Strange and deadly events in Cameroon crater lakes instigate inquiry and raise concern for the future.
by
Christie Morrison
GLG 201 The Dynamic Earth
Cameroon was the sight of two very unusual natural disasters in the summers of 1984 and 1986. The release of carbon dioxide from crater lakes in the northwestern region of the country resulted in substantial loss of human and animal lives. August 15, 1984, 37 people died when Lake Monoun discharged a large amount of carbon dioxide, and August 21, 1986 was the far more deadly release at Lake Nyos, which asphyxiated 1700 people and over 3000 cattle.
Cameroon (map) is located in western Africa, along the Atlantic Ocean. The Cameroon Volcanic Line is a northeast/southwest running zone of crustal weakness extending 1600 km from islands in the Atlantic into northwestern Cameroon and northeastern Niger. This region is home to a number of small and relatively young basaltic volcanoes. Numerous maars, which are circular craters produced by volcanic explosions, have formed in the area, due to the volcanic activity. Many of these maars have subsequently formed crater lakes, including Lake Nyos and Lake Monoun. The Lake Nyos crater may be only a few hundred years old, and the lake within it is 1925 m long, 1180 m wide, and 208 m deep at its maximum. The area is no longer demonstrates volcanic activity at the earth's surface, and bottom analysis of the lake also indicates no fresh volcanic constituents.
Where did the CO2 in the lakes come from?
Several hypothesis have been
designed to answer this question, but the most likely solution is that the carbon dioxide
gas was initially magmatic in origin. Pressure increases with depth into the Earth,
and the deep source region for magmas is at pressures great enough to stabilize carbon in
the mineral dolomite. This mineral eventually breaks down due to small degrees of
partial melting of the Earth's mantle, and this causes an addition of carbon dioxide into
the melt solution. The melt subsequently rises toward the surface, and as pressure
drops, the carbon dioxide is exsolved. The gas is also released as crystallization
of magma stored in the crust forces gas and other volatiles out of solution. In this
way, magma in place in the crust while Lake Nyos formed may still be a source of carbon
dioxide.
Carbon dioxide bubbles can ascend
through the melt and along fractures in overlying crustal rock, eventually escaping into
the atmosphere. If the access channels are blocked by water instead of air, the gas
dissolves into the liquid, and this is our source of carbon dioxide rich water. Such
an input system would provide for a rather steady, constant flow of carbon dioxide into
the reservoir.
It appears that the carbon dioxide
is not entering the lake through access channels directly underneath. The volcanic
rock with underlies the crater forming Lake Nyos would be a source of ions for any spring
percolating upward underneath the lake, and ion levels in the lake do not reflect such a
scenario. More likely, carbon dioxide rich groundwater feeds the lake with gas-laden
inputs. The groundwater may be in the form of a spring which supplies Lake Nyos,
though a spring connecting directly to the lake system has not yet been identified.
The countryside is littered with carbon dioxide rich springs, making this a very potential
hypothesis.
Though it has not been explicitly
demonstrated that the gas reaches Lake Nyos through a groundwater supply, other ideas on
the source have been discarded because they do not fit the available information.
Carbon dioxide gas can often be from volcanic origins, and as such is connected with high
temperatures and eruptive processes. If this was the origin of the Lake Nyos gas,
say from an injection of magma or gas from a phreatic explosion, then the gas would have
appeared in the lake suddenly, and there would have been an accompaniment of heat and an
increase in water temperature. Volcanic gases would also add sulfur, chlorine
compounds, and hydrogen fluoride. Lake Nyos demonstrates low levels of these and
similar compounds. On top of this, no fine sediments were detected below 10 m
following the gas release, and any injection of gas from underneath the lake would have
dispersed particles of sediment up into the water which would have taken a considerable
amount of time to settle.
Alternatively, the gas could be from
biogenic sources - produced by decomposition of organic matter, but carbon from this
source would be rich in relatively young C-14, whereas the carbon dioxide in the lake is
infinitely old in comparison, on the order of 35,000 years.
Evidence indicates that there has
been a slow accumulation of carbon dioxide gas in the deep waters of the lakes, and that
the source of this carbon dioxide is originally from magmatic origins, entering the lake
through carbon dioxide rich groundwater sources.
Why does the carbon dioxide stay in the lakes?
The lakes are vertically stratified due to water density and temperature differences. The hypolimnion is the lower strata of the lake, which is cooler and more dense than the water above it. The thermocline is a sharp change in temperature between the hypolimnion below and the warmer epilimnion above. A chemocline is a similar sharp gradient, only it indicates a change in chemical composition of the water, not a temperature relation. Water density is related to both temperature and chemical composition; the cooler and more saturated, the more dense the liquid. In Cameroon lakes, the dissolved carbon dioxide enters the lake bottoms through groundwater sources, and it remains in the hypolimnion because of the permanent stratification of the lake.
What triggered the gas release?
Any disturbance that vertically moved the carbon
dioxide rich waters, from the bottom, up to a point where supersaturation conditions were
met could have triggered the gas release. As the deep water rose, the dissolved
carbon dioxide bubbled out of solution like a pop bottle fizzing when you remove the cap;
as pressure drops, the gas exsolves. Carbon dioxide is denser than air, and as the
gas left the lake, it formed a cloud which maintained its integrity and spilled over the
crater rim. The cloud hugged the landscape as it sped downhill at some 20 m/s.
Any disturbance leading to stratification break down would have
triggered the gas release, culminating in a release of 2/3 of Lake Nyos' carbon dioxide
content over a land area of 10 km. Seismic activity is not considered a plausible
trigger simply because no such activity was recorded in the area prior to the event.
A rock slide could potentially have caused the disaster, but most likely high winds
initiated the disturbance. If the hypolimnion was saturated or near saturation, even
something usually as benign as strong winds would disturb the water column enough to
initiate degassing .
Mixing of stratified lake due to external input - in this case, the wind drawn by Christie Morrison -
To the source
It is interesting that both the Lake Monoun and Lake Nyos events
occurred in August, which is the coolest, rainiest time of the year in Cameroon, and that
they occurred two years apart in a decade exhibiting record low temperatures and high
precipitation. Studies of lakes in the area have lead to the observation that decreases in
air temperature and solar heat input lead to deep mixing of lakes in the winter season.
In many Cameroon crater lakes, shelter from the wind and great depth
has helped to create very stable stratification. Lake stability is controlled by the
amount of energy available to overcome the buoyancy forces maintaining stratification and
to mix the lake to uniform density. Minimums in energy, or heat content, of the
lakes correspond with minimums in air temperature, and in Cameroon lakes, these lows fall
during August and September. The atypical trends in climate preceding the events
probably reduced water column stability and increased the potential for gas release.
Much attention has been paid to Lake Nyos and its neighbors in Cameroon following the 1986 disaster, and the carbon dioxide recharge rate of the lake has been found to be tremendously fast. Previously, scientists had estimated that it would be quite a while before the lakes could recharge to a substantial level, but following 1990 and 1992 gas concentration measurements, it appears that the bottom waters of Nyos may be saturated again in less than 20 years, and Monoun in less than 10. The carbonated groundwater percolating into the bottom of the lakes has accumulated quickly because the lakes have returned to their stable natural stratification. The chemocline boundary, separating freshwater at the surface from deeper, denser water containing dissolved gases and minerals, is keeping the carbon dioxide from uniformly spreading out within the lake or steadily contacting the surface and air.
In 1992, testing began in Lake Monoun to pump deep water through a pipe to the surface, effectively pumping out the gas-laden water and lowering the carbon dioxide levels. Once pumping is initiated, the gas released as the water reaches the top would continue to drive the flow; the pump would be basically self-propelled. The degassed water then falls right back into the lake. Studies at Lake Monoun indicate that 3 pipes, each 14 cm wide, could drain the gas in only 3 years. Lake Nyos is more of a problem because it is deeper and therefore holds more gas, and it has a weak natural dam at one end. Should this break, the upper 40 m of the lake would spill into an area below inhabited by 10,000 people and a gas release would be triggered.
At Lake Monoun in 1992, dissolved gases propel deep water up through a pipe to the surface picture and caption from Science News, April 2, 1994, volume 145, page 215
Concerned groups, like the Commission on Volcanic Lakes, are beginning to coalesce and push for activism. Engineering could combat the problems of Lake Nyos, but thus far Cameroon has not funded a project to remedy its crater lakes, and no other country has volunteered. The time bomb continues to tick.
Web Sites of interest
Published materials used
Christie Morrison
May 4, 1998
