Microbial Carbonate Research Articles

Biohermal complex dominated by microbial carbonates from the early Miaolingian (lower Cambrian) Maozhuang Formation, North China

Biohermal complex dominated by microbial carbonates from the early Miaolingian (lower Cambrian) Maozhuang Formation, North China

Differences in thermal decomposition and crystallinity of dark organic laminae and light mineral laminae in same stromatolite

Stromatolite was a typical representative of microbial carbonate rocks, and also a unique microbial sedimentary structure in carbonate rocks. Previous studies on the characteristics of dark organic laminae and light mineral laminae in same stromatolite have carried out a lot of work, but there was less research in thermal decomposition and crystallinity. Here, X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) were used to determine the mineral composition and organic components of stromatolite and micrite. In addition, TG, DTG and were used to analyze their thermochemical characteristics and the non-isothermal decomposition of stromatolite and micrite at multi-heating rates of 5, 10, 20 and 30 °C min−1 in nitrogen atmospheres. Moreover, kinetic model function, kinetic parameters of apparent activation energy (E), and pre-exponential factor (A) were calculated by Popescu, Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS). The results showed that the mineral composition of two laminae structures was calcite, but there were organic matter and organic functional groups in the dark laminae. In addition, the results of thermal analysis showed that the crystallinity and thermal stability were significantly different, that was, crystallinity and thermal stability of dark laminae were significantly higher than that of light laminae. Moreover, the kinetic calculation results showed that the activation energy of dark laminae was also higher. These results provided an important reference for analyzing the thermochemical characteristics of stromatolite, especially the differences in dark organic laminae and light mineral laminae of the stromatolite.

No biological effect on magnesium isotope fractionation during stromatolite growth

The growth and morphology of stromatolites have been previously linked to diverse microbial activities. However, the role of microbial metabolisms on carbonate formation during stromatolite growth remains controversial. Magnesium isotopes have been proposed to serve as a tracer of microbial carbonate formation, implying a potential biological isotope fractionation. To further elucidate whether Mg isotope fractionation is modified during microbial carbonate formation, this study reports Mg isotope compositions of Holocene stromatolites and pore waters from Lagoa Salgada, a coastal ephemeral lake in Brazil. The stromatolitic carbonates are composed of high-Mg calcites characterized by extremely positive inorganic δ13C values, up to +20‰, and variable δ26Mg values, ranging from −2.98‰ to −0.68‰. Multiple pieces of evidence consistently demonstrate changes in microbial metabolism resulting from ecosystem fluid chemistry evolution. However, the direction of Mg isotope fractionation associated with carbonate precipitation remained invariable despite the changes in microbial activity. Mineralogical features indicate that the stromatolitic carbonates formed via 2-D nucleation. An isotopic mass balance calculation based on the observed variations in δ26Mg values and Mg concentrations of associated pore waters argues for an Mg isotopic equilibrium fractionation factor of −2.60‰ associating with the carbonate formation at Lagoa Salgada, well-matching experimental values for abiotic calcite precipitation. Thus, the observed δ26Mg variability of stromatolitic carbonates is primarily controlled by the changes of physicochemical conditions in the ambient fluid. We infer that during the formation of stromatolites, a consortium of different microbial communities produces extracellular polymeric substances, which serve as a substrate for carbonate nucleation from ambient fluid without significantly affecting the fractionation of Mg-isotopes. Our findings shed light on the effects of microbial processes on carbonate formation during stromatolite growth and improve the current understanding of the Mg isotope record in microbial carbonates.

Ediacaran cap carbonates with microbial build-ups capping barite-bearing methane seep networks in the Kaarta Mountains, Taoudeni Basin, Mali

Defining the variability and distribution of methane seeps and microbial activity in the aftermath of the Marinoan glaciation is a long-standing challenge in the field of Snowball models. Early diagenetic barite is commonly linked to tepee structures and associated breccias lacking microbial textures and fabrics, giving the impression that chemosynthetic microbes, or at least methane-tolerant microbes, did not participate in the carbonate production of their cap carbonate host. This apparent paradox has been an outstanding question in the lowermost Ediacaran cap carbonates of the Taoudeni Basin, NW Africa. In the Kaarta Mountains of Mali these carbonates exhibit, over short distances (<10 km), sharp facies-related environmental modifications with quiescent-dominated seafloor conditions, episodically interrupted by metre-scale disrupted substrates. The latter comprises fissure and fracture networks, occluded with tabular- and rosette-shaped barite cements, and sealed by decimetre-scale stromatolitic build-ups exhibiting intergrowths with barite needles. The strongly 13C-depleted carbon isotope values of the microbial carbonates (δ13C as low as −43.2 ‰ PDB) suggest the influence of methane, also preserved as fluid inclusions in barite crystals (documented with RAMAN spectroscopy) derived from a gas reservoir below the cap carbonate. Th of other fluid inclusions (Linkam microthermometry), ranging from 174 °C to 222 °C, provides minimum entrapment temperatures for barite precipitation. The microbially induced oxidation of methane and input of Ba-rich fluids were coupled to reduction of sulphate derived from seawater. The Sr/S isotope ratio and barite shape and size point to diagenetic barite crystals. The biomarkers yielded by the cap carbonate reflect a C29-dominant steroidal signature characteristic of stigmastanoid algal blooms. Although present-day microbial build-ups related to methane sources commonly occur in deep substrates and under anoxic bottom waters, the cap carbonate of the Kaarta Mountains is representative of shallower substrates, whereas its biomarkers point to deposition under episodic non-oxidising conditions.

Sedimentary environment and reservoir characteristics of microbial carbonate rocks in Leikoupo Formation of Jiangyoutongkou Section

As an important part of carbonate rocks, microbial rock is a good reservoir rock type, and its type characteristics and reservoir formation mechanism are different in different environments. By selecting Jiangyou Tongkou section in western Sichuan, the development environment and reservoir physical characteristics of typical microbial rock are analyzed, and the formation conditions of high-quality microbial rock reservoir are discussed. The results show that: The types of microbial rocks in the Leikoupo Formation of Jiangyou Tongkou Section are mainly tubbers and stromatolites, among which the second and fourth members of Leikoupo Formation are typical. They are developed in relatively warm and high salinity seawater, and their sedimentary environment is anaerobic environment with strong reductive property. The reservoir space types of microbial rocks are mostly intra (intergrain) void and grainy pore. In terms of reservoir physical properties, microbiolites are superior to non-microbiolites, and the 4th member of Lei Formation is the dominant reservoir of microbiolites.

Study on Depositional Fabric and Environmental Characteristics of Dendrolites in the Cambrian Zhangxia Formation: An Example from the Houziyu Section of Ezhuang Town, Zibo, Shandong Province, Eastern China

The dendrolites composed of light gray massive micrite limestone occur at the top of the Cambrian Zhangxia Formation in Houziyu Section, Zibo, Shandong Province. Because it is very difficult to study the genesis of microbial rocks through “the function of diagenesis filter,” this paper analyzes its genetic mechanism by characterizing the sedimentary fabric and formation environment of dendrolites and it is of great significance to study microbial carbonate deposited in geological history in oil exploration. The Cambrian Zhangxia Formation in the Houziyu section is a marine deposit. The macroscopic shrub-like fabric of centimeter scale can be seen on the surface of the tree-shaped stone, with small biological burrows and remnants of calcareous mud components after filling. The microstructure observation shows that the main fabric of the dendrolites is a dark micrite matrix, bright calcite cement, clastic particles, and a calcified cyanobacteria biological community. Detailed supplementary information on the diversity of calcified microorganisms in dendrolites has been made through the study of various types of calcified cyanobacteria and calcified residual substances in the microbial membrane of calcified cyanobacteria in the dendrolites of the study area. It is clear that dendrolites are the product of the calcification of biofilms or microbial mats dominated by cyanobacteria. With the deepening of this kind of research, it is bound to raise the research of microbial carbonate to a new level in China and provide great help to find oil and gas resources in marine strata of microbial origin in China.

Decompositional processes of microbial carbonates in Lagoa Vermelha, Brazil

ABSTRACTIn Lagoa Vermelha, Brazil, a lagoonal stromatolite and a saltpan microbial mat are investigated to understand the influence of environmental changes on the decomposition of microbial carbonates. The lagoonal stromatolite, composed mainly of magnesian calcite and aragonite, is developed on a dolomite-containing carbonate crust. While most stromatolites are eroded to the water surface level, some smaller, green stromatolites below the water surface retain a domal shape. The domal stromatolite surface is dominated by endolithic cyanobacteria with conspicuous microborings. In addition, microbial aerobic respiration causes carbonate dissolution in darkness, and metazoans grazing the inner surface of the stromatolite excrete fecal pellets. This suggests that the formational stage of lagoonal stromatolites has ceased and they are now decomposing, most likely because of environmental changes in recent years. The microbial mat, which is about 3 cm thick, developed in a saltpan pond precipitating carbonate and gypsum, and it contains quartz, magnesian calcite, aragonite, and gypsum. At the time of the investigation, the population of oxygenic phototrophs is low at the mat surface, and carbonate dissolution, rather than precipitation, is occurring by microbial metabolism deeper in the mat. This suggests that the formation of carbonate in the mat has ceased and is decomposing, probably due to the progressive salinity increase in the salt pan. This examination of two carbonate deposits in Lagoa Vermelha suggests that microbial metabolism is an important process for decomposing microbial carbonates in addition to grazing and microboring, and that environmental changes may alter microbial compositions from carbonate-constructive to carbonate-destructive communities.

Core scale investigation of fluid flow in the heterogeneous porous media based on X-ray computed tomography images: Upscaling and history matching approaches

In this paper, experiments and simulations were performed on outcrop samples from Lagoa Salgada in Rio de Janeiro, Brazil, as a possible analog to one of the most typical Brazilian Pre-salt carbonate reservoirs rocks. The rocks were microbial carbonates where plugs comprising two main facies were sampled, simplified as fine-grained and vugular facies. The plugs were utilized to study the impact of pore geometry with both experimental and simulation approaches on recovery factor, saturation profile, and relative permeability estimations. To provide direct visualization of the geometry, description of pore structure, and calculation of concentration profiles, computed tomography (CT) imaging was integrated with experimental measurements of petrography and core flooding. The injection of two pore volumes of formation water resulted in a recovery factor between 28 and 34 percent for the plug samples. Furthermore, based on porosity generated by dry and wet CT, as well as saturation profiles resulting from CT data collected during drainage and imbibition processes along the length of the plugs, it is revealed that the distribution of these properties was diverse and heterogeneous. An algorithm was used to process the 2D tomography images of the samples to remove the region related to the exterior parts. The images were then stacked to create a 3D fine-scale grid to simulate the porous media and the fluid flow by applying rules for the segmentation of rock types, porosity, and permeability estimations of each grid block. Course-scale grids were created by applying upscaling techniques to reduce computation time. Simulated produced fluid cuts for different upscaled models were compared with the experimental results from core flooding. A history-matching technique was then applied to match experimental and simulation results, calculating the relative permeability of two main defined facies and creating an updated model capable of assessing past and present performance and future forecasting. Since relative permeability is essential for accurate simulation, estimating these curves in the heterogeneous pre-salt reservoir considering different facies, greatly influences reasonable future prediction performance and the ability to make informed operational decisions.

Origin of Upper Cretaceous marine ironstones of Ayat Formation (Turgay depression, Northern Kazakhstan)

This work studies the mineralogical and geochemical features of ooidal ironstone formation of the Upper Cretaceous Ayat Formation located in the Turgay depression (Kazakhstan). The origin of ooidal ironstone deposits has been the subject of many discussions for a long time. These results present diagenetic conditions of the marine ironstone precipitation in the Ayat Formation. Various authigenic minerals, such as siderite, glauconite, goethite, pyrite, hydroxylapatite, wurtzite, and barite indicate fluctuations in the different geochemical conditions during the diagenesis of marine sediments of the Ayat Formation. The precipitation of in situ pyrite and wurtzite was controlled by an oxygen-deficient environment at the interface between water and sediment, which was accompanied by bacterial sulphate reduction under seabed conditions and, in consequence, by the concentration of sulfides. The authigenic assemblage of siderite, wurtzite, and barite indicates an integrated process of their input to site the sedimentation. The stable isotopic composition of siderite supports the microbial carbonate origin by decomposing the organic matter. However, geochemical features (the pattern Co/Zn vs (Co + Ni + Cu)) evidence the hydrogenous precipitation of iron. It involves the interaction of two central in situ processes to form Ayat ooidal ironstones. One of them is the iron precipitation from the bottom water, and the second is the microbial generation of hydrocarbons. Marine Ayat and channel Lisakovsk ironstones have similar features of bulk rock geochemistry. It reflects the exact environments of the ooidal iron ore formation, while the wall rocks have distinctive facies conditions.

A carbonate corrosion experiment at a marine methane seep: The role of aerobic methanotrophic bacteria.

Methane seeps are typified by the formation of authigenic carbonates, many of which exhibit corrosion surfaces and secondary porosity believed to be caused by microbial carbonate dissolution. Aerobic methane oxidation and sulfur oxidation are two processes capable of inducing carbonate corrosion at methane seeps. Although the potential of aerobic methanotrophy to dissolve carbonate was confirmed in laboratory experiments, this process has not been studied in the environment to date. Here, we report on a carbonate corrosion experiment carried out in the REGAB Pockmark, Gabon-Congo-Angola passive margin, in which marble cubes were deployed for 2.5 years at two sites (CAB-B and CAB-C) with apparent active methane seepage and one site (CAB-D) without methane seepage. Marble cubes exposed to active seepage (experiment CAB-C) were found to be affected by a new type of microbioerosion. Based on 16S rRNA gene analysis, the biofilms adhering to the bioeroded marble mostly consisted of aerobic methanotrophic bacteria, predominantly belonging to the uncultured Hyd24-01 clade. The presence of abundant 13 C-depleted lipid biomarkers including fatty acids (n-C16:1ω8c , n-C18:1ω8c , n-C16:1ω5t ), various 4-mono- and 4,4-dimethyl sterols, and diplopterol agrees with the dominance of aerobic methanotrophs in the CAB-C biofilms. Among the lipids of aerobic methanotrophs, the uncommon 4α-methylcholest-8(14)-en-3β,25-diol is interpreted to be a specific biomarker for the Hyd24-01 clade. The combination of textural, genetic, and organic geochemical evidence suggests that aerobic methanotrophs are the main drivers of carbonate dissolution observed in the CAB-C experiment at the REGAB pockmark.

Lacustrine-evaporitic microbial dolomite from a Plio-Pleistocene succession recovered by the SG-1 borehole in the Qaidam Basin, NE Tibetan Plateau

The Plio-Pleistocene evaporitic lacustrine succession in the Qaidam Basin, which was recovered by drill core SG-1, provides the material for understanding the maturation of dolomite since dolomite is present in the core from 7.6 to 870 m depth, spanning a time interval of 0.1–2.8 Ma. Twenty-two samples were collected at different burial depths. The samples were examined by thin-section petrography and SEM for textures, and analyzed for mineralogy (XRD), trace elements and carbon and oxygen isotopes. The main mineral compositions of each sample are dolomite, quartz, halite and gypsum. Dolomite is mostly scattered with spheroidal to ellipsoidal shapes, several microns in diameter, partially encapsulated in gypsum. Microbial micropores and calcified microorganisms are observed on and within the dolomite crystals and in the thin sections. The degree of ordering of the dolomite gradually increases with burial depth, which basically conforms to the first-order reaction function. The crystal constants of dolomite decrease with burial depth, indicating recrystallisation with increasing overburden. The trace elements of the dolomites are significantly different from those of hydrothermal dolomites, but close to lacustrine microbial carbonate. It is concluded that the dolomites in core SG-1 are mainly authigenic, with microbial processes a likely facilitator of dolomite precipitation. The dolomite formed and then evolved in crystal structure during burial, gradually approaching ordered stochiometric dolomite. This study gives clues to link microbial dolomite to the massive ideal dolomite rock encountered in the geological record.

Sedimentary structures of microbial carbonates in the fourth member of the Middle Triassic Leikoupo Formation, Western Sichuan Basin, China

In recent years, abundant natural gas has been found in microbial carbonates in the fourth member of the Leikoupo Formation in Western Sichuan Basin. In this study, from the observation of 626 microbial thin sections, four types of microbial carbonates are classified based on the differences of mesostructures. Among them, thrombolites and stromatolites are subdivided into eight types based on the differences of microstructures. Six types of microbial microstructure association (MSA) are identified, and are mainly developed in microbial mounds. The energy of sedimentary environment and hydrodynamic conditions of them from low to high is MSA-5, MSA-1, MSA-6, MSA-3, MSA-4 and MSA-2. Because of the arid climate in the Annie Period, a restricted platform are developed in the upper sub-member of the Leikoupo Formation in Western Sichuan Basin, and the sedimentary facies are lagoon (gypsiferous lagoon or salt lake in evaporate conditions), microbial mounds, shoals, inner platform shoals and open sea from the east to the west. Microbial microstructures not only affect the pore evolution of microbial carbonate reservoirs, but also affect the diagenesis of microbial carbonate reservoirs of the fourth member of the Leikoupo Formation.

Depositional framework and reservoir characteristics of microbial carbonates in the fourth member of the middle Triassic Leikoupo Formation, western Sichuan Basin, South China

Microbial carbonate reservoirs are the main petroleum exploration targets in the upper submember of the fourth member of the Middle Triassic Leikoupo Formation in the western Sichuan Basin. To improve understanding of the distribution and reservoir characteristics of microbial carbonates, detailed facies descriptions and reservoir characterizations were performed under the framework of cyclostratigraphy. Ten facies can be identified and grouped into four facies associations in two transgressive-regressive cycles, within an epeiric marine carbonate platform in the western Sichuan Basin. In Cycle 1, Emei taphrogeny caused the formation of the Longmenshan subaqueous paleo-uplift, which formed a relatively restricted tidal flat environment and developed large-scale microbialites on its eastern side. In the central area, wavy to domal stromatolitic dolowackestone (F3) and thrombolitic dolomudstone (F4) are developed in the intertidal and subtidal of the central area with steep substrate gradients and restricted seawater circulation. Laminated microbial dolowackestone (F2) is mainly developed in the intertidal of the southern area with low substrate gradients and restricted seawater circulation. Intraclast dolograinstone (F6) are widely developed in the subtidal shoals of the northern area, which is frequently disturbed by tides and waves with better seawater circulation. In Cycle 2, increasing relative sea level weakened the barrier of Longmenshan subaqueous uplifts to waves, the restricted subtidal environment developed the intraclast dolograinstone (F6) and intraclast packstone-grainstone (F7), occupying the platform with better seawater circulation and higher energy. The framework or inter-clot pores of F2, F3 and F4, interparticle pores of F6, and vugs resulting from syn-depositional dissolution are the dominant pore types. High-quality reservoirs are mainly distributed in the regressive microbialites (F2–F4) of Cycle 1 in the central area and intraclast dolograinstone (F6) of Cycle 1 and Cycle 2 in the northern area. The porosity of microbialites and intraclast dolograinstone reservoirs mainly ranges from 2.26% to 10.00%, with a mean value of 4.96%. The permeability mainly ranges from 0.01mD to 13.98mD, with a mean value of 4.63mD. This study proposed a depositional framework of microbialites and evaluated its control on reservoir quality, which is of great significance for the further study of microbial carbonate reservoirs.

Mitigating alkali-silica reaction in cement mortars through microbial carbonate precipitation technique

Alkali-Silica Reaction (ASR) is a major cause of durability reduction in cement mixtures. This reaction takes place between the reactive silica contained in aggregates and the alkaline elements contained in cement (sodium and potassium), producing a hydrophilic gel known as glass water. This gel swells upon absorbing water, causing a damaging internal pressure in the mortar. This study aimed to investigate the impact of Microbial-Induced Calcite Precipitation (MICP) with Sporosarcina pasteurii on the progress of ASR in cement mortars. The mortars were made using the accelerated mortar bar method (ASTM C1260) by substituting 0, 25, 50, 75, and 100 % of aggregates with waste glass, as the extreme material causing ASR. The glass was treated by two methods named Wet Calcite Precipitation (WCP) and Fixed Calcite Precipitation (FCP), which both involved spraying 0.5 ml/cm2 of bacterial suspension on the glass. After 28 days of exposure to alkaline conditions, the mortars were examined by Scanning Electron Microscopy, X-ray Diffraction spectroscopy, and Energy-dispersive X-ray spectroscopy to verify the occurrence of ASR. The mortars were subjected to the expansion test, the ultrasonic pulse velocity (UPV) test, and the compressive strength test to measure their physical and mechanical properties. Both MCP treatments managed to significantly reduce the adverse effects of ASR (compared to the control) at the 95 % confidence level. Between WCP and FCP, WCP was more effective in improving the results of UPV and expansion tests, and FCP was more effective in improving compressive strength. The findings showed that MICP can effectively inhibit the adverse effects of ASR in cement mortars.

Microbial sulphur-cycling and atmospheric signatures in the 2.52 Ga Gamohaan Formation, South Africa

The Late Archean atmosphere is thought to have been largely reducing as suggested by the preservation of large Mass-Independent fractionations of S isotopes (MIF-S) in sedimentary sulphides and sulphates. However, there is a growing body of evidence for transient oxygen accumulation in the Neoarchean oceans, especially from shallow marine environments. Here ∼2.52 Ga carbonates in the upper Campbellrand Group of South Africa are investigated from the shallow marine carbonate shelf Kogelbeen Formation to the deeper water basinal Gamohaan Formation. Previous studies of these rocks have found large positive Δ33S signatures, but with differing interpretations of the controls on, and pathways for, the preservation of Δ33S. Here bulk S isotope measurements from sedimentary sulphides and carbonate associated sulphate (CAS) are reported, combined with in-situ S isotope measurements by Secondary Ion Mass Spectrometry (SIMS) from well-preserved pyritised microbial mat microtextures of the Gamohaan Formation.Large positive Δ33S signatures are observed in sulphide grains of the Gamohaan and Kogelbeen formations (max Δ33SSIMS=12.6‰) confirming the isotopic effects of photolytic reactions in an anoxic atmosphere. Small, early pyrites in fenestrate microbialites record an atmospheric signature (average δ34SSIMS=7.6‰, Δ33SSIMS=8.5‰, Δ36SSIMS=−7.6‰) reflecting the preservation of atmospherically derived Sn particles in the microbial carbonates. Contorted microbial mat layers, also from the carbonate slope, exhibit large mass-dependent fractionations with δ34SSIMS of up to 30.6‰ plotting off the Archean reference array, suggesting oxygenated seawater on the shelf-slope. Pyrite concretions in organic rich carbonates from deeper water facies show S isotope values (average δ34SSIMS=3.1‰, Δ33SSIMS=−0.2‰, Δ36SSIMS=2.5‰) indicative of microbial sulphate reduction (MSR).The CAS data show positive Δ33S with values of up to 8.49‰ and positive δ34S values that are more similar to sulphide data measured from each sample than estimated late-Archean seawater sulphate. This is interpreted to record the early oxidation of sulphides with positive Δ33S by shallow oxygenated waters in the upper diagenetic zone that moved through the sediment and was then incorporated into the platform carbonates. In this way, we hypothesise that positive Δ33S can be inherited by the Neoarchean diagenetic sulphate reservoir and preserved in carbonates.

Microbial carbonate rocks: composition, structures, textures, mechanisms and environments of formation. Emergence of the doctrine of microbial formations and their forms. Article 1

Introduction. Concepts describing bacterial effects on mineral formation and, first of all, on the formation of carbonate deposits, started to appear late in the 19th century both in Russia and globally. The precipitation of materials was demonstrated to take place directly at the sedimentation stage and continue in the formed precipitate during diagenetic processes. Research in this direction has recently intensified. Rocks and their constituent parts formed as a result of bacterial activity have been referred to as “biosedimentary structures” or “microbiolites”.Aim. The paper presents the results of generalization and systematization of accumulated research data on the abovementioned forms.Materials and methods. In addition to literature materials, carbonate rocks of various ages – from the Vendian and Lower Cambrian of the Siberian platform to the Crimea Neogene and sediments of contemporary oceans – were examined using macro- and microscopic (mainly) methods of studying and describing specific objects. Article 1 considers the main morphological types of such carbonate formations, both in the form of individual structural components of sedimentary rocks and rocks themselves with peculiar structural characteristics.Results. Two main morphological groups of carbonate microbiolites were identified and characterized. The first group represents individual and independent forms of carbonate material, both of calcite, highly magnesian-dolomite and even pure magnesite composition. This type includes mineralized precipitates of bacterial forms represented by coccolites, tubules, filaments, as well as by plate and sheet structures – mineralized glycocalyx traces. In addition, this group features isolated carbonate objects, familiar to lithologists, including oolites, oncolites, microclusters of pelitomorphic carbonate materials, thrombolites and, apparently, such specific formations as tubiphytes, etc. The second group includes laminated forms of stromatolites with their specific internal structure, as well as mineralized microbial films and mats with a complex structure, etc.Conclusions. In the overwhelming majority of cases, microbiolites are formed at the sedimentation stage, and their morphology and type are determined by specific paleogeographic and paleoclimatic conditions.

Microbial carbonate rocks: composition, structures, textures, mechanisms and environments of formation. Microbiolite formation processes and environments. Article 2

Introduction. Until relatively recently, all carbonate rocks have been divided into organogenic and chemogenic rocks. The former group includes those half-composed of skeletal remains, thus representing a biological problem. The group of chemogenic carbonate rocks initially included all carbonate rocks with a crystalline structure. Later, the “chemogenicity” of formation was attributed only to pelitomorphic and microgranular varieties, while pure crystalline structures were recognised as the result of secondary recrystallization. However, the purely chemical precipitation of carbonate material from ocean waters by exceeding the solubility limit appears to be impossible. It occurs either biogenously during the formation of carbonate skeletons or biochemically as a result of changes in the acidity or alkalinity of the medium. The latter is typically implemented due to a carbonate disequilibrium between, on the one hand, the dissolved bicarbonates of calcium and magnesium and, on the other, water-dissolved carbon dioxide. The removal of the latter by photosynthetic organisms disturbs the dynamic balance and leads to an increased alkalinity and precipitation of carbonates, firstly calcium and then magnesium ones at higher pH values. Along with plants, microbial communities perform a similar function.Aim. The paper presents the results of generalization and systematization of accumulated research data on the abovementioned forms.Materials and methods. In addition to literature materials, carbonate rocks of various ages — from the Vendian and Lower Cambrian of the Siberian platform to the Crimea Neogene and sediments of contemporary oceans — were examined using macro- and microscopic (mainly) methods of studying and describing specific objects.Results. Сoccoid, tubular and fibrous formations, i.e. particular bacterial “skeletons”, represent one precipitation form. At the same time, bacterial communities produce glycocalyx, representing an extracellular polymer substance, which forms a basis for submicroscopic precipitates of carbonate minerals. In this case, specific sheet- and plate-like carbonate material precipitates can be formed. The environments for implementing such mechanisms are highly diverse. Microbiolites can be formed in water bodies, including from fresh to substantially and even abnormally saline waters, in suspended matter and at the bottom of basins, both at the stage of sedimentation and during further diagenetic processes. The most famous example of the latter are various nodules.Conclusions. The processes of sediment formation and material deposition are somewhat different. The formation of a solid phase is determined by the biochemical activity of microbiota and, first of all, the creation of geochemical conditions that contribute to the appearance of a solid carbonate material. However, the fixation of this material in a sediment is implemented either in “skeletal” ball-shaped (coccolites) and tubular (tubiphytes) forms, or by precipitation as a result of absorption on glycocalyx, or by the formation of various — both isolated individual (thrombolite microclusters, oolites and oncolites) and laminar (stromatolites) — forms.

Microbialites diversity from the Ediacaran of the Anti-Atlas (Morocco): A snapshot of microbial oases thriving in an alkaline volcanic lake

The Moroccan Anti-Atlas belt preserves an exceptional record of an Ediacaran microbial biosphere. The Amane Tazgart Formation of the Ouarzazate Group corresponds to an Ediacaran volcanic alkaline lake depositional system (ca. 571 Ma) where microbial buildups accreted in an extreme environment. These microbial accumulations are exceptional not only for their diverse range of extreme conditions but also for their significance in understanding the early biosphere and earth's habitability. A description of these buildups provides insights into their Spatio-temporal distribution. The lower part consists of thrombolitic limestones, usually displaying irregular to patchy mesoclots, associated with composite and stromatolitic buildups. The upper part dominated by clastic stromatolites exhibits a variety of morphotypes ranging vertically from planar wrinkly laminated to large domes. The transitional morphotypes are made of linked and vertically oriented or inclined columns, grading upward to cone-shaped domes. The increase in laminated fabrics and the decrease in clotted fabrics toward the top of the section indicate that environmental conditions were likely suitable for coexisting both fabrics during microbial carbonate accretion. The demise of carbonate production at the late stage coincides with riverine input of clastic sediments, subsequently followed during low sediment input by growth of siliciclastic stromatolites.

Dual eco-friendly application of silica fume and scoria in cement-based materials through the enhancement of microbially-induced carbonate precipitation

The partial replacement of cement or natural aggregates with industrial wastes and using microbially-induced carbonate precipitation to heal cracks are two developing methods to make more sustainable concrete. In this study, a combination of these two methods was used for the first time: Partial replacement of cement with silica fume to economically create a more favorable pH for bacterial activity and substituting natural sands partially with scoria aggregates as a novel carrier and protector for bacterial spores against the harsh and alkaline conditions of cement mortar. Preliminary investigations identified scoria as a suitable carrier since the activity of immobilized cells did not decrease considerably. Thermogravimetric and X-ray diffraction analyses confirmed that spores immobilized in scoria are capable of precipitating calcium carbonate. Different mortar mixtures were made with and without scoria immobilized healing agents and silica fume. In the absence of silica fume, adding biological healing agents to mortar mixture enhanced flexural and compressive strengths by 13% and 10%, respectively, and by 18% and 15% in the silica fume amended mortars. Healing agents lowered sorptivity coefficients by 49% when samples lacked silica fume and 63% when specimens included it. The presence of silica fume resulted in a more favorable pH environment for bacterial activity and increased precipitation of calcium carbonate, as confirmed by the scanning electron microscope. Replacing a part of cement with silica fume -as an industrial byproduct- has multiple ecological benefits: protecting the environment via waste reuse, reducing demand for energy and natural resources, decreasing greenhouse gas emissions, lowering cement manufacturing costs, and finally improving the performance of microbial carbonate precipitation leads to the production of more sustainable concrete.

Formation and preservation of Eocene lacustrine microbialites in the western Qaidam Basin (northeastern Qinghai-Tibetan Plateau, China): Petrological, mineralogical, and geochemical constraints

Studying the lacustrine microbialites in the Qaidam Basin may provide clues to the occurrence and mineralization processes of microbial communities in extremely harsh terrestrial environments. In the Eocene lacustrine systems of the Qaidam Basin, thick thrombolites (~0.4 to 1 m thick) generally formed in near-shore settings with rare to plentiful terrigenous admixtures and are characterized by four different clotted structures (i.e., maze-like, cortoid-like, tightly bound, and aggregated). In contrast, the small-scale stromatolitic, thrombolitic, and composite clotted-laminated microbialites (cm level) exhibit domical, columnar, and stratiform shapes, likely indicating that they developed in very shallow settings (e.g., a lagoon). The early lithification of the outer crusts and the internally laminated and clotted textures facilitated the preservation of the microbialites. The laminated and clotted textures of the Eocene microbialites analyzed in this study were composed of crypto- to micro-crystalline (mimetic) dolomite, which likely formed during the early mineralization process. This was likely caused by the strong evaporative lacustrine conditions on the Qinghai-Tibetan Plateau during the Eocene-Oligocene climate transition. The microbial mineralization process in dolomite-supersaturated pore fluids may also have contributed to early dolomitization. In contrast, the medium to coarsely crystalline components of the microbialites are composed of calcite minerals, implying that these components experienced cementation/recrystallization during diagenesis. In addition, large quantities of authigenic sulfate (celestite-barite) and sulfide (framboidal pyrite) minerals are exclusively distributed within the mimetically-dolomitized, laminated and clotted microbial structures. It is inferred that the saline lake conditions and the mineral dissolution (both carbonate and terrigenous particles) contributed to the SrSO4- and BaSO4-supersaturated fluid conditions and enabled the crystallization of celestite-barite. The development of Eocene lacustrine microbialites on the northeastern Qinghai-Tibetan Plateau provides a case for comparison with widely-distributed records of lake microbial carbonates, and the microstructural characteristics and analytical tools used in this study may offer a new perspective to explore the complex mineralization processes of microbialites.