This constant increase in salinity according to depth has two origins. Firstly, sedimentation. “The elements on the surface gradually sink. Once below 250 metres, these sediments are blocked and can no longer go up again, as though there were a lid at this depth. So, little by little, the deep waters become enriched with all these elements that were on the surface, especially nutrients." This is called the biogenic source, meaning that it results from the sedimentation of biological matter, which “dies” and sinks into the deep waters. Secondly, there is a geogenic source of salts in deep waters. At approximately 250 metres below the surface, Lake Kivu is supplied with spring water, which has probably infiltrated through faults in the earth of volcanic origin.  This water is much warmer and should therefore be lighter, owing to its warmth, and rise to the surface. But it has a high salt content. Therefore, its density is greater so it also sinks.

Where do the carbon dioxide and methane come from?

At present, the CH₄ and CO₂ are dissolved and stored in the layers of water below this "lid". The CO₂ is hypothetically of volcanic origin. Five sources of deep water have a sufficiently high water flow to allow observation, and they supply the majority of this gas. “As for the methane, it’s a bit more difficult to say”, the researcher explains. “Like the CO₂, the CH₄ could be the result of volcanic activity, but we have never been able to prove it. The surface sources of water don’t contain methane, so we can justifiably think that the deep sources don’t either. The CH₄ must therefore be biogenic, i.e., of biological origin. The most likely hypothesis is that this methane results from the anaerobic decomposition of organic matter. We know that during anaerobiosis, organic matter releases carbon as it decomposes, half of which is transformed into CO₂ and the other half into CH₄, according to the formula CH₃COOH (acetate) → CO₂ + CH₄.” In Lake Kivu, the anaerobic degradation of organic matter is responsible for approximately one third of the formation of the CH₄.

The remaining two thirds would therefore result from a bacterial reduction of the CO₂ present in the lake. “For this reduction to take place, there needs to be a reducer. In the case of the formation of methane, this is usually dihydrogen. The hydrogen in Lake Kivu could also have two origins. It could be volcanic, which would explain the formation of these two thirds of methane with no contribution from the organic matter in the lake. The other, most likely hypothesis would be, once again, the decomposition of organic matter. We know that before the formation of acetate, dihydrogen can form during the degradation of organic matter, which could subsequently produce methane through the reduction of carbon dioxide. Hence, although two-thirds of the carbon integrated in the methane seemingly result from volcanic activity, a biological production of organic matter would be necessary for it to occur.”

Dangerous prisoners of the deep layers

These waters with a high concentration of methane are potentially of a considerable richness. For instance, this gas can be used as fuel to produce electricity. But such a great quantity of gas dissolved by the pressure of the deep water, added to the even greater amount of CO₂, equates to a time bomb that could end in disaster. 

“Two lakes in Cameroon also contain large quantities of dissolved CO₂, though much less than in Lake Kivu.  Lake Monoun suffered from a gas eruption in 1984, and Lake Nyos, which erupted in 1986, killed approximately 2000 people.” There can be several causes behind a limnic eruption. The first is an accumulation and thus an excessive concentration of gas that cannot dissolve in water that is already saturated. Large quantities subsequently rise to the surface in the form of bubbles. A gas eruption could also be caused by a trigger (such as a volcanic eruption, a landslide or an earthquake), which would create underwater waves containing dissolved gas. These waters would rise into a zone of lower pressure and therefore suddenly release the supersaturated gas as the hydrostatic pressure decreases.