Red wood-ant nests are traps for fault-related CH4 micro-seepage

Berberich, G.; Ellison, A.; Hartmann, J.; Berberich, M.; Grumpe, A.; Becker, A.; Wöhler, C.

doi:10.1101/154245

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bioRxiv · 10.1101/154245

Red wood-ant nests are traps for fault-related CH4 micro-seepage

Berberich, G.·Ellison, A.·Hartmann, J.·Berberich, M.·Grumpe, A.·Becker, A.·Wöhler, C.

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Abstract

Methane (CH4) is common on Earth, forms the major commercial natural gas reservoirs, and is a key component of the global carbon cycle, but its natural sources are not well-characterized. We present a geochemical dataset acquired from a red wood-ant (RWA; Formica polyctena) nest in the Neuwied Basin, a part of the East Eifel Volcanic Field (EEVF), focusing on methane (CH4), stable carbon isotope of methane ({delta}13C-CH4), RWA activity patterns, earthquakes, and earth tides. Nest gas and ambient air were continuously sampled in-situ and analyzed to detect microbial, thermogenic, and abiotic fault-related micro-seepage. Methane degassing was not synchronized with earth tides. Elevated CH4 concentrations in nest gas appear to result from a combination of microbial activity and fault-related emissions moving via through fault networks through the RWA nest. Two {delta}13C-CH4 signatures were identified in nest gas: -69{per thousand} and -37{per thousand}. The -69{per thousand} signature of {delta}13C-CH4 within the RWA nest is attributed to microbial decomposition of organic matter. This finding supports previous findings that RWA nests are hot-spots of microbial CH4. Additionally, the -37% {delta}13C-CH4 signature is the first evidence that RWA nests also serve as traps for fault-related emissions of CH4. The -37{per thousand} {delta}13C-CH4 signature can be attributed either to thermogenic/fault-related or to abiotic/fault-related CH4 formation originating from e.g. low-temperature gas-water-rock reactions in a continental setting at shallow depths (microseepage). Sources of these micro-seeps could be Devonian schists (\"Sphaerosiderith Schiefer\") with iron concretions (\"Eisengallen\"), sandstones, or the iron-bearing \"Klerf Schichten\". We cannot exclude overlapping micro-seepage of magmatic CH4 from the Eifel plume. Given the abundance of RWA nests on the landscape, their role as sources of microbial CH4 and traps for abiotically-derived CH4 should be included in estimation of methane emissions that are contributing to climatic change.

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Berberich, G., Ellison, A., Hartmann, J., Berberich, M., Grumpe, A., Becker, A., Wöhler, C.. 2017-06-23. Red wood-ant nests are traps for fault-related CH4 micro-seepage. https://doi.org/10.1101/154245

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