Variations of gas compositions during a drilling process: A key study on the Hartoušov Mofette, Czech Republic

2020 ◽  
Author(s):  
Kyriaki Daskalopoulou ◽  
Heiko Woith ◽  
Martin Zimmer ◽  
Samuel Niedermann ◽  
Cemile D. Bağ ◽  
...  
Keyword(s):  
Czech Republic ◽  
Isotope Composition ◽  
Earthquake Swarms ◽  
Earthquake Activity ◽  
Drilling Process ◽  
Deep Biosphere ◽  
Eger Rift ◽  
Environmental Air ◽  
A Minor ◽  
Cheb Basin

<p>The Eger Rift (Czech Republic) is an intraplate region without active volcanism but with emanations of magma-derived gases and the recurrence of mid-crustal earthquake swarms with small to intermediate magnitudes (M<sub>L</sub> < 5) in the Cheb Basin. To understand the anomalous earthquake activity and CO<sub>2</sub> degassing, an interdisciplinary well-based observatory is built up for continuous fluid and earthquake monitoring at depth.</p><p>The fluid observatory is located at the Hartoušov Mofette (Cheb Basin), an area characterized by intense mantle degassing with a subcontinental lithospheric mantle (SCLM) contribution of He that increased from 38% in 1993 to 89% in 2016. Two drillings with depths of 30 and 108 m (F1 and F2, respectively) are being monitored since August 2019 for the composition of ascending fluids. Additionally, the environmental air composition is monitored. Gas concentrations were determined in-situ at 1-min intervals, while direct sampling campaigns took place periodically and samples were analyzed for their chemical and isotope composition. Samples of gases emerging in the mofette were also collected. During this period, a third borehole (F3) with a depth of 238 m was drilled.</p><p>At Hartoušov, carbon dioxide is the prevailing gas component (concentrations above 99.5%), with helium presenting a mantle origin (up to 90% considering a SCLM-type source). The atmospheric contribution is negligible, even though during drilling of F3 enrichments in atmospheric components such as Ar and N<sub>2</sub> have been observed. An increase in both CH<sub>4</sub> and He has been noticed in F2 (108 m borehole) at 40 m depth, whilst a decrease in He has been observed at 193 m depth in both F1 and the natural mofette. Enrichments in less soluble gases (eg. He and N<sub>2</sub>) at various depths accompanied by a minor CO<sub>2</sub> decrease have also been noticed. Such variations may have been caused by the different solubilities of gases in aquatic environments. Moreover, a decrease in CO<sub>2</sub> followed by a subsequent enrichment of CH<sub>4</sub> and C<sub>x</sub>H<sub>y</sub> during the first days after the initial drilling could promote the hypothesis of the generation of microbialy derived CH<sub>4</sub>. Diurnal variations were observed for the majority of the gas components during the last phase of the F3 drilling, when the well reached a depth >200 m.</p><p>This research is a part of the MoRe - “Mofette Research” project, which is included in the ICDP project “Drilling the Eger Rift: Magmatic fluids driving the earthquake swarms and the deep biosphere”). This work was supported by the DFG grant# WO 855/4-1 and BA 2207/19-1.</p>

Seismic structure of the Cheb Basin from high resolution surveying – data quality assessment and first results

2021 ◽  
Author(s):  
Natalia Banasiak ◽  
Florian Bleibinhaus
Keyword(s):  
High Resolution ◽  
Earthquake Activity ◽  
Seismic Structure ◽  
North West ◽  
The North ◽  
Eger Rift ◽  
First Results ◽  
Ground Roll ◽  
Variscan Granites ◽  
Cheb Basin

<p><span><span>In this study we present data and preliminary results from several shallow high-resolution seismic surveys in the Cheb Basin, CR, a small intracontinental basin in the North-West Bohemian Massif, located at the Western end of the Cenozoic Eger Rift. The area is well known for its intense earthquake activity, with the largest instrumentally recorded magnitude of M</span></span><span><sub><span>L</span></sub></span><span><span>=4.6. Macroseismic reports of local seismicity date back to the early 19</span></span><span><sup><span>th</span></sup></span><span><span> century, with magnitudes possibly above 5. Quaternary volcanoes, CO</span></span><span><sub><span>2</span></sub></span><span><span>-rich moffettes, and the swarm-like occurrence of the earthquakes suggest they are being triggered by crustal fluids. In contrast, most focal mechanisms show a dominant strike-slip component, indicative of tectonics. Investigating the role of fluids in triggering those earthquakes is one of the objectives of an ongoing ICDP program.</span></span></p><p><span>We expect high-resolution images of the basin structure to provide additional constraints regarding the importance of tectonic faulting. To that end, we surveyed several up to 3-km-long reflection and refraction profiles in the basin center across the putative Počátky-Plesná Fault, and at its edge, across the basin-bounding Mariánské Lázně Fault. The up to 350-m-thick basin sediments are mostly of Miocene and Quaternary origin, overlying Paleozoic Variscan units and post-Variscan granites. The main reflectors are around 200-400 ms. The data were collected with a 500-m-long split-spread of single geophones at 2 m spacing, and the raw shots are dominated by ground roll. In this presentation, we will show an overview of the field campaigns and present first results.</span></p>

scholarly journals Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an International Continental Drilling Program (ICDP) monitoring site to image fluid-related structures

Solid Earth ◽  
2019 ◽  
Vol 10 (6) ◽  
pp. 1951-1969 ◽  
Author(s):  
Tobias Nickschick ◽  
Christina Flechsig ◽  
Jan Mrlina ◽  
Frank Oppermann ◽  
Felix Löbig ◽  
...  
Keyword(s):  
Electrical Resistivity ◽  
Fault Zone ◽  
Large Scale ◽  
Monitoring Site ◽  
Earthquake Activity ◽  
Resistivity Tomography ◽  
Scientific Drilling ◽  
Eger Rift ◽  
Geophysical Surveys ◽  
Cheb Basin

Abstract. The Cheb Basin, a region of ongoing swarm earthquake activity in the western Czech Republic, is characterized by intense carbon dioxide degassing along two known fault zones – the N–S-striking Počatky–Plesná fault zone (PPZ) and the NW–SE-striking Mariánské Lázně fault zone (MLF). The fluid pathways for the ascending CO2 of mantle origin are one of the subjects of the International Continental Scientific Drilling Program (ICDP) project “Drilling the Eger Rift” in which several geophysical surveys are currently being carried out in this area to image the topmost hundreds of meters to assess the structural situation, as existing boreholes are not sufficiently deep to characterize it. As electrical resistivity is a sensitive parameter to the presence of conductive rock fractions as liquid fluids, clay minerals, and also metallic components, a large-scale dipole–dipole experiment using a special type of electric resistivity tomography (ERT) was carried out in June 2017 in order to image fluid-relevant structures. We used permanently placed data loggers for voltage measurements in conjunction with moving high-power current sources to generate sufficiently strong signals that could be detected all along the 6.5 km long profile with 100 and 150 m dipole spacings. After extensive processing of time series for voltage and current using a selective stacking approach, the pseudo-section is inverted, which results in a resistivity model that allows for reliable interpretations depths of up than 1000 m. The subsurface resistivity image reveals the deposition and transition of the overlying Neogene Vildštejn and Cypris formations, but it also shows a very conductive basement of phyllites and granites that can be attributed to high salinity or rock alteration by these fluids in the tectonically stressed basement. Distinct, narrow pathways for CO2 ascent are not observed with this kind of setup, which hints at wide degassing structures over several kilometers within the crust instead. We also observed gravity and GPS data along this profile in order to constrain ERT results. A gravity anomaly of ca. −9 mGal marks the deepest part of the Cheb Basin where the ERT profile indicates a large accumulation of conductive rocks, indicating a very deep weathering or alteration of the phyllitic basement due to the ascent of magmatic fluids such as CO2. We propose a conceptual model in which certain lithologic layers act as caps for the ascending fluids based on stratigraphic records and our results from this experiment, providing a basis for future drillings in the area aimed at studying and monitoring fluids.

ICDP project Drilling the Eger Rift – present status and further plans

2020 ◽  
Author(s):  
Torsten Dahm ◽  
Tomas Fischer ◽  
Heiko Woith ◽  
Pavla Hrubcova ◽  
Josef Vicek ◽  
...  
Keyword(s):  
Border Region ◽  
Small Scale ◽  
Earthquake Swarms ◽  
Fluid Composition ◽  
Mass Spectrometers ◽  
Monitoring Wells ◽  
Eger Rift ◽  
The Status ◽  
Cheb Basin ◽  
Natural Laboratory

<p><span>Within the ICDP-Eger drilling project we are developing one of the most modern and comprehensive laboratories at depth worldwide to study the interrelations between the flow of mantle-derived fluids through the crust and their degassing at the surface, the occurrence and characteristics of crustal earthquake swarms, and the relation to the geo-biosphere. The Cheb basin located in the western Eger Rift at the Czech-German border provides an ideal natural laboratory for such a purpose. In October 2016 the ICDP proposal was accepted for complementing two existing shallow monitoring wells with five new, distributed, medium depth (<400 m) drill holes F3 and S1-S4. </span></p><p><span>The resulting natural laboratory at depth will comprise five drilling sites for studying above mentioned phenomena. The F1-F3 drillings form a unique facility of three wells at one site within an active CO<sub>2</sub> mofette of Hartou</span><span>šov </span><span>for continuous recordings of fluid composition and fluid flow rate, as well as for intermittent GeoBio fluid sampling. Drillings S1-S4 are planned for seismological monitoring to reach a new level of high-frequency, near source observations of earthquake swarms and related phenomena such as seismic noise and tremors generated by fluid movements. Instrumentation of the seismic wells S1-S3 will include 8-element geophone chains and a bottom-hole broadband sensor. The borehole sensors will be complemented at S1 by small-scale surface array of approximately 400 m diameter to obtain truly 3D-array configurations. If possible, broadband surface stations and other sensors will be added to each drill location. </span></p><p><span>So far, we have completed drillings at sites S1, S2 and S3, with depth of </span><span>402, 480 and 400 m. </span><span>The drilling of S4 is planned in 2020 at one of the recently discovered Maars at the Czech-German border region. Drilling F3 was completed in September 2019 at a depth of 239 m. It has reached several over-pressurized, CO</span><span>2 </span><span>bearing layers. The three boreholes have been connected by underground tubes system to the nearby field laboratory equipped by flowmeters and mass spectrometers allowing for long time precise monitoring of the degassing process. The S1 borehole (Landwust) will be instrumented in January 2020 by a test geophone chain allowing, along with the DAS fibre-optic cable installed behind the casing, to carry out a VSP measurement.</span></p><p><span>In our presentation we provide information on the status of drillings, sensor installation and plans for the complete monitoring and data handling concept.</span></p>

scholarly journals Insight Into Hartoušov Mofette, Czech Republic: Tales by the Fluids

2021 ◽  
Vol 9 ◽  
Author(s):  
Kyriaki Daskalopoulou ◽  
Heiko Woith ◽  
Martin Zimmer ◽  
Samuel Niedermann ◽  
Johannes A. C. Barth ◽  
...  
Keyword(s):  
Complex System ◽  
Czech Republic ◽  
Heat Flow ◽  
Earthquake Swarms ◽  
Chemical Compositions ◽  
Chemical Concentration ◽  
Fischer Tropsch ◽  
Gas Fluxes ◽  
Cheb Basin ◽  
Insight Into

The Cheb Basin (Czech Republic) is characterized by emanations of magma-derived gases and repeated occurrences of mid-crustal earthquake swarms with small to intermediate magnitudes (ML < 4.5). Associated intense mantle degassing occurs at the Hartoušov Mofette, a representative site for the Cheb Basin. Here, we performed 14 sampling campaigns between June 2019 and March 2020. Gas samples of fluids ascending in two boreholes (F1, ∼28 m depth and F2, ∼108 m depth) and from a nearby natural mofette were analyzed for their chemical (CO2, N2, O2, Ar, He, CH4, and H2) and isotope compositions (noble gases and CO2). CO2 concentrations were above 99.1% in most samples, while O2 and N2 were below 0.6%. He ranged from 19 to 34 μmol/mol and CH4 was mostly below 12 μmol/mol. Isotope compositions of helium and carbon in CO2 ranged from 5.39 to 5.86 RA and from −2.4 to −1.3 ‰ versus VPDB, respectively. Solubility differences of the investigated gases resulted in fluctuations of their chemical compositions. These differences were accompanied by observed changes of gas fluxes in the field and at the monitoring station for F1. Variations in solubilities and fluxes also impacted the chemical concentration of the gases and the δ13C values that were also likely influenced by Fischer-Tropsch type reactions. The combination of (a) the Bernard ratio, (b) CH4/3He distributions, (c) P-T conditions, (d) heat flow, and (e) the sedimentary regime led to the hypothesis that CH4 may be of mixed biogenic and volcanic/geothermal origin with a noticeable atmospheric contribution. The drilling of a third borehole (F3) with a depth of ∼238 m in August 2019 has been crucial for providing insights into the complex system of Hartoušov Mofette.

Multi-level fluid monitoring to understand the origin of transients

2021 ◽  
Author(s):  
Heiko Woith ◽  
Kyriaki Daskalopoulou ◽  
Martin Zimmer ◽  
Tomáš Fischer ◽  
Josef Vlček ◽  
...  
Keyword(s):  
Temporal Variations ◽  
Primary Objective ◽  
High Temporal Resolution ◽  
Earthquake Swarms ◽  
Mantle Flow ◽  
Deep Biosphere ◽  
Eger Rift ◽  
The Earth ◽  
Multi Level ◽  
Tectonic Origin

<p>Anomalies in timeseries are frequently reported in the context of earthquake precursor studies. The state of knowledge can be summarized as follows: (i) significant anomalies exist, (ii) seismo-tectonically induced anomalies might exist, (iii) anomalies of non-tectonic origin exist and may look very similar to tectonic ones. Thus, presumably only a fraction of all reported precursors is real in the sense that they are of seismo-tectonic origin. A key problem in earthquake prediction research is to understand the origin of an anomaly and thus the separation of internal and external drivers like e.g. rainfall.  </p><p>State-of-the-art fluid monitoring techniques allow for a high temporal resolution compared to the low-resolution discrete sampling approach used in the last decades. A unique approach will allow to monitor ascending fluids along a vertical profile in a set of drillings from a depth of a few hundred metres to the surface. This setup can provide hints on the origin of temporal variations related to the opening of fault-valves, admixture of crustal fluids to a background mantle-flow or the release of hydrogen during fault rupturing. Gas migration velocities can thus be measured directly from the arrival times of anomalies at different depth levels. In addition, potential admixtures of mantle fluids with crustal or meteoric fluids during the ascent to the Earth’s surface can be quantified.</p><p>A prototype of a multi-level gas monitoring system has been implemented at a mofette. Mofettes are gas emission sites where CO2 ascends through long-lived, narrow channels from the deep crust and possibly the Earth’s upper mantle and thus provide natural windows to magmatic processes at depth. The primary objective of our research on mofettes is to clarify physical links between fluid properties, their pathways and the relation to swarm earthquakes. The Hartoušov mofette field with an estimated daily CO<sub>2</sub> flux between 23 and 97 t over an area of about 350,000 m<sup>2</sup> has been chosen as a key site in the frame of the ICDP project: “Drilling the Eger Rift: Magmatic fluids driving the earthquake swarms and the deep biosphere.” It is located in the Cheb Basin, which terminates the Czech part of the Eger Rift to the West and is known for recurring earthquake swarms and mantle degassing. Gas and isotope compositions will be continuously analyzed in-situ at different depth levels (30 m, 70 m, 230 m) reached by three adjacent boreholes.</p>

Near-surface conductivity structures of quaternary volcanic maars in the Western Bohemian Massif: 3D imaging using the Radio-Magnetotelluric method

2020 ◽  
Author(s):  
Gregor Willkommen ◽  
Radek Klanica ◽  
Světlana Kováčiková ◽  
Jan Mrlina ◽  
Anna Platz ◽  
...  
Keyword(s):  
Electrical Conductivity ◽  
Fault Zone ◽  
Bohemian Massif ◽  
Rift System ◽  
Earthquake Activity ◽  
Potential Core ◽  
Near Surface ◽  
Eger Rift ◽  
Cheb Basin ◽  
Scoria Cones

<p>As part of the Bohemian Massif, the Cheb Basin is one of the most active areas of the European Cenozoic Rift System. Separated from the ENE-WSW striking Eger Rift to the west by the morphological prominent Mariánské Lázne Fault Zone (MLF), the basin shows presently no active volcanism at the surface. Nonetheless it is characterized by degassing of mantle derived CO<sub>2</sub> in mofettes and mineral springs and by repeated occurrences of swarm earthquakes along the Pocátky-Plesná Zone (PPZ) and MLF near Nový Kostel. All these activities are vivid signs of ongoing magmatic processes in the lithospheric mantle. Over the last 15 years four potential maar diatreme structures were discovered and join the two known scoria cones Komorní hurka and Zelezná hurka in the western part of the Cheb Basin. Unlike scoria cones there are no prominent morphological indications for maar diatreme structures, why only modern approaches in remote sensing and systematic gravimetrical surveys led to the discovery of the Mýtina Maar in 2007 (Mrlina et. al., 2007), the Neualbenreuth Maar in 2017 (Rohrmüller et. al., 2017) and recently the two potentials Ztracený rybník maars close to Libá (Hosek et. al., 2019; Mrlina et. al. 2019). All these quaternary volcanic structures are located very close along the Tachov Fault Zone (TFZ), one of the major NNW-SSE striking fault zones of the Bohemian Massif. Maar volcanoes were formed when rising magma interacts explosively with groundwater. Advancing explosions left a cone-shaped diatreme that has been filled with post-eruptive sediments which could conduce as a climate archive for the last 300.000 years in central Europe. An interdisciplinary Project "Drilling the Eger Rift" within the International Continental Scientific Drilling Program (ICDP) targets the interactions between fluids, deep biosphere, CO<sub>2</sub> degassing and earthquake activity to shed light on the tectonic structure and related geodynamic processes. As a part of this project, Radio-Magnetotelluric (RMT) measurements were applied to image the near-surface electrical conductivity structure of these maar volcanoes. From May 2018 on, we conducted field experiments encompassing six 500 m RMT profiles across the Neualbenreuth maar, three 700 m profiles across Mýtina Maar and finally eight 400 - 1200 m long profiles over both Ztracený rybník maars. Compared with geo-electric resistivity tomography (ERT), our RMT measurements are more sensitive to conductors such as fluids or metallic compounds and were done with an areal coverage for 3D inversion and interpretation. With advanced and statistically robust data processing techniques typically applied to MT data resulted in impedance tensors in a period range of 10 kHz to 250 kHz. This RMT data sets are then modelled using inversion. The resulting 3D electrical conductivity models across the maar diatreme structures show distinct contrasts between the resistive rocks of the diatreme and the rather conductive post-eruptive sediments. The inversion results will be compared and discussed, in particular regarding a position for a potential core drilling in one of the maar structures. </p>

scholarly journals Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an ICDP monitoring drill site to image fluid-related structures

2019 ◽  
Author(s):  
Tobias Nickschick ◽  
Christina Flechsig ◽  
Jan Mrlina ◽  
Frank Oppermann ◽  
Felix Löbig ◽  
...  
Keyword(s):  
Electrical Resistivity ◽  
Fault Zone ◽  
Large Scale ◽  
Earthquake Activity ◽  
Resistivity Tomography ◽  
Near Surface ◽  
Scientific Drilling ◽  
Eger Rift ◽  
Geophysical Surveys ◽  
Cheb Basin

Abstract. The Cheb Basin, a region of ongoing swarm earthquake activity in the western Czech Republic, is characterized by intense carbon dioxide degassing along two known fault zones – the N-S-striking Počatky-Plesná fault zone (PPZ) and the NW-SE-striking Mariánské Lázně fault zone (MLF). The fluid pathways for the ascending CO2 of mantle origin are subject of an International Continental Scientific Drilling Program (ICDP) project in which several geophysical surveys are currently carried out to image the near-surface geologic situation, as existing boreholes are not sufficiently deep to characterize the structures. As electrical resistivity is a sensitive parameter to the presence of low-resistivity rock fractions as liquid fluids, clay minerals and also metallic components, a large-scale dipole-dipole experiment using a special type of electric resistivity tomography (ERT) was carried out in June 2017 in order to image fluid-relevant structures. We used static remote-controlled data loggers in conjunction with high-power current sources for generating sufficiently strong signals that could be detected all along the 6.5 km long profile with 100 m and 150 m dipole spacings. Extensive processing of time series and apparent resistivity data lead to a full pseudosection and allowing interpretation depths of more than 1000 m. The subsurface resistivity image reveals the deposition and transition of the overlying Neogene Vildštejn and Cypris formations, but also shows a very conductive basement of phyllites and granites that can be attributed to high salinization or rock alteration by these fluids in the tectonically stressed basement. Distinct, narrow pathways for CO2 ascent are not observed with this kind of setup which hints at wide degassing structures over several kilometers within the crust instead. We also observed gravity/GPS data along this profile in order to constrain ERT results. Gravity clearly shows the deepest part of the Cheb Basin along the profile, its limitation by MLF at NE end, but also shallower basement with an assumed basic intrusion in SW part of profile. We propose a conceptual model in which certain lithological layers act as caps for the ascending fluids, based on stratigraphic records and our results from this experiment, providing a basis for future drills in the area aimed at studying and monitoring fluids.

scholarly journals Drilling into an active mofette: pilot-hole study of the impact of CO<sub>2</sub>-rich mantle-derived fluids on the geo–bio interaction in the western Eger Rift (Czech Republic)

2017 ◽  
Vol 23 ◽  
pp. 13-27 ◽  
Author(s):  
Robert Bussert ◽  
Horst Kämpf ◽  
Christina Flechsig ◽  
Katja Hesse ◽  
Tobias Nickschick ◽  
...  
Keyword(s):  
Czech Republic ◽  
Active Faults ◽  
Pumping Test ◽  
Mineral Waters ◽  
Deep Biosphere ◽  
Pilot Hole ◽  
Microbial Life ◽  
Core Recovery ◽  
Eger Rift ◽  
The Impact

Abstract. Microbial life in the continental deep biosphere is closely linked to geodynamic processes, yet this interaction is poorly studied. The Cheb Basin in the western Eger Rift (Czech Republic) is an ideal place for such a study because it displays almost permanent seismic activity along active faults with earthquake swarms up to ML 4.5 and intense degassing of mantle-derived CO2 in conduits that show up at the surface in form of mofettes. We hypothesize that microbial life is significantly accelerated in active fault zones and in CO2 conduits, due to increased fluid and substrate flow. To test this hypothesis, pilot hole HJB-1 was drilled in spring 2016 at the major mofette of the Hartoušov mofette field, after extensive pre-drill surveys to optimize the well location. After drilling through a thin caprock-like structure at 78.5 m, a CO2 blowout occurred indicating a CO2 reservoir in the underlying sandy clay. A pumping test revealed the presence of mineral water dominated by Na+, Ca2+, HCO3−, SO42− (Na-Ca-HCO3-SO4 type) having a temperature of 18.6 °C and a conductivity of 6760 µS cm−1. The high content of sulfate (1470 mg L−1) is typical of Carlsbad Spa mineral waters. The hole penetrated about 90 m of Cenozoic sediments and reached a final depth of 108.50 m in Palaeozoic schists. Core recovery was about 85 %. The cored sediments are mudstones with minor carbonates, sandstones and lignite coals that were deposited in a lacustrine environment. Deformation structures and alteration features are abundant in the core. Ongoing studies will show if they result from the flow of CO2-rich fluids or not.

scholarly journals Microbial Signatures in Deep CO2-Saturated Miocene Sediments of the Active Hartoušov Mofette System (NW Czech Republic)

2020 ◽  
Vol 11 ◽  
Author(s):  
Qi Liu ◽  
Karsten Adler ◽  
Daniel Lipus ◽  
Horst Kämpf ◽  
Robert Bussert ◽  
...  
Keyword(s):  
Microbial Communities ◽  
Dna Analysis ◽  
Quantitative Polymerase Chain Reaction ◽  
Lacustrine Sediments ◽  
Deep Biosphere ◽  
Upward Migration ◽  
Eger Rift ◽  
And Migration ◽  
Cheb Basin ◽  
The Impact

The Hartoušov mofette system is a natural CO2 degassing site in the central Cheb Basin (Eger Rift, Central Europe). In early 2016 a 108 m deep core was obtained from this system to investigate the impact of ascending mantle-derived CO2 on indigenous deep microbial communities and their surrounding life habitat. During drilling, a CO2 blow out occurred at a depth of 78.5 meter below surface (mbs) suggesting a CO2 reservoir associated with a deep low-permeable CO2-saturated saline aquifer at the transition from Early Miocene terrestrial to lacustrine sediments. Past microbial communities were investigated by hopanoids and glycerol dialkyl glycerol tetraethers (GDGTs) reflecting the environmental conditions during the time of deposition rather than showing a signal of the current deep biosphere. The composition and distribution of the deep microbial community potentially stimulated by the upward migration of CO2 starting during Mid Pleistocene time was investigated by intact polar lipids (IPLs), quantitative polymerase chain reaction (qPCR), and deoxyribonucleic acid (DNA) analysis. The deep biosphere is characterized by microorganisms that are linked to the distribution and migration of the ascending CO2-saturated groundwater and the availability of organic matter instead of being linked to single lithological units of the investigated rock profile. Our findings revealed high relative abundances of common soil and water bacteria, in particular the facultative, anaerobic and potential iron-oxidizing Acidovorax and other members of the family Comamonadaceae across the whole recovered core. The results also highlighted the frequent detection of the putative sulfate-oxidizing and CO2-fixating genus Sulfuricurvum at certain depths. A set of new IPLs are suggested to be indicative for microorganisms associated to CO2 accumulation in the mofette system.

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