Limnic eruption

A limnic eruption, also termed 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 capable of suffocating wildlife, livestock, and humans. A limnic eruption may also cause tsunamis as the rising CO2 displaces water. Scientists believe earthquakes, volcanic activity, and other explosive events can serve as triggers for limnic eruptions. Lakes in which such activity occurs are referred to as limnically active lakes or exploding lakes. Some features of limnically active lakes include: A limnic eruption, also termed 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 capable of suffocating wildlife, livestock, and humans. A limnic eruption may also cause tsunamis as the rising CO2 displaces water. Scientists believe earthquakes, volcanic activity, and other explosive events can serve as triggers for limnic eruptions. Lakes in which such activity occurs are referred to as limnically active lakes or exploding lakes. Some features of limnically active lakes include: Investigations of the Lake Monoun and Lake Nyos casualties led scientists to classify limnic eruptions as a distinct type of disaster event, even though they can be indirectly linked to volcanic eruptions. Due to the largely invisible nature of the underlying cause (CO2 gas) behind limnic eruptions, it is difficult to determine to what extent eruptions have occurred in the past. In recent history, this phenomenon has been observed twice. The first recorded limnic eruption occurred in Cameroon at Lake Monoun in 1984, causing 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 m3 of CO2, killing around 1,700 people and 3,500 livestock, again by asphyxiation. A third lake, Lake Kivu, rests on the border between the Democratic Republic of the Congo and Rwanda, and contains massive amounts of dissolved CO2. Sediment samples from the lake taken by Professor Robert Hecky (University of Michigan) showed 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 detected and quantified on a CO2 concentration scale by taking air samples of the affected region. 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. For a lake to undergo a limnic eruption, the water must be nearly saturated with gas. CO2 was the primary component in the two observed cases (Lake Nyos and Lake Monoun). In Lake Kivu, scientists are concerned about the concentrations of methane gas as well. CO2 may originate 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 (e.g., a 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; when the pressure is released, 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 is at the bottom. Therefore, huge amounts of CO2 can be dissolved in large, deep lakes. CO2 also dissolves more readily in cooler water, such as that found at a lake bottom. A small rise in water temperature can lead to the release of a large amount of CO2. Once a lake is saturated with CO2, it is very unstable, but a trigger is needed to set off an eruption. In the case of the 1986 Lake Nyos eruption, landslides were the suspected triggers, but a volcanic eruption, an earthquake, or even wind and rain storms are potential triggers. Another possible cause of a limnic eruption is gradual gas saturation at specific depths which can trigger spontaneous gas development. For any of these cases, the trigger pushes some of the gas-saturated water higher in the lake, where pressure is insufficient to keep CO2 in solution. As bubbles start forming the water is lifted even higher in the lake (buoyancy), where yet more CO2 comes out of solution. This process forms a column of gas, at which point the water at the bottom of this column is pulled up by suction, and it, too, loses CO2 in a runaway process. This eruption discharges CO2 into the air and can even displace enough water to form a tsunami. Limnic eruptions are exceptionally rare for several reasons. First, a CO2 source must exist (regions with volcanic activity are most at risk). Second, the vast majority of lakes are holomictic (i.e., 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 only one meromictic lake exists for every 1,000 holomictic lakes. Finally, a lake must be deep enough to have sufficient pressure to dissolve large amounts of CO2. Once an eruption occurs, a large CO2 cloud forms above the lake and expands to the surrounding region. Because CO2 is denser than air, it has a tendency to sink to the ground, simultaneously displacing breathable air, resulting in asphyxia. CO2 can make human bodily fluids highly acidic and potentially cause CO2 poisoning. As victims gasp for air, they actually accelerate asphyxia by inhaling CO2 gas.