Limnic eruption

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A limnic eruption, also referred to as a lake overturn or exploding lake, is a rare type of natural disaster in which CO₂ suddenly erupts from deep lake water, posing the threat of 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. 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
• Close 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.

Contents 1 Historical occurrences 2 Causes 3 Consequences 4 A possible solution: Degassing lakes 5 Lake Kivu's potential danger 6 See also 7 External links

[edit] Historical occurrences

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 1700 and 1800 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.

[edit] Causes

For a limnic eruption to occur, a lake must be saturated with a gas. In the two known cases, the major component was CO₂. This CO₂ comes from volcanic gas emitted under the lake, or by decomposition of organic material accumulating on the lakebed. Before a lake is saturated, this is 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 bottle of pop 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. However, a few degrees of temperature difference are not enough to dissolve such large quantities of CO₂ as are necessary for an eruption.

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 turnover 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 be quite deep to have enough pressure to put enough CO₂ into solution too. 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.

[edit] 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 no breathable air. 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 toxic 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 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. 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 tsunami from the violent eruption.

[edit] A possible solution: Degassing lakes

Efforts have been underway 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 waters 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 bubbles natural buoyancy draws the water up the pipe at high velocity causing a large fountain to at the surface (see picture). The degassifying water acts as a pump, drawing more water into the bottom of the pipe, and the flow continues in a self-sustaining manner. 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 disolved 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, New Scientist magazine reported an 18 month project had been given approval to fully degas Lake Monoun. The project appears to have been subsequently cancelled.

[edit] Lake Kivu's potential danger

Lake Kivu is not only 2000 times larger than Lake Nyos - it is also located in a far more densely populated area, with over 2 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 degassing the lake in a similar manner as Lake Monoun and Lake Nyos could be implemented, due to the size of the lake and the volume of gas involved such an operation would be extremely expensive, running into millions of dollars. No plan has been initiated to reduce the risk posed by Lake Kivu.

[edit] See also

• Disasters
• Meromictic lakes
• Mazuku

[edit] External links

• BBC Horizon programme "Killer Lakes".
• Homepage of the team of French scientists working on degassing Lake Nyos.
• Lake's silent killer to be disarmed
• Lake Nyos (1986)
• Limnic Eruptions
• Investigations about the limnology of an active volcanic lake in Ecuador, Laguna Quilotoa
• 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/
• Article in russian
• Another article in Russian