Published: 26.09.08
Marine geology

Realistic models of Black Smokers

Along the mid-ocean ridge, where new ocean crusts are formed, there are fields of hydrothermal springs, or black smokers as they are commonly known. How these systems work – what drives them and keeps them alive – can now be revealed using a 3D model developed by ETH Zurich researchers.

Simone Ulmer
Thermal structure and flow patterns in hydrothermal patterns beneath the seabed: (A) Contours with the same temperature in the Earth’s crust below the seabed. The tubular structures envelop rock zones where hot, hydrothermal fluid rises. The open ends of the tubes correspond to areas with hot sources (“black smokers”). (B) Horizontal cuts at a depth of 100m and 500m. The colors indicate the intensity of the fluid stream. (C) Three-dimensional representation of the flow pattern.
Thermal structure and flow patterns in hydrothermal patterns beneath the seabed: (A) Contours with the same temperature in the Earth’s crust below the seabed. The tubular structures envelop rock zones where hot, hydrothermal fluid rises. The open ends of the tubes correspond to areas with hot sources (“black smokers”). (B) Horizontal cuts at a depth of 100m and 500m. The colors indicate the intensity of the fluid stream. (C) Three-dimensional representation of the flow pattern. (large view)

We all know from our school days that new oceanic crusts are formed on the mid-ocean range and that the oceanic plates move away from each other. Near the seam between the two edges of the plates, cold sea water penetrates deep into the crust, where it is heated by the subterranean magma chamber. So-called hydrothermal systems with black smokers result.

Model sheds light on dynamics

Dingeman Coumou, Thomas Driesner and Christoph Heinrich from the Department of Geosciences at ETH Zurich can now illustrate which processes call these systems into being and keep them alive, and what they might look like inside near the mid-ocean ridge using a three-dimensional model. The results, which were recently published in the scientific journal “Science”, give an accurate temporal and spatial idea of the heat flow and how the hydrothermal waters circulate from it.

Nutrient provider for unique ecosystems

On its journey through the crust, the water, which is heated to several hundred degrees in the system, releases chemical elements from the rock. When it is passed back out of the seabed, the enriched water serves as a nutrient basis for unique ecosystems. The metallic elements and sulfur that are transported from the fluids to the surface of the seafloor combine to form metal sulfides, such as copper and iron, thus allowing the black smokers and potential deposits to develop. Black smokers are circular towers up to ten meters high that release the hydrothermal water back into the ocean on account of their porous rock. It is in the course of this action that the black smoke to which the smokers owe their name appears. Until now, however, the structure of these hydrothermal systems where the sea water penetrates the crust and their dynamics had largely gone unexplained.

The model developed by the ETH Zurich researchers can now provide a more accurate insight into this process. The parameters that were preset for the model – such as heat flow and the permeability of the rock – are determined by specific known data. According to what we currently know, the temperature of the magma in this kind of region is about 1,200 degrees Celsius. Various observations, such as how fast the oceanic plates move or direct measurements, recorded an average heat flow of 350 megawatts per kilometer on the mid-ocean ridge. For the permeability of the basaltic oceanic crust, the value was automatically limited by the fact that the system would overheat with too low a permeability and that the heat would be too low if the permeability were too high.

First real values as modeling basis

They fed the parameters heat flow, permeability and water depth into a model. What is novel about the model is that the dependent factors such as the density and viscosity of the water flow into the model as non-linear values and are considered as realistic values independent of the temperature. “Up to now, we have always included them in models as linear values for the sake of simplicity”, explains Thomas Driesner. “This meant you always had to fiddle around with the other parameters until a useful model materialized”.

Every modeler’s dream

The ETH Zurich scientists have now used realistic physical parameters for the first time, without considering complex geology such as sediment layers and fissures in the rock. “The result was mind-boggling”, states Driesner. In a uniform block of stone that was permeable to water, without any cracks or fissures, a hydrothermal system like those found in nature developed all on its own. Circular black smoker fields with an average diameter of 20,000 square meters formed at intervals of 500 meters. “We are convinced that we have recorded the fundamental influential physical and geological data and therefore our model displays nature effectively”, explains Driesner.

Another idea also needs to be revised. It had always been assumed that sea water penetrated the crust furthest away from the black smokers. However, the model reveals that the temperature of the object decreases from 400 degrees Celsius at its core to 100 degrees Celsius in the periphery. The heating of the black smoker’s outer region results in the reduction of the water’s viscosity – tenfold at approximately 200 degrees Celsius. Consequently, in the regions directly next to the black smokers, new sea water can seep into the crust in large amounts with unaltered permeability.

The system thus creates ideal conditions for the self-perpetuating circulation of the water and precipitation of metal sulfides. According to the authors, for example, after 100 to 1000 years considerable copper deposits can form as a result. Moreover, the water would only remain in the system for three years for it to have circulated fully.

A jackpot for deep-sea mining?

Although these seemingly mysterious and ecologically colorful works of art from the deep sea were only discovered at the end of the 1970s, they are not only a jackpot for science. The supply of raw material that results from other geological occurrences that take place on the seabed at other hydrothermal springs have already drawn interest from companies looking to operate deep-sea mines there. Despite ecological concerns that the unique ecosystems in such regions might be destroyed and the organisms inhabiting the overlying water layers badly affected by the dirt, for example, the working of such deposits is due to begin off the coast of Papua New Guinea at a depth of 1,700 meters at the end of 2009. Nevertheless, the experts believe that the deep-sea mine will not come to pass as long as there are sufficient ore deposits onshore. The effort would be too great and the cost too high. Consequently, this is currently even less of an issue for the deeper black smoker regions on the mid-ocean ridge, explains Driesner. 

Literature reference

Coumou D, Driesner T, & Heinrich, CA: The Structure and Dynamics of Mid-Ocean Ridge Hydrothermal Systems, Science, doi: 10.1126/science.1159582

 
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