Precipitation Of Carbonate Minerals Research Articles

Sulphate reduction and carbonate precipitation in a high-energy algal rim framework

Algal ridges are protective features for coral reefs that form through the accretion and encrustation of reef rubble and debris by crustose coralline algae (CCA) and processes of diagenetic cementation. Carbonate precipitation and dissolution dynamics on and within algal ridge frameworks are poorly understood. We studied the surface and subsurface geochemistry of the algal ridge framework at One Tree Island, Australia. Measurable quantities of hydrogen sulphide were detected in most porewater samples collected from bores, indicating a largely anoxic ridge framework. Total alkalinity (TA) and pH measurements indicate that the precipitation of carbonate minerals within the interior of the ridge framework occurs under largely anoxic conditions and is likely to be driven by TA changes associated with sulphate-reducing bacteria. Modelling of porewater hydrogen sulphide concentrations in combination with TA and dissolved inorganic carbon (DIC) indicates anoxic respiration processes produce alkalinity within the algal ridge framework. However, significantly more TA is removed via the precipitation of mineral carbonate, resulting in porewater TA concentrations falling below the open seawater values. The precipitation of mineral carbonate also lowers interstitial water pH, such that pH changes are not solely from organic carbon diagenesis. The simultaneous precipitation and dissolution of carbonate minerals within the algal ridge framework are key to forming and cementing algal ridges, which are important physical protective features for coral reefs.

Microbial Autotrophy Recorded by Carbonate Dual Clumped Isotope Disequilibrium

AbstractThe proliferation of microbial carbon fixation is a key control on the evolution of the biosphere and global carbon cycle. Most records of these metabolisms in ancient rocks come from organic matter or fossils, which are not always preserved. Here, we present a potential proxy for microbial carbon fixation (autotrophy) based on the isotopic composition of carbonate minerals. Autotrophs influence carbonate chemistry in the cellular microenvironment by decreasing CO2 concentration and increasing the carbonate saturation state. This can induce rapid precipitation of carbonate minerals that are out of isotopic equilibrium with their environment. Recent work has identified disequilibrated dual clumped isotope compositions (∆47 and ∆48) in the skeletal fossils of marine calcifying organisms. Here we test whether the same is true of non‐skeletal carbonate fabrics associated with microbial autotrophs in modern and Eocene lakes. We found that microbial carbonate formed via autotrophic metabolism recorded lower ∆47 and higher ∆48 values (−∆47/+∆48) than predicted for thermodynamic equilibrium mineral formation. Our findings are supported by models of dual clumped isotope kinetics in the DIC system, and disequilibrium in the oxygen isotope system. We hypothesize that the inverse trajectory away from the equilibrium line (+∆47/−∆48) should be recorded by carbonates formed in association with alkalinizing heterotrophs, such as sulfate reducers. If so, carbonate dual clumped isotopes could be a powerful tool to identify the proliferation and rate of heterotrophic and autotrophic metabolisms in the carbonate rock record on Earth and (perhaps) other planets.

Time-calibration of carbonate diagenesis and regional tectonism in the Norwegian Barents Sea

Diagenesis plays a crucial role in carbonate reservoir properties, for example through the dissolution or precipitation of carbonate minerals, with burial history and fluid migration thought to play an important role in the timing of these events. To better understand these relationships and the local manifestation of regional events, we study carbonate sedimentary rocks and associated diagenetic cements from the Loppa High and Finnmark Platform using in-situ U–Pb carbonate geochronology and C–O stable isotope ratios, combined with burial history modelling. The results indicate a complex history of diagenesis: analyzed dolomicrite samples from the Loppa High typically yield ages that are older than their biostratigraphic age, in contrast to dolomicrite samples from the Finnmark Platform that yielded younger ages; this regional offset may be reflective of different styles of early diagenesis, as well as heterogeneous and re-deposited origin of some studied materials. While many diagenetic calcite cements coincide with modelled burial or uplift events, other events have no cements associated with them, although the possibility that some diagenetic carbonate phases were unsampled cannot be ruled out. Some calcite cements are not associated with burial events at all and may instead be related to hydrocarbon ‘charging’, supported by strongly negative δ13C values recorded in these cements. Broadly, these results highlight the value of integrating petrographic observations, burial history modelling, carbonate U–Pb geochronology, and C–O isotope ratios, as well as the complexity of untangling diagenetic histories.

Assessment of the in situ biomethanation potential of a deep aquifer used for natural gas storage.

The dihydrogen (H2) sector is undergoing development and will require massive storage solutions. To minimize costs, the conversion of underground geological storage sites, such as deep aquifers, used for natural gas storage into future underground hydrogen storage sites is the favored scenario. However, these sites contain microorganisms capable of consuming H2, mainly sulfate reducers and methanogens. Methanogenesis is, therefore expected but its intensity must be evaluated. Here, in a deep aquifer used for underground geological storage, 17 sites were sampled, with low sulfate concentrations ranging from 21.9 to 197.8µM and a slow renewal of formation water. H2-selected communities mainly were composed of the families Methanobacteriaceae and Methanothermobacteriaceae and the genera Desulfovibrio, Thermodesulfovibrio, and Desulforamulus. Experiments were done under different conditions, and sulfate reduction, as well as methanogenesis, were demonstrated in the presence of a H2 or H2/CO2 (80/20) gas phase, with or without calcite/site rock. These metabolisms led to an increase in pH up to 10.2 under certain conditions (without CO2). The results suggest competition for CO2 between lithoautotrophs and carbonate mineral precipitation, which could limit microbial H2 consumption.

Diagenetic alterations induced by lamina-scale mass transfer and the impacts on shale oil reservoir formation in carbonate-rich shale of the Permian Lucaogou Formation, Jimusar Sag

Diagenesis plays a crucial role in shale oil reservoir formation. However, complex organic-inorganic interactions result in pore system formation and heterogeneity changes at the centimeter, or even micrometer scale, which controls the evolution of shale oil reservoirs. In this paper, an integrated approach including core observation, thin section observation, cathodoluminescence thin section observation, scanning electron microscope, μ-XRF analysis, electron microprobe analysis, AMICS analysis, LA-ICP-MS analysis, and isotope analysis was used to clarify the laminae combination carbonate-rich shale, diagenetic alterations in different types of shale laminae and the controls on shale oil reservoir formation. According to the lamina combination, the carbonate-rich shale in the Permian Lucaogou Formation can be divided into three types. They are shale consists of dolomitic lamina and terrigenous felsic lamina (Type A shale), shale consists of calcite-rich tufaceous lamina and terrigenous felsic lamina (Type B shale) and shale consists of calcareous lamina and tufaceous lamina (Type C shale). Dolomite cementation is the major diagenesis in terrigenous felsic lamina of Type A shale. The dolomite can be identified as the early stage forming from recrystallization of micrite dolomite, and the late stage precipitate from adjacent dolomitic lamina. In the terrigenous felsic lamina of Type B shale, however, the calcite cementation is the dominated diagenesis, which was sourced from adjacent calcite-rich tufaceous lamina. Also, some feldspar dissolutions occurred in terrigenous felsic lamina during organic matter evolution. For the Type C shale, the main diagenetic alteration is micrite calcite recrystallization in calcareous lamina. The dissolution and precipitation of carbonate minerals between different laminae of carbonate-rich shale reveal that the organic matter evolution significantly influence the diagenetic alterations of inorganic minerals. Particularly, the diagenetic mass transfer and redistribution at the micro-scale in a relatively diagenetic system were confirmed by organic-inorganic interactions in laminated shale. In this process, dissolution pores and recrystallization pores have provided considerable reservoir pores for shale oil.

Hydrogeochemical characteristic and water quality index of groundwater and streamwater at Nam Mu River basin, Lai Chau province in northwest Vietnam

Assessing the quality of groundwater and upstream water in regions experiencing water scarcity during the dry season, such as the mountainous districts of Northern Vietnam, holds immense significance in the management, utilization, and preservation of natural water resources. In this study, we collected and analyzed 54 samples of groundwater and upstream streamwater within the Nam Mu river basin in Lai Chau province during the dry season. These samples underwent thorough analysis for physicochemical parameters, major ion concentrations, and trace elements. Some results revealed a close relationship between streamwater and groundwater, indicating a similar chemical composition. These waters could be classified based on geological formations, yielding two primary water types in the area. The first type, characterized by Ca2+ - Mg2+ - HCO3 − is prevalent in Triassic terrigenous sediments. The second type is a mixed water type, ranging from Ca2+ - Mg2+ - Cl− to Ca2+ - Na+ - HCO3 −, primarily distributed in Permian felsic volcanic formations. These distinct water types are primarily the result of interactions between water and rock, precipitation processes, and ion exchange phenomena. Geochemical simulation results demonstrated the supersaturation of aragonite, calcite, and dolomite minerals, leading to the precipitation of carbonate minerals. Concurrently, the simulation indicated a tendency to dissolve minerals such as fluorite, gypsum, halite, and sylvite. The assessment of water quality using the WQI indicates that, in accordance with QCVN 01-1:2018/BYT 2018 standards, the majority of groundwater and streamwater are suitable for domestic use. However, exceptions exist in certain sample locations where water quality exceeds permissible standards, primarily due to elevated levels of Fe, Mn, and NO3 − concentration. Hence, it is important to implement water treatment processes for the water sourced from these specific locations before their utilization in daily activities to ensure the safety and well-being of the local population.

The role of alkalinity on metals removal from petroleum produced water by dolomite

Alkalinity is a critical parameter for describing the composition, pH buffer capacity, and precipitation potential of petroleum produced water (PW). Besides salinity, alkalinity and metal concentrations are generally greater in PW than in freshwater (FW) and seawater. This study presents batch reaction experimental and simulation results showing that the removal of Ba, Sr, and Cd from PW by dolomite is mostly due to sorption reactions, with sorption reactions and thus removal levels being higher for Cd than for Ba and Sr. In contrast, we found that the removal of Pb and As from PW by dolomite is largely due to precipitation and coprecipitation reactions of carbonate minerals on dolomite. Analyses of changes in the morphology as well as in the elemental and mineral composition of dolomite surface, along with pH, alkalinity, and Ba, Sr, Cd, Pb, and As removal measurements using synthetic PW and FW containing high concentrations (∼100 mg/L) of single and mixture toxic metals and metalloids (Ba, Sr, Cd, Pb, and As) at different initial alkalinity and pH conditions, indicate that in addition to salinity, alkalinity and pH generated from the dissolution of dolomite controls the removal of Ba, Sr, Cd, Pb, and As from PW by dolomite. However, we found that their impact is different for each metal in PW and FW. Ba, Sr, and Cd removal by dolomite is 10, 2, and 4 times smaller in PW than in freshwater (FW), respectively. Whereas As removal is practically the same regardless of salinity. Moreover, this study reveals the need of thermodynamic data of complex carbonate minerals formed from the precipitation of Ba, Sr, Cd, Pb, and As to capture the effect of alkalinity on their removal from PW by dolomite.

Preservation and hydrocarbon generation potential of microalgal organic matter in product aggregates induced by Chlorella sp. MASCC-0008

Microalgae are one of the important sources of organic matter which, if preserved well in sediments, can generate hydrocarbons during burial diagenesis. However, the preservation mode of microalgal organic matter within product aggregates induced by marine microalgae has rarely been examined. Herein, a strain of eukaryotic marine microalgae, Chlorella sp. MASCC-0008, was used to conduct biomineralization experiments in an f/2 medium with a 0.02 mol/L Ca2+ ion concentration and various Mg2+ ion concentrations of 0.05, 0.06, 0.07 and 0.10 mol/L. It was found that the soluble organic matter of the microalgae contains amino acids and fulvic acids which can attract and absorb cations. With an increase in Mg2+ ion concentration in the experimental group with Chlorella sp., the aragonite content of the mineral precipitates increased whereas the calcite content decreased. In contrast, calcite was the sole mineral precipitated in the control experiments with no Chlorella sp.. Due to the porous structure of minerals, their organic matters were preferred to be preserved in aragonite aggregates compared to calcite aggregates. The TOC content of the minerals aggregates is the highest (0.45%) and the hydrocarbon generation potential (S1 + S2) of organic matter is the greatest (3.66 mg/g) in those precipitated in the medium with a Mg2+ ion concentration of 0.10 mol/L. Microalgal organic matter has significant hydrocarbon generation potential due to its effective preservation in the product aggregates formed by the microalgal biomineralization. Squalene is the most abundant alkene detected in the pyrolytic products. These results suggest that microalgae can induce precipitation of carbonate minerals, a process that facilitates the sequestration of organic matter within the product aggregates precipitated, thus potentially preserving this microalgal organic matter into the burial diagenetic realm where, if conditions are appropriate, hydrocarbons may be developed.

Spatial distribution and enrichment mechanisms of high fluoride groundwater in geotherm-affected Pliocene aquifers of the Guide basin, China

Native high fluoride (F−) groundwater is one of the most serious environmental problems facing China and even the international society, which seriously threatens human health. The confined aquifer in the Guide basin contains high concentrations of F− and is subject to multifaceted influences, stemming not only from the geothermal activity present at the basin's bedrock but also from distinctive attributes such as elevated alkalinity, significant burial depths, and limited water circulation. Consequently, the precise mechanisms governing the enrichment of F− within the groundwater in this confined aquifer remain inadequately understood. This study aims to elucidate the migration and enrichment mechanism of F− in aquifers and assess the relative contributions of different control mechanisms responsible for high F− groundwater through systematic sampling, analysis and statistical calculations. Chemical types of confined groundwater and unconfined groundwater are Na-HCO3-Cl and Ca-HCO3, respectively. High-F− groundwater mainly exists in confined aquifers, and the content of F− gradually increases along the flow path. High content of HCO3− in confined groundwater promotes the precipitation of Ca2+, then enhances the dissolution of F−-containing minerals, resulting in F− being continuously released into groundwater. High pH, high Na/Ca molar ratios, geothermal and long residence time of groundwater are the additional factors to accelerate the release and migration of F− in groundwater. Results of redundancy analysis showed that competitive adsorption is the primary mechanism for F− enrichment in confined groundwater, followed by the geothermal effect, precipitation of carbonate minerals and residence time of groundwater time. The results of potential health risk assessment indicate that the potential health risks associated with confined groundwater are notably higher across all age groups compared to unconfined groundwater. The research results are not only helpful for the local government in reducing the threat of drinking-water fluorosis, but also provide a reference for other high-F− groundwater research with similar geological conditions in China and around the world.

Removal of ammonium by‐products produced during biocementation soil improvement using rinse injection strategies

AbstractBio‐mediated soil improvement technologies leverage microbial enzymatic and metabolic processes to generate minerals, gases and biopolymers that can improve soil engineering behaviours with the potential to reduce detrimental environmental impacts when compared with conventional methods. Ureolytic biocementation is perhaps the most widely researched of these processes and relies on urea hydrolysis in the presence of calcium to initiate the precipitation of calcium carbonate minerals on soil particle surfaces and contacts, thereby improving soil behaviours. Although effective, urea hydrolysis generates aqueous ammonium by‐products that may result in undesirable environmental and human health impacts, if left unaddressed. Recent studies have shown the potential of rinse solution injections to effectively remove generated ammonium following biocementation through both advective flow and the removal of sorbed ammonium from soil surfaces; however, critical gaps remain in our understanding of the effect of rinse solution composition and injection strategies on ammonium removal efficacy. In this study, 16 soil column experiments were performed to investigate the effect of rinse solution cation types, cation concentrations, applied injection sequences and biocementation treatment variations on the removal of ammonium from a biocemented poorly graded sand. All columns receiving cation‐enriched rinse solutions achieved greater than 98% aqueous, 65% sorbed and 95% total ammonium removal after injecting 12 pore volumes, with no detectable impacts on cementation integrity. Cation‐enriched solutions specifically enhanced sorbed ammonium removal and achieved sorbed ammonium concentrations up to 2 orders of magnitude less than those observed in columns rinsed with deionized water alone. Columns receiving K+‐based rinse solutions and 12 daily 1‐PV rinse injections also achieved greater total ammonium removal when compared with comparable columns receiving rinse solutions containing other cations and continuous 12‐PV rinse injections.

Towards green carbon capture and storage using waste concrete based seawater: A microfluidic analysis

Carbon capture and utilization technology is the research stream dedicated to mitigating the pressing effect of rising atmospheric carbon dioxide (CO2). The present study investigates a potential environmentally conscious solvent to capture and utilize CO2 using waste concrete and seawater under reactor conditions. Although seawater's CO2 soubility is low due to salinity, waste concrete raises seawater's pH and alkalinity, acting as a feedstock for CO2 dissolution and offsetting the adverse effects of salinity. To evaluate the performance of the novel natural seawater-concrete solutions for CO2 capture, time-dependent pH changes of solutions exposed to CO2 were measured in a microchannel using fluorescence microscopy. The concentration of dissolved CO2 in the solution was derived from pH change, revealing a 4-fold increase in the total dissolved carbon from 0.034 to 0.13 M and a 57.54% increase in the CO2 dissolution coefficient from 530 to 835 μm2/s in seawater upon concrete addition. Electrolysis further enhanced the CO2 capture capacity of the seawater-concrete solution by increasing the pH, enabling the solid precipitation of carbonate minerals. Raman spectroscopy and scanning electron microscopy showed that electrolysis-driven precipitates are mainly amorphous calcium carbonates, useful building blocks for seashells and coral reefs.

Spectroscopic comparisons of two different terrestrial basaltic environments: Exploring the correlation between nitrogen compounds and biomolecular signatures

Life detection in the solar system relies on the unambiguous identification of signatures of life and habitability. Organic molecules are essential to life as we know it, and yet many organic compounds are ubiquitous in the solar system and can be synthesized abiotically; thus, their presence alone is not indicative of life. On Earth, chemical signatures of life's processes are often left behind in minerals through the biologically induced formation of secondary minerals or intermediary organic complexes. In natural rocks biomolecules and organic species often co-occur with minerals, and their overlapping peaks can create difficulties in interpretation. In the process of identifying the minerals and organic species in our basaltic samples we noticed signatures for cyanates co-occurring with organic molecules. Cyanates are an overlooked group of nitrogen compounds in which C is bonded to N (e.g., OCN− or SCN−) that often co-occur with urea and ammonium in environments where microorganisms are present. These compounds are common in many terrestrial and oceanic environments and play an important role in biogeochemical nitrogen cycling. In natural systems, these compounds form as the result of multiple biogeochemical pathways, often from the interaction of microbes with a chemically active environment. These interactions leave behind signatures in the form biotic breakdown products such as urea or ammonium and organic reaction byproducts that are observable with spectroscopic methods. To explore these relationships, we used field-portable Raman spectrometers and laboratory micro-Raman imaging to characterize and compare samples collected from two different terrestrial basaltic environments, a lava tube on Mauna Loa, Hawaii, dominated by the precipitation of sulfate minerals and a geothermal stream at Hveragil, Iceland dominated by the precipitation of carbonate minerals. The Raman (RS) measurements were complemented by laser induced breakdown spectroscopy (LIBS), Long-wave Infrared (IR) LIBS, with the addition of gas chromatograph mass spectrometry (GC–MS) and inductively coupled plasma-mass spectrometry (ICP-MS) to identify cyanate compounds, biomolecules, and other nitrogenous compounds related to the breakdown or production of cyanate in host basalts and secondary precipitates. The RS data suggest that the reason for RS cyanate signatures in the carbonate samples could be due to luminescence artifacts while those detected in the host basalts may be due to hydrolysis chemistry. The cyanate signatures detected in the lava tube samples dominated by sulfates do not seem to be luminescence artifacts but may in fact be evidence of an active microbial nitrogen cycle. Our results inform the spectroscopic detection of cyanates in planetary analog environments and the challenges in their identification. Further work is needed to understand their potential as biosignatures on other planetary bodies.

Microbial influence on dolomite and authigenic clay mineralisation in dolocrete profiles of NW Australia.

Dolomite (CaMg(CO3 )2 ) precipitation is kinetically inhibited at surface temperatures and pressures. Experimental studies have demonstrated that microbial extracellular polymeric substances (EPS) as well as certain clay minerals may catalyse dolomite precipitation. However, the combined association of EPS with clay minerals and dolomite and their occurrence in the natural environment are not well documented. We investigated the mineral and textural associations within groundwater dolocrete profiles from arid northwest Australia. Microbial EPS is a site of nucleation for both dolomite and authigenic clay minerals in this Late Miocene to Pliocene dolocrete. Dolomite crystals are commonly encased in EPS alveolar structures, which have been mineralised by various clay minerals, including montmorillonite, trioctahedral smectite and palygorskite-sepiolite. Observations of microbial microstructures and their association with minerals resemble textures documented in various lacustrine and marine microbialites, indicating that similar mineralisation processes may have occurred to form these dolocretes. EPS may attract and bind cations that concentrate to form the initial particles for mineral nucleation. The dolomite developed as nanocrystals, likely via a disordered precursor, which coalesced to form larger micritic crystal aggregates and rhombic crystals. Spheroidal dolomite textures, commonly with hollow cores, are also present and may reflect the mineralisation of a biofilm surrounding coccoid bacterial cells. Dolomite formation within an Mg-clay matrix is also observed, more commonly within a shallow pedogenic horizon. The ability of the negatively charged surfaces of clay and EPS to bind and dewater Mg2+ , as well as the slow diffusion of ions through a viscous clay or EPS matrix, may promote the incorporation of Mg2+ into the mineral and overcome the kinetic effects to allow disordered dolomite nucleation and its later growth. The results of this study show that the precipitation of clay and carbonate minerals in alkaline environments may be closely associated and can develop from the same initial amorphous Ca-Mg-Si-rich matrix within EPS. The abundance of EPS preserved within the profiles is evidence of past microbial activity. Local fluctuations in chemistry, such as small increases in alkalinity, associated with the degradation of EPS or microbial activity, were likely important for both clay and dolomite formation. Groundwater environments may be important and hitherto understudied settings for microbially influenced mineralisation and for low-temperature dolomite precipitation.

Mineralogical Evidence for Hydrothermal Uranium Mineralization: Discovery and Genesis of the Uranyl Carbonate Minerals in the BLS U Deposit, SW Songliao Basin, Northeast China

Diabase intrusions have been widely found in sandstone-type U deposits of the southwestern Songliao Basin, indicating diabase-related hydrothermal fluids might play an important role in this type of U mineralization. The first discovery of U-bearing carbonate minerals in the BLS U deposit provides an opportunity for understanding hydrothermal U mineralization and its relationship to diabase intrusions. U-bearing carbonate minerals occurred as thin shells generally ringing ankerite and then surrounded by colloidal pyrite through examination of scanning electron microscopy and energy dispersive spectroscopy. They can be interpreted as uranyl carbonate minerals, with the empirical formula of Ca2.7Fe0.9Mg0.4 (UO2) (CO3)5•9.6H2O, based on infrared absorption spectroscopy and electron microprobe. The formation of uranyl carbonate minerals is most likely related to the CO2-rich hydrothermal fluids from diabase intrusions according to its occurrence state, but the key factors are that the Ca-UO2-CO3 ternary complexes should have been produced in ore-forming hydrothermal fluids and adsorption of ankerite on ternary complexes. Thereby, a potential diabase-related hydrothermal U mineralization model for sandstone-type U deposits can be proposed. The ore-forming fluids that originated from diabase-related hydrothermal are formed through continuously extracting the adsorbed U6+ and dissolving the early U minerals. Then, U in the ore-forming hydrothermal fluids was migrated and transported probably either as uranyl carbonate ions or as Ca-UO2-CO3 ternary complexes. The former is easy to precipitate in the form of pitchblende dispersed in the fossil wood cells, ringing pyrite, and occurring along the edge of adsorbents at the site of reducing capacity change, while the latter will extremely inhibit the reduction of U6+, eventually leading to the precipitation of uranyl carbonate minerals ringing ankerite by adsorption. The above research results can provide mineralogical evidence for hydrothermal U mineralization in sandstone-type U deposits of the Songliao Basin.

Late diagenetic alteration of carbonate evidenced by carbonate-deficient siliciclastic laminae

The alteration of carbonate, caused by organic acid release during hydrocarbon generation, is usually neglected in carbonate chemistry, since the unaltered carbonate, which may reflect the (syn)depositional environments, can dilute and conceal the modification signals (e.g., δ13Ccarb and elemental compositions). In carbonate-deficient black shales, however, active hydrocarbon generation may provide sufficient organic acid to dissolve the original minerals, and the geochemical signals may remain evident in the altered carbonate. Moreover, laminae represent the smallest phase in sedimentary rock and can be distinguished by their chemical and mineral compositions, which reflect the alteration reactions and products. Thus, microscale investigation on laminae would help clarify alteration pathways during burial diagenesis and its geochemical effects. In order to understand the diagenetic reactions and organo-modifications of carbonate minerals, we examined both the mineral and geochemical characteristics of the carbonate-deficient, organic-rich, laminated black shales from the Yanchang Formation of the Ordos Basin, North China. Our study demonstrates that the carbonate content is negatively correlated with the ion concentrations within the carbonate lattice (e.g., Srcarb), providing evidence that carbonate phases have undergone considerable late diagenetic alteration during deep burial. This is consistent with the argument that stronger alteration indicators for carbonate should be found in rocks with lower carbonate content. In addition, the (re)precipitation of carbonate minerals, accompanied by corrosion of siliciclastic minerals, is also observed in thin sections, supporting the deep alteration scenario. By confirming the presence of deep diagenetic alteration of carbonate during deep burial and investigating this alteration in detail, this study provides new insights into interpretations of carbonate chemistry.

Stability of alkalinity in ocean alkalinity enhancement (OAE) approaches – consequences for durability of CO2 storage

Abstract. According to modelling studies, ocean alkalinity enhancement (OAE) is one of the proposed carbon dioxide removal (CDR) approaches with large potential, with the beneficial side effect of counteracting ocean acidification. The real-world application of OAE, however, remains unclear as most basic assumptions are untested. Before large-scale deployment can be considered, safe and sustainable procedures for the addition of alkalinity to seawater must be identified and governance established. One of the concerns is the stability of alkalinity when added to seawater. The surface ocean is already supersaturated with respect to calcite and aragonite, and an increase in total alkalinity (TA) together with a corresponding shift in carbonate chemistry towards higher carbonate ion concentrations would result in a further increase in supersaturation, and potentially to solid carbonate precipitation. Precipitation of carbonate minerals consumes alkalinity and increases dissolved CO2 in seawater, thereby reducing the efficiency of OAE for CO2 removal. In order to address the application of alkaline solution as well as fine particulate alkaline solids, a set of six experiments was performed using natural seawater with alkalinity of around 2400 µmol kgsw−1. The application of CO2-equilibrated alkaline solution bears the lowest risk of losing alkalinity due to carbonate phase formation if added total alkalinity (ΔTA) is less than 2400 µmol kgsw−1. The addition of reactive alkaline solids can cause a net loss of alkalinity if added ΔTA > 600 µmol kgsw−1 (e.g. for Mg(OH)2). Commercially available (ultrafine) Ca(OH)2 causes, in general, a net loss in TA for the tested amounts of TA addition, which has consequences for suggested use of slurries with alkaline solids supplied from ships. The rapid application of excessive amounts of Ca(OH)2, exceeding a threshold for alkalinity loss, resulted in a massive increase in TA (> 20 000 µmol kgsw−1) at the cost of lower efficiency and resultant high pH values > 9.5. Analysis of precipitates indicates formation of aragonite. However, unstable carbonate phases formed can partially redissolve, indicating that net loss of a fraction of alkalinity may not be permanent, which has important implications for real-world OAE application. Our results indicate that using an alkaline solution instead of reactive alkaline particles can avoid carbonate formation, unless alkalinity addition via solutions shifts the system beyond critical supersaturation levels. To avoid the loss of alkalinity and dissolved inorganic carbon (DIC) from seawater, the application of reactor techniques can be considered. These techniques produce an equilibrated solution from alkaline solids and CO2 prior to application. Differing behaviours of tested materials suggest that standardized engineered materials for OAE need to be developed to achieve safe and sustainable OAE with solids, if reactors technologies should be avoided.

Comparison of water-caprock and CO2-water-caprock geochemical reactivity in the Triassic Heshanggou Formation of Ordos Basin

This study investigates the influence of geochemical reactions among CO2, water and key minerals on caprock sealing using laboratory experiments and numerical simulations, based on mudstone caprock of the Triassic Heshanggou Formation at the Shenhua CCS demonstration project in Ordos Basin. The results show that there is an obvious dissolution occurring in caprock after long-term water-rock interactions without CO2 injection, with a maximum increase of 11% in caprock permeability. The dissolution of kaolinite, chlorite, K-feldspar and quartz results in the increase of caprock permeability, and the precipitation of illite and hematite is the key factor effectively slowing down the change of permeability. However, the self-sealing capability of the caprock is continuously enhanced under the condition of CO2 injection, with permeability reduced by 40% of the original permeability at maximum. The precipitation of carbonate minerals directly induces the change of caprock permeability, and the precipitation of Ca-smectite and quartz further promotes the caprock sealing. Dolomite, siderite, magnesite and dawsonite are the key CO2 trapping minerals for the enhancement of caprock sealing, and the maximum amount of CO2 trapped in mineral phase is 35.44 kg/m3 medium. The dissolution of albite, chlorite and calcite provides the necessary Na+, Mg2+, Ca2+ and Fe2+ for the precipitation of carbonate minerals, which is conducive to the enhancement of caprock sealing, while chlorite and calcite are the key minerals affecting the caprock sealing. The dissolution of K-feldspar is not conducive to the enhancement of caprock sealing. This study could provide a theoretical basis for selection of reservoir-caprock and safety evaluation of CO2 geological storage.

Spatio-temporal variation of major ion chemistry and nutrient stoichiometry in a tropical monsoonal estuary: insight into biogeochemical processes

Major ions and nutrient chemistry of surface water from the Devi river estuary, eastern India has been studied in order to understand the governing biogeochemical processes. Concentration of Na, K, Ca, Mg, Cl, SO4 and PO4 increases toward higher salinity; however, NO3 and dissolved silica (DSi) decreases from upper to lower estuary. All major ions, except NO3 and DSi, show the highest and lowest concentration in summer and monsoon season, respectively. Increased surface runoff has resulted in the highest concentration of NO3 and DSi in the monsoon period. The Na, Cl, SO4, NO3 and PO4 ions show conservative mixing behavior in the estuary throughout the year. Non-conservative behavior of DSi is attributed to the combined process of mineral weathering and biological removal by diatoms. Enrichment and depletion of Ca and Mg in the estuary is owed to carbonate mineral precipitation and dissolution processes. Decreasing carbonate saturation indices (SI) with reducing pH accompanied by lowering of DO% suggests the influence of organic matter degradation on the carbonate precipitation and/or dissolution potential. Nutrient stoichiometric ratios—NO3:PO4, DSi:PO4 and DSi:NO3—are found to vary spatially and seasonally as a result of differences in geochemical processes prevailing in the fresh- and marine-water environment. A prominent PO4 limitation is observed in the upper estuary during summer and winter. However, this limitation reduces toward the lower estuary owing to precipitation of Fe-sulfide under the influence of higher salinity. In contrast, the influx of PO4-limited freshwater into the estuary controls the nutrient stoichiometric ratios in the monsoon season. The spatial variation of chlorophyll concentration in the estuary is in accordance to the nutrient stoichiometric ratios. Thus, this study emphasizes on the role of unique biogeochemical processes in controlling the spatial and seasonal distribution of major ions and nutrient stoichiometry in a tropical estuarine system.

Calcium Carbonate Hexahydrate (Ikaite): History of Mineral Formation as Recorded by Stable Isotopes

Calcium carbonate hexahydrate (ikaite) is a rare mineral that forms as metastable species in the organic-carbon-rich sediments of the King George Basin, Bransfield Strait, Antarctica, as a consequence of early diagenetic decomposition of organic matter under cold water (−1.4 °C) and high pressure (200 bar) conditions. Large crystals grow in the sediment immediately below the diagenetic transition between microbial sulfate reduction and methanogenesis at ~320 cm below sea floor (bsf). This process is reflected in the dissolved sulfate, total carbon dioxide, and methane concentrations, as well as in the carbon, hydrogen, and oxygen isotope chemistries of the interstitial fluids and dissolved gases of the host sediment. The ikaite crystal faithfully records in its zonal structure the changing carbon isotope ratio of the total dissolved carbon dioxide pool as it gradually diminishes during methanogenesis (δ13Cikaite = −17.5 to −21.4‰). These changes in the crystal’s host environment follow general Rayleigh carbon isotope fractionation. The oxygen isotopes of the ikaite carbonate (δ18Oikaite = 1.46 to 4.45‰) also show a strong zonal distribution, unrelated to temperature of formation, but perhaps controlled by the degree of recrystallization of ikaite to calcite. The crystal water of the ikaite is depleted 11‰ in 2H/1H (VSMOW) relative to the coexisting interstitial water, which is in excellent agreement with the isotope fractionation of other hydrated minerals. In addition to the in situ temperature and pressure, nucleation of the ikaite crystals in the Bransfield Basin sediments may be induced by the high alkalinity, high phosphate concentrations, and dissolved organic compounds. Intense microbial metabolism generates such compounds; of these, aspartic acid and glutamic acid may play an important role, as they do in biological and extracellular carbonate mineral precipitation. All indications are that low temperatures (such as of polar environments), high calcium carbonate supersaturation caused by interstitial methanogenesis, and a sufficiently large supply of dissolved phosphate and amino acids favor metastable ikaite formation. These conditions, modified by recrystallization, may be preserved in calcite glendonites, thinolites, and other calcitic pseudomorphs derived from ikaite and found throughout the ancient sedimentary record.

Assessment the ability of water with different ionic strength to dissolve and precipitate carbonate minerals according to omega(Ω) and saturation index (SI)

The study included five locations in Kurdistan Region, Iraq, represented by the soils of Zawita, Zakho, Dukan, Smaquli and Aqrah, in order to know the solubility and precipitation of carbonate minerals based on the values of omega (Ω) and saturation index (SI) using water of different ionic strength (chloride water, sulfur water, Tigris river water).the results showed that the dolomite mineral was more precipitated in term of omega (9.38 * 108) and the saturation index (8.97) in the case of chloride water, while magnesite was soluble where the value of omega was)0.01) and saturation index (-1.85), then when using sulfur water the same behavior appeared for the same mineral (dolomite) in term of precipitation (SI = 9.08) and magnesite in term of solubility (SI = -0.94). The same applies to the water of the Tigris River. As a general conclusion, the dolomite was more precipitated and the magnesite was more soluble in the presence of chloride water compared with the water of the Tigris River and the sulfur water.