Stratification, mixing and transport processes in Lake Kivu

This chapter summarizes the knowledge on mixing and transport processes in Lake Kivu. Seasonal mixing, which varies in intensity from year to year, influences the top â¼65 m. Below, the lake is permanently stratified, with density increasing stepwise from â¼998 kg mâ3 at the surface to â¼1,002 kg mâ3 at the maximum depth of 485 m. The permanently stratified deep water is divided into two distinctly different zones by a main gradient layer. This gradient is maintained by a strong inflow of relatively fresh and cool water entering at â¼250 m depth which is the most important of several subaquatic springs affecting the density stratification. The springs drive a slow upwelling of the whole water column with a depth-dependent rate of 0.15â0.9 m yearâ1. This upwelling is the main driver of internal nutrient recycling and upward transport of dissolved gases. Diffusive transport in the deep water is dominated by double-diffusive convection, which manifests in a spectacular staircase of more than 300 steps and mixed layers. Double diffusion allows heat to be removed from the deep zone faster than dissolved substances, supporting the stable stratification and the accumulation of nutrients and gases over hundreds of years. The stratification in the lake seems to be near steady-state conditions, except for a warming trend of â¼0.01°C yearâ1

Are limnic eruptions in the CO2-CH4-rich gas reservoir of Lake Kivu (Democratic republic of the Congo and Rwanda) possible? Insights from physico-chemical and isotopic data

An overturn of Lake Kivu was seriously considered after the January 2002 Nyiragongo volcanic eruption, which erupted 20 × 106 m3 of lava from a NS-oriented fissure on the southern flank of the Congolese volcano. Part of this silica-undersaturated magma produced a lava flow that entered Lake Kivu down to a depth of 70-100 m. The possibility of a rollover comes from the fact that Lake Kivu is meromictic and below ≈250 m a CO2-CH4-rich gas reservoir is present. Thus, the riparian population (≈2,500,000 people) is endangered by a possible limnic eruption. During last 30 years several vertical profiles carried out by several researchers have evidenced a relatively pronounced vertical physico-chemical and isotopical variation. Nevertheless, saturation of CO2 and CH4 appears to be far from critical values, indicating that presently the hydrostatic pressure cannot presently be overcome. Recent studies have suggested an increase in dissolved gases (particular CH4) at depth and the uprising of the chemocline, a limnic eruption could possibly occur within 80-200 years from now. More studies are needed to follow up the lake evolution with time. Simulations will shed light on possible internal and external factors able to provoke the release a suffocating and, possibly, inflammable killer cloud. In this paper, we investigate using recent historical eruptive events of Nyiragongo volcano, we discuss the possibility that a limnic eruption may occur in a near future, although the stability of Lake Kivu is presently high: only exceptionally high magnitude events appear to be able to destabilize the 560 × 109 m3 water volume contained in its basin