Can a red wood-ant nest be a trap for fault-related CH4 micro-seepage? A case study from continuous short-term in-situ sampling

Methane (CH 4 ) 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 (CH 4 ), stable carbon isotope of methane (δ 13 C-CH 4 ), 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 δ 13 C-CH 4 signatures were identified in nest gas: −69‰ and −37‰. The −69‰ signature of δ 13 C-CH 4 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 CH 4 . Additionally, the −37% δ 13 C-CH 4 signature is the first evidence that RWA nests also serve as traps for fault-related emissions of CH 4 . The −37‰ δ 13 C-CH 4 signature can be attributed either to thermogenic/fault-related or to abiotic/fault-related CH 4 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 CH 4 from the Eifel plume. Given the abundance of RWA nests on the landscape, their role as sources of microbial CH 4 and traps for abiotically-derived CH 4 should be included in estimation of methane emissions that are contributing to climatic change.