Abiotic methane seepage in the Ronda peridotite massif, southern Spain

Abiotic methane in serpentinized peridotites (MSP) has implications for energy resource exploration,
planetary geology, subsurface microbiology and astrobiology. Once considered a rare occurrence on
Earth, reports of MSP are increasing for numerous localities worldwide in low temperature, land-based
springs and seeps. We report the discovery of six methane-rich water springs and two ponds with active gas bubbling in the Ronda peridotite massif, in southern Spain. Water is hyperalkaline with typical hydrochemical features of active serpentinization (pH: 10.7 to 11.7, T: 17.1 to 21.5 C, CaeOH facies). Dissolved CH4 concentrations range from 0.1 to 3.2 mg/L. The methane stable C and H isotope ratios in the natural spring and bubbling sites (d13CCH4: 12.3 to 37‰ VPDB; d2HCH4: 280 to 333‰ VSMOW) indicate a predominant abiotic origin. In contrast, springs with manmade water systems, i.e., pipes or fountains, appear to have mixed biotic-abiotic origin (d13CCH4: 44 to 69‰; d2HCH4: 180 to 319‰). Radiocarbon (14C) analyses show that methane C in a natural spring is older than ca. 50,000 y BP, whereas dissolved inorganic carbon (DIC) analysed in all springs has an apparent 14C age ranging from modern to 2334 y BP. Therefore most, if not all, of the CH4 is allochthonous, i.e., not generated from the carbon in the hyperalkaline water. Methane is also released as bubbles in natural ponds and as diffuse seepages (~101e102 mg CH4 m 2d 1) from the ground up to several tens of metres from the seeps and
springs, albeit with no overt visual evidence. These data suggest that the gas follows independent
migration pathways, potentially along faults or fracture systems, physically isolated from the hyperalkaline springs. Methane does not seem to be genetically related to the hyperalkaline water, which may only act as a carrier of the gas. Gas-bearing springs, vents and invisible microseepage in land-based peridotites are more common than previously thought. In addition to other geological sources, MSP is potentially a natural source of methane for the troposphere and requires more worldwide flux measurements.