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of carbondioxide and the water column is strongly and permanently density-stratified. Here,we want to explain the build-up of this unusual vertical stratification and itstemporal evolution. We also want to understand the build-up of the enormousmethane reservoir in Lake Kivu over centuries. Therefore, our results are alsoof economic interest: To avoid the risk of a gas eruption, the two riparian governmentshave decided to use the methane, worth > 20 billion US dollars. Therefore,it is of great interest to better quantify the methane production and verticalfluxes.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. This stratification is insofar unique, as the lakewater contains four components affecting water density. Whereas salt and carbondioxide increase the density in deeper layers, the temperature and methanereduce the density with greater depth. This close competition betweenincreasing and decreasing density leads to a so-called double-diffusivelayering, which causes an astonishing staircase of about 300 well-mixed layers(of ~1 m vertical thickness), separated by thin but very stable interfaces (of~1 dm vertical thickness).

 

So far we could demonstratethat it is possible to resolve - with specialized microstructure sensors - the verticalstructures of salinity down to 1mm and vertical structures of temperature downto 1 cm. With this high resolution we could show that the inner core of theinterfaces are laminar and the outer boundary of the interface form an unstableboundary layer of a few mm to cm thickness. This unstable boundary layer is theresult of the faster (and therefore broader) diffusion of temperature versusthe slower (and therefore narrowed spread) of salinity. Those unstable boundarylayers form the source of the convective mixing in the adjacent mixed layersabove and below any of those interfaces.

 

This project willmake a significant contribution to (1) reĀ­search of double-diffusive regimes wherefour components (salt, carbon dioxide, methane, and temperature) contribute tothe vertical density structure, and (2) to the benefit of an optimized planningof the methane extraction.

 

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