Methane and hydrogen in hyperalkaline groundwaters of the serpentinized Dinaride ophiolite belt, Bosnia and Herzegovina

Abstract

Methane (CH4) in continental serpentinized peridotites (MSP) has been documented in numerous hyperalkaline (pH > 9) springs and gas seeps worldwide. With a dominantly abiotic origin, MSP is often associated with considerable amounts of hydrogen (H2), produced by serpentinization. Both gases may fuel microbial activity in igneous rocks and may have played roles in the origin of life. MSP is also a natural CH4 source for the atmosphere, not included in the global greenhouse-gas budget, yet. Here we document a new and major case of MSP, in the Dinaride ophiolite belt in Bosnia and Herzegovina. CH4 is dissolved (83-2706 mM) in low temperature (13-30 °C), hyperalkaline (pH 10 to 12.8) waters in six sites, sampled through springs and boreholes. Four sites (Slanac, Vlajici, Kulasi and Ljesljani) show CH4 isotopic signatures typical of abiotic MSP (d13C: -18.5 to -35.7‰; d2H: -221 to -335.4‰); two sites (Vaiceva and Kiseljak) show a dominantly biotic signature (d13C: -58.8 and -65.1‰; d2H: -310.8 and -226.8‰), probably due to mixing with gas from coal-beds adjacent to the ultramafic rocks. H2 concentration is highly variable (up to 348 mM), ethane, propane and butane reach 0.13 vol.% in total, and helium isotopic composition (R/Ra: 0.12 to 0.48) reflects a dominant crustal signature. The Ljesljani site features the highest pH (12.8) and CH4 emission (~9 ton y-1) in peridotite-hosted hyperalkaline groundwater documented so far. Geological and geochemical data converge towards the hypothesis that, as proposed in similar cases, CH4 was mainly generated by Sabatier reaction between H2 (from serpentinization) and CO2 (from C-bearing rocks, in tectonic contact with the ophiolite, or other CO2 sources). CH4-H2-H2O disequilibria and Sabatier reaction constraints suggest that CH4 is not formed in the hyperalkaline water, but in water-free or unsaturated rocks hosting opportune metal catalysts (e.g., chromitites). The amount of methane released to the atmosphere from individual springs is comparable to that of conventional biotic gas seeps/springs in sedimentary basins.