How did Ida die?
The Messel lake, where Ida was found, lay in a crater which had formed after a volcanic eruption. Not far below the 300 m deep water there was a pocket of magma, i.e., the melted rock in the Earth’s interior. From this pocket, gases – especially carbon dioxide – may have seeped out and become dissolved in the water, similar to what you have in a pop bottle.
And in the same way that the fizz can escape when we open the bottle, the dissolved gases in the Messel lake may have been released when the water was disturbed in some way.
Today we only know of three lakes with large amounts of carbon dioxide dissolved in the water. All three are located in Central Africa, and in 1986 1700 people died when one of them, the Nyos lake, released a cloud of gas. 1.6 million tonnes were released, sufficient to lower the water level of the lake by one meter. It is unclear what triggered the disaster. It may have been an avalanche, a small volcanic eruption at the bottom of the lake, or some other cause.
Geological conditions in Messel 47 million years ago resembled those we find in these three African lakes today. Similar gas eruptions may have occurred there, but evidently volcanic gases were released smoothly, because no indications of mass death turned up until now. Birds and bats as well as flying insects were highly endangered by CO2 while flying close to the water surface. Therefore, they appear so abundant in the fossil record.
What we think happened is that Ida was down by the water’s edge just when the lake released its deadly gas. Overwhelmed by the gas, she immediately lost consciousness, fell in the water and drowned. The dead body sank to the bottom, where it was gradually buried in the silt.
The Nyos lake shortly after the gas eruption in 1986. Along the shore we can clearly see how areas which used to be covered by water have become visible because the water level has dropped. Photo: United States Geological Survey