What lake turnovers?

Answer 1

There actually exists a process known as Lake Turnoverand an event known as Lake Overturn (Limnic Eruption). The first, known as lake turnover occurs on many large and/or deep bodies of water. As air temperatures drop so does the water temperature of the upper layers of a body of water. As these upper layers cool they become denser and heavier. Eventually they become cold enough and heavy enough to the point they begin to sink. As this heavy dense layer sinks it displaces the water at the lake bottom forcing the lower layers to the surface. It is the unique properties of the water molecule which causes water to contract down to 40 degrees F and then to begin expanding at 39 degrees F to 32 degrees F that makes this process possible without freezing the lake through. The second, known as lake overturn is also known as limnic eruption and, by all accounts, is much more rare and can be deadly. Limnic eruption occurs when a layer of carbon dioxide rich water at the bottom of a deep body of water is displaced or disturbed and rises rapidly and erupts from the surface releasing the gas into the atmosphere. These eruptions have been identified as the cause of mass death events in villages at Lake Nyos in Cameroon, Africa when these areas were overcome with the heavier-than-air carbon dioxide resulting in the suffocation of those close to the ground or unable to flee. As currently understood, carbon dioxide rich springs seep into the lake bottom and over time a thick layer of this carbon dioxide builds up in the lower portion of the lake. Pressure from the upper layers of water keeps the gases from escaping to the surface similar to how the cap on a bottle of soda water keeps the gases from escaping. This gaseous layer continues to build until it is displaced, perhaps by vulcanism, seismic activity or landslides. When any of these events occur and bring the carbon dioxide rich bottom lake layer to the top of the lake you have what is know as lake turnover.