Subterranean air in karst cavities has often low methane contents in comparison to atmosphere and so karst systems
have been considered a sink of atmospheric methane. Methane-oxidizing bacteria have been hypothesized as
responsible for CH4 depletion in many caves around the world, although ionization radiation was also proposed as
possible mechanism for this process. We measured CO2 and CH4 concentration and their C isotopic composition
(d13C-CO2 and d13C-CH4) over 2 years within the Nerja cave system (South Spain) and in 9 boreholes drilled
into the vadose zone (Triassic carbonate aquifer) surrounding the cave. According to d13C-CO2 and d13C-CH4
vadose zone of this karst system is a source of biogenic methane, produced both by acetate fermentation and CO2
reduction. Biogenic and atmospheric methane flows (along fractures from the vadose zone, and through ventilation,
respectively) into the cave, where it is oxidized by methanotrophic bacteria that we detected on soil samples
into the cave.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech
