Geochemical Fingerprinting of Rising Deep Endogenous Gases in an Active Hypogenic Karst System

Figure 7

Relationship between CH₄ and δ¹³C-CH₄ in underground air of Vapour Cave (VC; closed circles). The standard composition of the local background atmosphere is indicated by the green crosshairs (as a reference, the δ¹³C_CH4 value of this crosshairs separates the CH₄ involved in bacterial oxidation from CH₄ coming from other sources). Continuous curves show the locus of compositions formed by methanotrophic consumption of atmospheric methane modelled as a Rayleigh process using several kinetic fractionation factors (: from 1.005 to 1.017). Horizontal dotted lines with labels show the percentage of deep endogenous methane remaining in cave air after methanotrophic consumption. Curves for isotopic fractionation by diffusion and molecular fractionation by advection are only inferred, but not modelled.