Methane formation and future extraction in Lake Kivu

This chapter summarises the current knowledge on the vertical distribution of methane (CH4) and its formation in Lake Kivu. Additionally, we review the objectives and restrictions under consideration for sustainable extraction (safe, environmentally acceptable, and economically effective) of the enormous amount of CH4 from the lake. The harvested CH4 will be used to produce electricity which is desperately needed in both neighbouring countries: the Democratic Republic of the Congo and Rwanda. From a system-analysis point of view, the following processes need to be included as the minimum for adequately evaluating the vertical and temporal development of the lake CH4 during extraction: (1) in situ CH4 formation occurring in the permanently stratified, anoxic deep-water, (2) CH4 oxidation in the oxic surface water, (3) natural lake-water upwelling caused by subaquatic springs, (4) artificial lake-water up- and downwelling due to extraction- and reinjection-related flows, and (5) upward diffusion caused by double diffusive convection and weak turbulence. Water density is parameterised as a function of temperature, salinity, and the two gases carbon dioxide and CH4. For the sake of clarity of the presentation, we use a simplified 4-box analysis and are neglecting the diffusion process (5). This allows for the essence of the CH4 extraction challenge to be conveyed while avoiding excessive complexities. The system analysis for different CH4 extraction concepts clearly reveals that the depth of reinjection of the CH4-depleted deep-water is critical for the sustainability of the extraction and an optimal CH4 harvesting plan. Here, the suitability of different reinjection scenarios is compared by evaluating each of them in terms of the objectives "safety" (water column stability), "lake ecological integrity" (nutrient upward fluxes), and "economic viability" (amount of harvestable CH4). Comparison of model simulations, run over 50 years, revealed that (1) using lake surface (dilution) water for adjusting the density of the reinjection water and (2) reinjecting the nutrient-rich deep-water in the top 190 m are both unacceptable in terms of sustainability

Lack of steady-state in the global biogeochemical Si cycle: emerging evidence from lake Si sequestration

Weathering of silicate minerals releases dissolved silicate (DSi) to the soil-vegetation system. Accumulation and recycling of this DSi by terrestrial ecosystems creates a pool of reactive Si on the continents that buffers DSi export to the ocean. Human perturbations to the functioning of the buffer have been a recent research focus, yet a common assumption is that the continental Si cycle is at steady-state. However, we have no good idea of the timescales of ecosystem Si pool equilibration with their environments. A review of modelling and geochemical considerations suggests the modern continental Si cycle is in fact characterised in the long-term by an active accumulation of reactive Si, at least partially attributable to lakes and reservoirs. These lentic systems accumulate Si via biological conversion of DSi to biogenic silica (BSi). An analysis of new and published data for nearly 700 systems is presented to assess their contribution to the accumulating continental pool. Surface sediment BSi concentrations (n = 692) vary between zero and > 60 % SiO2 by weight, apparently independently of lake size, location or water chemistry. Using sediment core BSi accumulation rates (n = 109), still no relationships are found with lake or catchment parameters. However, issues associated with single-core accumulation rates should in any case preclude their use in elemental accumulation calculations. Based on lake/reservoir mass-balances (n = 34), our best global-scale estimate of combined lake and reservoir Si retention is 1.53 TMol year(-1), or 21-27 % of river DSi export. Again, no scalable relationships are apparent, suggesting Si retention is a complex process that varies from catchment to catchment. The lake Si sink has implications for estimation of weathering flux generation from river chemistry. The size of the total continental Si pool is poorly constrained, as is its accumulation rate, but lakes clearly contribute substantially. A corollary to this emerging understanding is that the flux and isotopic composition of DSi delivered to the ocean has likely varied over time, partly mediated by a fluctuating continental pool, including in lakes