What is a hydrothermal plume?
Hydrothermal plumes are a mixture of seawater and hydrothermal fluids, which are generated by the circulation of seawater through the oceanic crust. The magmatic chamber, a deep heat source, and the permeable structure of the oceanic crust are two of the factors that contribute to underground circulation.
Plumes can be found on the ocean floor in the form of mounts or chimneys formed by the precipitation of certain minerals composed of sulfates (anhydrite, barite) and sulfurs (minerals rich in metals), and by the formation in the water column of hydrothermal plumes enriched in He, CH4, Mn and H2S.
Hydrothermal plumes, which depend closely on the nature of the fluids released at the ocean floor, are often, “black” or “white” due to the presence of mineral particles which precipitate rapidly when high temperature fluids (350°C) mix with cold seawater (around 2°C). Whilst certain low-temperature fluids contain practically no particles, the well-known black plumes contain fluids dense in metals and are commonly known as “black smokers”.
Why do we study hydrothermal plumes?
Hydrothermal plumes rise up through the water column above active zones as their density is lower than that of seawater. Once released from the source, mixed with seawater and transported by advection (the transfer of heat or matter horizontally by the flow of a fluid), the fluids disperse laterally through the currents to reach a state of stable equilibrium.
In addition to detecting active sites along mid-ocean ridges, through hydrothermal plumes we are able to monitor the dispersion, mixing and evolution of fluids and associated particles over time and at different distances from the source, thereby contributing to the study and better understanding of the geochemical behaviour and cycle of elements in seawater.
How do we detect hydrothermal plumes?
Hydrothermal plumes are detectable in the water column in active fields, along ridges and their flanks and at the bottom of ridges. They present very different physical and geo-chemical properties to those of seawater.
A number of physical and geochemical tracers may be applied to hydrothermal activity exploration. Several methods and strategies have been developed to map hydrothermal plumes. They are all based on the detection of physical (temperature, nephelometry) and geochemical (Mn, CH4, H2S, Hélium, Radon, ...) anomalies.
The strategy applied to hydrothermal exploration
Research programmes related to hydrothermal circulation along mid-ocean ridges developed over the last 20 years have among others, the following objectives:
- Develop exploration methods to locate, map and identify the distribution of hydrothermal plumes associated with active hydrothermal fields;
- Record and monitor temporal variations and the evolution of hydrothermal systems, and in particular their behaviour after tectonic and volcanic events;
- Increase knowledge of the links between hydrothermal, magmatic and tectonic processes along the mid-ocean ridge system in its broad sense (magmata-tectonic cycle);
- Study the physical, geochemical and thermodynamic processes controlling hydrothermal circulation.
The search for active zones using physical and chemical tracers has three distinct phases:
- Regional exploration, to a range of 1000 km of ridge, to detect major geochemical anomalies,
- A more detailed study of a segment of ridge (10 to 100 km) to study the variability of hydrothermal activity and to more precisely locate the active zones. These first two steps are conducted during oceanographic surface expeditions.
- The third step, probably the most challenging, consists of identifying and studying in detail a defined hydrothermal site with submersibles and ROVs. Generally, this exploration is conducted on a scale of “boxes” of 10x10 km.
This adopted scientific approach, implying tracers, has proved efficient and recently successful in identifying new active sites such as Menez Gwen, Broken Spur, Rainbow and Ashadze and the Mid-Atlantic Ridge.
Once a precise bathymetric map has been made of the area, the scientists look for physical and chemical anomalies (tracers) created in the water column through hydrothermal inflow. The presence of these anomalies, which form hydrothermal plumes, is synonymous with hydrothermal activity.
Measurements are made with a CTD probe equipped with different sensors (temperature, conductivity, nephelometry, in-situ chemical analyser). The analytical equipment is set up in a portable laboratory installed on board the ship. On-board processing of physical data enables us to detect in real-time physical anomalies (temperature, conductivity, density), particle plumes (e.g. transmissometry, nephelometry, …) and geochemical anomalies (e.g. methane, manganese).
The information obtained directly on-board also helps to give direction to the expedition, identify and define active zones, and prepare future submersible dives, generally scheduled over successive years on the newly discovered sites.