Authigenic Research Articles

Mitigation Measures of Swelling Characteristics in Alkali-Contaminated Red Earth using Fly Ash and GGBS Blends

Alkali contamination induces swelling in the inherently non-swelling soils, resulting in heave severely damaging the sub and superstructures. Stabilization measures must be taken in vulnerable areas to prevent such colossal losses. Considerable research has been conducted regarding the swelling behavior of soils contaminated with alkalis and the impact of subsequent stabilization techniques utilizing different industrial byproducts and pozzolanic materials. However, there is a paucity of such studies on the Red Earth (RE) of the Visakhapatnam region. Furthermore, limited attention was given to the effects of extremely low concentrations of alkalis on the swelling behavior of soils. RE in the study area of the Visakhapatnam region predominantly consists of quartz, kaolinite, and hematite. In phase 1 of the experimental analysis, RE was contaminated with a highly dilute NaOH solution (0.05 N), and the resulting swell characteristics were investigated. A free swell of 10% was observed. One-dimensional standard oedometer tests revealed an equilibrium swelling of 5.6%, indicating significant potential for substantial heave and subsequent damage. X-ray diffraction (XRD) analysis revealed the formation of natrolite and analcime zeolites with a hollow structure, which caused the swell. Peaks of silicate minerals like paragonite and ussingite were also observed, indicating the dissolution of authigenic minerals and new precipitations even at lower concentrations of NaOH. In phase 2, swell tests were conducted on a mixture of contaminated red earth (50% w/w), fly ash (25% w/w), and ground granulated blast furnace slag (GGBS, 25% w/w), and it was observed that the swelling was completely arrested. XRD analysis unveiled that even though siliceous zeolites were formed, the development of chrysotile, attapulgite, tobermorite, and dicalcium silicate effectively inhibited the swelling with their pozzolanic behavior.

Discerning the sulfur geochemical features of turbidites and methane-rich sediments from the South China sea

Upward diffusion of dissolved methane and associated anaerobic oxidation of methane (AOM) coupled with microbial sulfate reduction (MSR) are widespread in normal hemipelagic sediments of continental margins. The occurrence of authigenic sulfur enrichment in such sediments is usually regarded as an indicator of AOM; however, extensive MSR and authigenic iron sulfide generation also occur in turbidites. Therefore, research attention should be directed toward determining the geochemical features of authigenic sulfur mineralization in methane-rich background sediments and turbidites. Here we report the sedimentology and geochemical composition of sediments from four piston cores (CL30, CL44, CL47, and CL3A) containing turbidite layers from methane-rich areas of the South China Sea. The turbidite layers were deposited at 17–15 kyr BP or earlier during the last glacial period and were located above the present sulfate–methane transition zone (SMTZ). The median grain size, proportion of sand-sized grains, and Si, Ti, K, and Zr contents show marked elevation in the turbidite layers compared with the background sediments, indicating the input of large amounts of terrigenous detrital matter with coarser grains. High total sulfur (TS) contents and low δ34SAIS values (AIS: acid-insoluble sulfur) (−35.2‰ to −26.1‰) are observed within turbidite layers relative to the background hemipelagic sediments in core CL44. These results suggest that intensive sulfidization occurred as a result of enhanced organoclastic sulfate reduction rates in turbidite horizons, which were probably caused by the resultant decrease in downward oxygen flux and waning bioturbation after rapid deposition of turbidites. In comparison, TS contents are higher and δ34SAIS values are positive (0.2‰–46.8‰) within the SMTZ on account of the rapid consumption of sulfate and build-up of isotopically heavier hydrogen sulfide pools via intensive AOM. Moreover, authigenic barium (Ba) enrichments commonly occur in the depth interval immediately above the SMTZ rather than within turbidite layers; therefore, they serve as a useful indicator of the position of the SMTZ. This study presents distinct differences in the geochemical compositions of turbidity deposits and methane-rich background sediments and highlights that a multi-proxy approach including sedimentological and geochemical analyses should be used to constrain the types of MSR in deep-sea sediments, given the common occurrence of turbidites and subsurface methane release in marginal deep-sea areas. It is also suggested that if applied with caution, S–C–Ba geochemical patterns can be used to identify palaeo-SMTZs and palaeo methane-rich zones in turbidite-containing strata.

Seismogenic Structures in Sediments of Different Lithological Composition and their Position in Mesozoic–Cenozoic Sections of the Northern Caucasus

A comparative analysis of the morphological features of rock horizons affected by paleoearthquakes showed that seismites from lithologically different deposits differ significantly. Powerful earthquakes determine not only the destruction of the sedimentary structure of near-surface deposits and their chaotic mixing, but also actively affect the underlying layers to a depth of several tens of meters; since the degree of diagenetic lithification of rocks in different horizons varies with depth, the nature of their seismogenic changes also changes. It was found that major seismic events were associated with the time intervals of tectonic and paleogeographic restructuring of sedimentation basins, which consisted in the intensification of the process of bowing of the reservoir bed, changes in the material composition of sediments accumulated in the paleobasin. A special type of seismites was formed during the impact of earthquakes on siliceous deposits, in which the processes of diagenetic redistribution of authigenic mineral matter, which passed through the gel stage during diagenesis, actively proceeded.

GEOCHEMICAL CONDITIONS OF SULLAR SPRING: FORMATION OF AUTHIGENIC MINERALS AND METHANE FLUXES

Link for citation: Soldatova E.A., Sidkina E.S., Kiriukhin B.A., Maximov P.N., Krivenok L.A., Ivanov V., Tananaev N.I. Geochemical conditions of Sullar spring: formation of authigenic minerals and methane fluxes. Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2023, vol. 334, no. 10, рр. 16-33. In Rus. Relevance and the research object. Interpermafrost aquifers and areas of their discharge with seasonal ice covers are special permafrost-hydrogeological and hydrochemical systems that are not typical for the thick continuous permafrost zone of Central Yakutia. Differences in chlorofluorocarbon content indicate the anaerobic conditions of some springs suggesting microbial degradation of these compounds by methanogenic bacteria. In the discharge area of such springs, a sharp change in the geochemical conditions takes place leading to a transformation in the water chemical composition and boosting the processes of authigenic mineral formation in this buffer area. In current work we considered the geochemical system of the Sullar spring – the most northern and the least studied among interpermafrost groundwater discharge zones of the Lena River right bank. Intensive accumulation of authigenic minerals was found in the Sullar discharge area during field research. It was Fe compounds with bacterial mats as an opalescent film. Some researchers state the connection between the transformation of Fe compounds during permafrost melting and the migration and transformation of organic matter, including its consumption by microorganisms with further methane emission. We assumed that there is a relationship between the authigenic mineral formation and the content and emission of methane in the discharge area of the spring. This research is the first comprehensive description of the hydrochemistry of the interpermafrost water of the Sullar spring. The aim of the research was to explore influence of geochemical conditions and microbial communities on formation of authigenic minerals and methane fluxes, in particular the effect of the change of geochemical conditions due to groundwater discharge on Fe precipitation and to analyze the relation of that process with methane fluxes. For this purpose, we study water chemical composition of the Sullar spring, measure methane fluxes from water surface and analyze mineral and microbial composition of secondary mineral crusts and bottom eluvium collected near water sampling points. Methods. The main element content was determined by ion chromatography. The concentrations of the bicarbonate ion and dissolved carbon dioxide were calculated by the equilibrium modeling method based on the pH and Eh values of the system. The content of trace elements was measured by mass spectrometry with inductively coupled plasma. To identify seasonal and annual variability, a retrospective analysis of the water chemical composition of the Sullar spring for the period from 1962 to 2020 was carried out. Methane fluxes were measured using the chamber method. The methane concentration was determined by gas chromatography with a flame ionization detector. The features of the secondary mineral crusts and bottom eluvium were determined using a petrographic microscope. The chemical composition of individual mineral phases was evaluated using a scanning electron microscope equipped with a detector for energy dispersive X-ray spectral microanalysis. The DNA concentration was measured on a fluorimeter. Amplicon libraries were generated by polymer chain reaction with universal primers for the V4 region. Equilibrium modeling was carried out using the HCh software.

Semiarid Lakes of Southwestern Siberia as Sentinels of On-Going Climate Change: Hydrochemistry, the Carbon Cycle, and Modern Carbonate Mineral Formation

Towards a better understanding of factors controlling carbon (C) exchange between inland waters and atmosphere, we addressed the inorganic carbon cycle in semiarid lakes of Central Eurasia, subjected to the strong impact of on-going climate change. As such, we assessed the hydrochemical variability and quantified its control on the formation of authigenic carbonate minerals, occurring within the upper layer of sediments in 43 semiarid lakes located in the southwest of Western Siberia (Central Eurasia). Based on measurements of pH, total dissolved solids (TDS), cationic and anionic composition, dissolved organic and inorganic C, as well as textural and mineralogical characterization of bottom sediments using X-ray diffraction and scanning electron microscopy, we demonstrate that lake water pH and TDS are primarily controlled by both the lithological and climatic context of the lake watershed. We have not revealed any direct relationships between lake morphology and water chemistry. The most common authigenic carbonates scavenging atmospheric CO2 in the form of insoluble minerals in lake sediments were calcite, aragonite, Mg-calcite, dolomite and hydromagnesite. The calcite was the most common component, aragonite mainly appears in lakes with sediments enriched in gastropod shells or artemia cysts, while hydromagnesite was most common in lakes with high Mg/Ca molar ratios, as well as at high DIC concentrations. The relationships between mineral formation and water chemistry established in this study can be generalized to a wide suite of arid and semiarid lakes in order to characterize the current status of the inorganic C cycle and predict its possible modification under on-going climate warming such as a rise water temperature and a change in hydrological connectivity, primary productivity and nutrient regime.

Microbial communities and mineral assemblages in sediments from various habitats at the Haima Cold Seep, South China Sea

Cold seeps create diverse habitats in the deep sea and play an important role in the global carbon cycling. Anaerobic oxidation of methane (AOM) and biogenic mineralization are essential carbon pathways of methane and carbon transformation in cold seeps, however, the effects of habitat heterogeneity on the processes are still poorly understood. In this study, we investigated the microbial communities and mineral assemblages at distinct habitats in the Haima cold seep and their relationships with environmental factors. These habitats were classified as methane seep site (MS), seep-free faunal habitat (FH), and control site (CS). Bacterial communities were significantly different among the three habitats. ANME-3 archaea, Sulfurovum bacteria, and mineralization-associated microbes (e.g., Campylobacterales) were detected in high relative abundances at ROV2. Mineralogical analysis revealed abundant calcite minerals at the seep site, indicating that authigenic carbonate minerals were formed at highly active seep. Multivariate statistical analysis demonstrated that the concentrations of SO42–, Ca2+, and Mg2+ were significantly correlated with the presence of calcite minerals and bacterial communities. These results suggested that AOM-accompanied authigenic carbonate formation is an important factor influencing the mineral assemblages in seep habitats. This finding improves our understanding of marine microbial carbon cycling.

Diagenesis as a control on the tight sandstone reservoir quality of the upper Carboniferous strata in the northeastern Ordos basin, China

The upper Carboniferous tight sandstone strata are considered the most promising targets for gas exploration in the northeastern Ordos Basin. Reservoir quality is critical for the successful commercial exploration and development of tight sandstone gas. Reservoir quality of deeply buried tight sandstones is controlled by diagenesis, yet there is sparse systematic documentation on upper Carboniferous tight sandstone reservoir quality and its diagenetic link. In this study, we utilized core analysis and wireline log data, and core samples were studied using X-ray diffraction, optical light microscopy, scanning electron microscopy, porosity–permeability, mercury intrusion capillary pressure and nuclear magnetic resonance measurements. Dolomite, siderite, quartz, kaolinite, and illite are the main diagenetic cements. The upper Carboniferous sandstones pore system consists of residual intergranular pores, intragranular dissolution pores associated with feldspar grains and micropores mainly related to diagenetic minerals. Due to differences in diagenetic alterations, sandstones have significant differences in pore types and pore-filling components, ultimately leading to differences in reservoir quality. The residual intergranular porosity is closely related to the reservoir quality. Quartz cement and authigenic clay minerals lead to reduced residual intergranular porosity, but their impact on reservoir quality is relatively limited due to their relatively low content. The most significant factors contributing to reservoir quality deterioration are ductile (clay-rich) grain-influenced compaction and pervasive pore-filling carbonate cement. When sandstone contains large numbers of ductile grains, it loses a large amount of intergranular porosity due to compaction, and the pore space contains only a small number of dissolution pores and micropores, resulting in reservoir quality deterioration. Similarly, when a large amount of carbonate cement is developed in sandstone, it leads to intergranular porosity loss. The best reservoir quality sandstones preserve a percentage of intergranular porosity because they (i) have few ductile grains, resulting in low compaction, and (ii) do not have significant amounts of carbonate cements. Finally, the diagenetic variations were successfully correlated with wireline logs to evaluate the reservoir quality of subsurface upper Carboniferous sandstones. The data sets provided in this research offer insights to better evaluate and predict reservoir quality in the northeastern Ordos Basin.

Source of quartz cement and its impact on reservoir quality in Jurassic Shaximiao Formation in central Sichuan Basin, China

Quartz cement is an essential authigenic mineral in the tight sandstones of the Shaximiao Formation in the Sichuan Basin, China. Accurately identifying its sources and investigating its impact on reservoir quality is important for understanding diagenetic fluid evolution and pore development in these reservoirs. To address this, we conducted a systematic analysis of the silicon sources in quartz cement employing mineralogical, fluid inclusion, and geochemical data. Quartz cement is characterized by two phases of quartz overgrowth and pore-filling authigenic quartz. Homogenization temperatures of fluid inclusions range from 60 to 150 °C, indicating that quartz cementation occurred from the early diagenetic phase (B stage) to the late mesodiagenetic phase. During the early diagenetic stage, volcanic material was dissolved by meteoric water, resulting in the formation of the earliest quartz cement and smectite. At temperatures of approximately 70–100 °C, smectite illitization, accompanied by K-feldspar consumption and SiO2 precipitation. In the mesodiagenetic stage (temperature approximately 100–130 °C), organic acids led to the dissolution of minerals like feldspar and laumontite, releasing SiO2. In the deep burial stage, clastic quartz grains experienced dissolution due to high stress, providing a limited source of SiO2. Smectite alteration and the dissolution of aluminosilicate minerals represent the primary sources of silicon for quartz cementation, while contributions from volcanic material hydrolysis and pressure solution are relatively minor. The presence of chlorite films inhibits the development of quartz overgrowths, and the limited precipitation of authigenic quartz within the primary porosity has a minimal impact on reservoir quality. Consequently, sandstones rich in chlorite films represent a favorable lithology for the development of high-quality reservoirs.

Reactivity of glauconitic clasts during burial diagenesis

Palaeogene glauconitic reservoir sandstones in the produced Nini West oilfield in the Siri Canyon, Danish North Sea, are chosen as the first pilot CO2 storage site in Denmark. Since glauconitic clasts comprise up to 35 vol% of the sandstones, their stability is crucial for the reservoir permanence. The likely reactivity of glauconitic clasts during CO2 injection is evaluated from their changes during burial diagenesis (long-term reactions) as a contribution to experimental testing (short-term reactions) of sandstone resilience towards CO2. Cored wells penetrate the Palaeogene gravity-flow sandstone units in depth intervals of 1700 m (<52 °C) to 3100 m (107 °C). The depth range allows a detailed microscopic and geochemical investigation of diagenetic changes in mineralogical composition with increasing burial depth. Although the glauconitic clasts show a large variation in chemical composition, clear average trends of increasing K2O and decreasing Fe2O3 in glauconitic clasts are recorded with increasing burial depth down to 2600 m. This trend is caused by the gradually reduced number of expandable layers (smectite) in the mixed-layer glauconitic mica/Fe-smectite. This shift in smectitic content is accompanied by a change in authigenic minerals from berthierine, siderite, microquartz and opal coatings at shallow burial depth to chlorite, calcite and quartz overgrowths at burial > 2200 m. Chloritisation of glauconitic clasts is intensive in the central parts of the gravity-flow sandstone units in the deepest (2800–3000 m) and most distal settings. Consequently, authigenic chlorite precipitation in pores occurs at shallower burial depth than chloritisation of glauconitic clasts. The microporous structure of the glauconitic clasts controls the slow gradual illitisation during increased burial. Abrupt lowering of pore-water salinity is considered the main reason for expansion of smectite components in the mixed-layer glauconitic mica/Fe-smectite, opening of the micropores and thereby intense chloritisation of glauconitic clasts in the deep and distal gravity-flow units. Hence, glauconitic clasts are potentially highly reactive, although slow to react due to their microporous structure unless the water composition is distinctly changed.

Surface reactivity of the iron and manganese-oxidizing bacterium Leptothrix cholodnii SP-6

Surfaces of prokaryotic cells play a significant role in the adsorption of metals from aqueous solution and the formation of authigenic minerals (Konhauser 2006). Although most studies focus on the cell wall, it is known that many bacteria synthesise an extracellular layer of polysaccharides and proteins, including what are known as sheaths. It has been shown that the cyanobacterium Calothrix sp. produces as sheath which is neutrally charged at circumneutral pH values, and it was hypothesized that such a sheath might allow the cyanobacterium to survive in geothermal settings with high silicification rates (Phoenix et al. 2002). Specifically, the dominance of hydroxyl sites on Calothrix’s sheath surface facilitates hydrogen bonding with aqueous silica species, inducing the precipitation of amorphous silica on the sheath and thus protecting the underlying cell (Phoenix et al. 2002). Leptothrix cholodnii is a sheathed, iron and manganese-oxidizing bacterium that frequently inhabits minerals seeps, where Fe2+ and Mn2+ discharge into oxygenated surface waters (Spring et al. 1996). As a result, the sheath becomes encrusted with Fe(III) and Mn(IV) oxyhydroxides while the underlying cells are protected from mineralization (Emerson and Ghiorse 1992, Emerson et al. 2010). However, unlike Calothrix, Leptothrix’s sheath composition suggests that it might behave differently at circumneutral pH (Emerson and Ghiorse 1993). To investigate the surface reactivity of Leptothrix's sheath and cell wall we analyzed isolated sheaths, sheathless cells, and intact filaments of L. cholodnii SP-6. We studied these components using potentiometric titration, zeta-potential, Cd-adsorption, and Fourier transform infrared (FTIR) spectroscopy to elucidate changes in surface charge between the cell wall and sheath. For the isolated sheaths and intact filaments, titration data were fit using a two-site protonation model, resulting in the following pKa values: 6.05 (±0.29) and 9.34 (±0.11); and 7.77 (±0.17) and 10.50 (±0.20), respectively. For the sheathless cells, the best fit was obtained by using a three-site protonation model, resulting in the following pKa values: 5.40 (±0.59), 8.11 (±1.64) and 10.73 (±0.45). Total proton-active site concentrations were lower in isolated sheaths compared to intact filaments. Additionally, at circumneutral pH, net negative charge was lower for sheathless cells compared to intact filaments and isolated sheaths (Fig. 1). This information agrees with the Cd adsorption behaviour found for the three materials (Fig. 2). Thus, our preliminary results suggest that Leptothrix’s sheath is less reactive than the intact filaments at circumneutral pH, leading us to hypothesize that the outermost layer would sequester relatively lower amounts of cations, including Mn2+, from solution and potentially would protect the underlaying cell from deleterious mineralization. In addition to that, the less reactive sheath’s surface would also contribute to cell attachment, which is important for a species commonly found in streams (Phoenix et al. 2002, Emerson et al. 2010).

Authigenic Mineral Formation in Aquifers near the Uranium Sludge Storage Facility of Chepetsky Mechanical Plant during the Formation of a Biogeochemical Barrier in a Laboratory and Field Experiment

In this work, authigenic microbial mineral formation in groundwater near the uranium sludge storage at SC Chepetsky Mechanical Plant (ChMP) (Glazov, Russia) was analysed in field and laboratory experiments using thermodynamic modelling when the microbial community was activated by a mixture of acetate, glucose and whey. It was found that the mineral basis of the barrier consisted of aggregated soil particles with freshly deposited phases of carbonate and sulphide minerals of different degrees of crystallinity. An important factor in the formation of calcium phases is microbial denitrification, which is accompanied by an increase in pH values of the medium. The main factors of uranium immobilisation in the biogeochemical barrier were revealed, including its reduction to insoluble forms of uranium dioxide, adsorption on ferrous and sulphide-ferrous minerals, as well as the formation of phosphate phases through the addition of phosphorus-containing whey and co-precipitation or co-crystallisation in calcite phases.

Phosphate burial in aquatic sediments: Rates and mechanisms of vivianite formation from mackinawite

Excess phosphorus abundance often drives eutrophication and affects surface water quality. Formation of vivianite (Fe3(PO4)2 • 8H2O) in aquatic sediments acts as a significant sink for phosphate (P), crucial for resorting surface waters. Authigenic vivianite formation, however, can be limited by other ferrous iron containing phases, in particular iron sulfides. Although thermodynamically feasible under suitable conditions, the formation of vivianite from mackinawite has been widely disregarded for authigenic phosphate mineral formation. Here we investigated the formation of vivianite from mackinawite (FeS) in batch experiments in which dissolved sulfide was continuously removed, at P levels between 0 – 5 mM in a pH of 6 to 8. Solid characterizations by electron microscopy, X-ray diffraction as well as Mössbauer and X-ray absorption spectroscopy demonstrates that vivianite was formed at all pH values in P amended experiments. The temporal evolution of dissolved Fe(II) concentrations indicates that the transformation proceeds via the release of the dissolved Fe(II) by FeS dissolution and subsequent vivianite precipitation, over time scales of days. The kinetics of the transformation are controlled by the dissolution rates of FeS. Aging and transformation of FeS, however, compete with vivianite formation. Aging is more pronounced at higher pH but is inhibited by P adsorption. Hence, the effect of pH and P concentration on aging is the main reason for the influence on these parameters on the rates and extent of vivianite formation. Our findings demonstrate that FeS can be an effective iron source for vivianite formation in aquatic sediments when sulfide concentrations decrease due to, for example, changes in external forcing or microbial sulfide oxidation. Formation of vivianite from FeS as an Fe source can also open new perspectives in P recovery in water treatment, for example when Fe is added to digesters to bind H2S.

Microbial pyrite formation: mineral morphology and precipitation kinetics

Pyrite (FeS2) is a mineral of wide interest due to its importance in the biogeochemical cycling of Fe and S, which is tied to those of carbon and other trace metals and nutrients. The mineral itself has potential applications as biosignatures, for environmental remediation and as semiconductors. Despite being a common authigenic mineral in sedimentary environments, most laboratory-based experiments have failed to form pyrite in microbial cultures at ambient temperatures. To fill this knowledge gap, we have employed cultivation-based approaches, using different combinations of microorganisms (Fe(III)- and S0-reducers; S0-disproportionaters) and initial Fe and S sources to identify the conditions that are optimal for pyrite formation. Scanning electron microscopy (SEM), Raman spectroscopy and X-ray diffraction (XRD) demonstrate that pyrite is not formed within ~3 years at circumneutral pH when oxidants (e.g., Fe(III) minerals, elemental sulfur, S0) are exhausted. In contrast, pyrite is formed within months when S0 and Fe(III) minerals are both present. The precipitation timescale is consistent with those expected in sedimentary environments Mansor and Fantle 2019), albeit some natural pyrite framboids are expected to form even faster Rickard 2019). The pyrite crystals formed in our experiments are in the low micrometer range, with either a spherical or euhedral morphology depending on the initial crystallinity of the supplied Fe(III) minerals. Framboids are not formed in our system, perhaps due to the relatively slow precipitation kinetics. Time-based sampling indicates mackinawite (FeS) as the first product, with rare (and minor) detection of greigite (Fe3S4) and vivianite [Fe3(PO4)2.8H2O]. We hypothesize that pyrite formation and growth proceed mainly from transformation of precursor minerals, with potential contributions via dissolution-precipitation and/or particle attachment pathways. Our preliminary study is the first step in optimizing a system for microbial pyrite formation, with the intention of investigating the properties and the reactivity of the mineral with implications for biosignatures, environmental remediation and industrial applications.

Biomineralization of amorphous Fe-, Mn- and Si-rich mineral phases by cyanobacteria under oxic and alkaline conditions

Abstract. Iron and manganese are poorly soluble elements in oxic and alkaline solutions, whereas they are much more soluble under anoxic conditions. As a result, the formation of authigenic mineral phases rich in Fe and/or Mn has traditionally been viewed as diagnostic of global or local anoxic conditions. Here we reveal that some specific cyanobacteria of very small size (&lt; 2 µm, i.e., picocyanobacteria) can biomineralize abundant, authigenic Fe(III)-, Mn(IV)- and Si-rich amorphous phases under oxic conditions in an alkaline lake in Mexico. The resulting biominerals cluster as small globules arranged as rings around the division septum of cyanobacterial cells. These rings are enveloped within an organic, likely polysaccharidic envelope and are partially preserved, at least morphologically, upon sedimentation. Based on their 16S rDNA sequence, these cyanobacteria were affiliated with the Synechococcales order. The high Fe and Mn enrichment of the biominerals questions the systematic inference of anoxic conditions based on their detection. Moreover, this process scavenges iron from the water column, an overlooked biological contribution to the Fe cycle. Finally, it reveals a new case of controlled biomineralization of Si-rich phases by bacteria.

Аутигенные минералы железа в речных отложениях Восточного Донбасса

Актуальность работы. Восточный Донбасс – территория на западе Ростовской области, где на протяжении многих десятков лет в процессе разработки угольных месторождений и последующей ликвидации нерентабельных шахт мощному техногенному воздействию подвергались все компоненты окружающей среды, но особенно речные системы. Важнейшей составляющей последних являются, как известно, донные отложения, которые несут информацию о степени загрязнения водной среды, об экологическом состоянии обширных водосборных площадей, об условиях седиментации, характере и направленности постседиментационных, в первую очередь диагенетических процессов. Цель исследований.Изучение аутигенных минералов железа в речных отложениях Восточного Донбасса, оценка влияния техногенного фактора в диагенетическом минералообразовании. Методы исследований. Изучение ассоциации аутигенных минералов железа выполнено методами электронно-зондовых исследований на растровом электронном микроскопе VEGA II LMU с системой энергодисперсионного микроанализа. Результаты исследований. Аутигенные минералы железа представлены в донных отложениях гидроксидами (ферригидритом) и оксидами (магнетитом) железа, реже сульфидами (пиритом) и карбонатами (сидеритом). Они образуются в существенно пелитовых илах, обогащенных современным органическим веществом, и являются членами одного минералогического ряда. Наблюдаемая ассоциация аутигенных минералов железа формируется при совокупном влиянии природных и антропогенных факторов. Среди последних важную роль играют техногенные шахтные воды – основной источник железа, сульфатов и других компонентов в речных отложениях. Формирование аутигенных минералов железа происходит в нестабильных физико-химических условиях, что характерно для раннего этапа диагенеза. Об этом свидетельствуют: присутствие в ассоциации аутигенных минералов наряду с оксидами и гидроксидами железа сульфидов и железосодержащих карбонатов; рентгеноаморфное состояние минеральных масс, наличие псевдоморфоз, отсутствие четко выраженной структуры кристаллов, свидетельствующее о незавершённости процессов минералообразования в донных осадках. Подчеркнуто, что на раннем этапе диагенеза изученных речных отложений наиболее предпочтительным с энергетической точки зрения является формирование из гидроксидов железа магнетита. Именно «магнетитовая реакция» начинается раньше, протекает быстрее и с меньшими затратами органического вещества, чем «сидеритовая» и тем более «пиритовая». Relevance. Eastern Donbass is a territory in the western part of the Rostov region, where all environmental components , but especially river systems were subjected to heavy technogenic impact for many decades in the process of developing coal deposits and the subsequent liquidation of unprofitable mines. As it is known, the most important component of the river systems river sediments, which carry information about the pollution degree of the aquatic environment, the ecological state of entire catchment areas, the sedimentation conditions, the nature and direction of post-sedimentation processes, primarily diagenetic ones. Aim. Investigation of authigenic iron minerals in river sediments of the Eastern Donbass, assessment of the technogenic factor effect on diagenetic mineral formation. Methods. The study of the authigenic iron minerals assemblage have carried out using electron probe methods on a scanning electron microscope VEGA II LMU with an energy dispersive microanalysis system. Results. Authigenic iron minerals are represented in river sediments by iron hydroxides (ferrihydrite) and oxides (magnetite), and, less frequently, sulfides (pyrite) and carbonates (siderite). They are formed in essentially pelitic silts enriched in organic matter of modern origin, and these minerals are members of the same mineralogical series. The observed authigenic iron minerals assemblage is produced under the combined influence of natural and anthropogenic factors. Among the latter technogenic mine waters play an important role as the main source of iron, sulfates and other components in river sediments. The formation of authigenic iron minerals occurs under unstable physicochemical conditions, which is typical for the early stage of diagenesis. This is evidenced by the presence of authigenic minerals (iron oxides and hydroxides along with sulfides and ferruginous carbonates) in mineral assemblage, X-ray amorphous state of mineral masses, the presence of pseudomorphs, the absence of a clearly defined crystal structure indicating the incompleteness of mineral formation processes in river sediments. It is emphasized that the magnetitite formation from iron hydroxides is most preferable from the energetic point of view at the early stage of diagenesis of the river sediments studied. It is the “magnetite reaction” that starts earlier, proceeds faster and requires less organic matter than the “siderite” and even more so “pyrite” reaction.

Organic Matter of the Wufeng–Longmaxi Formation Shales Using Scanning Electron Microscopy

Fine-grained organic matter (OM) particles are commonly widely dispersed in shale deposits. However, carrying out investigations of pores hosted by OM particles and the nature of grain interactions in OM particles and associated detrital grains using optical microscopy is difficult at best. Scanning electron microscopy (SEM) is much better suited for characterizing the microstructure of dispersed OM particles and has found wide application in the study of unconventional oil and gas systems. Scanning electron microscopy was used to define the types of OM contained in marine shale deposits of the Wufeng and Longmaxi Formations spanning the Ordovician–Silurian transition in South China. Of particular interest was the development of OM-hosted pores and the identification of the factors that controlled their formation. The dominant OM type contained in the studied deposits is pyrobitumen, with subordinate graptolitic OM and sparse OM of unknown origin. Pyrobitumen is present in four forms, including pore fillings among authigenic quartz grains, within framboidal pyrite, and between authigenic clay grains and massive material. Diagenetic alteration has given rise to OM pores of differing morphology, size, and time of formation. Common small, equisized circular or oval OM pores are most developed and appear to have formed in association with the generation of hydrocarbons. Shale deposits containing abundant pyrobitumen filling interparticle pores among authigenic quartz crystals display robust reservoir and fracturing capacity. A sedimentary environment appears to have been the main factor affecting the type of OM and the nature of its association with detrital and authigenic minerals. Results of this study demonstrate that a sedimentary environment is a primary requisite for the formation of highly prospective/high-yielding hydrocarbon shale reservoir deposits.

Fluid evolution and related fluid–rock interactions of the Oligocene Zhuhai sandstones in the Baiyun Sag, northern margin of the South China Sea

Pore fluids control the diagenetic processes and storage spaces of deep clastic rock reservoirs and have become a major area of interest within the fields of sedimentology and petroleum geology. This paper aims to relate the diagenetic processes of the Oligocene Zhuhai sandstones in the Baiyun Sag to pore fluids varying with burial depth. The types and distribution patterns of authigenic minerals are investigated through analysis of petrographic, mineralogical, and geochemical features to illustrate the origin and flow patterns of pore fluids and their influences on reservoir diagenesis. Strong cementation of eogenetic carbonate cement near the sandstone–mudstone interface was a consequence of material migration from adjacent mudstones on a large scale. The pore fluids were mainly affected by microbial methanogenesis and carbonate mineral dissolution in adjacent mudstones during eogenesis. The pore fluids were diffusively transported in a relatively open geochemical system within a local range. Support for this model is provided by the heavier stable isotopic values present in eogenetic calcite and dolomite. Feldspar dissolution during early mesogenesis was spatially accompanied by the precipitation of authigenic quartz and ferroan carbonate cement. Pore fluids in this period were rich in organic acids and CO2, and their migration mechanism was diffusive transport. The obviously lighter carbon and oxygen isotopic compositions of the ferroan calcite support this inference. During late mesogenesis, the input of deep hydrothermal fluid might have been partly responsible for the precipitation of ankerite, barite and authigenic albite. Oil charging may have inhibited carbonate cementation and compaction, accordingly preserving porosity, and together with authigenic kaolinite, might have promoted the transition of the reservoir from water wet to oil wet to the benefit of oil entrapment. The findings reported here shed new light on the evaluation and prediction of sandstone reservoirs that have experienced multiple periods of fluid flow.

Maceral evolution of lacustrine shale and its effects on the development of organic pores during low mature to high mature stage: A case study from the Qingshankou Formation in northern Songliao Basin, northeast China

Organic matter (OM) hosted pores are crucial for the storage and migration of petroleum in shale reservoirs. Thermal maturity and macerals type are important factors controlling the development of pores therein. In this study, six lacustrine shale samples with different thermal maturities from the first member of the Qingshankou Formation in the Songliao Basin, of which vitrinite reflectance (Ro) ranging from 0.58% to 1.43%, were selected for a comparative analysis. Scanning electron microscopy (SEM) and reflected light microscopy were combined to investigate the development of organic pores in different macerals during thermal maturation. The results show that alginite and liptodetrinite are the dominant primary macerals, followed by bituminite. Only a few primary organic pores developed in the alginite at the lowest maturity (Ro = 0.58%). As a result of petroleum generation, oil-prone macerals began to transform to initial-oil solid bitumen at the early oil window (Ro = 0.73%) and shrinkage cracks were observed. Initial-oil solid bitumen cracked to oil, gas and post-oil bitumen by primary cracking (Ro = 0.98%). Moreover, solid bitumen (SB) was found to be the dominant OM when Ro > 0.98%, which indicates that SB is the product of oil-prone macerals transformation. Many secondary bubble pores were observed on SB, which formed by gas release, while devolatilization cracks developed on migrated SB. Additionally, at the late oil window (Ro = 1.16%), migrated SB filled the interparticle pore spaces. With further increase in temperature, the liquid oil underwent secondary cracking into pyrobitumen and gas, and spongy pores developed on the pyrobitumen at higher levels of maturity (Ro = 1.43%), which formed when pyrobitumen cracked into gas. Vitrinite and inertinite are stable without any visible pores over the range of maturities, verifying their low petroleum generation potential. In addition, it was concluded that clay minerals could have a catalytic effect on the petroleum generation, which may explain why organic-clay mixtures had more abundant pores than single OM particles. However, after Ro > 0.98%, authigenic minerals occupied the organic pore spaces on the organic-clay mixtures, resulting in fewer pores compared to those observed in samples at the early to peak oil window.

Modes of occurrence of critical metals (Nb-Ta-Zr-Hf-REY-Ga) in altered volcanic ashes in the Xuanwei Formation, eastern Yunnan Province, SW China: A quantitative evaluation based on sequential chemical extraction

Altered volcanic ash with highly-elevated concentrations of Nb(Ta), Zr(Hf), Ga, and rare earth elements and yttrium (REY) is an untraditional type of critical metal deposit that has been discovered in Yunnan Province, southwest China. Previous studies have briefly described the modes of occurrence of Nb(Ta)-Zr(Hf)-REY-Ga in the ore deposit, showing that Zr and Hf are mostly hosted in zircon; light rare earth elements (LREE) mainly occur in REY-bearing carbonates and phosphates; Nb and Ta are primarily hosted in the former Fe-Ti oxide phases and zircon; and Ga is predominantly associated with clay minerals. However, previous studies of the modes of occurrence of critical metal elements (Nb-Ta-Zr-Hf-REY-Ga) have been limited to qualitative or semi-quantitative studies of individual mineral grains, and quantitative research of the modes of occurrence of these elements in whole rocks is absent. This paper quantifies the modes of occurrence of critical elements (Nb-Ta-Zr-Hf-REY-Ga) using a sequential chemical extraction method. The results show that about 65.3–86.7% Nb is associated with hydrofluoric acid (HF)-soluble aluminosilicates (clays) and 13.3–34.7% in the insoluble phases such as Nb-rutile. Approximately 87.6–90.2% Zr is the primary hosted in zircon and 9.5–12.0% Zr appears to be associated with HF-soluble aluminosilicates in the altered volcanic ash. In felsic altered volcanic ashes (Al2O3/TiO2 ratio between 21 and 70), the REY are predominantly in the authigenic minerals such as REY-bearing aluminum phosphates and sulfates of the alunite supergroup (APS minerals), carbonates or fluorocarbonates. About 74.0–89.4% LREE, 26.5–34.2% middle rare earth elements (MREY), and 22.1–24.8% heavy rare earth elements (HREE) occur in HF-soluble aluminosilicates. In intermediate altered volcanic ash samples (Al2O3/TiO2 ratio between 8 and 21), 93.0% LREE are associated with insoluble phases such as monazite and zircon. Gallium is predominantly associated with HF-soluble aluminosilicates (73.3–80.5%) in all altered volcanic ash samples.

Authigenic mineral phases as a driver of the upper-ocean iron cycle.

Iron is important in regulating the ocean carbon cycle1. Although several dissolved and particulate species participate in oceanic iron cycling, current understanding emphasizes the importance of complexation by organic ligands in stabilizing oceanic dissolved iron concentrations2-6. However, it is difficult to reconcile this view of ligands as a primary control on dissolved iron cycling with the observed size partitioning of dissolved iron species, inefficient dissolved iron regeneration at depth or the potential importance of authigenic iron phases in particulate iron observational datasets7-12. Here we present a new dissolved iron, ligand and particulate iron seasonal dataset from the Bermuda Atlantic Time-series Study (BATS) region. We find that upper-ocean dissolved iron dynamics were decoupled from those of ligands, which necessitates a process by which dissolved iron escapes ligand stabilization to generate a reservoir of authigenic iron particles that settle to depth. When this 'colloidal shunt' mechanism was implemented in a global-scale biogeochemical model, it reproduced both seasonal iron-cycledynamics observations and independent global datasets when previous models failed13-15. Overall, we argue that the turnover of authigenic particulate iron phases must be considered alongside biological activity and ligands in controlling ocean-dissolved iron distributions and the coupling between dissolved and particulate iron pools.