Mofette Field Research Articles

Effect of Pressure Perturbations on CO2 Degassing in a Mofette System: The Case of Hartoušov, Czech Republic

Mofettes are gas emission sites where high concentrations of CO2 ascend through conduits from as deep as the mantle to the Earth’s surface and as such provide direct windows to processes at depth. The Hartoušov mofette, located at the western margin of the Eger Graben, is a key site to study interactions between fluids and swarm earthquakes. The mofette field (10 mofettes within an area of 100 m × 500 m and three wells of 28, 108, and 239 m depth) is characterized by high CO2 emission rates (up to 100 t/d) and helium signatures with (3He/4He)c up to 5.8 Ra, indicating mantle origin. We compiled geological, geophysical, geochemical, and isotopic data to describe the mofette system. Fluids in the Cheb basin are mixtures between shallow groundwater and brine (>40 g/L at a depth of 235 m) located at the deepest parts of the basin fillings. Overpressured CO2-rich mineral waters are trapped below the mudstones and clays of the sealing Cypris formation. Drilling through this sealing layer led to blow-outs in different compartments of the basin. Pressure transients were observed related to natural disturbances as well as human activities. External (rain) and internal (earthquakes) events can cause pressure transients in the fluid system within hours or several days, lasting from days to years and leading to changes in gas flux rates. The 2014 earthquake swarm triggered an estimated excess release of 175,000 tons of CO2 during the following four years. Pressure oscillations were observed at a wellhead lasting 24 h with increasing amplitudes (from 10 to 40 kPa) and increasing frequencies reaching five cycles per hour. These oscillations are described for the first time as a potential natural analog to a two-phase pipe–relief valve system known from industrial applications.

ICDP drilling of the Eger Rift observatory: magmatic fluids driving the earthquake swarms and deep biosphere

Abstract. The new in situ geodynamic laboratory established in the framework of the ICDP Eger project aims to develop the most modern, comprehensive, multiparameter laboratory at depth for studying earthquake swarms, crustal fluid flow, mantle-derived CO2 and helium degassing, and processes of the deep biosphere. In order to reach a new level of high-frequency, near-source and multiparameter observation of earthquake swarms and related phenomena, such a laboratory comprises a set of shallow boreholes with high-frequency 3-D seismic arrays as well as modern continuous real-time fluid monitoring at depth and the study of the deep biosphere. This laboratory is located in the western part of the Eger Rift at the border of the Czech Republic and Germany (in the West Bohemia–Vogtland geodynamic region) and comprises a set of five boreholes around the seismoactive zone. To date, all monitoring boreholes have been drilled. This includes the seismic monitoring boreholes S1, S2 and S3 in the crystalline units north and east of the major Nový Kostel seismogenic zone, borehole F3 in the Hartoušov mofette field and borehole S4 in the newly discovered Bažina maar near Libá. Supplementary borehole P1 is being prepared in the Neualbenreuth maar for paleoclimate and biological research. At each of these sites, a borehole broadband seismometer will be installed, and sites S1, S2 and S3 will also host a 3-D seismic array composed of a vertical geophone chain and surface seismic array. Seismic instrumenting has been completed in the S1 borehole and is in preparation in the remaining four monitoring boreholes. The continuous fluid monitoring site of Hartoušov includes three boreholes, F1, F2 and F3, and a pilot monitoring phase is underway. The laboratory also enables one to analyze microbial activity at CO2 mofettes and maar structures in the context of changes in habitats. The drillings into the maar volcanoes contribute to a better understanding of the Quaternary paleoclimate and volcanic activity.

Multi-Level Gas Monitoring: A New Approach in Earthquake Research

Fluid anomalies were often considered as possible precursors before earthquakes. However, fluid properties at the surface can change for a variety of reasons, including environmental changes near the surface, the response of the superficial fluid system to loads associated with the mechanical nucleation of earthquake fractures, or as a result of transients in fluid flow from the depths. A key problem is to understand the origin of the anomaly and to distinguish between different causes. We present a new approach to monitor geochemical and geophysical fluid properties 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, as the migration direction and speed of properties can be measured. In addition, potential admixtures of fluids from a deep crustal or mantle origin with meteoric fluids can be better quantified. A prototype of a multi-level gas monitoring system comprising flow and pressure probes, as well as monitoring of fluid-geochemical properties and stable isotopes is being implemented in a mofette field with massive CO2 (up to 97 tons per day) degassing. The mofette is believed a gas emission site where CO2 ascends through crustal-scale conduits from as deep as the upper mantle, and may therefore provide a natural window to ongoing magmatic processes at mantle depth. Fluids from three adjacent boreholes - 30 m, 70 m, and 230 m deep - will be continuously monitored at high sampling rates.

Real-time gas monitoring at the Tekke Hamam geothermal field (Western Anatolia, Turkey): an assessment in relation to local seismicity

This study presents the results of a real-time gas monitoring experiment conducted, via the use of a quadrupole mass spectrometer, in a mofette field within the Tekke Hamam geothermal site in western Anatolia (Turkey), a tectonically active region hosting several east–west trending grabens. The study is aimed to establish a baseline gas profile of the region. Within the framework of the experiment, gas compositions (CO2, N2, O2, H2, H2S, CH4, He, and Ar) and flow rate of a mofette were monitored during two observation periods: November 2007–January 2008 and April–July 2008. During the course of monitoring, the major gas component was CO2 with concentration changing around 96 volume percent. Other gases, from the most abundant to the least, were N2, CH4, O2, H2S, Ar, H2, and He. The study produced a short-term, baseline gas profile of the region with daily/diurnal variations and temporal gas fluctuations appearing as instant signals. Although the temporal gas fluctuations did not reach the anomaly level (variations staying within the mean ± 2σ), some of the variations in more than one parameter in the gas compositions (exceeding the mean ± 1σ), accompanied by changes in the diurnal gas pulses lasting for long durations, were correlated with the seismic events selected according to the adopted seismic event elimination criteria. The variations were mainly attributed to changing gas mixing ratios in relation to porosity/permeability modifications possibly related to seismicity. Studies involving the continuous monitoring of meteorological parameters are necessary to assign these variations to geogenic events.

Nonvolcanic Carbon Dioxide Emission at Continental Rifts: The Bublak Mofette Area, Western Eger Rift, Czech Republic

This study presents the results of gas flux measurements of cold, mantle-derived CO2release at the Bublák mofette field (BMF), located inside of the N-S directed Počátky Plesná fault zone (PPFZ). The PPFZ is presently seismically active, located in the eastern part of the Cheb Basin, western Eger Rift, Central Europe. The goal of the work was to identify the linkage between tectonics and gas flux. The investigated area has a size of 0,43 km2in which 1.115 locations have been measured. Besides classical soil CO2gas flux measurements using the closed chamber method (West Systems), drone-based orthophotos were used in combination with knowledge of plant zonation to find zones of high degassing in the agriculturally unused part of the BMF. The highest observed soil CO2gas flux is 177.926,17 g m-2 d-1, and the lowest is 0,28 g m-2 d-1. Three statistical methods were used for the calculation of the gas flux: arithmetic mean, kriging, and trans-Gaussian kriging. The average CO2soil degassing of the BMF is 30 t d-1for an area of 0,43 km2. Since the CO2soil degassing of the Hartoušov mofette field (HMF) amounts to 23 t d-1for an area of 0,35 km2, the average dry degassing values of the BMF and HMF are in the same magnitude of order. The amount of CO2flux from wet mofettes is 3 t d-1for the BMF and 0,6 t d-1for the HMF. It was found that the degassing in the BMF and HMF is not in accordance with the pull-apart basin interpretation, based on the direction of degassing as well as topography and sediment fill of the suggested basins. En-echelon faults inside of the PPFZ act as fluid channels to depth (CO2conduits). These structures inside the PPFZ show beginning faulting and act as tectonic control of CO2degassing.

3-D fluid channel location from noise tremors using matched field processing

SUMMARY Presently ongoing geodynamic processes within the intracontinental lithospheric mantle give rise to different natural phenomena in the NW Bohemia/Vogtland region (Czech Republic, Germany), amongst others: earthquake swarms, mineral springs and degassing zones of mantle-derived fluids as well as highly concentrated CO2 (mofettes). Their interaction mechanisms and relations are not yet fully understood, but fluid pathways within the crust are assumed, that allow efficient fluid transport between the main hypocentral swarm quake region and the degassing areas at the surface. Here, we focus on the location of the presumed fluid channels as well as on the investigation of their near-surface spatio-temporal variability, targeting a depth of a few hundreds of metmetres. We applied a 3-D matched field processing (MFP) approach in the frequency band of 10–20 Hz considering the fluid flow as seismic noise source. Within three campaigns in 2015/2016, we recorded continuous seismic noise data on the Hartoušov Mofette Field within the Cheb Basin (NW Bohemia, CZ), which is a key site to study fluid flow as it is characterized by strong and continuous surface degassing of CO2. We used temporary arrays varying in extent (70-600 m aperture) and in the amount of stations (25–95 units). Assuming a homogeneous velocity model and applying conventional MFP phase-matching over a 3-D grid search, we located two channel-like structures beneath the test site, which could be traced down to a common source area down to 2000 m depth. We thereby evaluated the influence of amplitude normalization of the measured noise signal on the MFP location considering water-filled or dry mofette channels. Additionally, a spatio-temporal analysis using time windows with a length of 10 min during 5 hr of noise record shows variability of fluid flow activity in space and time and hence, its migration beneath the test site on a short timescale.

Influence of CO2 Degassing on the Microbial Community in a Dry Mofette Field in Hartoušov, Czech Republic (Western Eger Rift).

The Cheb Basin (CZ) is a shallow Neogene intracontinental basin filled with fluvial and lacustrine sediments that is located in the western part of the Eger Rift. The basin is situated in a seismically active area and is characterized by diffuse degassing of mantle-derived CO2 in mofette fields. The Hartoušov mofette field shows a daily CO2 flux of 23–97 tons of CO2 released over an area of 0.35 km2 and a soil gas concentration of up to 100% CO2. The present study aims to explore the geo–bio interactions provoked by the influence of elevated CO2 concentrations on the geochemistry and microbial community of soils and sediments. To sample the strata, two 3-m cores were recovered. One core stems from the center of the degassing structure, whereas the other core was taken 8 m from the ENE and served as an undisturbed reference site. The sites were compared regarding their geochemical features, microbial abundances, and microbial community structures. The mofette site is characterized by a low pH and high TOC/sulfate contents. Striking differences in the microbial community highlight the substantial impact of elevated CO2 concentrations and their associated side effects on microbial processes. The abundance of microbes did not show a typical decrease with depth, indicating that the uprising CO2-rich fluid provides sufficient substrate for chemolithoautotrophic anaerobic microorganisms. Illumina MiSeq sequencing of the 16S rRNA genes and multivariate statistics reveals that the pH strongly influences microbial composition and explains around 38.7% of the variance at the mofette site and 22.4% of the variance between the mofette site and the undisturbed reference site. Accordingly, acidophilic microorganisms (e.g., OTUs assigned to Acidobacteriaceae and Acidithiobacillus) displayed a much higher relative abundance at the mofette site than at the reference site. The microbial community at the mofette site is characterized by a high relative abundance of methanogens and taxa involved in sulfur cycling. The present study provides intriguing insights into microbial life and geo–bio interactions in an active seismic region dominated by emanating mantle-derived CO2-rich fluids, and thereby builds the basis for further studies, e.g., focusing on the functional repertoire of the communities. However, it remains open if the observed patterns can be generalized for different time-points or sites.

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)

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.

Architecture and temporal variations of a terrestrial CO2 degassing site using electric resistivity tomography and self-potential

The Hartousov mofette field in NW Bohemia, Czech Republic, is characterized by strong CO2 degassing from the Lithospheric Mantle. In a test survey using electrical resistivity tomography, we observed changes over time in the subsurface structure beneath heavily CO2 degassing spots to depths of about 40 m and compared them to CO2 gas flux and soil gas mappings from previous studies. Changes in the electrical resistivity were measured between September 2012 and October 2013 at irregular intervals and highlight large variations in the resistivity distribution where the CO2 degassing was strongest (56 × 103 g m−2 day−1), indicating a fluid-induced change of properties of the underlying clayey sediments. Positive and negative anomalies in the electric self-potential parallel to the ERTs can be found where CO2 degassing occurs which indicates varying ascent or descent of fluids within these spots. Some degassing spots seem not to be actively degassing continuously over time, which can also be observed by other studies in the same area. We suggest that future gas mappings are accompanied by methods that observe the state of fluid systems at subsurface over time, e.g. electric resistivity tomography and self potential.

Spiders in mofette fields—Survival of the toughest in natural carbon dioxide springs?

In mofette fields, natural carbon dioxide springs, organisms have to stand extreme CO2 concentrations up to 100%. These hostile conditions are spatially small-scaled and further influenced by earth tides, wind and temperature. The present project investigated the influence of increased atmospheric CO2 concentration on spiders as representatives of above-ground organisms by means of pitfall traps in three mofette fields, differing in habitat conditions in the Plesná valley, eastern Cheb Basin, Czech Republic.Among the 71 recorded spider species four were rarely found in the Czech Republic. A canonical correspondence analysis revealed significant influences of environmental parameters on the spider assemblages. Two groups of spiders are clearly distinguishable, one being positively influenced by humidity and the second by temperature. A cluster analysis showed distinct and congruent results: spider assemblages of pitfall traps at spots with a mean CO2 concentration above 7.6% grouped close together and this grouping was independent of site. At >7.6% CO2 significantly fewer individuals and species were found in comparison to areas with lower CO2 concentration. Between 2.5 and 10% CO2, spiders indicated increased CO2 concentrations much more sensitively than endogeic organisms (Nematoda, Collembola) in a nearby mofette field. Unlike in nematodes, collembolans and plants, no mofettovageous or mofettophilous spiders were detected. In contrast to humidity, CO2 concentration and temperature, the vegetation cover was not among the factors, which significantly influenced spiders. This is explained by the fact that mofettophilous plants occurred at spots where almost no spiders could live. In a field experiment, most Pardosa pullata males tested passed a 30cm long corridor with increased carbon dioxide concentration. These results and that of pitfall traps showed that relatively large and wandering specimens respectively were able to transit moderately hostile spots. Further experiments are necessary to find out if there is any active avoidance of high-CO2 areas by spiders.

Locating mofettes using seismic noise records from small dense arrays and matched field processing analysis in the NW Bohemia/Vogtland Region, Czech Republic

ABSTRACTThe NW Bohemia/Vogtland region is characterized by currently ongoing geodynamic processes within the intracontinental lithospheric mantle. Among others, this activity results in the occurrence of mid‐crustal earthquake swarms and CO2 degassing zones called mofettes. These two natural phenomena are related to each other since it is considered that fluid flow and fluid‐induced effective stress can trigger earthquake swarms. At the Earth’s surface, they appear spatially separated, but their connection could be explained by the existence of pathways within the crust that allow efficient and permanent fluid transport. However, neither the structure nor the position of such pathways has been imaged yet.With this background, we used a matched field processing analysis within the NW Bohemia/Vogtland region to locate mofettes and investigate their characteristics. Considering the CO2 degassing process as a high‐frequency noise source, we chose two different test sites: the Dolní Částkov Borehole, which is an artificial mofette that we used to validate the method, and the South Hartoušov mofette field, a natural CO2 degassing area. On both sites, we measured seismic noise in a continuous mode for several hours (7 hours to 9 hours), with a sampling frequency of 250 samples/second, in multiple campaigns using an array of about 60 × 60 m2 with approximately 30 randomly distributed stations.For the matched field processing computation, the phase velocity of the study area is required, which we obtained from active seismic experiments with vertical hammer‐blow as the source. The phase velocity varies between 200 m/s and 420 m/s for Dolní Částkov and between 100 m/s and 280 m/s for South Hartoušov, both in a frequency range of 7 Hz–60 Hz.With the matched field processing analysis at the artificial mofette in Dolní Částkov, we could relocate the noise source successfully. In the South Hartoušov mofette field, we detected one dominant vertically extended noise source, probably a fluid pathway, as well as a small matched field processing maxima at the surface that can be related to a dry mofette.

Laboratory experiments on nematodes from natural CO2 springs reveal species-specific tolerance to extreme CO2 concentrations

This study focuses on the CO2-tolerance of soil nematodes from natural CO2 springs (mofette fields). In laboratory experiments, we compared survival, reproduction, activity and reactivation of a CO2-sensitive species with a CO2-tolerant species. Both species survived even 100% CO2, but in an inactive state. The higher the CO2 concentration the more individuals entered inactivity. We found significant differences between the two species: more adults of the CO2-tolerant species maintained activity and reproduction at higher CO2 concentrations. Moreover, reactivation after inactivity was faster. Together with a higher juvenile mortality of the CO2-sensitive species, these interspecific differences are conclusive to explain the niche separation of the two species that was observed in the mofette field.

CO2 degassing in the Hartoušov mofette area, western Eger Rift, imaged by CO2 mapping and geoelectrical and gravity surveys

Strong, subcontinental mantle-dominated CO2 degassing occurs in the Hartousov and Bublak mofette fields in the western Eger Rift. The combination of CO2 gas flux and soil gas measurements as well as gravity and geoelectric surveys provides insight into the surface and subsurface of this unique mofette area. CO2 soil gas and gas flux measurements reveal that large amounts of carbon dioxide are released via channels with diameters below 1 m. Carbon dioxide emissions of several tens and up to more than 100 kg day−1 are ejected via these small seeps. Measurements with small spacings are necessary to account for the point like, focused gas discharge in the lesser degassing surrounding. We estimate that between 23 and 97 tons of CO2 are released over an area of about 350,000 m2 each day in the Hartousov mofette field. The application of widely used geostatistical tools leads to estimations of the CO2 discharge with very high standard deviations due to the strong positive skewness of the data distribution. Geophysical investigations via electrical resistivity tomography and gravity measurements were carried out over areas of strong seepage and reveal distinct anomalies in the subsurface below mofettes, indicating rock and sediment alterations and/or sediment transport by pressurised, ascending CO2 and water mobilised by it. This study reveals that the gas emanations only occur west of a morphological step which is related to a N–S-oriented fault zone, the Pocatky-Plesna fault zone. The results of CO2 mapping and the geophysical studies can track the course of this fault zone in this area. Our results fit into a tectonic model in which the mofette fields are in the centres of two independent pull-apart basin-like structures. We hypothesise that the sinistral strike-slip movement of the Pocatky-Plesna fault zone leads to a pull-apart basin-like opening, at which the strong, mantle-derived CO2 degassing occurs nowadays. Since the Hartousov and Bublak mofette fields only illustrate examples along the N–S-striking Pocatky-Plesna fault zone, its role and other N–S-striking faults’ roles of the Regensburg–Leipzig–Rostock zone for upper mantle degassing might have been underestimated previously.

Geodynamic processes in the NW Bohemian swarm earthquake region, Czech Republic, identified by continuous gas monitoring

AbstractThe time series of two continuously operating gas monitoring stations at Oldřišská and Nový Kostel located along seismoactive faults in the epicentral area of the NW Bohemian swarm earthquakes (Czech Republic) are compared with water level fluctuations in two boreholes positioned along these faults and with gas flux variations of a mofette at the Soos mofette field at 9 km distance. The seasonal trend of the monitored CO2 concentration with a maximum in November and a minimum in March/April is governed by groundwater temperatures, superimposed in spring by soil temperatures. CO2 concentration variations identified at Oldřišská are also reflected in gas flux variations in the Soos mofette and/or water level fluctuations of two boreholes. Variations in the gas monitoring recordings of station at Nový Kostel are also linked with variations at Oldřišská. In all data sets, diurnal variations generated by earth tides occur, reflecting a daily stress – fault permeability cycle. Additional stress interferes with this cycle. Significant, abrupt changes are attributed to geodynamic processes linked with seismic events, as revealed by local seismicity or by the transient of waves of a strong remote earthquake. Simultaneous variations of the gas concentrations in the Nový Kostel area and in the gas flux in the Soos point to an interconnected hydraulic conductive fault systems present in the northern part of the Cheb Basin. Sharp falls in gas concentration, during or subsequent to, earthquake swarms may reflect fault compression associated with impeded gas migration. However, gas variations also occur in periods without seismic activity, indicating changes in fault permeability were caused by local aseismic fault movements, as revealed by events with opposite trends in the gas recordings at Oldřišská, Nový Kostel and the Soos. Therefore, a mathematical approach to establish a correlation between seismicity and gas geochemical variations is not possible.

CO2 discharge in an active, non-volcanic continental rift area (Czech Republic): Characterisation (δ13C, 3He/4He) and quantification of diffuse and vent CO2 emissions

In the western Eger Rift (ER) area along the Počatky–Plesná fault zone (PPZ) CO2 degassing occurs predominantly within two mofette fields Bublák and Hartoušov. We studied 27 wet mofettes belonging to these mofette fields for gas emission rates repeatedly between 2007 and 2009 and selected mofettes for gas composition and isotope ratios (δ13C, 3He/4He). Detailed ground mapping along the PPZ provided further two separated diffuse degassing structures (DDS) within the mofette fields Hartoušov and Bublák. The DDS Hartoušov was studied in detail by measurements of 682 CO2 soil gas concentrations, 762 CO2 soil gas fluxes (max. 10 grid spacing) and partly by analyses of isotope ratios (13C/12C, 3He/4He) of soil gas. At the DDS Hartoušov the total CO2 soil flux yielded 1.559tm−2d−1 in spring 2009 and the CO2 emission rate at the wet mofettes of Hartoušov mofette field yielded 0.62td−1. The total CO2 discharge of the 27 wet mofettes was 3.75t d−1.At sites with high CO2 soil flux, the portion of mantle-derived helium is in the same range as the releasing at wet mofettes; both cover the signature of the subcontinental mantle. Also, the δ13C values analysed in the gas releasing from wet mofettes and those analysed in soil gas are nearly the same. Taking in account a biogenic soil CO2 flux of 25gm−2d−1 as background, the mantle-derived CO2 flux yielded approximately 1566tm−2d−1. As a result of the CO2 flux mapping of the DDS Hartoušov, it could be proved that 97.4% of the released soil CO2 (1.518tm−2d−1) stems from sites with CO2 fluxes higher than 500gm−2d−1‐pointing to dominantly fault-related CO2 release.At the central mofette Bublák (B2), the gas emission rate was determined for the first time in 1993. Measurements repeated between 2007 and 2009 showed a clear increase in the gas emission rate of more than 40%, correlating well with the increase of the 3He/4He ratios from 5Ra to approximately 6Ra between 1993 and 2008 at this location (Bräuer et al., 2009).The Bublák mofette field is characterised by the highest CO2 emission rate along the PPZ, and in combination with the helium isotope signature of the European subcontinental mantle, this area was identified as a deep-reaching fluid injection zone.