Impact of geogenic CO2 on the depth distribution of and feedstock provision for deep microbial communities in the Hartousov mofette system in NW Bohemia

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The Hartoušov mofette system located in the northern Cheb Basin (NW Bohemia, Czech Republic) represents a diffuse degassing structure of exhaling mantle derived gases dominated by CO2 with a relative heavy d13C signature (-2 ‰). From a biogeochemical and microbiological point of view these CO2 seeps form an interesting and unique life habitat for microbial communities with intense geogenic CO2 fluxes forming a particular carbon and energy source for the indigenous microbial community. Thus, the aim of the current study is the investigation of the impact of degassing CO2 on the shallow and deep microbial communities in the mofette system and their carbon and energy sources using organic-geochemical, isotope geochemical and biogeochemical approaches. In spring 2016 the GFZ drilled a 108.5 m deep borehole into the Hartoušov mofette field recovering the entire sedimentary successions down to the weathered basement. In 78.5 m depth at the transition from terrestrial Oligocene to lacustrine Miocene claystones a CO2 blow out occurred suggesting a kind of “CO2 reservoir” at this depth interval, whereas the lacustrine claystones might act as a kind of seal. Investigations on the mineral matrix below this “seal” suggest an interaction with the ascending CO2. In the deeper terrestrial Oligocene deposits the geogenic CO2 appears to stimulate the precipitation of zoned calcium-magnesium siderites, while in the “CO2 reservoir” section carbonates are dissolved due to the high CO2 concentrations in the pore water. The d13C signal of the bulk organic matter and the compound specific d13C signal of the first investigated past microbial biomarkers reveal significantly heavier d13C signals for the deposits below the lacustrine Miocene claystones, indicating an influence of the ascending geogenic CO2 also on the organic matrix in the deeper core interval. Although our first results provide indications on the interaction between the geogenic CO2 and the mineral and organic matrix in the deep subsurface, further investigations are necessary to verify these interactions and to understand the involved processes.