Lead
The 485 m deep Lake Kivu (Rwanda, DR Congo) is a fascinating and unique aquatic system: The water contains ~60 km3 methane and ~300 km3 carbon dioxide and the water column is strongly density-stratified. The aim of this project is to understand the evolution and the maintenance of this unusual natural stratification of the lake. Due to sub-aquatic inputs of salty and carbon dioxide-rich water, Lake Kivu is strongly and permanently stratified. This stratification is unique, as the lake water contains four components affecting water density. Whereas salt and carbon dioxide increase the density in deeper layers, the temperature and methane reduce density in the layers of greater depth. This close competition leads to a so-called double-diffusive layering regime, which causes an astonishing staircase of about 300 well-mixed layers (of ~m large scale), separated by thin but very stable interfaces (of ~dm large scale).

Lay summary

of carbondioxide and the water column is permanently density-stratified. Here, we aim (1)at explaining the build-up of this unusual stratification and its temporal evolutionand (2) at understanding the build-up of the enormous methane reservoir in LakeKivu over centuries. Our results will be of economic interest as the value ofthe methane is worth > 20 billion US dollars and it is important to betterquantify the methane production and the vertical fluxes.3 of methane and ~300 km3The 485 m deepLake Kivu (Rwanda, DR Congo) is a fascinating and unique natural water body: Itcontains ~60 km

 

Due to sub-aquaticinputs of salty and carbon dioxide-rich water, Lake Kivu is strongly andpermanently stratified. Salt and carbon dioxide increase the density in deeperlayers, the temperature and methane reduce the density with greater depth. Thisclose competition between increasing and decreasing density leads to aso-called double-diffusive layering, which causes an astonishing staircase ofabout 300 well-mixed layers (of ~1 m vertical thickness), separated by thin butvery stable interfaces (of ~1 dm vertical thickness).

 

In this project, wecould demonstrate that it is possible to resolve - with specialized microstructuresensors - the vertical structures of salinity down to 1mm and verticalstructures of temperature down to 1 cm. With this high resolution we could showthat the inner core of the interfaces are laminar whereas the outer boundary ofthe interface form an unstable boundary layer of a few mm to cm thickness. Thisunstable boundary layer is the result of the faster (and therefore broader)diffusion of temperature versus the slower (and therefore narrowed spread) ofsalinity. Those unstable boundary layers form the source of the convective mixingin the adjacent mixed layers above and below any of those interfaces. We havebeen able to simulate this production of turbulence with Direct NumericalSimulation for real conditions of Lake Kivu.

 

For background information on Lake Kivu

http://www.eawag.ch/forschung/surf/gruppen/kivu/methane_harvesting/index_EN

 

For an introduction to double diffusion

http://www.eawag.ch/forschung/surf/gruppen/kivu/publications/2012_carpenter.pdf