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Structural Geological Interpretation of Settlement Patterns of Red Wood Ants (Formica rufa group) in Southwest Germany between the Black Forest and the Swabian Alb

Doi: 10.20944/preprints202312.1696.v1

Website: https://www.preprints.org/manuscript/202312.1696/v1

Am 14. April 2023 verstarb unser lieber Kollege Prof. Dr. Dietrich Klimetzek vom Institut für Forstwissenschaften, Biometrie und Umweltsystemanalyse der Universität Freiburg im Alter von 79 Jahren. Sein plötzlicher Tod hinterlässt eine große Lücke in unserer Forschung über die Standortbedingungen hügelbauender Waldameisen auf gaspermeablen tektonischen Störungszonen. Er war einer von den Ameisenforschern, die neugierig genug waren, um weit über den Tellerrand zu schauen und weit ausgetretene Pfade zu verlassen. Erst im hohen Alter lernte er die von geologischer Seite gemachte Beobachtung kennen, die einen deutlichen Zusammenhang zwischen Bruchzonen in der Erdkruste mit austretendem CO₂ und der Standortwahl von Formica belegen. Er musste immer wieder gegen seine Überzeugung eingestehen, dass es diesen Zusammenhang gibt, die Belege im Gelände mit treffsicherer Vorhersage waren einfach zu deutlich. Allerdings diskutierten wir bis zuletzt die Ursachen, bei denen er sich nicht sicher war. Leider schaffte er es nicht mehr, wichtige Daten für eine gemeinsame Veröffentlichung aufzubereiten. Trotzdem ist viel von seinem Wissen in unsere Arbeit der letzten Jahre östlich des Schwarzwaldes eingeflossen (Maresch & Schreiber, 2023). Dietrich Klimetzek war ein Entomologe mit einem ausgeprägten Forscherdrang und einem „Spezies“-eigenen Humor. Wir werden ihn vermissen.

Structural geological interpretation of settlement patterns of red wood ants (Formica rufa group) in Southwest Germany between the Black Forest and the Swabian Alb

Maresch, L., Schreiber, U.

Abstract

Based on the theory that certain geological fault zones surpass the influence of other abiotic and biotic site factors for mound-building red wood ants (Formica rufa group), an area northwest of Tuttlingen with extremely high ant density was systematically surveyed, taking into account official geological maps. Indicators of active fault zones were recorded. These were additionally examined in a model area using a high-resolution digital terrain model (DTM 1-meter). The study area is located in the Swabian Jura at the western edge of the Albstadt shear zone, where recent seismic activity is documented by frequent earthquakes. The earthquakes are considered to be the cause of gas permeability in the strike-slip faults, around which the clustered nesting sites were mapped. More than 2900 nests of the Formica rufa group were mapped and their site conditions analysed in different geomorphological positions. In addition to recording forestry site conditions, a geological classification was carried out, as well as an analysis of soil gases/mofettes in areas with the highest nest densities. The correlation between forestry data and nest sites did not reveal a consistent pattern. In contrast, the relationships with gas-permeable strike-slip faults, though predominantly covered with Quaternary sediments, could be identified through CO₂- and CH₄-rich gas emissions, mofettes, and doline features. The interpretations were complicated due to different mapping ages and signatures of the geological maps. Fortunately, this issue could be analysed and evaluated in a sub-area with mappings from 1936/71 and 1995 regarding presumed or confirmed fault trajectories. New insights were gained regarding the relationships between Formica nest sites, gas-permeable faults, and groundwater levels on narrow mountain ridges with steep relief. Here, the lateral emission of CO₂ depends on the groundwater level and does not occur vertically directly above the fault. C- and H-isotope data of CO₂ and CH₄ do not show a consistent source. While methane is mainly derived from biological production by Archaea in the upper crust and in swamp areas, CO₂ mostly originates from the mantle. The results of the investigations revealed that a multitude of geological and ecological factors must be considered for the interpretation of the occurrences of Formica sites northwest of Tuttlingen. These factors confer a distinct originality to this region, contributing not only to an enhanced understanding of site conditions but also furnishing information regarding environmentally relevant gas emissions.

Beispiel aus der Veröffentlichung:

Fig. 11a, b: Section CE2 (Fig. 1, 3) from the map sheet GK 7918 Spaichingen (State Geological Survey, Mineral Resources, and Mining Baden-Württemberg) southeast of Dürbheim, showing different observations of mapped faults (black, partially dashed lines) from a) 1936/1971 (GM 7918 Spaichingen, 1971) and b) 1995 (GM 7918 Spaichingen, 1995), along with the locations of RWA nests in 2022 (red squares) in the immediate vicinity of the faults (dashed fault lines: inferred). Smaller parallel displacements, cross faults, and Riedel shears, which always occur in association with strike-slip faults (here overprinted by normal faults), are not captured in the available resolution.