Impact of geogenic CO2 on the depth distribution of and feedstock provision for deep microbial communities in the Hartousov mofette system in NW Bohemia
Mineralogy, Petrology and Geochemistry
Final Report Abstract
The study area, the Hartoušov mofette system, is located in the northern Cheb Basin (Eger Rift, Czech Republic). The area is characterized by active seismicity in form of periodically occurring swarm earthquakes and lithospheric mantle derived gas emanations (> 99 % CO2) with a comparatively heavy δ13C signal (ca. -2 ‰) compared to atmospheric CO2 (ca. -8 ‰). In spring 2016 a 108.5 m deep borehole was drilled into the Hartoušov mofette field down to the weathered basement. During drilling a CO2 blow out occurred in 78.5 m depth indicating a CO2 reservoir in the deep subsurface of the Hartoušov mofette system. This CO2 reservoir was located in a low-permeable deep aquifer and was suggested to stimulate deep microbial communities due to its substrate potential. Thus, the basic scientific aims of this study were to investigate the impact of geogenic CO2 on deep microbial communities and to examine its influence on the sedimentary matrix (dissolution and precipitations of carbonates) forming the life habitat of the indigenous microorganisms. The CO2 reservoir was located at the transition between two main geological successions: the Main Seam and Cypris Formation both deposited during Early Miocene time. For a better understanding of the geological background bulk elemental data and paleovegetational biomarkers were investigated to elucidate the paleoenvironmental history. The data show that the Main Seam FM. is characterized by terrestrial sediments deposited at least partly in a swamp environment. In contrast, for the Cypris FM. the data indicated a change into a lacustrine environment with a mixed aquatic and terrestrial organic matter source. Present and past microbial life was examined using a microbial biomarker approach. As past microbial biomarkers hopanoids and glycerol dialkyl glycerol tetraethers (GDGTs) have been analysed, since these lipids are already partly degraded. The compositional change of the different past markers with the transition from the Main Seam to the Cypris FM. suggests that these biomarkers rather reflect changes during time of deposition than being a degradation signal of a currently present deep biosphere. As markers for living microbial communities intact polar lipids (IPLs) and phospholipid fatty acids (PLFAs) have been investigated, since these lipid biomarkers are usually stable only in living microorganisms. Common intact phospholipidesters (PLs) and their corresponding fatty acids (PLFAs) have not been detected in significant amounts in the investigated sediments. Their overall low abundance in the Hartoušov mofette system suggests that the CO2-saturated aquifer interval does not reflect a hotspot for deep microbial life as might be expected from a substrate point of view. However, two clusters of previously unknown bacterial IPLs likely with a sulfonic head group and different ether-linked side chains, have been identified with their highest abundance in the CO2 reservoir. This suggests the presence of specifically adapted bacteria in the mofette system, which is confirmed by the results of the microbiological partner project. In order to investigate the impact of the geogenic CO2 on the mineral matrix siderites (iron carbonates) in the Main Seam and Cypris FM have been examined. The metal ion composition indicated for the main part of the siderites early diagenetic incorporation and a syn-sedimentary siderite formation. However, changes of the metal compositional at the rim of the siderites might suggest reactivation of siderite precipitation with the onset of CO2 migration during the Late Pleistocene and the lack of siderites in the CO2 reservoir might point to carbonate dissolution processes. These results indicate possible impact of the geogenic CO2 on the microbial life habitat in the early phase of mofette activity. However, nowadays neither formation nor dissolution of carbonates are indicated as predicted by PhreeqC modeling using the current frame conditions.
Publications
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2017. Impact of geogenic CO2 in the Hartoušov mofette system in NW Bohemia. 28th International Meeting on Organic Geochemistry 2017, Florence, Italy, September 17-22, 2017
Adler, K., Alawi, M., Liu, Q., Bussert, R., Vylita, T., Schulz, H.-M., Kämpf, H., Plessen, B., Wagner, D., Mangelsdorf, K.
