Limnic eruption

File:Lake nyos.jpg

Lake Nyos, silty after a limnic eruption

A limnic eruption, also referred to as a lake overturn, is a rare type of natural disaster in which carbon dioxide (CO₂) suddenly erupts from deep lake water, suffocating wildlife, livestock and humans. Such an eruption may also cause tsunamis in the lake as the rising CO₂ displaces water. Scientists believe landslides, volcanic activity, or explosions can trigger such an eruption. Lakes in which such activity occurs may be known as limnically active lakes or exploding lakes. Some features of limnically active lakes include:

• CO₂-saturated incoming water
• A cool lake bottom indicating an absence of direct volcanic interaction with lake waters
• An upper and lower thermal layer with differing CO₂ saturations
• Proximity to areas with volcanic activity

Scientists have recently determined, from investigations into the mass casualties in the 1980s at Lake Monoun and Lake Nyos, that limnic eruptions and volcanic eruptions, although indirectly related, are actually separate types of disaster events.

Historical occurrences

Cow killed by Lake Nyos

To date, this phenomenon has been observed only twice. The first was in Cameroon at Lake Monoun in 1984, causing the asphyxiation and death of 37 people living nearby. A second, deadlier eruption happened at neighbouring Lake Nyos in 1986, this time releasing over 80 million cubic meters of CO₂ and killing between 1,700 and 1,800 people, again by asphyxiation.

Due to the nature of the event, it is hard to determine if limnic eruptions have happened elsewhere. However, a third lake — Lake Kivu — containing massive amounts of dissolved CO₂ exists on the border between the Democratic Republic of the Congo and Rwanda. Sample sediments from the lake were taken by Professor Robert Hecky from the University of Michigan which showed that an event caused living creatures in the lake to go extinct approximately every thousand years, and caused nearby vegetation to be swept back into the lake.

The Messel pit fossil deposits of Messel, Germany, show evidence of a limnic eruption there in the early Eocene. Among the victims are perfectly preserved insects, frogs, turtles, crocodiles, birds, anteaters, insectivores, early primates and paleotheres.

Causes

For a limnic eruption to occur, the lake must be nearly saturated with a gas. In the two known cases, the major component was CO₂. This CO₂ may come from volcanic gas emitted under the lake, or from decomposition of organic material. Before a lake is saturated, it behaves like an unopened carbonated beverage (soft drink): the CO₂ is dissolved in the water. In both the lake and the soft drink, CO₂ dissolves much more readily when it is at a higher pressure. This is why bubbles in a can of soda only form after the drink is open; the pressure is released and the CO₂ comes out of solution. In the case of lakes, the bottom is at a much higher pressure; the deeper it is, the higher the pressure at the bottom. This means that huge amounts of CO₂ can be dissolved in large, deep lakes. Also, CO₂ dissolves more readily in cooler water, such as that at the bottom of a lake. A small rise in temperature can lead to the release of a large amount of CO₂.

Once the lake is saturated with CO₂, it is at a critical and very unstable point, and a trigger is all that is needed to set off an eruption. In the case of the 1986 eruption at Lake Nyos, landslides were the suspected triggers, but an actual volcanic eruption, an earthquake, an explosion, or even wind and rain storms are other possible triggers. In any case, the trigger pushes some of the saturated water higher in the lake, where the pressure is insufficient to keep the CO₂ in solution. Then bubbles start forming, and the water is lifted even higher in the lake (buoyancy), where even more of the CO₂ comes out of solution. This process forms a column of gas. At this point the water at the bottom of this column is pulled up by suction, and it too loses its CO₂ in a runaway process. This eruption pours CO₂ into the air, and also displaces water to form a tsunami.

There are several reasons this type of eruption is very rare. First of all, there has to be a source of the CO₂, and so regions with no volcanic activity are not prone to this kind of eruption. Also, temperate lakes, such as North America's Great Lakes region, turn over each spring and fall, mixing water from the bottom and top of the lake, so CO₂ that builds up at the bottom of the lake is brought to the top where the pressure is too low for it to stay in solution and it escapes into the atmosphere. A lake must also be quite deep to have enough pressure to put enough CO₂ into solution. So only deep, stable, tropical, volcanic lakes such as Lake Nyos are prone to limnic eruptions. As for Lake Kivu, scientists are concerned about the concentrations of methane gas along with CO₂ and the possibility of a volcanic eruption.

Consequences

Once an eruption occurs, a large CO₂ cloud forms above the lake and expands to the neighbouring region. Because CO₂ is denser than air, it has a tendency to sink to the ground while pushing breathable air up. As a result, life forms that need to breathe oxygen suffocate once the CO₂ cloud reaches them, as there is very little oxygen in the cloud. The CO₂ can make human bodily fluids very acidic, potentially causing CO₂ poisoning. As victims gasp for air they actually hurt themselves more by sucking in the CO₂ gas.

At Lake Nyos, the gas cloud descended from the lake into a nearby village where it settled, killing nearly everyone. In this eruption, some people as far as 25 km (15.5 miles) from the lake died. A change in skin color on some bodies led scientists to think that the gas cloud may have contained a dissolved acid such as hydrogen chloride as well, but that hypothesis is disputed. Many victims were found with blisters on their skin. This is believed to have been caused by pressure ulcers, which likely formed from the low levels of oxygen present in the blood of those asphyxiated by the carbon dioxide. Thousands of cattle and wild animals were also asphyxiated, but no official counts were made. On the other hand, vegetation nearby was mostly unaffected except for that which grew immediately adjacent to the lake. There the vegetation was damaged or destroyed by a 5-meter (16.4 ft.) tsunami from the violent eruption.

A possible solution: Degassing lakes

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A siphon used by French scientists in an attempt to degas Lake Nyos.

Efforts have been under way for several years to develop a solution to remove the gas from these lakes and prevent a build-up that could lead to another catastrophe. A team of French scientists began experimenting at Lake Monoun and Lake Nyos in 1990 using siphons to degas the waters of these lakes in a controlled manner. A pipe is positioned vertically in the lake with its upper end above the water's surface. Water saturated with CO₂ enters the bottom of the pipe and rises to the top. The lower pressure at the surface allows the gas to come out of solution. Interestingly, only a small amount of water has to initially be mechanically pumped through the pipe to start the flow. As the saturated water rises, the CO₂ comes out of solution and forms bubbles. The natural buoyancy of the bubbles draws the water up the pipe at high velocity causing a large fountain at the surface. The degassifying water acts as a pump, drawing more water into the bottom of the pipe, and creating a self-sustaining flow. This is the same process that leads to a natural eruption, but in this case it is controlled by the size of the pipe.

Each pipe has a limited pumping capacity and several would be required for both Lake Monoun and Lake Nyos to degas a significant fraction of the deep lake water and render the lakes safe. The deep lake waters are slightly acidic due to the dissolved CO₂ which causes corrosion to the pipes and electronics, necessitating ongoing maintenance. There are also fears that the CO₂ from the pipes could settle on the surface of the lake forming a thin layer of unbreathable air and thus causing problems for wildlife.

In January 2001, a single pipe was installed on Lake Nyos. A second pipe was installed at Lake Monoun in late 2002. These two pipes are thought to be sufficient to prevent an increase in CO₂ levels, removing approximately the same amount of gas as that naturally entering at the lake bed. In January 2003, an 18-month project had been given approval to fully degas Lake Monoun. The project appears to have been subsequently cancelled.

Lake Kivu's potential danger

Satellite image of Lake Kivu.

Lake Kivu is not only 2,000 times larger than Lake Nyos — it is also located in a far more densely populated area, with over two million people living along its shores. Fortunately, it has not reached a high level of CO₂ saturation yet. If the water were to become heavily saturated, it could become an even greater risk to human and animal life, as it is located very close to a potential trigger, Mount Nyiragongo, an active volcano that erupted in January 2002. It is also located in an active earthquake zone and close to other active volcanoes.

While the lake could be degassed in a manner similar to Lake Monoun and Lake Nyos, due to the size of the lake and the volume of gas involved such an operation would be expensive, running into millions of dollars. A scheme initiated in 2010 to utilise methane trapped in the lake as a fuel source to generate electricity in Rwanda has led to a degree of CO₂ degassing. During the procedure for extracting the flammable gas used to fire power stations on the shore, some CO₂ is removed in a process known as scrubbing. It is as yet unclear as to whether enough of the gas will be removed in this way to ensure the danger of a limnic eruption posed by Lake Kivu will be completely eliminated.

See also

• Natural disasters
• Mazuku
• Meromictic
• Lake Nyos
• Lake Monoun
• Lake Kivu

External links

• Page of the team degassing Lake Nyos
• Lake's silent killer to be disarmed
• Lake Nyos (1986)
• Limnic Eruptions
• Degassing Lake Nyos
• Cracking the Killer Lakes of Cameroon
• Development and sensitivity analysis of a model for assessing stratification and safety of Lake Nyos during artificial degassing
• Response of Lake Kivu stratification to lava inflow and climate warming
• Lake Monoun
• http://www.nyos.lv/?l=2&m=1&c=14&p=/ Russian language website about Lake Nyos