Fear The Evil Wind

On August 15, 1986, 1,746 people and 3,500 head of cattle near Lake Nyos, Cameroon, died within minutes of one another. They didn't die to violence or an explosion- they just dropped dead. Why? The Evil Wind did it. A mazuku, Swahili for evil wind, is a pocket of carbon dioxide rich air, that, to put it bluntly, kills you by suffocation. They're invisible, scentless, and silent. The Lake Nyos mazuku erupted from Lake Nyos itself, rising into the air at 100 kilometers per hour (62 miles per hour), then descended downwards onto nearby villages, displacing the lighter air and suffocating nearly everyone within 25 kilometers (16 miles) of the lake.

A cow in Cameroon killed by a mazuku.

When it occurred, experts quickly figured out what had caused it, as a similar event had happened two years earlier at another lake, Lake Monoun, about 62 miles away. The Lake Monoun event had killed 37 people, and had at first been thought to be a terrorist attack. It was next thought to be the result of a volcanic outgassing through the lake, but that also proved not to be the case. It was, rather, a limnic eruption.

A limnic eruption, also known as a lake overturn, is when dissolved gasses (carbon dioxide, mainly, though it can include other gases as well) erupts out of the lake. Those gases then flood out into the surrounding area, suffocating any nearby animals. Plants are generally unaffected, unless the mazuku remains in a low-lying area for some time. The mechanism is, in essence, the exact same as that of a shaken soda fizzing up. The actual trigger can vary, but it must deliver some sort of shock to the system. Earthquakes, volcanic activity, landslides, or man-made explosions could all be possible triggers. Tsunamis in the lake can also be triggered by the outgassing. All known lakes vulnerable to limnic eruptions are meromicritic- that is, they have layers of water that don't intermix. (Not all meromicritic lakes are vulnerable to limnic eruptions, however.) There have, thankfully, only been two observed liminal eruptions- the one at Lake Monoun and the one at Lake Nyos. The Lake Nyos disaster seems most likely to have been caused by a landslide, though other other explanations exist, like a small volcanic eruption or a temperature differential caused by rainfall. The landslide explanation seems most likely, however.

Lake Nyos eight days after the limnic eruption. The waters are still reddish from the eruption, whereas formerly they had beed a deep blue. [Image source]

So what, exactly, can be done about limnic eruptions? Detectors aren't a lot of use- the eruptions happen too quickly to evacuate, and give little warning ahead of time. There is, however, a practical solution- degassing. Degassing involves planting a siphon in the lake- a pipe with openings at the bottom of the lake and at the surface. You start by mechanically pumping water from the bottom of the pipe to the top, and then the pressure differential/ siphon craziness takes over (seriously, siphons are nuts and barely obey the laws of physics- which speaks more for our understanding of physics than of the actual properties of siphons, of course), and essentially starts pumping itself. In the lower pressure conditions at the surface, the dissolved CO2 readily comes out, allowing for gradual degassing of the lakes, rather than a full catastrophic eruption. Lake Monoun has actually been rendered safe by degassing, and Lake Nyos is well on its way to being safe as well.

A heavy search for other at-risk lakes was performed, and another candidate was found on the border of the Democratic Republic of the Congo and Rwanda. Lake Kivu is one of the African Great Lakes, and is around 2,000 times the size of Lake Nyos. Along with CO2, massive amounts of methane are also present in the lake waters. Volcanic activity is most likely responsible for the buildup of gases in Lake Kivu, and it seems to pose a definite threat of limnic eruption. In fact, geologists have discovered evidence that it does so about every thousand years or so, causing complete extinctions of local wildlife every time it does so. The trigger seems to be volcanic activity heating the lake, forcing outgassing of methane, leading to a methane explosion that releases the CO2. If it did so today, around 2 million people would die. Unfortunately, however, simply degassing the lake would be much more problematic. It would be far more expensive than degassing the smaller Lakes Nyos and Monoun, for one. More importantly, there are 500 million tons of CO2 in the lake, equivalent to around 2 percent of the annual CO2 released by human activity- degassing it would have hugely detrimental effects on climate change. That doesn't even address the methane, an even more severe (if shorter lived) greenhouse gas than CO2. Some methane extraction, however, is occurring for reasons of profit- it's basically just natural gas. It's unknown whether the methane extraction will significantly alleviate the risks, however- it's the CO2 that's the real problem, and the methane concentration would have to be reduced an absurd amount to prevent it from triggering the CO2 eruption.

A methane extraction platform on Lake Kivu. [Image source]

We've also found fossil evidence of liminal eruptions in the Messel fossil pit in Germany. The Messel pit is made of deposits of shales and other sedimentary rocks deposited at the bottom of a huge lake bed. The lower levels of the lake were practically anoxic and had little movement, so offered ideal conditions for fossil preservation, making it a critically important site. Fossil deposits like this one that preserve soft tissues as well as bones are known as laggerstaten, and include the famous Burgess Shale, arguably the most important fossil deposit of all time. Evidence indicates that tectonic activity in the region periodically released volcanic gas into the lake, killing fish, birds and bats near the surface, and terrestrial creatures near the shore. This would account for the rock layers containing far greater than normal numbers of perfectly preserved organisms.

Limnic eruptions aren't the only cause of mazukus. Mammoth Mountain, a lava dome complex in California, is currently outgassing huge amounts of CO2 from its southern flank. This is creating mazuku in the region- enough that it's actually killing off local plantlife. So far only three people have died from them- three members of the area ski patrol on duty fell into an outgassing fumarole and suffocated. Fumaroles are openings in the crust of the planet that outgas a wide range of gases- those vents on the sides of volcanoes always emitting smoke, for instance, are fumaroles.

Trees killed by Mammoth Mountain's CO2 outgassing. [Image source]

Apart from being a serious geological hazard, mazukus are a serious risk involved in carbon sequestration efforts. Artificial carbon sequestration is the deliberate removal of CO2 from the Earth's atmosphere to combat anthropogenic climate change. (Natural carbon sequestration happens all the time, but is unable to keep up with our carbon emissions. In addition, we're doing immense amounts of damage to the Earth's carbon sequestration abilities by deforestation, destruction of peat bogs, and other environmentally destructive methods. While carbon sequestration is looking like it will become easier and cheaper than previously thought, storing CO2 in gaseous or liquid form leaves a huge risk of leaks causing fatal mazukus in their regions. This is one of the many reasons I generally advocate for carbon sequestration through reforestation, peat bog restoration, and other techniques rather than by simple CO2 storage. Even more strongly, however, I advocate for just not emitting as many greenhouse gases in the first place.

The Cave of Dogs was a disturbing Italian tourist attraction. The cave had a naturally occurring fumarole releasing huge quantities of CO2 into the cave, which accumulated in the deeper sections of the cave. Guides would suspend dogs or other small animals in it, rendering them unconscious, then restoring them in the waters of nearby Lake Agnago. The attraction was closed in 1870 when the lake was drained. [Image source]

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Bibliography:

• http://nowiknow.com/bubbling-over/
• https://en.wikipedia.org/wiki/Mazuku
• https://en.wikipedia.org/wiki/Limnic_eruption
• https://en.wikipedia.org/wiki/Lake_Nyos
• https://en.wikipedia.org/wiki/Lake_Nyos_disaster
• https://en.wikipedia.org/wiki/Meromictic_lake
• https://en.wikipedia.org/wiki/Lake_Kivu
• https://en.wikipedia.org/wiki/Lake_Monoun
• https://en.wikipedia.org/wiki/Carbon_sequestration
• https://en.wikipedia.org/wiki/Cave_of_Dogs
• https://en.wikipedia.org/wiki/Mammoth_Mountain
• https://en.wikipedia.org/wiki/Fumarole
• https://en.wikipedia.org/wiki/Messel_pit