Berberich, G. | bioRxiv Science

bioRxiv2017

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

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.

ecology↗

bioRxiv2017

Nests of red wood ants (Formica rufa-group) are positively associated with tectonic faults: a double-blind test

Ecological studies aim to better understand the distribution and abundances of organisms. Yet ecological works often are subjected to unintentional biases thus an improved framework for hypothesis testing should be used. Double-blind ecological studies are rare but necessary to minimize sampling biases and omission errors and improve the reliability of research. We used a double-blind design to evaluate associations between nests of red wood ants (Formica rufa, RWA) and the distribution of tectonic faults. We randomly sampled two regions in western Denmark to map the spatial distribution of RWA nests. We then calculated nest proximity to the nearest active tectonic faults. Red wood ant nests were eight times more likely to be found within 60 meters of known tectonic faults than were random points in the same region but without nests. This pattern paralleled the directionality of the fault system, with NNE-SSW faults having the strongest associations with RWA nests. The nest locations were collected without knowledge of the spatial distribution of active faults thus we are confident that the results are neither biased nor artefactual. This example highlights the benefits of double-blind designs in reducing sampling biases, testing controversial hypotheses, and increasing the reliability of the conclusions of research.

ecology↗