The topic of CO2 sequestration is very much in the news these days. When chatting recently with Don Lawton about the role geophysics will play in this area, I asked him what the worst case CO2 sequestration scenario was. He replied that if there was a large scale leak of CO2 to the surface, it would pool in low lying areas as it is heavier than air. If volumes were sufficient, humans and indeed all animal life in the area would suffocate. Of course it goes without saying that the role of geophysics and other disciplines will be to reduce to zero the risk of such a disaster happening. The reason I mention this exchange is that it jogged my memory regarding a story I saw in the news some time back, about some lakes in Africa that periodically belch forth poisonous gases, killing local inhabitants and their livestock. Could these poisonous gases be CO2? Could these naturally occurring incidents be examples of what could happen with future large scale CO2 sequestration projects if due diligence isn’t followed? The following article documents my discoveries.
Introduction
There are three known “exploding” lakes in Africa that periodically burp up poisonous gases generated by volcanic activity beneath the lakes. They are Lakes Nyos and Monoun in Cameroon, and Lake Kivu in Rwanda. The lakes in the two different countries behave in a similar manner, but the reasons for the explosive behaviour differ somewhat.
Lake Kivu is a very large lake of 2700 km2, situated within the western arm of the great East African Rift Valley, north of Lake Tanganyika. The volcanic activity underneath it is directly related to the rifting.
Lakes Nyos and Monoun on the other hand, are much smaller – Nyos is approximately 1.6 km2, and Monoun is even smaller. They are actually perched on the sides of inactive volcanoes, within the Oku volcanic field, which is associated with the Cameroon Volcanic Line and its tectonics.
Lakes Nyos and Monoun
Around eighty million years ago the African Plate was rotating counter-clockwise, following its split from South America around 110 mya. This created an aulacogen, or failed rift, that began in the Atlantic west of Gabon, and extended up into the African continent almost to Lake Chad. This corridor of volcanic activity, named the Cameroon Line, is manifested as a string of volcanic islands in the Atlantic – including São Tomé, famous for its chocolate – that continues onshore as an extended mountain range that forms the border between Cameroon to the SE and Nigeria to the NW (Figure 1). Lake Nyos (indicated by letter ‘A’ inside red eyedropper symbol on figure 1) and Lake Monoun are situated near the northeastern extent of this line of tectonic activity in an area known as the Western Highlands.
In August of 1984, 137 people were killed on the shores of Lake Monoun by what appears to have been a sudden release of CO2. To pathologists the appearance of the victims resembled that of people who have experienced a domestic gas oven explosion. Two years later, in August of 1986, there was a larger and more deadly event 100 km to the north, at Lake Nyos – the death toll was 1,700 people and 3,500 domesticated animals. The knowledge gained in the Lake Monoun tragedy allowed scientists to understand what had happened at Lake Nyos more quickly, plus they had far more eye witness accounts as data points.
Approximately one cubic kilometre of CO2 gas, about 1.6 million tonnes, was released (fairly easy to calculate from the one metre drop in the lake’s water level) at Lake Nyos, and moved up to the surface at about 100 km/h. This exploded out of the lake into a ninety metre high plume of foaming gas and water. The gas spilled over the lip of the dam, and dropped down into the populated valleys below in a fifty metre thick blanket of death, that moved at about twenty to thirty kilometres per hour. After about twenty kilometres the gases had dissipated enough to no longer be deadly, but survivors who fled or were caught near the edge of the deadly zone suffered from a variety of symptoms such as lesions and respiratory ailments. A tsunami scoured some parts of the shore to bedrock (right side, Figure 2), and the lake turned a deep red colour as iron-rich lake bottom water brought to the surface oxidized.
Neither of these two lakes is actually situated above an active volcano. Instead, they are perched on the sides of inactive volcanoes, inside secondary craters known as maar. This type of crater is formed by an explosion created when hot lava comes into contact with cold groundwater. Presumably there is a sudden build up of steam; if the near surface geology is such that the steam is contained, the pressure created can blow a large chunk of the near surface rock away, leaving a crater. In the case of Lake Nyos, it is estimated the maar was created 400 years ago.
At any rate, the volcanoes these lakes are situated on have been dormant for some time. The chemistry of the lake water indicates that there is no direct release of volcanic gases into the lakes, the main indicators being a lack of acid and sulfur. Instead, the waters are CO2-rich, indicating a magmatic origin. Scientists have concluded that the CO2 is coming from remnant magma chambers about 80 km below the lakes. Gas is slowly but continually released by the magma, and rises up through the rock until it comes into contact with groundwater, which absorbs it. Water chemistry again shows that Lakes Nyos and Monoun are fed by groundwater springs. In other words there is a continual injection of gas-rich groundwater into the lakes.
Limnologists refer to lakes such as Nyos and Monoun, where the water layers don’t mix or rarely mix, as meromictic. They are stratified and stable – meaning that the deep, cold, gas-rich water in Lake Nyos stays where it is. Unless….unless there is a trigger. Given that the CO2 is continually building up, the trigger could be simple oversaturation, or it could also be an earthquake, high rainfall levels, or anything else that disrupts the stability of the lake’s equilibrium. As soon as the pressure on the CO2- rich water is reduced enough, the gas effervesces, or comes out of solution (exsolution). A convenient domestic example is a large two litre bottle of pop – when first opened the drop in pressure allows some of the dissolved gas to come bursting out of solution. Most scientists who have studied the 1986 Lake Nyos event believe that a landslide into the lake stirred up the layering, causing some of the deep gas-rich water to move up into a lower pressure zone, which triggered a local oversaturation, which led to the release of the large volume of gas.
These were not one time events. Current overall gas levels and gas concentrations in the lakes are higher than they were in the mid ‘80s. This has led some scientists to estimate that gas releases every 10 to 30 years are inevitable. Furthermore, at Lake Nyos the natural dam holding the lake in shows signs of instability, with earth tremors and high rainfall common in the area. If the dam breached it would create a two-headed monster: villages below the lake would suffer flash floods and landslides, and the change in pressure due to a smaller water column would likely cause the lake to overturn, releasing a huge volume of deadly gas.
There has been a proactive attempt to degas the lakes (Figure 3). At Nyos a French idea has been put into place, in 2001, funded by a US agency. A pump initially brings water up a pipe from the bottom to the surface . Soon the gradual depressuring of the water as it travels upwards in the pipe and the resulting exsolution of gas drives the process so that the pump is no longer needed . One such installation is operating at Lake Nyos, and the scientists involved think an additional four will provide enough degassing to make the lake safe. A similar pipe was installed at Monoun in 2003, and they think only one additional pipe is required at that lake. Early warning CO2 detection devices are in place at both lakes, and the degassing systems can be remotely controlled via satellite.
Lake Kivu
Lake Kivu is located at the highest point in the Rift Valley, 1,460m above sea level. The border between Rwanda and the Democratic Republic of the Congo runs down the middle of the lake (Figure 4). It is one of the largest lakes in Africa, and its 480m depth makes it one of the deepest lakes in the world. The fertile volcanic soil in the region and the Lake Kivu-based fishery make it a very attractive place for human habitation; between two and three million people live in the larger lake basin. Much of the recent and well documented Rwandan violence occurred on its shores, and the lake itself gained notoriety as a convenient place to dump dead bodies. Note the twin border cities of Goma and Gisenyi – their combined population is between one and oneand- a-half million people, but goes up and down depending on the current refugee situation; more on these cities later. Lake Kivu is approximately two thousand times larger than Lake Nyos, with proportionally larger volumes of gas trapped in its depths. The prospect of a sudden, large outgassing in this much more densely populated area is truly horrifying.
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Lake Kivu used to be part of a larger lake that fed north, but the volcanic Virunga Mountains to the north pushed up in the middle and created a dam between Lake Kivu and Lake Edward; Lake Kivu now flows south through the Ruzizi River into Lake Tanganyika. Some of the Virunga volcanoes can be seen in Figure 4 directly north of the lake. These mountains are home to the threatened mountain gorilla. The entire area is extremely tectonically active. Two of the eight major volcanoes are active, with the most recent major eruption occurring in 2006.
Sediment cores from the bottom of Lake Kivu show that approximately every one thousand years there have been local mass extinctions in the lake and in the surrounding onshore basin, and sudden and massive influxes of terrestrial vegetation into the lake. The natural explanation is that the lake turns over and degasses violently every thousand years or so. That said, there have been no occurrences in recent history, or at least not documented in the western sense. I have not been able to find any references to such disasters in local folklore, which I find strange, although if I knew where to look I have no doubt I’d find them. Usually if an area has experienced natural disasters there is no shortage of local folklore; in fact there is no shortage of local folklore even when nothing has happened (e.g. Loch Ness monster), but that’s another matter!
There are many similarities between the two Cameroon lakes and Lake Kivu. However, Lake Kivu is far larger, it is sitting directly on top of current volcanic activity, and there are huge volumes of methane (CH4) in solution at depth in addition to CO2. It is estimated that the lake contains 65 km3 of methane and 256 km3 of CO2. The methane is created by the breakdown of biomass at the bottom of the lake and is also directly and indirectly released into the lake from lava. Scientists hypothesize that underwater volcanic activity (i.e. direct or indirect heat from a volcanic source) could heat the water sufficiently to cause the methane to exsolve and explode, which would then release a huge volume of CO2 to the surface. The CO2 would blanket the area in a similar fashion to what happened at Nyos, albeit on a much larger scale. This would result in a mass kill of all animal life in the area. Note that the deep anoxic water coming to the surface would also kill off most or all marine life. There would almost certainly also be a large tsunami, which would swamp the shorelines creating landslides into the lake made up of massive volumes of top soil and surface vegetation.
In January of 2002 the nearby volcano Mount Nyiragongo erupted and some lava flowed through Goma, destroying about 40% of the city and killing many, many people; huge numbers of refugees swarmed across the border into Gisenyi. Lava flows extended about 150m into Lake Kivu. Figure 5 is a NASA LAndsat image created by combining C-Band Interferometric Radar and infrared data (note, vertically exaggerated). Bright red indicates hot lava, magenta is recent lava, and the grey area on the lakeshore in the foreground is the city of Goma with lava running right through its middle and into the lake. Scientists familiar with the lake and its deadly properties were on tenterhooks, hoping that the lava flows would not trigger a lake overturn. As it turns out there was no overturn, yet. Since then cracks in the volcano have formed, creating a tunnel carrying lava deeper and further into the lake. On February 3rd, 2008 there was an earthquake of magnitude 5.9 with an epicenter 20 km from the lake. Again, no gas explosion. However, one cannot help but feel that Mother Nature has created a ticking geological time bomb at Lake Kivu.
Lake Kivu has also attracted attention as an energy source. In fact, methane was produced from the lake bottom for many years before the Lake Nyos explosion raised concerns about a similar explosion at Kivu. Until 2004 a Gisenyi brewery was powered by methane from the lake. As with Lake Nyos, the depth to surface pressure differential combined with the pressure effects related to exsolution is sufficient that no external power source is required to bring the methane to the surface. It is estimated that there is enough methane added each year to the lake to satisfy Rwanda’s current energy needs, so there is hope that the methane will be commercially exploited, and the country can become a net energy exporter. Negotiations between the government and companies are under way to make this a reality, and this would be a real boon to a country that has experienced more than its share of tragedy in recent times.
The topic of natural release of CO2 can be taken to a broader scope, and I will leave readers some directions they can explore on their own.
There are many, many places on earth where volcanic activity results in periodic releases of gases, some dangerous; the ones I described in this article are more unusual because they occur under meromictic lakes. In Iceland between 1783 and 1784 eruptions in the Laki volcano system released, along with massive lava flows, huge volumes of fluorine and sulfur dioxide gases, which killed approximately half of the island’s livestock, which in turn created a famine that killed one quarter of the human population. In many locations within the Great East African Rift system there are fumaroles that give off various gas cocktails. The CO2 gas can enhance plant growth, which in turn attracts more animal life. Occasionally some of these vents burp out more than normal volumes of CO2, causing localized mass animal kills. Similar occurrences happen in any of Earth’s volcanically active areas, on various scales.
Looking back in time, many geologists explain mass extinctions, such as that which occurred at the Permian – Triassic boundary, with theories that involve natural CO2 release. For example, Gregory Ryskin of Northwestern University has written on a scenario that involves, “a water column eruption caused by the interplay of buoyancy forces and exsolution of dissolved gas.” He draws a direct comparison to the 1986 Lake Nyos eruption, but on a much larger scale. Instead of happening in a small lake, he has it happening in large silled oceanic basins, on a periodic basis at the end of the Permian. In his mind, and indeed in the minds of many scientists, theories like this are more than adequate to explain the huge global scale extinctions seen at various geological boundaries.
Lastly, there is the contentious topic of anthropogenic CO2 emissions and the effect on global warming, etc. If the Earth is continually giving off CO2, and sometimes giving it off suddenly in huge volumes, why are people so worried about CO2 emissions related to small-scale human affairs? Hasn’t Earth survived just fine through worse? I won’t comment on this topic as I am eminently unqualified to do so. All I can leave readers with is three thoughts: first, going back to my introduction, I am in no way suggesting that CO2 sequestration is setting us up for Lake Nyos type occurrences, and in fact I suspect that scenario is highly implausible; second, that I sincerely hope that rational scientific methods will determine the best strategy for humans and the Earth with regard to dealing with atmospheric emissions; and third, that I definitely do not want to experience a massive CO2 release firsthand and become a small bit of carbon within a future mass extinction geological boundary! (Or at least not until my options go up a bit in value.)
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