Weak mixing in Lake Kivu: New insights indicate increasing risk of uncontrolled gas eruption

Abstract

[1] The deep waters of the East African Rift Lake Kivu contain large amounts of dissolved carbon dioxide and methane. The release of a fraction of these gases, which could be triggered by a magma eruption within the lake, would have catastrophic consequences for the two million people living on its shore. Up to now the safety assessment of the lake was based on the assumption that the gas concentrations in the deep waters are in a steady state with a residence time of 400 years. Turbulent transport was regarded as the main pathway of vertical exchange. Recent measurements and the analysis of the vertical transport processes in the lake radically change this evaluation. The vertical turbulent exchange is negligible, as documented by a spectacular set of several hundred double-diffusive layers. Gases are mainly transported out of the deep zones by a slow upwelling with a residence time of 800-1000 years. Our results indicate that the methane production within the sediment has recently increased, leading to a gas accumulation in the deep waters and consequently decreasing the heat input needed to trigger a devastating gas release. With the estimated current CH 4 production, the gas concentrations could approach saturation within this century.

Primary production in a tropical large lake: The role of phytoplankton composition

“…Huge amounts of CO 2 and methane (CH 4 ) (300 km 3 and 60 km 3 , respectively, at 0°C and 1 atm) are dissolved in the deep water layers of Lake Kivu (Degens et al, 1973;Schmid et al, 2005). Around one third of CH 4 originates from acetoclastic methanogenesis of sedimentary organic material, the other two thirds is produced by reduction of geogenic CO 2 (Schoell et al, 1988;Pasche et al, 2011).…”

“…Around one third of CH 4 originates from acetoclastic methanogenesis of sedimentary organic material, the other two thirds is produced by reduction of geogenic CO 2 (Schoell et al, 1988;Pasche et al, 2011). Deep gas concentrations increased by 15-20% for CH 4 and 10% for CO 2 in the past 30 years (Schmid et al, 2005). In an attempt to explain the increase of CH 4 concentrations in deep waters, Schmid et al (2005) simulated the Table 4 Volumetric (average in the euphotic zone) and total areal chlorophyll a (Chla) concentrations, mean annual primary production (PP) in the pelagic zone of the East African great lakes.…”

“…General physico-chemical characteristics of the lake's water are described elsewhere (e.g. Borges et al, 2011;Descy et al, 2012;Sarmento et al, 2006;Schmid et al, 2005). Limnological observations were made twice a month from March 2002to November 2009.…”

“…By contrast, the monimolimnion, i.e. the lower layer that never mixes with surface waters, is rich in nutrients and dissolved gases (Degens et al, 1973;Schmid et al, 2005). Pelagic primary production in Lake Kivu exhibits substantial variation at the seasonal scale, but also between years , as shown by the variable height of the dry season Chla peak.…”

Vertical Distribution of Functional Potential and Active Microbial Communities in Meromictic Lake Kivu

“…Lake Kivu in East Africa is a meromictic lake with a permanent density stratification separating the oxicmixolimnion from a deep anoxic monolimnion rich in dissolved salts, carbon dioxide (CO 2 ), and methane (CH 4 ) [17]. In spite of the presence of high amounts of CH 4 at the bottom of Lake Kivu, the CH 4 concentration in the oxic zone is surprisingly low compared to other lakes globally, due to intense microbial methane oxidation [18,19].…”