located in the volcanic arc of Greece which stretches from the east end of the Peloponnese peninsula to Santorini Island. This part forms the tip and is the active part of the arc. It then stretches up through Kos into Turkey. Hydrothermal vent activity has mainly been studied in the south of Milos, in a place called Palaeohori. The chimney structures seen there are different to the traditional chimneys of hydrothermal vents. |
The entire island of Milos is a circular caldera (a crater formed by the collapse of the centre of a volcano) formed during the island's past volcanism. Although no longer active, a lot of magma remains underneath the island. This is what drives the present day hydrothermal activity. |
The volcanic activity of the past is still visible in many extinct craters throughout Milos, whereas modern geothermal activity on land is expressed as discharges of sulphur-rich steam. |
The hot water generated by the hydrothermal activity has many therapeutic uses, and a public spa has been constructed in the main port town of Adhamas. Elsewhere on the island, another expression of hydrothermal activity is brown staining of the water, indicating the richness of iron. |
Samples of hydrothermal discharge have been taken from a cave on Milos two or three times a year for the last five years. The locality is ideal, because the discharge occurs just above seawater level. As a results the samples collected are not contaminated by seawater. Hydrothermal waters discharge from two distinct fissures in the cave, and form small pools, before flowing into the sea. The water's acidity from both points of discharge is around 1.7 pH units, with a temperature of 80-90°C. |
Hydrothermal discharges underwater are a lot more spectacular. Streams of bubbles come through the sea floor over a wide area in Palaeohori Bay. In some localities the hot waters discharge through rocks and the leaching of the rock surrounding the discharge point is highly visible. |
Some of the vents located on the sandy sea floor around Milos are extremely vigorous, so much so that their discharge resembles mini-explosions which
 can be heard for some distance. It can be difficult for scientists to take samples because of the explosive style of discharge and the scalding temperatures of the water. Some of these vents form hydrothermal edifices, from the fallout of particles precipitating as a result of water cooling. These structures are the closest analogues of black smokers, although the vents are much cooler, with a maximum temperature of about 120°C. Most vents are irregularly shaped because of their formation in shallow high-energy environments. Occasionally the vents are cone shaped, closely resembling black smoker chimneys. The discharge hole is made of hydrothermal precipitate (mostly iron sulphides). |
Researchers use a special probe to measure temperature and acidity in situ. The detailed chemistry of the waters discharging from the vents is assessed in the laboratory. Water is syringed out of the vent and preserved for analysis. The samples taken underwater also contain some seawater which gets mixed in with the vent water. |
Another spectacular and unusual aspect of the hydrothermal activity is that it can cause the formation of brine pools. Some of the
 hydrothermal waters in Milos are extremely saline, they have about three times the amount of salt as normal seawater. This makes them very dense and allows them to settle on the sea floor and form brine pools when the sea is calm. The acidity and salinity of these pools make it extremely difficult for sea animals to survive if they accidentally fall in them. However, these brine pools are short-lived because they are destroyed by even the slightest wave action. |
The density and variety of life forms from mid-ocean ridges does not appear to be present in Milos.
 However there is a clear relationship between the submarine activity and the formation of bacteria mats, the only form of life that thrives in such conditions. The mats, which can be seen from above seawater, form white patches on the seafloor from tens of centimetres to several metres across. They are made of bacteria cells and exopolymer--organic 'slime' generated by the bacteria as a means of conditioning the environment surrounding them. |
 Pyrite (iron sulphide) is the main hydrothermal precipitate found in Milos. Interestingly these pyrites have been found to contain high levels of arsenic. There are indications that these pyrites may be biological in origin, formed by micro-organisms that have a fantastic ability to cope with arsenic. |
The deep and the shallow
Are the Milos vents really a good analogue for mid-ocean ridge vents? There is a lot of similarity between Milos and typical deep-sea hydrothermal vent chemistry, including their metal and alkaline earth content. There are both high and low salinity fluids discharging from the Milos vents. A detailed study of the chemistry of these two fluid types suggest that they were originally a single seawater-derived phase that split into the components of different salinities (a 'brine' and a 'steam') as the fluids depressurised during their ascent. |
Each of the two fluids have distinct metal contents. This reveals much about how metals are transported up to the surface. The high-salinity fluid transports one set of metals, including cadmium, zinc, manganese, arsenic and lead. The low salinity fluid transports another set which includes cobalt, nickel, aluminium, iron and chromium. |
Microbiology of hydrothermal samples
Bacteria cells have been found attached on the surfaces of the Milos pyritic precipitates. These cells have the
 typical appearance of bacteria, i.e. elongated bodies, about one micrometre across (a millionth of a metre). The cells appear to be dividing and excreting a conditioning polymer around them, both indications that the bacteria are thriving in the vent water, even though it contains extremely high levels of arsenic. |
There is still much research to be done at Milos, but the work carried out so far has shown that the chemistry of the shallow hydrothermal vents there is fascinating. Other key areas for future research include the microbiology of the vents and the prospect of associating the chemistry of the vents with seismic activity. |
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