Vadose zone air as a biogenic source of methane in Nerja Cave system (South of Spain)

Abstract

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.