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 per 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 uniform in terms of water density. In fact, most lakes are sufficiently deep that layers of lower density water heated by the sun seasonally form and “float” atop a deep, denser layer. When these layers don’t intermix, the lake is said to be stratified.
In most lakes, however, this effect is temporary. Seasonal temperature changes and circulation driven by surface winds ultimately allow the lake water to mix and the stratification degrades or collapses. 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, don’t experience regular water column turnover. Eventually, their prolonged incomplete mixing of the water column develops into a stable stratification called meromixis.
In these lakes, the denser layer of water (the monimolimnion) 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?
Ectogenic meromixis is an externally-driven phenomenon caused by an influx of water of a different salinity than that of the water in the lake. For example, it might involve a sudden surge of saltwater into a freshwater lake. That happened to Hemmelsdorfer See in Germany in 1872 when it was flooded with seawater by a storm in the Baltic Sea. Conversely, an input of freshwater into a saline lake can produce the same effect. This is what happened in Mono Lake in the United States when the natural tributaries were re-established.
In endogenic meromixis, internal processes within the lake itself lead to the formation of the layers. This can occur when large amounts of dissolved organic material (from dead organisms) sink to the lake bottom. It can also occur as a result of calcium or iron cycling.
Thermal meromixis can 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. In order to happen, the processes that normally mix lake water layers (e.g. wind) are must be diminished enough to maintain the thermal separation.
Lake Nyos was Primed to Erupt
Now, all that talk about meromixis is fascinating, but I’m sure you’re wondering what it has to do with Lake Nyos?
The ground beneath Lake Nyos is volcanically active. At the bottom of the lake are volcanic vents that have been pumping carbon dioxide, iron, and other substances into the lake over a long period of time. When these materials seeped up from the ground, they dissolved into the monimolimnion and stabilized the stratification.
Now you might be thinking, “but shouldn’t most of the carbon dioxide just immediately bubble up from the bottom and into the air?” That would definitely be the case in a shallow lake, but the depth of Lake Nyos is sufficient enough to allow the upper water column to exert substantial downward pressure on the monimolimnion below.
Over time, the deep waters of Lake Nyos became supersaturated with carbon dioxide, much like a can of soda. All it would take to release the gas was a disturbance great enough to overcome the hydrostatic pressure. Once disturbed, all that CO2 will begin to degas out of the lake in an event known as a limnic eruption.
This is exactly what happened on August 21, 1986 at Lake Nyos.
The Lake Bubbles Up
While experts disagree on the root cause of the degassing (landslide, upwelling, water temperature change, etc.), what is absolutely clear is that a very large amount 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. Lake Nyos was locked in a runaway release of gas.
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 plunged into the surrounding forest.
Being denser than the surrounding air, the carbon dioxide quickly coalesced into a blanket of gas (estimated to be 50 meters high at one point). That blank hugged the ground and silently slipped 16 kilometers (10 miles) into the valley below. Hundreds of people, livestock, and wildlife in its path suffocated as a result.
Since that last major eruption, 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. As long as their volcanic activity and stratification remain, there is a risk of future catastrophic events.
References and Further Reading
- 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