A limnic eruption, also referred to as a lake overturn, is a rare type of natural disaster in which dissolved carbon dioxide (CO2) suddenly erupts from deep lake waters, forming a gas cloud that can suffocate wildlife, livestock, and humans. Such an eruption may also cause tsunamis in the lake as the rising CO2 displaces water. Scientists believe earthquakes, volcanic activity, or explosions can be a trigger for such a phenomenon. Lakes in which such activity occurs may be known as limnically active lakes or exploding lakes. Some features of limnically active lakes include:
Scientists have recently determined, from investigations into the mass casualties in the 1980s at Lake Monoun and Lake Nyos, that although limnic eruptions can be indirectly related to volcanic eruptions, they are actually separate types of disaster events.
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 38 people living nearby. A second, deadlier eruption happened at neighbouring Lake Nyos in 1986, this time releasing over 80 million m3 of CO2 and killing around 1,700 people and 3,500 livestock, again by asphyxiation.
Due to the nature of the event, whether limnic eruptions have happened elsewhere is hard to determine. A third lake, Lake Kivu, exists on the border between the Democratic Republic of the Congo and Rwanda, and contains massive amounts of dissolved CO2. 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 around every 1000 years, and caused nearby vegetation to be swept back into the lake. Limnic eruptions can be measured on a scale using the concentration of CO2 in the surrounding area.
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.
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For a limnic eruption to occur, the lake must be nearly saturated with gas. In the two known cases, the major component was CO2; however, in Lake Kivu, scientists[who?] are concerned about the concentrations of methane gas, as well. This CO2 may come from volcanic gas emitted from 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 at higher pressure (Henry's law). This is why bubbles in a can of soda form only after the can is opened; 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 water temperature can lead to the release of a large amount of CO2.
Once the lake is saturated with CO2, it is very unstable. Only a trigger 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, or even wind and rain storms are other possible triggers. Another possible cause of a limnic eruption is the gradual development of saturation at certain depths which can trigger the spontaneous development of gas. 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. 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 can also displace water to form a tsunami.
This type of eruption is very rare for several reasons. First, a source of the CO2 must exist, so regions with volcanic activity are most at risk. Second, the vast majority of lakes are holomictic, meaning their layers mix regularly, preventing a buildup of dissolved gases. Only meromictic lakes do not mix and remain stratified, allowing CO2 to remain dissolved. It is estimated that only one meromictic lake exists for every 1000 holomictic lakes. Finally, a lake must be quite deep to have enough pressure to dissolve large volumes of CO2.
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 very little oxygen is 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 inhaling 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 (16 mi) 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 are likely to have 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. However, 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 m (16 ft) high tsunami from the violent eruption.
The carbon dioxide released from the lake is at a very low temperature believed to cause frostbite, which is consistent with blisters found on the skin of survivors of the incident at Lake Monoun. The survivors also reported a smell of rotten eggs and feeling warm before passing out; this is explained by the fact that at high concentrations, carbon dioxide acts as a sensory hallucinogenic. Studies with jet pilots regarding carbon dioxide report similar effects as reported by survivors, i.e. smell of rotten eggs and gunpowder, and feeling of warmth.
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 led by the French scientist Michel Halbwachs 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. Only a small amount of water has to be mechanically pumped initially 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 by the French-Cameroun team on Lake Nyos and two more pipes have been installed in 2011 with funds from the United Nations Development Programme. A pipe was installed at Lake Monoun in 2003 with two more being added in 2006. These three pipes are thought to be sufficient to prevent an increase in CO2 levels, removing approximately the same amount of gas that naturally enters at the lake bed. In January 2003, an 18-month project had been given approval to fully degas Lake Monoun, and it has since been rendered safe.
Lake Kivu is not only 2,000 times larger than Lake Nyos, but is also located in a far more densely populated area, with over two million people living along its shores. It has not reached a high level of CO2saturation yet; if the water were to become heavily saturated, it could become an even greater risk to human and animal life. Two main changes in Lake Kivu's physical state that have brought attention to the possible limnic eruption are the high rates of methane dissociation and the rising surface temperature. Thorough research of historical and present temperature monitoring show that Lake Kivu's surface temperature is increasing by about 0.12 °C per decade. It is located very close to a potential trigger, Mount Nyiragongo, an active volcano that erupted in January 2002, 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 use 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 fuel power stations on the shore, some CO2 is removed in a process known as catalyst scrubbing. It is unclear whether enough of the gas will be removed to eliminate the danger of a limnic eruption at Lake Kivu.