The accumulation of huge amounts of methane and carbon dioxide in Lake Kivu is only possible because density increases strongly with depth below ~60 m. Thus the seasonal convective mixing which happens in most other lakes is inhibited. Upward transport occurs through slow upwelling (< 1 m per year) caused by sub-aquatic inflows, and in part by “double-diffusion”. Investigating upward transport is important for a general understanding of Lake Kivu and in particular for a sustainable extraction of the methane.
Double-diffusion occurs because both temperature and salinity (in Lake Kivu also dissolved gases) are increasing with depth. In regions where the stabilizing gradient of salinity is two to eight times stronger than the destabilizing gradient of temperature, mixed layers (m-scale) form, which are separated by stable interfaces (dm-scale). Investigating the vertical fluxes of temperature, salts and dissolved gases through such double-diffusive staircases is the goal of this project.
Three field campaigns in 2010 (see movie for impressions), 2011 and 2015 have so far been conducted and the main results are:
- We showed that the most common parameterization for vertical fluxes through double-diffusive systems works well for the step sizes observed in Lake Kivu, but underestimates the heat flux in systems with larger steps (like in parts of the Arctic Ocean) by up to a factor of four.
- We conducted direct numerical simulations of double-diffusive interfaces under Lake Kivu conditions and showed that the interface structure is well reproduced by the simulations and that the heat flux through double-diffusive interfaces is close to molecular.
- The results above rely on measuring large gradients within the temperature and salinity interfaces. Even though microstructure sensors are extremely fast (ms) they are not fast enough to accurately resolve double-diffusive interfaces. We therefore developed an in-situ method to improve the current knowledge of the sensor responses.
Currently we are working on linking double-diffusion to the large-scale dynamics of a water body by studying the horizontal coherence of the layering in various systems including Lake Kivu, Powell Lake (Canada) and the Arctic Ocean.