Methane sources and sinks in Lake Kivu

TLDR: A recent study reported that CH 4 concentrations in Lake Kivu have increased by up to 15% in the last 30 years and that accumulation at this rate could lead to catastrophic outgassing by ∼2100 as mentioned in this paper.

Abstract: Unique worldwide, Lake Kivu stores enormous amounts of CH 4 and CO 2 . A recent study reported that CH 4 concentrations in the lake have increased by up to 15% in the last 30 years and that accumulation at this rate could lead to catastrophic outgassing by ∼2100. This study investigates the present-day CH 4 formation and oxidation in Lake Kivu. Analyses of 14C and 13C in CH 4 and potential carbon sources revealed that below 260 m, an unusually high ∼65% of the CH 4 originates either from reduction of geogenic CO 2 with mostly geogenic H 2 or from direct inflows of geogenic CH 4 . Aerobic CH 4 oxidation, performed by close relatives of type X CH 4 -oxidizing bacteria, is the main process preventing CH 4 from escaping to the atmosphere. Anaerobic CH 4 oxidation, carried out by CH 4 -oxidizing archaea in the SO 4 2--reducing zone, was also detected but is limited by the availability of sulfate. Changes in 14C CH4 and 13C CH4 since the 1970s suggest that the amount of CH 4 produced from degrading organic material has increased due to higher accumulation of organic matter. This, as well as the sudden onset of carbonates in the 1960s, has previously been explained by three environmental changes: (1) introduction of nonnative fish, (2) amplified subaquatic inflows following hydrological changes, and (3) increased external inputs due to the fast growing population. The resulting enhancement of primary production and organic matter sedimentation likely caused CH 4 to increase. However, given the large proportion of old CH 4 carbon, we cannot exclude an increased inflow of geogenic H 2 or CH 4 . Copyright 2011 by the American Geophysical Union.