On the evening of August 21, 1986, the village of Lower Nyos in Cameroon was startled by a deep rumbling beneath nearby Lake Nyos. Within an hour, approximately 1,700 nearby people would be dead along with around 3000 livestock and countless wildlife and outside experts had no idea why. Even more unsettling was the fact that the local government had received reports of a similar occurrence almost 2 years earlier in the area of nearby (roughly speaking) Lake Monoun in which 37 people suddenly perished under almost the same mysterious circumstances. What was the cause of their deaths? Was it a set of terrorist attacks like the Cameroonian government initially suspected? Evil spirits, as suggested by a local legend? After a thorough international investigation, the cause of death was ruled to be asphyxiation caused by carbon dioxide. But what was the source? To understand that, you must first understand the phenomenon of lake stratification (skip ahead if you’re already familiar).
Lakes, like all large bodies of water, are not always uniform in terms of their water density. In fact, most lakes are sufficiently deep that layers of lower density water heated by the sun can seasonally form and “float” atop a deeper, denser layer which does not completely mix with the water above. When this happens, the lake is said to be “stratified”. In most lakes, however, this stratification is only temporary and the combined effect of seasonal temperature changes and circulation driven by surface winds is sufficient to allow the lake water to mix and the stratification to at least degrade if not collapse entirely. Such lakes are called holomictic lakes and it is generally accepted that they account for the majority of lakes on the planet.
Meromictic lakes on the other hand do not experience this regular turnover and (as a result of the prolonged incomplete mixing of the two layers) develop a more long-term and stable stratification called meromixis. In a these lakes, the denser layer of water (the monimolimnion) effectively becomes more or less inescapably trapped between the groundwater and the less dense upper layer (the mixolimnion). So how do these stable layers form in the first place?
In ectogenic meromixis there is an influx of water which is of a different salinity than the water in the lake. This could mean that there was a sudden input of saltwater into a freshwater lake (as in the case of Hemmelsdorfer See in Germany which was flooded by a storm from the Baltic Sea in 1872) or it could mean that there was an input of freshwater into a saline lake (as in the case of Mono Lake in the United States when the natural tributaries were re-established). In endogenic meromixis, internal processes within the lake itself eventually lead to formation of the layers. This could be the result of large amounts of dissolved organic material (from dead organisms) sinking to the lake bottom or, in some cases, it could be the result of calcium or iron cycling. Thermal meromixis can also occur if the bottom layer of the lake remains sufficiently cold and dense for a long enough period of time to maintain its separation from the warmer layer above. This also assumes that the processes which normally mix lake water layers (e.g. wind) are also diminished and can’t sufficiently degrade the thermal separation.
Now all this is fascinating, but I’m sure you’re wondering what it has to do with Lake Nyos? Well, the ground beneath Lake Nyos is volcanically active and as such, volcanic vents have been feeding stuff like carbon dioxide, iron, and other substances into the lake over a long period of time. As they seeped up from the ground, these substances dissolved into monimolimnion and stabilized the stratification. Now you may be thinking, “but shouldn’t most of the carbon dioxide just immediately bubble up from the bottom and into the air?” While this would likely be the case in a shallow lake, the depth of Lake Nyos is sufficient enough to allow the upper water column to exert a great deal of pressure on the high density monimolimnion below. Thus the deep waters of Lake Nyos can become supersaturated with carbon dioxide much like a pressurized can of soda. All it takes is some disturbance great enough to overcome the hydrostatic pressure and all that CO2 will begin to degas out of the lake in a limnic eruption. This is what happened on August 21, 1986 at Lake Nyos.
While experts disagree on the root cause of the degassing (landslide, upwelling, water temperature change, etc.), what is clear is that large amounts of CO2 began to bubble up from the dense, pressurized monimolimnion and rush toward the surface. The resulting displacement of the surrounding water created a sort of CO2-driven-siphon, forcing more and more of the dense water layer toward the surface where it depressurized and fueled a runaway release of gas. Soon, the entire Lake essentially erupted, blasting as much as 1.3 billion cubic meters of CO2 into the air above. (Just to give you an idea of the power of that eruption: the force of the sudden water displacement was great enough to produce a water surge approximately 80 meters high which ravaged the nearby forest.) Being denser than the surrounding air, the carbon dioxide quickly settled into a cloud (estimated to be 50 meters high at one point) which hugged the ground and silently descended 16 kilometers (10 miles) into the valley below; causing the deaths of hundreds of people, livestock, and wildlife in its path.
Since then, efforts have been made to mechanically degas the lake using specialized siphons and pumps, but the threat seems to still remain. Lake Nyos (and the few lakes like it) could experience similar eruptions in the future. Fortunately, such events appear to be rare. Regardless, Lake Nyos provides a remarkable example of an extreme consequence of meromixis and reminds us that still waters can run deep.
- Boehrer, B., & Schultze, M. (2008). Stratification of lakes. Reviews of Geophysics, 46(2).
- Evans, William C., Gregory Tanyileke, and George W. Kling. “Evolution of CO2 in Lakes Monoun and Nyos, Cameroon, before and during controlled degassing.” (2008).
- Hakala, A. (2004). Meromixis as a part of lake evolution; observations and a revised classification of true meromictic lakes in Finland. Boreal Environment Research, 9(1), 37-53.
- Patrick O’Sullivan, C. S. Reynolds, The Lakes Handbook: Limnology and Limnetic Ecology
Volume 1. John Wiley & Sons, 2008, ISBN: 9780470999264