Pore Water Composition Research Articles

Effects of Fe oxides on organic carbon variation in the evolution of clayey aquitard and environmental significance

As an important part of groundwater systems, aquitards may have a considerable impact on the quality of groundwater in an aquifer. Organic carbon (OC) serves as an important component in biogeochemical processes which affects the hydrochemical composition of pore water; the association of OC with (Iron) Fe-containing minerals is recognized as an important stabilization mechanism for OC. However, the characteristics of the process forming complexes between OC and Fe oxides in subsurface aquitards and the contribution of OC to aquifers has been poorly understood. In this study, the content, speciation and reaction types of OC and Fe oxides were investigated to reveal the characteristics and mechanisms of OC-Fe interaction in subsurface aquitards at different depths in two typical sedimentary facies of the Jianghan Plain in central China. The total organic carbon (TOC) contents of aquitard sediments in alluvial-lacustrine and residual slope facies were 7.25 ± 5.10 and 2.79 ± 0.86 mg g−1, respectively. In general, the proportion of the heavy fraction of OC (HFOC) to TOC gradually increased with increasing depth because consumption of the light fraction of OC (LFOC) that caused more HFOC to remain in sediment. On average, Fe bound-OC contributed 30.3% and 31.6% of TOC by adsorption and coprecipitation, respectively. The adsorption-stabilized OC has not changed obviously but the coprecipitation-stabilized OC increased gradually with depth. Coprecipitation stabilized more OC in a stable environment (residual slope facies) when compared with an unstable environment (alluvial-lacustrine facies), which can be supported by the greater average ratio of FePP bound-OC:TOC ratio and increased enrichment of carboxylates and aromatics in a stable environment. The transformation of OC-Fe complexes could affect the transport of As, Cr and ammonium which chemically bind to organic matter (OM) and Fe minerals from sediment to pore water by reductive dissolution. The findings of this study are helpful in understanding the interaction between OC and Fe oxides in subsurface aquitard environments, in which reaction products may affect adjacent aquifers.

Predicting required time for stabilising Norwegian quick clays by potassium chloride

Salt wells filled with potassium chloride (KCl) were installed in a highly sensitive, low-salinity, glaciomarine quick clay at Dragvoll, Trondheim, Norway, to investigate the improvement in geotechnical properties and the required time to stabilise the clay volume sufficiently to inhibit retrogressive landslide development. Improving the remoulded shear strength (c ur) beyond 1 kPa reduces the risk for successive back-scarp failures. When more than 20% of the cations in the pore water consist of potassium (K+), magnesium (Mg2+) and calcium (Ca2+) ions, c ur is improved beyond 1 kPa. Optimisation of the well grid is necessary prior to installation to ensure that c ur > 1 kPa within 2–3 years. PHREEQC simulations predicted a diameter of stabilised clay of minimum 1·5 m within 3·3 years after installation, which fits reasonably with observed pore-water compositions and improved geotechnical properties in situ. Salt wells may be used as landslide mitigation, preventing backward successive, retrogressive development, and PHREEQC can be used to give a rough estimate of the required time to stabilise the quick-clay volume around the wells.

Improved accuracy and precision of water stable isotope measurements using the direct vapour equilibration method

A method to measure the δ2 H and δ18 O composition of pore water in soil samples using direct vapour equilibration and laser spectrometry was first described in 2008, and was rapidly adopted. Here, we describe an improved setup to measure pore water δ2 H and δ18 O values through direct vapour equilibration with a laser spectrometer, combining a liquid and a vapour mode for water isotope analyses, and resulting in improved accuracy. We first tested new gas sampling bags as part of the equilibration protocol. Then, to assess measurement accuracy, vapour samples from equilibrated liquid waters of known isotope composition were measured in the liquid mode of the analyser using the new setup as well as the manufacturer's vapour mode. Various modes of preparing liquid water standards, namely equilibration, nebulisation, and vapourisation, were tested to determine the best calibration in terms of accuracy. Finally, the proposed modified liquid setup was validated by analysing water vapour equilibrated from soil pore water of a known composition. The δ2 H and δ18 O measurements were found to be more accurate by the modified liquid mode than by the factory-setup vapour mode. The strong and non-linear dependence of measured δ2 H and δ18 O values on H2 O concentration in vapour mode, especially at concentrations equal to the vapour pressure saturation typically found in laboratories, is problematic for corrections. Regarding calibration and standards, the use of two equilibrated liquid water standards was found to best calibrate measurements in the modified liquid setup. Finally, the modified liquid mode setup and its calibration, as described here, were shown to be appropriate for soil pore water analysis. The proposed modified setup results in more precise δ2 H and δ18 O soil pore water values than the usual protocols. An average standard deviation of 0.04‰ for δ18 O values and 0.3‰ for δ2 H values, based on 228 soil sample analyses, was obtained.

Insights into the retention behaviour of europium(III) and uranium(VI) onto Opalinus Clay influenced by pore water composition, temperature, pH and organic compounds

In the scientific community, a consensus exists to store high-level radioactive waste (HLW) in deep geological formations for several hundred thousand years to protect humanity and the environment. After a container damage followed by the release of HLW components, the retention of the radionuclides can be affected by diverse chemical reactions in the geosphere. In this study, geochemical effects possibly influencing the retention of the radionuclides were investigated. Therefore, the retention behaviour of U(VI) and Eu(III) (as homologue for trivalent actinides) onto Opalinus Clay (OPA) as a potential host rock for a HLW disposal was analysed at different geochemical conditions. For these experiments, a wide range of metal concentrations in presence and absence of natural clay organic matter (NOM) (lactate or humic acid) in different types of background electrolytes (0.01 mol L−1 sodium perchlorate and 0.4 mol L−1 synthetic OPA pore water) were investigated. Inductively coupled plasma mass spectrometry (ICP-MS) was used as a high specific and very sensitive method for elemental trace analysis. For the speciation of Eu3+-NOM-complexes, a method with high separation performance, capillary electrophoresis, was hyphenated with ICP-MS. The performed batch experiments simulate a potential water intrusion into the waste disposal. The retention (determined as log Kd values) of the metal ions strongly depends on the pH value, the presence of competing cations, temperature and NOM. At a pH value of pH 7.6 in synthetic OPA pore water, which is relevant for a disposal in OPA formations, the sorption of Eu(III) onto OPA (4 g L−1) is significantly higher (log Kd = 3.35 ± 0.04 at 25 °C and 4.85 ± 0.08 at 60 °C) than the retention of uranyl (log Kd = 2.17 ± 0.21 at 25 °C and 2.86 ± 0.01 at 60 °C) due to the formation of an uncharged aqueous calcium-uranyl-carbonate-complex (Ca2UO2(CO3)3(aq)) which does not sorb onto OPA. Modelling metal retention data onto OPA with PHREEQC reveals a very good agreement with most measurement results. At elevated temperatures, PHREEQC modelling could not be performed due to lack of data, especially for possible secondary minerals. The observed increased Kd values at elevated temperature revealed in this study might be assumed advantageous for the safe storage of HLW in a future repository.

Fluid migrations at the Krasny Yar methane seep of Lake Baikal according to geochemical data

Sediments of the deep water Krasny Yar methane seep area of Lake Baikal contain channels of 1 cm in diameter and up to a depth of 150 cm that have a high water content and are characterized by higher oxidized and reduced conditions than the surrounding sediment. New data of lithological and geochemical studies, including the full anionic and cationic composition of pore waters, indicate that chemical flow at the sediment surface of the seep is heterogeneous in space with both areas of inflow of fluids and areas with outflow of fluids. We suggest that much of this flow is modulated by more complex hydrological processes that may include aqueous pumping driven by gas expulsion- and changes in the permeability of the conduits due to gas hydrate formation and dissociation. We discuss the seep output fluid chemistry and heat flow in relation to the above hydrological processes and propose a seep model where fluid and gas migrations act at different spatial scales partly favored by hydrate accumulation and the fresh water conditions of Lake Baikal.

Hydrogeochemistry of Urban Territories in the Southern Fore-Ural Areas

Data of field studies elucidate how the chemical composition of groundwaters is formed at urban territories in the southern Fore-Ural areas. These data were used to determine the changes in the salt composition of pore-waters in the clay rocks, their exchange–adsorption characteristics, and the distribution and accumulation of supertoxicants in the rocks and waters. The effect of the chemical composition of the waters on hazardous geological processes is estimated. It is determined that the geological environment is most strongly impacted within the depth range from the surface to 15–20 m. Dioxins and heavy metals are concentrated in the ground at the sites of industrial facilities near the surface (at depths up to 5–7 m), and their concentrations drastically decrease at depths from 5–7 to 20 m. Liquid organic contaminants and water-soluble salts penetrate through almost the whole circulation zone. The self-clarification of the aquifers and the recovery of the natural parameters takes dozens to hundreds of years, even after the contamination source is eliminated, i.e., the characteristic time of this process exceeds the lifetime of one generation of humans.

A diagenetic origin for isotopic variability of sediments deposited on the margin of Great Bahama Bank, insights from clumped isotopes

Abstract The clumped isotope temperature proxy has been used to investigate the diagenetic history of carbonate sediments in two cores recovered during ODP Leg 166 on the margin of Great Bahama Bank. While periplatform sediments constitute a tempting archive of paleo ocean chemistry as they are unlikely to be subducted, their primary limitation is a well-documented susceptibility to post-depositional diagenetic reworking. The crystallization temperatures reconstructed using the clumped isotope proxy, as well as the mineralogy and δ13C and δ18O values have been used to determine the relative effects of sediment mixing and sediment recrystallization. Results show that as sediments undergo diagenetic alteration at the seafloor, their initially “warm” clumped isotope composition is overprinted at cooler benthic temperatures. This process appears to occur in an environment with sufficient fluid exchange to overprint carbon isotopes; an observation confirmed in a separate study by analyses of calcium, a similarly rock-buffered element. This early reactive exchange between carbonates and fluids is likely driven by the conversion of metastable aragonite to calcite. This partially occurs within the “flushed zone”, where porewater compositions remain compositionally similar to seawater throughout the upper ∼40 meters of the sediment column. Sediments dominated by open system isotopic compositions correspond to a period of minimal sediment accumulation between 2 and 3 Ma. More deeply buried Miocene sediments of the more platform-proximal Site 1003 show evidence of subsequent recrystallization, incorporating the warmer geothermal TΔ47 values, and more as well as modified water δ18O values, likely driven by co-evolving porewater and carbonate oxygen isotopes. Reconstructed water δ18O values of these deeper sediments at Site 1003 are considerably more positive than the measured modern values, suggesting that porewater δ18O values were more positive during the Miocene. Sediments deposited at the platform distal Site 1006 between the early and middle Miocene did not show evidence for this deeper recrystallization. Differences in diagenetic behavior between the two sites cannot be solely accounted for by differences in sediment accumulation rate. To illustrate this, time-integrated models were constructed which simulated the burial of identically reactive material through the depositional history of each site, the sediments deposited at Site 1006 appear to “stabilize” after an initial phase of neomorphism, whereas the more platform proximal Site 1003 continues to recrystallize during deeper burial, apparently in the presence of a parent fluid with a more positive δ18O value than observed today. We conclude that despite consisting of the same end-member sediment sources, and being spatially separated by less than 30 km, the difference in clumped and oxygen isotopic composition between Sites 1003 and 1006 can be predominantly attributed to differences in the rate and duration of recrystallization during burial.

An active microbial community in Boom Clay pore water collected from piezometers impedes validating predictive modelling of ongoing geochemical processes

Safe geological disposal of radioactive waste requires a detailed understanding of the geochemical conditions present in the host formation. Consequently, analysis of pore water is essential, as its composition determines among others, the speciation and solubility of radionuclides. Boom Clay is considered a potential host formation in Belgium. Although the elemental composition of Boom Clay pore water is relatively well known, the real mechanisms controlling the pCO2 (g) and the pH, the two most important parameters, are not completely understood. Currently, these parameters are under investigation based only on inorganic chemistry. Borehole waters of different Underground Research Facilities (URF) harbour an active microbial community; however, their possible impact on the geochemistry of Boom Clay pore water extracted from piezometers is not yet examined. The present study discusses the evolution of the geochemistry and the microbial community in the pore water from the piezometers around the PRACLAY gallery of the HADES URF during 7 years after installation of the piezometers. Overall, the elemental composition seemed to vary during the first 4 years, while afterwards it remained quite stable. However, the pCO2 values varied substantially over time, while the pCH4 increased in all filters. The presence of an active microbial community in the piezometers, could explain why experimental pCO2 – pH data do not correspond to the data obtained by predictive modelling, hampering validation of current predictive models of the ongoing geochemical processes. Moreover, the nature of the sampling equipment and the sampling procedure possibly stimulated the present microbial community, resulting in increased methane production rates. To improve predictive modelling, microbial processes are needed to be taken into account together with inorganic geochemistry considered at the current stage, which necessitates detailed microbial and geochemical monitoring in future studies.

The retarding effect of phosphoric acid during CAC hydration

The hydration of CA-cement under the influence of low concentrations of phosphoric acid was investigated. The research was focused on the developing phase content and phase equilibria by QXRD and pore water analysis. To obtain a quantitative phase content the G-Factor method was employed, which was established and applied in the last years for different cementitious systems. Pore water extraction was enforced through centrifugation of cement paste and by high pressure squeezing of cement stone. Data could be obtained up to 48 h. The overall heat flow during this time was also recorded by heat flow calorimetry. We found strong influence of phosphoric acid on the pore water composition and thus determining the initially and later formed hydrate phases. The theoretical data of oversaturated phases from thermodynamic point of view were compared to the actual phases detected by QXRD.

Spatial and Temporal Patterns of Pore Water Chemistry in the Inter-Tidal Zone of a High Energy Beach

Submarine groundwater discharge (SGD) is a ubiquitous source of meteoric fresh groundwater and recirculating seawater to the coastal ocean. Due to the hidden distribution of SGD, as well as the hydraulic- and stratigraphy-driven spatial and temporal heterogeneities, one of the biggest challenges to date is the correct assessment of SGD-driven constituent fluxes. Here, we present results from a 3-dimensional seasonal sampling campaign of a shallow subterranean estuary in a high-energy, meso-tidal beach, Spiekeroog Island, Northern Germany. We determined beach topography and analyzed physico-chemical and biogeochemical parameters such as salinity, temperature, dissolved oxygen, Fe(II) and dissolved organic matter fluorescence (FDOM). Overall, the highest gradients in pore water chemistry were found in the cross-shore direction. In particular, a strong physico-chemical differentiation between the tidal high water and low water line was found and reflected relatively stable in- and exfiltrating conditions in these areas. Contrastingly, in between, the pore water compositions in the existing foreshore ridge and runnel system were very heterogeneous on a spatial and temporal scale. The reasons for this observation may be the strong morphological changes that occur throughout the entire year, which affect the exact locations and heights of the ridge and runnel structures and associated flow paths. Further, seasonal changes in temperature and inland hydraulic head, and the associated effect on microbial mediated redox reactions likely overprint these patterns. In the long-shore direction the pore water chemistry varied less than the along the cross-shore direction. Variation in long-shore direction was probably occurring due to topography changes of the ridge-runnel structure and a physical heterogeneity of the sediment, which produced non-uniform groundwater flow conditions. We conclude that on meso-tidal high energy beaches, the rapidly changing beach morphology produces zones with different approximations to steady-state conditions. Therefore, we suggest that zone-specific endmember sampling is the optimal strategy to reduce uncertainties of SGD-driven constituent fluxes.

Exploring sediment porewater dissolved organic matter (DOM) in a mud volcano: Clues of a thermogenic DOM source from fluorescence spectroscopy

Mud volcanoes (MVs) are potential conduit migration pathways for deep thermogenic DOM. In this study, we investigated the dissolved organic matter (DOM) of porewater in a MV in the Canadian Beaufort Sea and compared dissolved organic carbon (DOC) and fluorescent DOM (FDOM) between the MV and a reference site (RS). The chemical and isotopic compositions (Cl−, δ18O and δD) of porewater from the MVs indicated that these fluids are derived from a mixture of seawater, meteoric water, and clay dehydration, causing a freshening of the porewaters. Interestingly, the porewaters in the MV exhibited DOC concentrations up to 14 times higher than those in the RS. This high DOC concentration was attributed to a higher concentration in the deep fluid moving upwards through the MV, and in minor part to processes such as particulate organic matter sulfate reduction, anaerobic oxidation of methane and higher biological activity in the MV sediments. The fluorescence results showed the presence of four components in both MV and RS sites, which included two humic-like, one microbial humic-like, and a protein-like component. All the four fluorescent components increased with depth, showing a good linear relationship with DOC. However, the DOC-normalized fluorescence in the porewater DOM was on average 3 to 7 times lower in the MV, suggesting that the DOM molecules have undergone thermogenic processes in the deep sediments, and that shallow processes do not affect significantly the FDOM composition. Our results highlight that fluids migrating from the deep sediment through the MV can be an important source of thermogenically altered DOM to the shallow sediments and overlaying water column.

Isotopic Fingerprint of Uranium Accumulation and Redox Cycling in Floodplains of the Upper Colorado River Basin.

Uranium (U) groundwater contamination is a major concern at numerous former mining and milling sites across the Upper Colorado River Basin (UCRB), USA, where U(IV)-bearing solids have accumulated within naturally reduced zones (NRZs). Understanding the processes governing U reduction and oxidation within NRZs is critical for assessing the persistence of U in groundwater. To evaluate the redox cycling of uranium, we measured the U concentrations and isotopic compositions (δ238U) of sediments and pore waters from four study sites across the UCRB that span a gradient in sediment texture and composition. We observe that U accumulation occurs primarily within fine-grained (low-permeability) NRZs that show active redox variations. Low-permeability NRZs display high accumulation and low export of U, with internal redox cycling of U. In contrast, within high-permeability NRZs, U is remobilized under oxidative conditions, possibly without any fractionation, and transported outside the NRZs. The low δ238U of sediments outside of defined NRZs suggests that these reduced zones act as additional U sources. Collectively, our results indicate that fine-grained NRZs have a greater potential to retain uranium, whereas NRZs with higher permeability may constitute a more-persistent but dilute U source.

Responses of microbial activity in hyporheic pore water to biogeochemical changes in a drying headwater stream

Abstract Microbial heterotrophic activity is a major driver of nutrient and organic matter processing in the hyporheic zone of headwater streams. Additionally, the hyporheic zone might provide refuge for microbes when surface flow ceases during drought events. We investigated chemical (organic and inorganic nutrients) and microbiological parameters (bacterial cell concentration, live–dead ratios, and extracellular enzyme activities) of surface and interstitial pore water in a period of progressive surface‐hyporheic disconnection due to summer drying. The special situation of the chosen study reach, where groundwater mixing is impeded by the bedrock forming a natural channel filled with sediment, allowed as to study the transformation of these parameters along hyporheic flow paths. The chemical composition of the hyporheic pore water reflected the connectivity with the surface water, as expressed in the availability of nitrate and oxygen. Conversely, microbiological parameters in all hyporheic locations were different from the surface waters, suggesting that the microbial activity in the water changes rapidly once the water enters the hyporheic zone. This feature was principally manifested in higher live–dead ratios and lower leucine aminopeptidase (an activity related to nitrogen acquisition) in the hyporheic pore waters. Overall, bacterial cell concentration and extracellular enzyme activities increased along hyporheic flow paths, with a congruent decrease in inorganic nutrients and dissolved organic matter quantity and apparent molecular size. Our findings show two important functions of the hyporheic zone during drought: (1) deeper (−50 cm) water‐saturated layers can act as a refuge for microbial activity; and (2) the hyporheic zone shows high rates of carbon and nitrogen turnover when water residence times are longer during drought. These rates might be even enhanced by an increase in living microbes in the remaining moist locations of the hyporheic zone.

Soil moisture influences the avoidance behavior of invertebrate species in anthropogenic metal(loid)-contaminated soils

Water availability is paramount in the response of soil invertebrates towards stress situations. This study aimed to evaluate the effects of forecasted soil moisture scenarios on the avoidance behavior of two invertebrate species (the arthropod Folsomia candida and the soft-bodied oligochaete Enchytraeus crypticus) in soils degraded by different types of anthropogenic metal(loid) contamination (mining soil and agricultural soil affected by industrial chemical wastes). Different soil moisture contents (expressed as % of the soil water holding capacity, WHC) were evaluated: 50% (standard soil moisture conditions for soil invertebrates' tests); 75% (to simulate increasing soil water availability after intense rainfalls and/or floods); 40%, 30%, 25% and 20% (to simulate decreasing soil water availability during droughts). Invertebrates’ avoidance behavior and changes in soil porewater major ions and metal(loid)s were assessed after 48 h exposure. Soil incubations induced a general solubilization/mobilization of porewater major ions, while higher soil acidity favored the solubilization/mobilization of porewater metal(loid)s, especially at 75% WHC. Folsomia candida preferred soils moistened at 50% WHC, regardless the soils were contaminated or not and the changing soil porewater characteristics. Enchytraeus crypticus avoided metal(loid) contamination, but this depended on the soil moisture conditions and the corresponding changes in porewater characteristics: enchytraeids lost their capacity to avoid contaminated soils under water stress situations (75% and 20–25% WHC), but also when contaminated soils had greater water availability than control soils. Therefore, forecasted soil moisture scenarios induced by global warming changed soil porewater composition and invertebrates capacity to avoid metal(loid)-contaminated soils.

Hydrogeochemical features of pore-waters of bottom sediments of the shelf and limans of the Northwestern Black Sea region

The article presents information on the chemical composition and salinity of pore-waters of the Holocene bottom sediments in the shelf and limans from the Northwestern Black Sea region. The specific features of the water quality distribution laterally and in depth are considered. The peculiarities of the structure of hydrogeochemical sections confirm the transgressive-regressive character of the sea level change. Analysis of the geological structure, hydrogeochemical and paleoclimatic features allowed to substantiate the hypothesis of the use of limans as sources of salt in the Holocene period. Based on the information on the hydrochemical features of bottom sediments and changes in sea level, the reconstructions of paleogeographic sedimentation environments during the late Pleistocene-Holocene time on the northwestern shelf and limans of the Black Sea are presented.

Wine origin authentication linked to terroir – wine fingerprint

Global wine and alcohol trade faces a serious economic problem linked to counterfeiting of these commodities. Recently applied authentication methods and techniques pose more difficulties for counterfeiters but they are apparently not effective once we consider economical losses identified by EU legal authorities. The presented solution links isotopic characteristics of the soil, plant, technological intermediate product and the final food product (wine, grapes) on the basis of 87Sr/86Sr isotopes ratios. For the isotopic signature of wines, the average isotope composition of the substrate cannot be a reliable indicator. Only the isotopic composition of pore water can, as it leaches various mineral phases at different stages and passes into vine root system. Instead of complicated sampling of pore water, an original method of preparing and processing soil samples and consequently must & wine samples was developed. Based on both, soil and biological material analysis, we can unquestionably determine not only geographical but also regional and local authenticity of the wine. Determination of red wines isotopic signature is more straightforward process in comparison to white wines, because of technologically different processing of grapes. That is the reason why, in case of white vines, the 87Sr/86Sr ratio of bentonites (natural purifier and absorbent useful in the process of winemaking) must also be taken into consideration. Results of analyses of Slovak wines from geographically diverse regions as well as from sites in close-by distances have clearly established reliability of presented concept, in which the soil is linked to the plant and to the final food product (wine or table grapes).

New results of Boda Claystone research: Genesis, mineralogy, geochemistry, petrophysics

Abstract Boda Claystone is a very tight clayey rock with extreme low porosity and permeability, nano-size pores and small amounts of swelling clays. Due to this character it is ideal as a potential host rock for research into the possibilities of high-level waste deposition in geological formation. Though the research started more than 30 years ago, the genesis, the geotectonic history of the Boda Claystone Formation (BCF) and the geology of surrounding areas has only been sketched out recently. On the basis of research of the past few years the process of sedimentation of different blocks was able to be reconstructed. Equipment and methodological developments were needed for the investigation of reservoir geological and hydrodynamic behaviour of this rock, which began in the early 2000s. Based on them the pore structure and reservoir could be characterized in detail. Only theoretical approaches were available for the chemical composition of free porewater. Traditional water-extracting methods were not adaptable because of excessively low porosity and nano-scale pore size distribution. Hence, new ways have to be found for getting enough water for analysis. These new results of BCF research help to prepare more sophisticated and directed experiments, in which there is a great interest internationally.

Recent, deep-sourced methane/mud discharge at the most active mud volcano in the western Mediterranean

Active mud volcanism in the West Alboran Basin (WAB) is closely associated with tectonically mobilized, overpressurized shales and shale-diapirism. This appears to control mud expulsion at Carmen mud volcano, a cone-shaped structure 65 m high and 1 km in basal diameter.The presence of gas-rich mud breccia, living chemosynthetic fauna, the absence of hemipelagic draping and the abrupt transition that occurs between high dissolved sulfate in the uppermost interval and low sulfate together with high methane concentrations in the lowermost sediment interval all point to a recent expulsion of mud breccia at the summit of Carmen MV.For the lowermost interval, the depletion of major elements (i.e., Ca2+ and Mg2+) and the enrichment of trace species (i.e., Li and B) in the pore water all indicate a deep fluid source. The δ18Opw (5.7‰ VSMOW) and δDpw (−10‰ VSMOW) of pore water in the lowermost interval correspond with smectite dehydration as the main pore-water freshening mechanism. Water-formation temperatures calculated with empirical geo-thermometers (K-Na, K-Mg; δ18Opw, δDpw, and dissolved B) reveal that fluids were generated at temperatures of ~140 ± 20 °C. Taking a regional geothermal gradient for the WAB of 25–27 °C/km, this points to a fluid source from ~5 ± 1 km sediment depth. This is not only consistent with the depth of overpressurized shales and megabreccia of Lower to Middle Miocene age, but it also fits nicely with the Upper/Middle-Miocene seawater value for the porewater 87Sr/86Sr derived from dissolving carbonates. The stable carbon and hydrogen isotopic composition of methane (δ13Cmethane ~ −59.4‰ VPDB and δDmethane −184‰ VSMOW) for the deepest samples of summit-core GP05PC is consistent with the mentioned deep origin.Mud breccia expulsion of overpressurized deep sedimentary units would be accompanied by rigorous degassing, leading to rapid, ‘instantaneous’ replacement of pore fluid by bottom water in the upper sediments. The absence of oxidized sediment draping, the seawater-like pore-water composition in the uppermost part of the mud breccia interval, and the abrupt methane to sulfate transition all provide evidence for a very recent mud expulsion.The distinctively kink-shaped pore-water Cl− profile in core GP05PC has been used in a numerical transport-reaction model to derive the timing for this event. This eruptive event appears to have taken place very recently, namely 12 ± 5 yrs prior to the 2012 coring, thus in the year 2000 CE.

Solute Reservoirs Reflect Variability of Early Diagenetic Processes in Temperate Brackish Surface Sediments

Coastal marine sediments are a hotspot of organic matter degradation. Mineralization products of early diagenetic processes accumulate in the pore waters of the sediment, are subject of biological uptake and secondary biogeochemical processes and are released back into the water column via advective and diffusive fluxes across the sediment-water interface. Seven representative sites in the shallow coastal area of the southern Baltic Sea (15–45 m water depth), ranging from permeable sands to fine grained muds, were investigated on a seasonal basis for their key mineralization processes as well as their solid phase and pore water composition to identify the drivers for the variability of early diagenetic processes in the different sediment types. The sandy sediments showed about one order of magnitude lower organic carbon contents compared to the muds, while oxygen uptake rates were similar in both sediment types. Significantly higher oxygen uptake rates were determined in two near-shore muddy sites than in a deeper coastal muddy basin, which is due to higher nutrient loads and the corresponding addition of fresh algal organic matter in the near-shore sites. Pore water concentration profiles in the studied sediments were usually characterized by a typical biogeochemical zonation with oxic, suboxic, and sulfidic zones. An up to 15 cm thick suboxic zone was sustained by downward transport of oxidized material in which dissolved iron and phosphate indicate an intensive reduction of reactive Fe with the release of adsorbed phosphorus. While the geochemical zonation was stable over time in the muds of the studied deeper basin, high variability was observed in the muds of a near-coastal bay probably mainly controlled by sediment mixing activities. The sediments can be characterized by essentially two factors based on their near-surface benthic solute reservoirs: (1) their organic matter mineralization and solute accumulation efficiency and (2) their redox-state. Benthic solute reservoirs in the pore waters of the top decimeter were generally higher in the muddy than in the sandy sediments as the more permeable sands were prone to an intensive exchange between pore water and bottom water. The three studied muddy sites showed great dissimilarities with respect to their predominating redox-sensitive metabolites (dissolved iron, manganese, and sulfide). Surface-near advective transport like irrigation of permeable sands and rearrangement of cohesive muds had a particularly strong influence on early diagenetic processes in the studied sediments and were probably the most important cause for the spatiotemporal variability of their benthic solute reservoirs.

Salinity and Macrocomponent Composition of Pore Water of the Bazhenov, Achimov, and Georgiev Formations from Analysis of Aqueous Extracts

This paper presents a reliable dataset on formation water salinity (mineralization) and salt composition for low-permeable (tight) shale rocks of the Bazhenov, Achimov and Georgiev formations for the first time. The data were obtained via laboratory analysis of aqueous extracts from rock samples before and after hydrocarbon extraction (cleaning). Thermodynamic modeling of the obtained solutions with a number of minerals was carried out and a correlation analysis of the Na/Cl, Ca/SO4, Mg/SO4, Ca/Cl, Ca/HCO3, and Fe/SO4 ratios was performed. It was established that the concentrations of certain macro-components, including Ca, Mg, Fe, hydrocarbonate, and sulfate ion in pore water cannot be reliably determined using aqueous-extract data, while at the same time, a reliable estimate of the minimal formation water salinity could be made. The need to use non-extracted core samples with the maximum preserved residual water saturation to study a pore water composition of tight shale rocks is shown.