A red wood ant supercolony as a bioindicator for neotectonic fault structures at the peninsula Bodanrueck (southwest Germany)

Red wood ants (RWA) can be used as bioindicators for the identification of neotectonic fault systems as there is a correlation of soil gas anomalies and spatial distribution of RWA mounds along those faults. For myrmecologists, the causes and stringency of such a linkage are paramount, because linear distribution patterns have mostly been associated with edge effects of forest stands and/or roads. The study area at the peninsula Bodanrueck (Lake of Constance, South West Germany) is located at the crosscut zone of two major complex tectonic structures with several faults systems that control the gas leakage from deep reservoirs in combination with uplift and subsidence. Geogenic gas analyses (carbon dioxide, methane, helium, radon and hydrogen sulphide) were performed in order to evaluate the relationship between neotectonically caused degassing and mound distribution within a RWA supercolony (Formica spp., Hymenoptera: Formicidae) of 2,300 mounds. It is divided into three clusters (1,276, 790 and 120 mounds) which are separated by two RWA-free corridors. Significantly increased gas anomalies provide evidence of gas vents along fault zones, spotty degassing anomalies, and neotectonic stress. In contrast, the RWA-free corridors show no soil gas anomalies. They are interpreted as non-gaspermeable normal faults. Additionally, the Hough transform was applied to the spatial distribution of RWA mounds and correlated with known neotectonic fault systems. They denote the preferential alignment directions matching well known fault systems. The three RWA clusters are corresponding to three tectonic blocks which directly map the main stress field and the conjugated shear system in hierarchically succession. Considering all aspects, we interpret our findings as a possible double transpressional pop-up structure. This new findings complement the neotectonic fault regime at the Bodanrueck and confirm the close correlation between RWA mounds and gaspermeable fault structures.