Wednesday, 1 September 2010

When Lakes Burp

In August 1986 a huge cloud of CO2 was emitted from Lake Nyos in Cameroon. The suffocating gas cloud killed 1700 villagers and thousands of livestock. Scientists were initially puzzled and locals blamed a spirit woman named ‘Mammy Water’ who lives in the lakes and rivers. Lake Nyos lies within the Oku Volcanic Field and occupies a maar crater formed from an eruption some 400 years ago. The volcanic activity beneath the lake continues and CO2 seeps through the lake bed and dissolves into the water body where it exists as a dense cold layer trapped below warmer layers of water above. It is the intense pressure from the water above that allows the CO2 to dissolve into the bottom layers without escaping to the surface.

If this layer becomes disturbed then it can rapidly rise to the surface and spread to the surrounding area with disastrous consequences for human and animal life. The disturbance could be a landslide, an earthquake, a volcanic eruption or even increased surface runoff after a very heavy rain storm. In the case of Lake Nyos, scientists believe that it was a rapid accumulation of rainwater in the lake following a storm that caused the disturbance. The rainwater may have been blown to one side of the lake due to the strong August winds. The heavier rainwater sank to the bottom of the lake causing a convective overturn which destabilised the CO2 rich bottom layer. The result was a massive explosion of CO2 form the lake causing the surface to drop by 1 metre.

Images of Lake Nyos before (left) and after (right) the 1986 gas eruption. The rising of carbon dioxide to the surface brought up iron which oxidised at the surface to give the brown colour.

Lake Nyos is still heavily studied and monitored. Pipes have been placed in the lake to siphon off the CO2 slowly over time to avoid another catastrophic build up.

Lake burping also has an impact on biodiversity. In the waters of the crater lake Barombi Mbo, in Cameroon, 11 species of fish are highly threatened and live a precarious existence as deforestation increases the risk of lake ‘burping’, where large levels of carbon dioxide are released from deep within the lake, suffocating the fish. Without management intervention these species, some of which are important food sources, may be lost forever. I’m still not entirely sure how the deforestation increases the risk. It could be due to increased risk of landslides and increased surface runoff during severe storms.

 Lake Barimbo Mbo

Perhaps of more concern than lake burping is ocean burping. Bacteria produce methane as they decompose organic matter in the ocean sediments, and in cold, high-pressure environments, methane clathrate compounds will form. This is an ice-like solid that consists of methane surrounded by water molecules in a lattice structure. However, if the temperature warms, or the pressure is reduced (for instance if local sea level decreases), the clathrate will break up and release the methane as gas which can bubble up through the ocean and enter the atmosphere. Methane is a much more powerful greenhouse gas than carbon dioxide. Some scientists believe this form of methane emission could have played a key role in the Paleocene-Eocene Thermal Maximum (PETM). This process which is also known as the clathrate gun hypothesis may have been a key cause of past mass extinctions such as the Permian extinction event 251 million years ago. Below is an excerpt from Ryskin (2003)
“The consequences of a methane-driven oceanic eruption for marine and terrestrial life are likely to be catastrophic. Figuratively speaking,the erupting region ‘‘boils over,’’ ejecting a large amount of methane and other gases (e.g., CO2, H2S) into the atmosphere, and flooding large areas of land. Whereas pure methane is lighter than air, methane loaded with water droplets is much heavier, and thus spreads over the land, mixing with air in the process (and losing water as rain). The air-methane mixture is explosive at methane concentrations between 5% and 15%; as such mixtures form in different locations near the ground and are ignited by lightning, explosions and conflagrations destroy most of the terrestrial life, and also produce great amounts of smoke and of carbon dioxide. Firestorms carry smoke and dust into the upper atmosphere, where they may remain for several years, the resulting darkness and global cooling may provide an additional kill mechanism. Conversely, carbon dioxide and the remaining methane create the greenhouse effect, which may lead to global warming.”