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The different environmental conditions (temperature, oxygen and water availabilities), microbial composition, availability of fresh organic inputs and textural and mineralogical properties of soil layers with the depth result differences between the origin, composition, C/N ratio and stability of the dissolved organic matter (DOM) of topsoils and subsoils.This research examines the content and chemical composition of the DOM of topsoil and subsoil layers of a silty Luvisol and a sandy Arenosol. Both soils are derived from oak forests from Hungary. The soils were collected as composite samples (10 random subsamples within a 20 m × 20 m area) from the 0–20 and 30–50 cm layers.The DOM was extracted with ultrapure water for 12 h at room temperature with a tumbling shaker. The sample was centrifuged for 35 min (1400 × g) and the supernatant was decanted and passed through a 250 µm-sieve. The fraction that passed through the sieve was filtrated through a 0.45 μm membrane filter to obtain the DOM samples. The filtered samples were acidified to pH 2 with HCl, passed through a solid phase extraction cartridge using a styrene divinyl benzene polymer sorbent (Agilent Mega Bond Elut PPL), eluted with methanol and dried.The dried DOM samples were analyzed with a Bruker Vertex 70 FT-IR spectrometer. For each sample a spectral range of 4000–400 cm–1, a resolution of 4 cm–1, 128 scans, and three replicates were recorded. Relative absorbances were calculated for six peaks (2920, 2850, 1730, 1640, 1515, or 1420 cm–1) representing characteristic organic matter compounds.The dried DOM samples were dissolved in 0.05 M NaHCO3 solution in order to determine the C and N content and the fluorescence and UV-VIS-NIR spectroscopical properties. The total organic carbon and nitrogen content of the DOM samples were analysed using a TOC/TN analyser (Shimadzu TOC-L). The chemical composition of the DOM samples was determined using fluorescence (Shimadzu RF6000) and UV-VIS-NIR (Shimadzu UV3600) spectrometry. Excitation-emission matrices were obtained by measuring fluorescence intensity excitation wavelengths ranging from 230–450 nm and emission wavelengths ranging from 260–600 nm with 2 nm increments. Fluorescence, humification and biological indices were determined from the fluorescence spectra in order to determine the sources, structural complexity and humification degree of the DOM samples. Synchronous fluorescence spectra were recorded with a fixed wavelength difference (Δλ=18) to separate SOM components with different molecular weights. Specific UV absorption (SUVA254 and SUVA280, L mg-1 m-1) was calculated by dividing the absorption at 254 and 280 nm by the DOC concentration.The study aimed to assess the differences between the sources and the structural and chemical variability of the DOM samples from varying soil depths with different textural properties.This work was supported by the Development and Innovation Fund of Hungary [grant No. NKFIH 142936] and the Eötvös Loránd Research Network [grant No. SA41/2021].

Purpose Small-scale runoff and soil redistribution processes are important factors in rainfall simulation studies. Therefore, the main objective of this study is to examine the feasibility of rare earth oxide (REO) tracking combined with 3D surface modelling and soil crust analysis by scanning electron microscopy. Materials and methods Four 40 mmh−1 rainfall simulations (divided into two blocks) were conducted on a Luvisol sample at 9% slope steepness. In a block, two successive simulations were run on a tilled, and then, on a crusted surface. Before the first rainfall simulation of a block, the tilled surface was prepared by hoeing and application of four REO tracers (Pr6O11 Sm2O3, Ho2O3 and Yb2O3) to the freshly tilled surface. REOs divided the parcel into two back, and two front sub-parcels. The REO runoff content was measured by XRF, while the redistributed REOs were measured by SEM on polished crust samples taken after the second experiment in each block. Additionally, before and after 3D models of the surface were created for determining runoff direction and redistribution pattern. Results and discussion According to the REO content of the soil loss samples, the soil washed down from the front sub-parcels, while back parcels started contributing to soil loss only during the second block experiments. The surface microtopography changed between the experiments. The runoff path from the back sub-parcels headed to one side of the parcel. This strong, cross-side runoff pattern explained the lack of the back sub-parcel REOs in the soil loss. Meanwhile, in the crust samples, several forms of the redistribution were identified. The REOs of the back sub-parcels were found in the samples that were collected in runoff paths, and the leaching pattern became traceable with REOs. Moreover, we were able to reconstruct the original surface easily on SEM images. Conclusions Small-scale redistribution and the role of the microtopography of the surface should be considered as an erosional factor in erosion studies in a more detailed way. Behaviour of the REO as a soil sediment tracer has great potential, although questions remain. REO tracing applied with runoff direction modelling and SEM analysis of soil crust samples was suitable to monitor the runoff path, and explain the soil redistribution pattern horizontally and vertically.

A temporary solution to massive anthropogenic CO2 emissions can be the capture of industrial CO2 from flue gas and sequestering it in geological formations. For safe and effective storage of CO2, interaction processes in the rock-pore fluid–CO2 system should be known. Investigation of natural CO2 accumulations provides valuable examples to what physical and chemical effects could be expected during CO2 influx at future CO2 storage sites. One of the key controlling factors of the processes occurring in natural CO2 reservoirs is the lithology of the storage rocks, which is primarily determined by the formation conditions of these rocks. In this respect, the lithologies of individual CO2 accumulation areas influence the processes between the host rock, the pore fluid, and the CO2 in different ways. In the current study, we focus on a well-studied natural CO2 storage reservoir, namely the Mihályi-Répcelak area, NW Hungary. We provide insight into the so far unstudied conglomerate reservoirs that represent a stratigraphically deeper reservoir unit with significantly different lithology and pore water compositions compared to the sandstone reservoirs. Our results indicate that dawsonite /NaAlCO3(OH)2/ formation also affected the conglomerate reservoirs, which indicates that at least part of the CO2 could be trapped in mineral form. An important role of salinity in reducing the CO2 mineral trapping capacity of the storage system is also demonstrated. Furthermore, H isotope analysis of diagenetic kaolinite was applied to trace the origin of the pore water that was present during the rock formation. Based on the data, dawsonite formation was induced by the flux of meteoric water that infiltrated during a warm and humid period and mixed with ascending CO2.

Particle size and shape are among the most important properties of sedimentary deposits. Objective and robust determination of granulometric features of sediments is a challenging problem, and has been standingin the focal point of sedimentary studies for many decades. In this study, we provide an overview of a new analytical approach to characterize particles from thin sections of sandstones by using 2D automated optical static image analysis. The analysed samples are originated from the turbiditic Lower Pannonian (Upper Miocene) sediments of Szolnok Formation. Sandstone samples were analysed from 1,500 to 2,250 m depth range. According to the previous studies: the detrital components are quartz, muscovite, dolomite, K-feldspar and plagioclase. Diagenetic minerals are mostly carbonates (calcite, Fe-dolomite, ankerite, siderite), clay minerals (illite, kaolinite), ankerite, siderite and kaolinite. As the discussed Szolnok Formation is considered as a potential CO2 storage system (to reduce atmospheric CO2 concentration), special attention has to be paid on grain size and shape alteration evaluation, since pore water-rock interactions affected by CO2 injection may cause changes in particle properties. The primarily aim of the present study was to develop a method for effective characterization of the particle size and shape of sandstones from thin sections. We have applied a Malvern Morphologi G3SE-ID automated optical static image analyser device, what is completed with a Raman spectrometer. Via the combined analysis of granulometry and chemical characterization, it was obvious that there were specific relationships among various grain shape parameters (e.g., circularity values correlate to width and length ratios, as well as to convexity) and the results indicated that based simply on particle shapes, muscovites can be effectively separated from other minerals. Quartz and feldspar grains showed the highest variability in shapes as these are detrital ones, and sometimes arrived as lithic fragments from which other parts were dissolved The size and shape of carbonate minerals depends highly on the original pore size and shape because these minerals are mainly diagenetic. The shape of detrital dolomites depends on diagenetic ankerite, as it replaces the rim of dolomites.

Sedimentary transport processes and pore water- rock interaction may cause dissolution and precipitation of different minerals. These processes influence the particle shapes and grain size. In this paper we analyzed thin sections of sandstones with 2D image analysis (Morphologi G3ID) to specify the most characteristic shapes of the different minerals and pores.The studied samples originated from the Pannonian Basin, which was filled by delta and fluvial sediments in the Late Miocene. The previous studies show that the detrital minerals are quartz, muscovite, dolomite, K-feldspar and plagioclase. The main diagenetic minerals are carbonates (calcite, Fe-dolomite, ankerite, siderite) and clay minerals (illite, kaolinite).We measured the sandstone samples by Morphologi G3ID, which is a method of 2D image analysis combined with chemical characterization (Raman spectrometer). The results show that HS circularity (high sensitive) is in correlation with aspect ratio, furthermore, convexity and HS circularity also showed interdependency. According to the results, based on morphological parameters muscovites form a well separable group (HS circularity ~ 0.36; aspect ratio ~0.36, convexity ~0.86). The quartz and feldspar grains show high variability of the shapes (HS circularity: ~0.2-0.9, aspect ratio: 0.2-1, convexity: 0.8-1), because these minerals are detrital minerals and sometimes arrived as lithic fragments, from which one part dissolved. The shape of carbonate minerals can be described with the following parameters: HS circularity 0.3-0.8, aspect ratio 0.3-0.8 and convexity 0.8-1. The shape of carbonate minerals depends on the original pore size and shapes, because these minerals are mainly of diagenetic origin. Dolomite is detrital, however the shape of these grains depends on the diagenetic ankerite, because ankerite replaces the rim of dolomite.

One of the challenges of the present century is to limit the greenhouse gas emissions for the mitigation of climate change which is possible for example by a transitional technology, CO2 geological storage. Clay minerals are considered to be responsible for the low permeability and sealing capacity of caprocks sealing off stored CO2. However, their reactions are not well understood for complex simulations. This work aims to create a kinetic geochemical model of Na-montmorillonite standard SWy-2 supported by solution and mineral composition results from batch experiments. Such experimentally validated numerical models are scarce. Four 70-hours experiments have been carried out at atmospheric conditions, and with CO2 supercritical phase at 100 bar and 80 °C. Solution samples have been taken during and after experiments and their compositions were measured by ICP-OES. The treated solid phase has been analyzed by XRD and ATR-FTIR and compared to in-parallel measured references (dried SWy-2). Kinetic geochemical modelling of the experimental conditions has been performed by software PHREEQC. Experiments and models show fast reactions under the studied conditions and increased reactivity in presence of scCO2. Solution composition results cannot be described by the change of the uncertain reactive surface area of mineral phases. By considering the clay standard’s cation exchange capacity divided proportionally among interlayer cations of Na-montmorillonite, the measured variation can be described on an order of magnitude level. It is furthermore indicated that not only the interlayer cations take part in this process but a minor proportion of other, structural ions as well, differently in the reference and scCO2 environments.

We investigated CO2-free and naturally CO2-flooded sandstone samples from a deep saline aquifers formation, which represents potential carbon storage reservoirs. A descriptive geochemical model is also coupled to the laboratory study for the better understanding of geochemical interaction between sandstone and CO2. The studied area is located in the, Western Hungary in the Little Hungarian Plain where one of the largest CO2-producing fields in Europe can be found. In this region, we have the opportunity to compare rocks of the same sandstone formation without CO2 (not affected by natural CO2 flooding) and naturally CO2 flooded sandstone, where the CO2 was trapped around 7-4 million years ago. As boreholes sampled not only the parts of the formation, which were flooded by CO2 (Mihályi-Répcelak), but also the parts which were not affected at all by this flooding (Ölbő).Besides petrographic observations, scanning electron microscopy and mineral chemistry analyses, X-ray diffraction and infrared spectroscopy were used to determine 7 CO2-free and 6 CO2-flooded samples textural features, mineral compositions and the presence of OH-bearing minerals. We carried out thermodynamic-batch modelling with PHREEQC geochemical modelling software and compared to the laboratory results.The sandstone samples from the CO2 bearing reservoirs contain quartz, mica, kaolinite, K-feldspar and carbonates such as dolomite, calcite, ankerite and siderite. The CO2-free samples also contain chlorite, plagioclase and pyrite and all mentioned above. In the CO2-flooded samples a carbonate phase, dawsonite (NaAlCO3(OH)2) could be also observed in significant amounts (3-16 w/w%). This is an indicator mineral of large amount of CO2 inflow in the CO2-water-rock system. In addition, chlorite is apparently missing in the CO2-flooded samples. According to the petrographic observations and X-ray diffraction (XRD) results, it is clear that the plagioclase content is higher (∼ 11 w/w%) in the CO2-free samples compared to the CO2-flooded ones (2 flooded reservoir rocks. The lower amount of K-feldspar and plagioclase in the CO2-flooded samples can be explained by precipitation of dawsonite. These minerals can dissolve as a result of CO2-flooding and serve Na+ and/or Al3+ ion for dawsonite formation. The amount of the carbonate minerals also reveal systematic differences between the CO2-free and CO2-flooded sandstone, the amount of ankerite is higher (from 6 to 12 w/w%) in the later ones implying that some parts of the ankerite formed after the CO2 flooding event. The investigation of this unique area provides opportunity to study sandstone before interaction with CO2 and after millions of years being in contact with CO2.Acknowledgements:This research was financed by Hungarian Scientific Research Fund (K131353).Dóra Cseresznyés’ work is supported by the Cooperative Doctoral Programme granted by The Ministry for Innovation and Technology (ITM), National Research, Development and Innovation Office.

Eleven unique core samples from the National Radioactive Waste Repository of Hungary, Bataapati were studied in this work. The samples all cross the granite–concrete interface and have been drilled from around 275 m depth from the surface, 1–15 months after concrete injection. Phase analytical techniques, optical microscopy, SEM-EDS and Raman-spectroscopy were used for the analysis of interactions between granitic rock and cementitious building material. Newly formed phases, Ca-carbonates and titanite, were observed at the interface. Carbonation may reduce the porosity and permeability in the contact zone. The presence of titanite indicates the changing geochemical and thermodynamical constrains along the reaction front of granite–concrete, furthermore, it may help in the validation of future geochemical models. The cementitious material is seen to penetrate among the sheets of biotite mineral in granite which process is probable to cause the attachment of granite and concrete.

Soil organic matter (SOM) is in the focus of research as it plays crucial role in soil fertility, carbon sequestration, and all adsorption related processes in the soil. Nevertheless, its compound and the methods to investigate it are rather diverse. Some approach prefers to define different theoretical carbon pools in the soil based on input and mineralization dynamics using mean residence times. Other studies apply physical and/or chemical fractionations of the soil to separate the various eg. mineral phase associated or aggregate occluded carbon pools to gain less heterogeneous material. However, in practice, these two approaches are hardly met each other. As a considerable part of SOM is strongly associated with the mineral colloid fraction or even cations its investigation reveals the question of extractions. Traditional methods aimed to extract pure SOM fractions such as fulvic and humic acids (FA; HA) and characterized the whole SOM based on them, even though these pure fractions represented only a small part of the total SOM and were not present under natural conditions. Recent methods try to characterize the SOM using in situ samples where the role of organic mineral complexes is still not fully understood. As a result, findings based on several approaches are hardly comparable with each other. The present study aims to characterize SOM based on parallel in situ solid-phase investigation FA separation, and water dissolved organic matter extraction. The study site is a haplic Luvisol under plowing and conservation tillage. Fourier transform infrared spectroscopy on the solid phase fractions resulted in an inverse proportion between organic carbon content and aromaticity independently from tillage. The aggregate occluded SOM was characterized by the lack of aliphatic components, whereas the fine fraction, and the bulk soil associated SOM seemed to be rich in them. The water-soluble SOM revealed molecular size increase in both the fine fraction related and the aggregate occluded organic matter owing to plowing, nonetheless, aggregates occluded the same sized OM molecules as those attached to the fine fraction. In general, FA fractions provided more humified organic matter, whereas water dissolved SOM showed a more intensive microbiome origin. The photometric properties of the FA fractions did not differ between the tillage systems, except for the SUVA254, which provided higher aromaticity under conservation tillage due to the lack of plowing. Also, the water-soluble part of SOM showed more humified composition and increased aromaticity under conservation tillage compared to plowing tillage. As a consequence, beneath the fingerprint of recent microbial activity, DOM reflects soil organic matter composition as well, therefore it seems to be suitable as a direct SOM proxy. The present research was supported by the Hungarian National Research and Innovation Office (NKFIH) K-123953, which is kindly acknowledged.

Laser diffraction grain size data have been widely used in paleoenvironmental reconstructions as physicochemical alteration-related proxies. Many studies are available on comparison of different laser diffraction devices, optical theories and optical settings. The ignorance of some uncertainty factors can lead to poorly comparable granulometric datasets. Other important factor leading to the aforementioned effect is the inadequate chemical pretreatment procedures which are often overlooked, but are capable to basically affect the results. In this study we examine a few past and recent sediment types from different geomorphological environments from the Carpathian Basin: lake and fluvial sediments, paleosols and loess. Our aim is to review and create a reliable methodology for laser diffraction particle size analysis and optical particle shape investigations. We compare widely used pretreatment methods -which can be found in the literature- with each other. We are also taking into account that different sediment types need different pretreatment methods. We can state that the duration of chemical pretreatment can affect the optical properties (color), the texture and the mineral composition of the sediments, as well as the size and shape of mineral particles in the samples. The changes in these significant parameters can mislead the researcher’s main objectives. The study is supported by the ÚNKP-19-3 New National Excellence Program of the Ministry for Innovation and Technology. Support of the National Research, Development and Innovation Office NKFIH K120620 is gratefully acknowledged.

The top metre of the soil is one of the largest terrestrial carbon reservoirs. More than 50% of the soil carbon is stored as soil organic matter (SOM). Several papers report about the SOM losses due to tillage and land-use change. On the other hand, a huge amount of papers focus on the environmental potential of various min-till, no-till and other techniques for regenerative agriculture. The change of the fertilization regime also has an influence on SOM so it also can influence the humus status of the soils. This presentation focuses on the effects of different kinds of fertilization and abandonment of arable lands on the quantity and quality of the SOM.The present study is based on Martonvásár Experimental Station (Hungary) which was established in 1958. The research focused on maize monoculture with the following treatments: (a) no fertilization, (b) NPK, (c) NPK with manure addition. The soil of the plots is Chernozem. Two controls were selected: (a) a natural Grassland and a secondary grassland. The secondary grassland was an arable land until 1990. Five repetitions of soil samples were taken from each plot and times. Soils were fractionated to silt and clay associated OM (s+c), aggregate associated OM (S+A), dissolved organic matter (DOM) and particulate organic matter (POM) according to Zimmermann’s method (4). Quality parameters of the DOM were studied by CN analyser, UV-Vis spectrometer, spectrofluorometer, zetasizer and size exclusion chromatograph. Solid SOM fractions were studied by CHNS analyser, ATR-FTIR and DRIFT FTIR. The V3-V4 regions of the 16S rRNA gene obtained from the soil samples were sequenced on the Illuma platform for the description of microbial diversity.Twenty years were enough to restore the natural SOM content of the soils (land-use change from arable land to grassland). Labile fractions of the SOM were higher in case of secondary than the primary grasslands. We have found differences in weight ratios of SOM fractions between fertilization regimes, as well. The proportion of microbial contribution to SOM were higher in the arable soils than the grasslands based on the C:N ratios of the SOM. However, the predominance of phyla Proteobacteria, Acidobacteria, Bacteriodetes, Actinobacteria and Verrucomicrobia in all studied soils, microbial diversity is generally higher in the grasslands than in the arable plots. The DOM of different fertilization regimes and land uses have shown the most characteristic differences. The difference between arable plots (with various fertilization regimes) and grasslands can be characterized by humic substances (HS) with higher condensation degree and molecular mass. The application of manure has result same proportion of peptide-like components and HS with lower molecular as the DOM of grassland soils.The microbial diversity of abandoned arable land remained similar to that of the arable lands over twenty years. The major part of the growth of SOM occurred in the labile fractions. The change of the fertilization regime also has limited potential to grow a total mass of SOM.Support of the GINOP 2.3.2-15-2016-00056 and National Research, Development and Innovation Office under contracts K123953 are gratefully acknowledged.

In spite of its considerable size and the presence of mature oil source rocks, the Neogene Danube Basin is characterized by the absence of commercial oil accumulations. However, important [Formula: see text] fields have been discovered in the basin, which have been possibly explained recently by displacement of oil by later migration of [Formula: see text] into the reservoirs. Previous studies performed independently in the southern, Hungarian part of the basin (known as the Little Hungarian Plain) have attempted to model petroleum generation/migration history, to identify sources of the [Formula: see text], and to understand the tectonic evolution of the basin and its deep-water flow regime. We were attempting to combine the results of these studies to interpret the fluid-migration history and reveal the influencing processes. This work supports the hypothesis that [Formula: see text] could have played a key role in preventing the formation of oil accumulations. During the latest Miocene, early mature oil and saline water, the latter formed by dissolution of up to now unidentified halites, moved together toward the Mihályi High, a regional uplifted structure in the central part of the basin. Modest amounts of oil could have been trapped there while saline water mixed with the low-salinity water of the reservoirs, with the [Formula: see text] arriving later and displacing the oil.

Natural CO2 reservoirs provide an opportunity to study long-term fluid-rock interactions, which are essential to reassure the safety of mineral storage of carbon-dioxide. The Mihalyi-Repcelak field (Pannonian Basin, Central Europe) is one of the largest natural CO2-bearing reservoirs in Europe (25 Mt). The CO2 was trapped in Neogene sandstones, which contain various carbonate minerals (dolomite, ankerite, siderite, dawsonite). To reveal the origin of the parent fluid, from which these minerals precipitated, dawsonite and siderite were separated by a new physical method to minimise the uncertainties in the analysis of their stable isotope composition. The δ13CDaw values range from +1.3‰ to +1.6‰ and the calculated δ13CCO2 values in equilibrium with dawsonite (−4.8‰ - –2.0‰) overlap with the carbon isotope compositions of the local CO2 and the European Subcontinental Lithospheric Mantle (−3.9‰ - –2.1‰). This indicates that the dawsonite-forming CO2 had a magmatic origin. The siderite data indicates that some formed from the magmatic CO2, possibly simultaneously with dawsonite (−6.0‰ - –3.9‰), whereas the rest (−8.4‰ - –6.1‰) formed either from a fractionated CO2 with magmatic origin or before the CO2 invasion. The hydrogen isotope composition of structural OH− of dawsonite (−57‰ to −74‰) was determined and was used to estimate the origin of the interacting porewater. The calculated porewater data (δD: −69‰ - –103‰ and δ18O: −1.4‰ - +4.7‰) indicate that the parent fluid was meteoric water modified by water-rock interaction. Our data allows estimation of the total amount of CO2 stored in the dawsonite-bearing sandstone reservoir to be 25 kg/m3, well in line with previous modelling works, which gives a total of 2.01 × 106 t of CO2, higher than previous estimates. We suggest that individual mineral analysis complemented by hydrogen isotope analysis is to be employed to effectively trace in-reservoir fluid-rock interactions in CO2 reservoirs and provide valuable input data for geochemical modelling for better predicting conditions for mineral storage of CO2.

When CO2 is injected into a subsurface reservoir, it undergoes structural-stratigraphic, residual, dissolution and mineral trapping mechanisms. Among these, the most permanent and preferable is the mineral trapping, in which carbonate minerals build the CO2 into their structures. In this study the amount of CO2 trapped in minerals was estimated from both analytical data of and thermodynamic geochemical models for a natural CO2 occurrence. Models underestimate the amount of trapped CO2 to be 0.004 tons/m3 reservoir in comparison with measured amount of dawsonite and other carbonates in rock samples indicating that 0.02-0.18 tons CO2 is trapped in one m3 reservoir.

Stable isotope systems provide exceptional insight into fluid-solid interactions occurring in the sub-surface. We provide detailed insight into the stable isotope distribution of carbonate minerals in a natural CO 2 reservoir system from Mihalyi-Repcelak area, NW-Hungary. Several types of measurements and calculations have been applied for the estimation of CO 2 and H 2 O isotopic composition in equilibrium with dawsonite. The estimated δ 13 C of CO 2 exhibit values around -4.5 to -2.6 ‰ overlapping with assumed magmatic values and the presently measured gas data confirming magmatic origin, whereas we may assume that the water present during carbonate formation likely had meteoric origin.

Low permeability caprock prevents migration of injected CO 2 towards the surface and potable water reservoirs. As a result of geochemical reactions among caprock minerals and CO 2 -saturated pore water, physical properties such as porosity, permeability and tortuosity may change, which could affect the sealing capability of caprocks. Due to the common high clay mineral content of caprocks, the reactivity of these minerals should be well-studied at CO 2 -saturated reservoir conditions. The main aim of this study is to understand better the geochemical behavior of CO 2 –brine–clay mineral system by using laboratory experiments at reservoir conditions on reference clays.

The caprock is the only physical barrier in the migration path of CO2 during CO2 geological storage. For this reason, caprock integrity is one of the most important factors regarding the long-term safe underground storage of CO2. As a result of geochemical reactions, among the caprock minerals and CO2 saturated pore water, the physical properties of caprocks such as porosity and permeability may change, which could affect their sealing capacity. Natural CO2 occurrences can help to understand these long-term reactions under storage conditions on geological timescale.

The purpose of this study was to investigate the results of re-interpreted old well-logs and porosity and permeability measurements of core samples from a natural CO 2 occurrence in NW-Hungary. The reservoirs formed in Neogene sediments over a basement high. We compared the estimated physical properties based on well logging and the results of core sample analysis. The goal of our investigation was to give a distribution of hydrogeological properties in the in Mihalyi –Repcelak area. We outlined the paleoenvironment of the reservoirs and caprocks searching for the explanation of the formation of this natural CO 2 storage.

The Special Collection of papers in this issue of Astrobiology provide an overview of the characteristics and potential for future exploration of the Ojos del Salado volcano, located in the Andes Mountains in front of the Atacama Desert in northern Chile. The main benefits of this site compared with others are the combination of strong UV radiation, the presence of permafrost, and geothermal activity within a dry terrain. The interaction between limited snow events and wind results in snow patches buried under a dry soil surface. This leads to ephemeral water streams that only flow duringdaytime hours. On this volcano, which has the highest located subsurface temperature monitoring systems reported to date, seasonal melting of the permafrost is followed by fast percolation events. This is due to the high porosity of these soils. The results are landforms that shaped by the strong winds. At this site, both thermal springs and lakes (the latter arising from melting ice) provide habitats for life; a 6480m high lake heated by volcanic activity shows both warm and cold sediments that contain a number of different microbial species, including psychrophiles. Where the permafrost melts, thawing ponds have formed at 5900m that is dominated by populations of Bacteroidetes and Proteobacteria, while in the pond sediments and the permafrost itself Acidobacteria, Actinobacteria, Bacteroidetes, Patescibacteria, Proteobacteria, and Verrucomicrobia are abundant. In turn, fumaroles show the presence of acidophilic iron-oxidizers and iron-reducing species. In spite of the extreme conditions reported at Ojos del Salado, this site is easily accessible., Comment: Accepted in Astrobiology Special Issue 2020. 04. 10

The volume of soil organic matter (SOM) changes, owing to variations in tillage systems. Conservation tillage (CT) is a useful method for recovering the SOM content of crop fields. However, little is known about the SOM composition of silt- and clay-associated and aggregate-occluded organic matter (OM). The present study aimed at determining the SOM compositions of various SOM fractions in the same Luvisol in a native forest and under ploughing and CT. SOM fractions (silt and clay associated, sand and aggregates associated, restricted OM) were characterized using diffuse reflectance Fourier transform infrared (FTIR) spectroscopy. The size of both the aggregate-occluded and resistant SOM pools increased, owing to the shift in the tillage system to CT for 15 years. As a general trend, the soil organic carbon content was inversely proportional to aromaticity under both crop fields, which supported the preferential mineralization of aliphatic components in each fraction. The shift in the tillage system could trigger rapid qualitative changes even in the stable restricted carbon pools, nevertheless, it was difficult to distinguish between the role of OM and the mineral composition in the FTIR spectra. In particular, the clay-related organic-mineral complexes could trigger difficulties in the traditional interpretation methods.

Prediction of the fate of the injected CO2 in geological formations strongly relies on geochemical modeling, laboratory and field testing. However, all of these approaches have their major drawbacks, either being much too simplistic and/or out of realistic timescales (1–100 days vs. 100–1000 years) for deep subsurface storage. Natural CO2 occurrences provide an exceptional opportunity to study long-term behavior of the scCO2-porewater-reservoir rock system. Despite their complex geologic evolution, these occurrences have been used lately to identify CO2-related reaction features as well as to test and fine-tune geochemical models. The Mihalyi-Repcelak occurrence in NW Hungary, with core material available, not only provides a general insight into subsurface reaction processes related to CO2, but represents one of the largest deep saline aquifers in the Pannonian Basin, it also behaves as a natural laboratory for this potential Late Miocene saline reservoir. The multi-layered structure of the occurrence with separated hydrodynamic units enables the simultaneous analysis and the identification of reaction features both in the reservoirs and their sealing cap rocks providing a further step in understanding the long-term performance of storage complexes.

Potential disaster scenarios of CO2 geological storage (CGS) are CO2 escape and brine displacement to protected freshwater aquifers via unexpected pathways. Present study evaluates the geochemical aspects of such events in the vicinity of potential CGS sites in Hungary. This is primarily possible by reactive transport modelling using the open source PHREEQC. Solution and mineral compositions, electrical conductivity and porosity are simulated in time and space. Main results predict an increase of ion-concentrations in the freshwater; some ions may even exceed drinking water limit values either in the case of CO2 and brine leakage. Finally, electrical conductivity of water is suggested to record for a cost-effective, early detection of both scenarios.

A study was conducted on the sorption of 17α-ethynylestradiol (EE2) on five soils formed under different redox conditions: an Arenosol (A_20) with fully aerobic conditions, two Gleysol samples (G_20 and G_40) with suboxic and anoxic conditions and two Histosols (H_20 and H_80) with mostly anoxic conditions. The soils were characterized on the basis of total organic carbon (TOC), specific surface area (SSA) and the Fourier transform infrared spectra of the humic acid and humin fractions (the soil remaining after alkali extraction) of the soil. The maximum adsorption capacity of the soils (Qmax) ranged from 10.7 to 83.6 mg/g in the order G_20 > H_20 > G_40 > A_20 > H_80, which reflected the organic matter content of the soils. The sorption isotherms were found to be nonlinear for all the soil samples, with Freundlich n values of 0.45–0.68. The strong nonlinearity found in the adsorption of the H_80 samples could be attributed to their high hard carbon content, which was confirmed by the high aromaticity of the humin fraction. The maximum sorption capacity (Qmax) of the soils did not increase indefinitely as the organic carbon content of the soils rose. There could be two reasons for this: (i) the large amount of organic matter may reduce the number of binding sites on the surface, and (ii) the decrease in SSA with increasing soil OC content may limit the ability to adsorb EE2 molecules. In anaerobic soil samples, where organic matter accumulation is pronounced, the amount of aromatic and phenolic compounds was higher than in better aerated soil profiles. Strong correlations were found between the amount of aromatic and phenolic compounds in the organic matter and the adsorption of EE2 molecules, indicating that π-π interaction and H-bonding are the dominant sorption mechanisms.

Caprock integrity is one of the most important factors regarding the long-term safe underground storage of CO2. As a result of geochemical reactions among the caprock mineralogy and CO2 saturated pore water, the physical properties of caprock such as porosity, permeability may change, which could affect its sealing capacity. Natural CO2 occurrences can help to understand these long term reactions under storage conditions on geological timescale. Our study area, the Mihalyi-Repcelak natural CO2 occurrence, is believed to be leak-proof system on geological timescale. To identify and understand the mineral reactions in the caprocks we applied XRD, FTIR-ATR and SEM analysis of drill cores derived from the study area. The petrophysical properties of the studied rock samples were determined from the interpretation of geophysical well-logs and grain size distribution. The effective porosity (~4 %), permeability (0.026 mD) and clay content (~80 %) of the drill cores imply that the studied clayey caprocks represent an adequate physical barrier to the CO2. Our analytical results show that dawsonite has formed within the caprocks. In most cases the dawsonite crystallized after albite dissolution. This implies that CO2 or CO2-saturated brine can penetrate into the caprock resulting in mineral reactions and most likely changing the porosity and permeability of the sealing lithology. On the other hand the caprock may react as a geochemical buffer for the CO2 and, at least part of it, can be stored within the caprock as solid phase, thereby increasing the storage capacity of the system.

The research of natural analogues for future CCS projects is a dynamically growing sector of geoengineering. In our work this newly discovered aspect of a CO2 field is presented. In NW Hungary there is a field utilized for more than 60 years. Thus there is a large amount of data from hydrocarbon exploration, gas measurements, seismic sections, well logs, and porewater analysis. Modelling work is done to describe the interactions resulting in the experienced situation to provide an estimate for a planned CO2 reservoir similar to this natural analogue in pressure, temperature, hydrogeology, tectonics, and composition.