Pore Fluid Chemistry Research Articles

Evaluation of the mechanical behaviour of brine+CO2 saturated brown coal under mono-cyclic uni-axial compression

CO2-sequestration and ECBM techniques necessitate injection of CO2 into coal reservoirs that are saturated under various pore fluid conditions, resulting in alterations in mechanical properties. In this study, we evaluate the effect of pore fluid chemistry and the interaction time on the coal mechanical degradation. Uni-axial compressive strength (UCS) tests, ARAMIS photogrammetric analysis, micro-CT imaging, and Fourier transform infrared spectroscopy (FT-IR) are combinedly used to evaluate such alterations and to interpret the causative factors in water, water+CO2, 10% brine+CO2 and 20% brine+CO2 saturated coal. The results indicate that irrespective of the pore fluid chemistry, mechanical parameters including UCS, Young’s modulus and brittleness index reduce significantly in saturated coal, due to moisture adsorption-induced softening effect. The presence of CO2 in pore fluids causes additional alterations in each mechanical property, due to the corrosive chemical interactions occur in acidic environments, CO2 adsorption-induced energy reduction and plasticization-induced alterations in coal macromolecular structure. The direct comparison of FT-IR spectrums of natural and CO2-interacted coal concludes that CO2 interaction causes alterations in the coal macromolecular structure possibly causing the mechanical degradation of coal mass due to plasticization effect and the extraction of pore-constricting hydrocarbons. The volumetric strain analysis and micro-CT image-based 3D-reconstruction infer that water, water+CO2 and 10% brine+CO2 saturation increase the ductile properties of coal, resulting in a dilatancy deformation and extensive fracturing, upon mechanical loading. In contrast, the higher order of NaCl concentrations (i.e. 20% brine) in pore fluid causes NaCl crystallization in coal, resulting in an elevated brittleness and consequently altering the sample deformation, failure pattern and fracturing mechanism. Although, softening effect and chemical interactions cause continues mechanical degradation with increasing saturation time in all saturation conditions, a significant strength reduction occurs at a short-term saturation period and the latter longer saturations have caused only gradual strength reductions, probably due to the rapid CO2 adsorption process on to coal matrix.

Modeling of active multiphase materials

AbstractIn this contribution, material modeling and experimental investigations towards a chemically‐active multiphase material are presented. The material under consideration consists of a solid skeleton, while the pore space is filled with multiple reactive pore fluids. Here, the pore space is simultaneously filled with a pore liquid and a hydrogel made of a granulate of superabsorbent polymers (SAP). The three‐phase material is modeled within a continuum mixture theory based on the Hybrid Mixture Theory (HMT). Due to the hydrophilic character, the hydrogel is able to absorb large amounts of the pore liquid and binds it to the polymer structure. As a consequence, the saturation of free liquid in the void space decreases during the absorption, which also affects the permeability with respect to seepage flow. Since the absorption dynamics as well as the maximum water capacity of the hydrogel depend highly on external stimuli, e.g. the pore fluid chemistry, temperature, electric and magnetic fields, hydrogels are usually called active or functional materials. The hydrogel material used in this study is sensitive with respect to the ion concentration of the surrounding pore liquid.

Soil consistency and interparticle characteristics of xanthan gum biopolymer–containing soils with pore-fluid variation

Biopolymer–soil technology is currently recognized as an environmentally friendly soil improvement method for geotechnical engineering practices. However, concerns exist regarding biopolymer fine-soil applications because the performance of biopolymers is based on an electrical interaction with clay or a pore fluid. Thus, the effect of water content and pore-fluid chemistry on biopolymer behavior in soil must first be clarified in terms of biopolymer applications. In this study, the liquid limits of xanthan gum biopolymer–treated clay–sand mixtures (clayey silt, kaolinite, montmorillonite, and sand) were obtained using three chemically distinct pore fluids (deionized water, 2 mol/L NaCl brine, and kerosene). Xanthan gum has contrary effects to the soil consistency, where the liquid limit can decrease via xanthan gum–induced particle aggregation or increase due to xanthan gum hydrogel formation. The clay-mineral type governed the xanthan gum behavior in the deionized water, while the pore-fluid chemistry governed the xanthan gum behavior in the brine and the kerosene.

Impact of Chemical Environment on Compaction Creep of Quartz Sand and Possible Geomechanical Applications

AbstractInduced seismicity and surface subsidence are adverse effects of natural gas and geothermal energy production that may present barriers to their use as low‐carbon alternatives to coal and oil. The driving force for these unwanted effects is compaction of the reservoir, which can potentially be mitigated by injecting (pressurized) fluids that restore the pore pressure and chemically inhibit compaction. We conducted uniaxial compaction experiments on quartz sand aggregates to investigate the effect of pore fluid chemistry on time‐dependent compaction (creep). In addition to a low‐vacuum (dry) environment, supercritical fluids (N2, CO2, and wet CO2), simple aqueous solutions (three HCl solutions and a NaOH solution), and complex aqueous solutions with additives (AlCl3, AMP, and washing detergent) were employed. N2, CO2, and fluids containing scaling inhibitor (AMP), as well as wastewater (detergent solution) are generally considered for injection. Compaction creep was enhanced in fluid‐saturated environments compared to dry. Wet CO2 caused more creep with faster strain rates than the relatively dry CO2 and N2 environments. Experiments conducted with simple aqueous solutions exhibited a clear pH dependency. The complex aqueous solutions enhanced creep compared to their simple solution counterpart with similar pH. Based on acoustic emission data and microstructural analyses, we inferred that compaction creep was controlled by subcritical crack growth, aided by water, hydroxyl ions, and additives. If microcracking also controls compaction in reservoir sandstones, these results indicate that injection of supercritical fluids or acidic solutions may mitigate reservoir compaction.

Sensitivity of clumped isotope temperatures in fossil benthic and planktic foraminifera to diagenetic alteration

Applying the clumped isotope (Δ47) thermometer to foraminifer microfossils offers the potential to significantly improve paleoclimate reconstructions, owing to its insensitivity to the isotopic composition of seawater (unlike traditional oxygen isotope (δ18O) analyses). However, the extent to which primary Δ47 signatures of foraminiferal calcites can be altered during diagenesis is not well known. Here, we present Δ47 data as well as high-resolution (∼10 kyr) δ18O and δ13C middle Eocene time series, measured on benthic and planktic foraminifera from ODP/IODP Sites 1408, 1409, 1410, 1050, 1260 and 1263 in the Atlantic Ocean. The sites examined span various oceanographic regimes, including the western tropical to mid-latitude North Atlantic, and the eastern mid-latitude South Atlantic. Comparing data from contemporaneous foraminifera with different preservation states, we test the effects of diagenetic alteration on paleotemperature reconstructions for the deep and surface ocean. We find that overall, primary Δ47 signatures appear similarly sensitive to diagenetic overprinting as δ18O, with differences in sensitivity depending on pore fluid chemistry and the amount of secondary calcite. Where planktic foraminifera are significantly altered, sea surface temperatures derived from Δ47 and δ18O values are biased towards cool temperatures. In comparison, Δ47 and δ18O values of benthic and well preserved planktic foraminifera are less affected by diagenesis and thus likely to yield robust foraminiferal calcification temperatures. With independent estimates of diagenetic calcite fractions, secondary overprints could be corrected for, using end-member modeling and Δ47-based temperatures from benthic foraminifera.

Experimental and Modeling Study of Soil Water Retention Curves of Compacted Bentonite considering Salt Solution Effects

Due to its low hydraulic conductivity, high swelling capacity, and good adsorption properties, the Gaomiaozi (GMZ) bentonite has been selected as potential buffer/backfill materials for construction of engineered barriers in the deep geological repository for disposal of high-level nuclear waste (HLW) in China. Investigation of salt solution effects on the water retention properties of compacted bentonite is of great importance in the context of geological disposal of HLW based on the multibarrier concept. In this study, amended specimens were obtained through a spray of different concentrations of salt solutions to reach target salt contents, respectively. With employment of the vapor phase technique for suction control, water retention tests were conducted on densely compacted GMZ bentonite (1.7 Mg/m3) with different salt contents under confined conditions. Corresponding soil water retention curves (SWRCs) were obtained. Analysis indicates that, for a given suction, the measured water content of GMZ bentonite specimen increases as the salt content (or pore fluid concentration) increases. The influencing rate depends on suction. For lower suctions (lower than 38 MPa), the water retention capacity increases as the salt content increases, while for higher suctions (higher than 38 MPa), the influence can be negligible. Based on the Fredlund and Xing (1994) equation, a soil water retention model was proposed for simulation of the SWRCs of compacted GMZ bentonite with consideration of pore fluid chemistry. Parameters were analyzed and determined with consideration of influences of the pore fluid concentration. Verification indicates that the SWRCs simulated by the proposed model are well agreed with the measured ones.

Inelastic Deformation of the Slochteren Sandstone: Stress‐Strain Relations and Implications for Induced Seismicity in the Groningen Gas Field

AbstractPore pressure reduction in sandstone reservoirs generally leads to small elastic plus inelastic strains. These small strains (0.1%–1.0% in total) may lead to surface subsidence and induced seismicity. In current geomechanical models, the inelastic component is usually neglected, though its contribution to stress‐strain behavior is poorly constrained. To help bridge this gap, we performed deviatoric and hydrostatic stress cycling experiments on Slochteren sandstone samples from the seismogenic Groningen gas field in the Netherlands. We explored in situ conditions of temperature (T = 100 °C) and pore fluid chemistry, porosities of 13% to 26% and effective confining pressures (≤320 MPa) and differential stresses (≤135 MPa) covering and exceeding those relevant to producing fields. We show that at all stages of deformation, including those relevant to producing reservoirs, 30%–50% of the total strain measured is inelastic. Microstructural observations suggest that inelastic deformation is largely accommodated by intergranular displacements at small strains of 0.5%–1.0%, with intragranular cracking becoming increasingly important toward higher strains. The small inelastic strains relevant for reservoir compaction can be described by an isotropic, Cam‐clay plasticity model. Applying this model to the depleting Groningen gas field, we show that the in situ horizontal stress evolution is better represented by taking into account combined elastic and inelastic deformation than it is by representing the total deformation behavior using poroelasticity (up to 40% difference). Therefore, inclusion of the inelastic contribution to reservoir compaction has a key role to play in future geomechanical modelling of induced subsidence and seismicity.

Effect of frequency on cyclic response of marine clay saturated with various pore fluids

ABSTRACT In the present study, the cyclic triaxial tests and resonant column (RC) tests were carried out, to study the effect of frequency on reconstituted marine clay saturated with distilled water and NaCl solutions. Slurry consolidated samples were prepared at 50 kPa vertical stress. Samples were isotropically consolidated at three different effective confining pressures i.e. 80, 150 and 300 kPa. The loading frequencies adopted for the study were 0.1 Hz and 1 Hz with a number of 100 loading cycles. The results show that the effect of frequency could affect the shear modulus of clay soil reconstituted with varying pore fluid chemistry. The shear modulus increases and damping ratio decreases with increase in loading frequency for clay soil reconstituted with concentrated saline solution. Furthermore, the seismic response analysis was conducted to study the effect of frequency on marine clay soil reconstituted with distilled water and electrolyte solutions using one dimensional equivalent linear DEEPSOIL software.

Exploring the micromechanics of non-active clays by way of virtual DEM experiments

The micromechanical behaviour of clays cannot be investigated experimentally in a direct fashion due to the small size of clay particles. An insight into clay mechanical behaviour at the particle scale can be gained by way of virtual experiments based on the discrete-element method (DEM). So far, most DEM models for clays have been designed on the basis of theoretical formulations of inter-particle interactions, with limited experimental evidence of their actual control over the clay's macroscopic response. This paper presents a simplified two-dimensional DEM framework where contact laws were inferred from indirect experimental evidence at the microscale provided by Pedrotti and Tarantino in 2017 (particle-to-particle interactions were probed experimentally by varying the pore-fluid chemistry, and the resulting effect was explored by way of scanning electron microscopy and mercury intrusion porosimetry). The proposed contact laws were successfully tested against their ability to reproduce qualitatively the compression behaviour of clay with pore fluids of varying pH and dielectric permittivity. The DEM framework presented in this work was intentionally kept simple in order to demonstrate the robustness of the micromechanical concept underlying the proposed contact laws. It is anticipated that a satisfactory quantitative prediction would be achieved by moving to a three-dimensional formulation, by considering polydisperse specimens and by refining the contact laws.

Low-grade evolution of clay minerals and organic matter in fault zones of the Hikurangi prism (New Zealand)

ABSTRACTClay minerals and organic matter occur frequently in fault zones. Their structural characteristics and their textural evolution are driven by several formation processes: (1) reaction by metasomatism from circulating fluids; (2) in situ evolution by diagenesis; and (3) neoformation due to deformation catalysis. Clay-mineral chemistry and precipitated solid organic matter may be used as indicators of fluid circulation in fault zones and to determine the maximum temperatures in these zones. In the present study, clay-mineral and organic-matter analyses of two major fault zones – the Adams-Tinui and Whakataki faults, Wairarapa, North Island, New Zealand – were investigated. The two faults analysed correspond to the soles of large imbricated thrust sheets formed during the onset of subduction beneath the North Island of New Zealand. The mineralogy of both fault zones is composed mainly of quartz, feldspars, calcite, chabazite and clay minerals such as illite-muscovite, kaolinite, chlorite and mixed-layer minerals such as chlorite-smectite and illite-smectite. The diagenesis and very-low-grade metamorphism of the sedimentary rock is determined by gradual changes of clay mineral ‘crystallinity’ (illite, chlorite, kaolinite), the use of a chlorite geothermometer and the reflectance of organic matter. It is concluded here that: (1) the established thermal grade is diagenesis; (2) tectonic strains affect the clay mineral ‘crystallinity’ in the fault zone; (3) there is a strong correlation between temperature determined by chlorite geothermometry and organic-matter reflectance; and (4) the duration and depth of burial as well as the pore-fluid chemistry are important factors affecting clay-mineral formation.

Seawater recirculation through subducting sediments sustains a deeply buried population of sulfate-reducing bacteria.

Subseafloor sulfate concentrations typically decrease with depth as this electron acceptor is consumed by respiring microorganisms. However, studies show that seawater can flow through hydraulically conductive basalt to deliver sulfate upwards into deeply buried overlying sediments. Our previous work on IODP Site C0012A (Nankai Trough, Japan) revealed that recirculation of sulfate through the subducting Philippine Sea Plate stimulated microbial activity near the sediment-basement interface (SBI). Here, we describe the microbial ecology, phylogeny, and energetic requirements of population of aero-tolerant sulfate-reducing bacteria in the deep subseafloor. We identified dissimilatory sulfite reductase gene (dsr) sequences 93% related to oxygen-tolerant Desulfovibrionales species across all reaction zones while no SRB were detected in drilling fluid control samples. Pore fluid chemistry revealed low concentrations of methane (<0.25mM), while hydrogen levels were consistent with active bacterial sulfate reduction (0.51-1.52nM). Solid phase total organic carbon (TOC) was also considerably low in these subseafloor sediments. Our results reveal the phylogenetic diversity, potential function, and physiological tolerance of a community of sulfate-reducing bacteria living at ~480m below subducting seafloor.

Effects of Pore Fluid Chemistry and Saturation Degree on the Fracability of Australian Warwick Siltstone

Fracability of unconventional gas reservoirs is an important parameter that governs the effectiveness of subsequent gas extraction. Since reservoirs are saturated with various pore fluids, it is essential to evaluate the alteration of fracability of varyingly saturated rocks. In this study, varyingly saturated (dry, water, and brine with 10%, 20% and 30% NaCl by weight) siltstone samples were subjected to uniaxial compressive loading to evaluate their fracability variation. Acoustic emission (AE) and ARAMIS photogrammetry analyses were incorporated to interpret the crack propagation. SEM analysis was carried out to visualize the micro-structural alterations. Results show that siltstone strength and brittleness index (BI) are reduced by 31.7% and 46.7% after water saturation, due to water-induced softening effect. High NaCl concentrations do not reduce the siltstone strength or brittleness significantly but may contribute to a slight re-gain of both values (about 3–4%). This may be due to NaCl crystallization in rock pore spaces, as confirmed by SEM analysis. AE analysis infers that dry siltstone exhibits a gradual fracture propagation, whereas water and brine saturated specimens exhibit a hindered fracturing ability. ARAMIS analysis illustrates that high NaCl concentrations causes rock mass failure to be converted to shear failure from splitting failure, which is in favour of fracability.

Effects of steel pile corrosion on permeability of hydraulic barriers

An increasing economic interest leads to construction on old landfills and waste deposits, raising questions about potential contamination of groundwater due to installation of piles through clay barriers. The current consensus in the geotechnical practice, that penetration of clayey hydraulic barriers by driven steel piles with closed conical tips should not cause pollution of the underlying aquifers, is challenged in this paper. Consolidometer and flow-column tests on clays with iron-rich pore fluid were performed for the assessment of pile corrosion effects on hydraulic barriers. Based on the results of these tests, a constitutive model for the coupled chemomechanical behaviour was modified to account for different pore-fluid chemistry and incorporated into a finite-element code to solve the problem of iron diffusion originating from a corroding source in a case study on groundwater pollution. The combined action of diffusion, corrosion and increased convection resulted in an overall increased transport rate and therefore a significantly earlier arrival of critical concentrations of the pollutant in the aquifer that was initially protected by a hydraulic barrier. Once this barrier is perforated by foundation piles subjected to corrosion, small concentrations can arrive in the aquifer up to four times earlier than implied by the initially low hydraulic conductivity.

2D micromodel study of clogging behavior of fine-grained particles associated with gas hydrate production in NGHP-02 gas hydrate reservoir sediments

Fine-grained particles (fines) commonly coexist with coarse-grained sediments that host gas hydrate. These fines can be mobilized by liquid and gas flow during gas hydrate production. Once mobilized, fines can clog pore throats and reduce reservoir permeability. Even where particle sizes are smaller than pore-throat sizes, clogs can form due to clusters of fines. For certain types of fines, particularly swelling clays, cluster sizes depend on pore-fluid chemistry, which changes as pore-fluid freshens during gas hydrate dissociation. Fines can also be concentrated by a moving gas/liquid interface, increasing the chances of pore-throat clogging regardless of fines type. To test the relative significance of these clogging mechanisms, 2D micromodel experiments have been conducted with different pore-throat widths (20, 40, 60 and 100 μm), single-phase pore-fluids (deionized water and 2M-sodium-chloride solution), and moving gas/liquid interfaces on specimens from Sites NGHP-02-09 and NGHP-02-16 (NGHP-02: National Gas Hydrate Program Expedition 02) as well as a selection of pure fines (silica silt, mica, calcium carbonate, diatoms, kaolin, and bentonite). Clogging depended on the ratio of particle-to-pore throat size, and also on pore-fluid chemistry because the pore-fluid chemistry changes effectively increased or decreased the fine's cluster size relative to the pore-throat width. These interactions can be predicted based on the fine's electrical sensitivity (defined by Jang and Santamarina, 2016). The fine-grained sediment component (grain size < 75 μm) from the primary gas hydrate reservoir layers at Sites NGHP-02-09 and −16 show clogging via blocking or size exclusion (sieving) due to the large particles. Clogs also formed due to bridging or blocking by clusters of the smaller particles. Clogging generally occurred for pore-water sediment concentrations so low (0.2% by mass or less), that it was difficult to resolve the enhanced clogging in the presence of the gas/liquid meniscus.

Adhesive forces between two cleaved calcite surfaces in NaCl solutions: The importance of ionic strength and normal loading

The mechanical strength of calcite bearing rocks is influenced by pore fluid chemistry due to the variation in nano-scale surface forces acting at the grain contacts or close to the fracture tips. The adhesion of two contacting surfaces, which affects the macroscopic strength of the material, is not only influenced by the fluid chemistry but also by the surface topography. In this paper, we use Atomic Force Microscope (AFM) to measure the interfacial forces between two freshly cleaved calcite surfaces in CaCO3-saturated solutions with varying NaCl concentration. We show that calcite contacts become stronger with increasing NaCl concentration (>100 mM), as a result of progressively weaker secondary hydration and increasing attraction due to instantaneous ion-ion correlation. Moreover, we discuss the effect of normal applied force (Fn) and surface roughness on the measured adhesion forces (Fad). We show that the measured pull-off force (adhesion) is linearly correlated with the magnitude of Fn, where an increase in applied force results in increased adhesion. This is attributed to a larger number of contacting surface asperities and thus increase in real contact area and the contact-bond strength. We discuss that the possible variation in local topography at contacts, together with strong dependence on ionic strength of the solution, can explain the inconsistent behavior of calcite rocks in NaCl solutions.

Impact of Pore Fluid Chemistry on Fine‐Grained Sediment Fabric and Compressibility

AbstractFines, defined here as grains or particles, less than 75 μm in diameter, exist nearly ubiquitously in natural sediment, even those classified as coarse. Macroscopic sediment properties, such as compressibility, which relates applied effective stress to the resulting sediment deformation, depend on the fabric of fines. Unlike coarse grains, fines have sizes and masses small enough to be more strongly influenced by electrical interparticle forces than by gravity. These electrical forces acting through pore fluids are influenced by pore fluid chemistry changes. Macroscopic property dependence on pore fluid chemistry must be accounted for in sediment studies involving subsurface flow and sediment stability analyses, as well as in engineered flow situations such as groundwater pollutant remediation, hydrocarbon migration, or other energy resource extraction applications. This study demonstrates how the liquid limit‐based electrical sensitivity index can be used to predict sediment compressibility changes due to pore fluid chemistry changes. Laboratory tests of electrical sensitivity, sedimentation, and compressibility illustrate mechanisms linking microscale and macroscale processes for selected pure, end‐member fines. A specific application considered here is methane extraction via depressurization of gas hydrate‐bearing sediment, which causes a dramatic pore water salinity drop concurrent with sediment being compressed by the imposed effective stress increase.

Wetting-induced collapse behavior of kaolinite: influence of fabric and inundation pressure

Collapsible soils are known to withstand high normal stresses without undergoing a significant volume change in an air-dry state. The soil is, however, susceptible to a large volume change upon wetting. The volumetric collapse behavior of these soils is of great engineering interest to address the problem of ground subsidence. Kaolinite was found to be a collapsible soil similar to the loess soil. The collapse mechanism in kaolinite was due to changes in particle association (fabric) with the interaction with different pore fluids. Several physicochemical parameters strongly influenced the particle association by altering the charges on the particle surfaces and edges. The collapse nature of the kaolinite was investigated with great detail in this work. Wetting-induced collapse behavior of kaolinite was studied under the influence of pore-fluid chemistry using a multi-scale approach. The influence of pH, salt concentration, and dielectric pore-fluid environment on the clay behavior was analyzed using sedimentation and collapse tests. The collapse test results were well corroborated with the sedimentation test data, scanning electron microscopy images of lyophilized specimens, and edge isoelectric point (IEPedge). The influence of inundation fluid and inundation pressure on the fabric changes and collapse potential was elucidated. The collapse potential was found to depend on the inundation pressure and pore-fluid characteristics. Maximum collapse potential was observed to be due to water inundation and minimum due to inundation with kerosene. The mechanical pressure at higher inundation pressures helped to bring the clay platelets into better face–face association during the inundation.

Enhanced carbon-sulfur cycling in the sediments of Arabian Sea oxygen minimum zone center

Biogeochemistry of oxygen minimum zone (OMZ) sediments, which are characterized by high input of labile organic matter, have crucial bearings on the benthic biota, gas and metal fluxes across the sediment-water interface, and carbon-sulfur cycling. Here we couple pore-fluid chemistry and comprehensive microbial diversity data to reveal the sedimentary carbon-sulfur cycle across a water-depth transect covering the entire thickness of eastern Arabian Sea OMZ, off the west coast of India. Geochemical data show remarkable increase in average total organic carbon content and aerial sulfate reduction rate (JSO42−) in the sediments of the OMZ center coupled with shallowing of sulfate methane transition zone and hydrogen sulfide and ammonium build–up. Total bacterial diversity, including those of complex organic matter degraders, fermentative and exoelectrogenic bacteria, and sulfate-reducers (that utilize only simple carbon compounds) were also found to be highest in the same region. The above findings indicate that higher organic carbon sequestration from the water-columns (apparently due to lower benthic consumption, biodegradation and biotransformation) and greater bioavailability of simple organic carbon compounds (apparently produced by fermetative microflora of the sediments) are instrumental in intensifying the carbon-sulfur cycle in the sediments of the OMZ center.

Diagenesis in tephra-rich sediments from the Lesser Antilles Volcanic Arc: Pore fluid constraints

We present sediment pore fluid and sediment solid phase results obtained during IODP Expedition 340 from seven sites located within the Grenada Basin of the southern Lesser Antilles Volcanic Arc region. These sites are generally characterized as being low in organic carbon content and rich in calcium carbonate and volcanogenic material. In addition to the typical reactions related to organic matter diagenesis, pore fluid chemistry indicates that the diagenetic reactions fall within two broad categories; (1) reactions related to chemical exchange with volcanogenic material and (2) reactions related to carbonate dissolution, precipitation, or recrystallization. For locations dominated by reaction with volcanogenic material, these sites exhibit increases in dissolved Ca with coeval decreases in Mg. We interpret this behavior as being driven by sediment-water exchange reactions from the alteration of volcanic material that is dispersed throughout the sediment package, which likely result in formation of Mg-rich secondary authigenic clays. In contrast to this behavior, sediment sequences that exhibit decreases in Ca, Mg, Mn, and Sr with depth suggest that carbonate precipitation is an active diagenetic process affecting solute distributions. The distributions of pore fluid 87Sr/86Sr reflect these competitive diagenetic reactions between volcanic material and carbonate, which are inferred by the major cation distributions. From one site where we have solid phase 87Sr/86Sr (site U1396), the carbonate fraction is found to be generally consistent with the contemporaneous seawater isotope values. However, the 87Sr/86Sr of the non-carbonate fraction ranges from 0.7074 to 0.7052, and these values likely represent a mixture of local arc volcanic sources and trans-Atlantic eolian sources. Even at this site where there is clear evidence for diagenesis of volcanogenic material, carbonate diagenesis appears to buffer pore fluid 87Sr/86Sr from the larger changes that might be expected given the high abundance of tephra in these sediments. Part of this carbonate buffering, at this site as well as throughout the region, derives from the fact that the Sr concentration in the non-carbonate fraction is generally low (<200 ppm), whereas the carbonate fraction has Sr concentrations approaching ∼1000 ppm.

Calcium adsorption on clays: Effects of mineralogy, pore fluid chemistry and temperature

Abstract The present study describes the calcium adsorption behavior of clays exhibiting distinct mineralogical composition. The adsorption characteristics were determined using conventional batch-equilibrium sorption method, and different theoretical models were applied to describe the equilibrium sorption isotherms. The variation in calcium adsorption capacity was determined as a function of clay mineralogy, temperature and pore fluid chemistry. Further, the thermodynamic parameters were also calculated to describe the nature of adsorption mechanisms. Significant variation in calcium adsorption potential was observed among the clays, primarily attributed to their mineralogical diversities and related unique surface charge properties. The adsorption density escalated with rise in calcium concentration, temperature and pH of the adsorption system. These observations can be attributed to surface charge modifications and mineral dissolution properties of the clays, which in turn resulted in higher electro-negativity of the clay surface and thereby enhancing the affinity for calcium ions.