Can a Red Wood-Ant Nest Be Associated with Fault-Related CH(4) Micro-Seepage? A Case Study from Continuous Short-Term In-Situ Sampling

SIMPLE SUMMARY: Methane (CH(4)) is common on Earth but its natural sources are not well-characterized. We investigated concentrations of CH(4) and its stable carbon isotope (δ(13)C-CH(4)) within a red wood-ant (RWA; Formica polyctena) nest in the Neuwied Basin, a part of the East Eifel Volcanic Field (EEVF), and tested for associations between methane concentration and RWA activity patterns, earthquakes, and earth tides. Methane degassing was not synchronized with earth tides, nor was it influenced by a micro-earthquake or RWA activity. Elevated CH(4) concentrations in nest gas appear to result from a combination of microbial activity and fault-related emissions. The latter could result from micro-seepage of methane derived from low-temperature gas-water-rock reactions that subsequently moves via fault networks through the RWA nest or from 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 biological indicators for abiotically-derived CH(4) should be included in estimations of methane emissions that are contributing to climatic change. ABSTRACT: We measured methane (CH(4)) and stable carbon isotope of methane (δ(13)C-CH(4)) concentrations in ambient air and within a red wood-ant (RWA; Formica polyctena) nest in the Neuwied Basin (Germany) using high-resolution in-situ sampling to detect microbial, thermogenic, and abiotic fault-related micro-seepage of CH(4). Methane degassing from RWA nests was not synchronized with earth tides, nor was it influenced by micro-earthquake degassing or concomitantly measured RWA activity. Two δ(13)C-CH(4) signatures were identified in nest gas: −69‰ and −37‰. The lower peak was attributed to microbial decomposition of organic matter within the RWA nest, in line with previous observations that RWA nests are hot-spots of microbial CH(4). The higher peak has not been reported in previous studies. We attribute this peak to fault-related CH(4) emissions moving via fault networks into the RWA nest, which could originate either from thermogenic or abiotic CH(4) formation. Sources of these micro-seepages could be Devonian schists, iron-bearing “Klerf Schichten”, or 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 biological indicators for abiotically-derived CH(4) should be included in estimation of methane emissions that are contributing to climatic change.