Scientific sheet 1

Thirty years of oceanic ridges exploration: Diversity and location of the hydrothermal sites

Yves Fouquet, Daniel Desbruyères, Jean-Luc Charlou

White smokers photographed by Nautile during the Nautilau expedition in the volcanic areas near the Tonga islands (Southwestern Pacific). Depth: 1700 m.
© Ifremer

The first deep-sea high temperature hydrothermal vents (350°C) were discovered in 1978 on the East Pacific Rise. Through this spectacular discovery, scientists showed for the first time that a large proportion of the Earth's heat is extracted by superheated sea-water. We currently estimate that 25% of that heat is evacuated by hydrothermal fluids.

Mineral and biological diversity

The repercussions of this discovery rapidly went beyond the domain of geophysics and plate tectonics. The chemical energy that is contained in the fluids allows the development of an intense microbial activity, and of unique and specialized animal communities that live from the geothermic activity.

In addition, the metals that are extracted from the depth of the Earth's crust and mantle accumulate to form mounds of metal sulfides that are potential mineral resources. Under some conditions, rocks from the Earth's mantle can reach the surface of the ocean's bottom. Fluids resulting from the reaction of sea-water with these rocks are very enriched in hydrogen, methane, and heavier hydrocarbons.

Ridges and underwater volcanoes: areas rich in hydrothermal vents

In 35 years of exploration, scientists have discovered 90 active hydrothermal fields.
The vents are mainly located along oceanic ridges, at the boundary of tectonic plates, where the volcanic and tectonic activities are the most intense.

The oceanic crust is formed at the ridges, where the plates that form the Earth's crust spread. After the discovery of hydrothermal fields on fast-spreading ridges (spreading speed between 6 and 18 cm/year), exploration in the 80's and 90's revealed vents on slow-spreading ridges (speed of less than 6 cm/year).

At the end of the 80's and during the 90's, exploration also showed that hydrothermal vents were very common on underwater volcanic ridges located behind the great troughs (in particular in the Western Pacific, behind the Mariana and Tonga troughs).
Some "intra-plate" volcanoes -not located at the boundaries of plates but rather within the area of the plates- are also a site of underwater hydrothermal activity. At all these locations, scientists have observed faunal and microbial communities that are associated with the venting of hydrothermal fluid. To date, 580 animal species have been described from these habitats.

All the volcanic systems currently known are associated with an intense hydrothermal activity. A large proportion of the 60,000 km that circle the Earth remains to be explored. Research is still ongoing on Atlantic, Indian, and Pacific ridges. Numerous studies are also carried out at specific study sites in order to understand the processes, interactions, and temporal variations of hydrothermal activity, and its effects on biological activity.

Hydrothermal systems: when rocks and water interact

We now know that there is a great variability of the composition of the fluids, of the hydrothermal precipitates, and animal communities associated with the vents. This diversity mainly depends on two parameters : the pressure and the type of the rocks that react with the sea-water.
The pressure is directly linked to the temperature at which the fluids boil. This is especially meaningful because hydrothermal vents are found from the littoral zone to 4,100 m depth. If the temperature of the fluids exceeds the boiling temperature of sea-water, the fluids can then be more (brine) or less charged in salts (condensed vapor) than sea-water. For instance, at 3,000 meters, the boiling temperature of sea-water is higher than 400°C.

The type of rocks that are washed out by the hydrothermal circulation is the second factor that affects the composition of the vent fluids and hydrothermal deposits. These fluids and deposits resulting from the interaction of sea-water with basaltic lava (silica-poor), rhyolithic lava (silica-rich), sediments, or ultra-basic rocks coming from the mantle have widely different compositions.

In the long term, the exploration of the deep-sea is an adventure that must be built in the context of international cooperation. France, along with the United States, has been at the origin of the discovery of hydrothermal systems. During some 20 years, the countries that have the technology of inhabited deep-sea submersibles (France, United States, Japan, Russia, Canada) had a privileged access to the study of hydrothermal systems. The development of remotely-operated vehicles has widened this group to include Germany, England, China, and South Korea.

30 years of French-Russian cooperation

There has been a strong cooperation for over 30 years between France and Russia for the exploration and the knowledge of the Mid-Atlantic Ridge, place of spreading between the American and African plates. In this context, several joint cruises have been on French and Russian research vessels.

The goal of this cooperation is to better know the ocean floor and its relationships with the inside of the Earth, to inventory the potential mineral resources and to understand the ecology of hydrothermal systems. This knowledge will be essential for a reasoned utilization of the mineral resources of the deep-sea and the protection of our environment.

During the 70's and the 80's, the cooperation was focused on the study of volcanic rocks. The goal was to understand the mantle fusion mechanisms that produced the lava that is at the origin of the oceanic crust.
Then, at the beginning of the 90's, the research was targeted toward the mechanisms of chemical transfers by hydrothermal circulation paths. More specifically, French and Russian scientists focused on the transfer of metals and to their accumulation as mounds of polymetallic sulfides, these latter representing a potential mineral resource.
Finally, the cooperation widened to include other research areas that include the study of deep-sea biodiversity and the study of extremophilic bacteria.