Microbial Carbonate Research Articles

ВЛИЯНИЕ МИКРОБНОЙ КАРБОНАТНОЙ МИНЕРАЛИЗАЦИИ НА ГИДРОФОБНОСТЬ ПОВЕРХНОСТИ ЦЕМЕНТНОГО КАМНЯ

The effect of microbial carbonate mineralization on increasing the hydrophobicity of the cement stone surface is theoretically justified and experimentally confirmed. Based on experimental studies and analysis of literature data, the main stages of microbial-induced sedimentation of calcium carbonate on the surface of a cement stone are formulated. Changes in the morphology of the cement stone surface due to crystalline new formations formed during biomineralization in the postgenetic period are described. It is shown that biocolmatation of the interstitial space of a cement stone by crystalline new formations leads to an increase in hydrophobicity, as evidenced by an increase in the contact wetting angle. The relationship between the change in the concentration of calcium in neoplasms and the average value of the contact wetting angle on the surface of the cement stone from the type of bacterial culture is established. According to chemical and morphological analysis of bioinduction tumors on the surface of samples and also evaluation of changes of the contact angle compared to the original index for cement, bacterial cultures are ranked in order to increase the efficiency of use for sedimentation of calcium carbonate with their participation, as well as increase of cement materials hydrophobicity in the following sequence: Sporosarcina pasteurii  Bacillus megaterium  Lysinibacillus sphaericus  Bacillus pumilus.

Recycled aggregate mortar enhanced by microbial calcite precipitation

The reuse of construction and demolition waste (CDW) is hampered by its high water absorption and weak bonds with new cement paste. This problem is more severe for recycled fine aggregates (RFAs) that are involuntarily generated during the production of recycled coarse aggregates. To explore the feasibility of reusing RFAs in construction, microbial carbonate precipitation (MCP) is presented in this study to consolidate the RFAs derived from CDW. The effects of calcium sources on bacterially induced MCP were investigated, and the efficiency of MCP for improving RFAs was evaluated in terms of physical and mechanical properties. Results show calcium chloride is an optimal calcium supplement that is useful for generating calcium carbonate, as compared with the effect of calcium nitrate. MCP treatment has improved the quality of RFAs, with decreases in water absorption, crushing value and the powder content. Furthermore, mortars containing 100% treated RFAs exhibited better workability and improved compressive strength, with a maximum increment of 32% after 28 d of curing. The pore structures of mortars containing modified RFAs were densified, with decreased porosities detected by the mercury intrusion porosity method. X-ray diffraction and scanning electron microscopy revealed the precipitation of calcium carbonate on the RFAs.

Pedogenic and subaerial exposure microfabrics in a late Carboniferous-early Permian carbonate-volcanic lacustrine-palustrine system (San Ignacio Formation, Frontal Cordillera, Argentina)

In the Argentinian Andes (Frontal Cordillera) the upper part of the late Carboniferous-early Permian San Ignacio Formation is made up of lacustrine-palustrine microbial carbonates and interbedded volcanic deposits. In this lacustrine-palustrine environment a natural monospecific forest was developed. The deposits of this sedimentary-volcanic succession were repeatedly subjected to subaerial exposure and modified by pedogenesis to varying degrees giving rise to paleosoils development. Diagenetic microfabrics were well preserved in the carbonates and volcanic rocks. The carbonate microfabrics comprise a wide spectrum of features consisting of root marks and stumps-related structures (rhizoliths, alveolar texture, tunnel-like structures and coprolites of arthropods), pisoids, coated grains and pseudomicrokarst, cracking, brecciated and nodular fabrics, and grainification also occur corresponding to different stages in the pedogenic evolution. Meteoric dissolution and cementation processes are observed; examples are well identified by scanning electron microscope showing silica-filled voids in partially dissolved carbonates and growths of inorganic carbonate microcrystals or of microbial origin in voids. Other different types of cements can be seen such as discontinuous carbonate crusts, ribbon spar, cavities with silt infillings and pendant cements. The whole set of these microfabrics are indicative of wetting, desiccation and meteoric conditions (vadose and phreatic). The abundance of plant roots and associated micro-organisms mainly of bacterial origin (micro-rods, short rod-shapes, nano-fibres, filaments and nano-spheres) played an important role in the pedogenic and subaerial diagenetic processes affecting these deposits. The immature character of the paleosoils and absence of calcretes point out to short intervals of subaerial exposure due to oscillating fluctuations in water level, intermittent volcanic supply, tectonic subsidence and oscillating climatic conditions. The whole of the macro and microfabrics reveals that the prevailing weather could correspond to an intermediate between semi-arid to sub-humid, however the alternating wetting and drying conditions in which the fossil forest developed and the abundance and diversity of micro-organisms, suggest a transition to sub-humid climate conditions.

Induction of microbial carbonate precipitation as a sustainable strategy for post-improvement and remediation of cold mix asphalt

Cold Mix Asphalt (CMA) is highly susceptible to moisture damage, manifesting itself in stripping, raveling, rutting, and cracking faults. This study aimed to tackle this concern with bio-treatment. In doing so, this effort investigates the efficacy of microbial calcium carbonate precipitation in (i) the post-bio-strengthening (PBS) of mechanical properties and (ii) bio-remediation (BR) of moisture damage in cold-mix asphalt emulsion mixtures. To study the post-bio-strengthening potential, a set of cold-mix asphalt specimens was sprayed and injected with bacterial culture medium and nutrient solution to induce calcium carbonate precipitation for the purpose of post-strengthening. Then, the effect of curing times of 7, 14, 28, and 56 days on the fixation of precipitates in the mixtures was examined. To study the bio-remediation potential, another set of cold-mix asphalt specimens was subjected to a freeze-thaw cycle (modified Lottman test) intended to cause moisture damage and bacterial culture medium and nutrient solution were injected, afterward. For these specimens, the appropriate curing time of the fixation process for the bio-remediation was determined. All specimens were subjected to the destructive and non-destructive tests of mechanical properties, semi-circular bending (SCB), moisture susceptibility and also Cantabro test to assess indirect tensile strength (ITS), Tensile strength ratio, Indirect tensile fracture energy ratio, percent of toughness loss, crack propagation by SCB fracture energy, ultrasonic pulse velocity ratio, electrical resistivity, as well as the abrasion resistance. The Analysis of Variance (ANOVA) of the results was performed at 95% confidence level. A Field Emission Scanning Electron Microscope (FESEM) examination was also performed to study the microstructure and precipitates formed in the mixtures. The results showed that the post-bio-strengthened specimens had significantly better mechanical and resistance to moisture damage properties than those of control. The moisture-damaged specimens that underwent the bio-remediation also showed significant improvements in mechanical properties, and in some cases were similar to the undamaged control specimens. The results indicate that microbial calcium carbonate precipitation (MCP) can serve as an effective and sustainable method for both post-strengthening and remediation of cold mix asphalt mixtures.

Sedimentary and Diagenetic Features and Their Impacts on Microbial Carbonate Reservoirs in the Fourth Member of the Middle Triassic Leikoupo Formation, Western Sichuan Basin, China

In recent years, the discovery of two gas fields in the fourth member of the Leikoupo Formation in the Western Sichuan Basin of SW China confirmed the exploration potential of microbial carbonates. The aim of the present study is to clarify the formation mechanism of the microbial reservoirs in the Leikoupo Formation. For this purpose, lithofacies, depositional environments, and diagenesis analyses were performed in samples collected from cores of 12 wells. The climate of study area was arid during Anisian time, and the water body was restricted. In such a climate, an evaporitic environment was developed, where ten types of lithofacies, dominated by microbial carbonates and gypsum rocks, were recognized. Thrombolites and stromatolites are the main high-quality reservoirs rock types in the fourth member of the Leikoupo Formation in the Western Sichuan Basin of SW China, which developed as microbial mounds, with reservoir space of microbial inter-clot pores, intra-clot pores, fenestral pores, inter-crystalline pores, and cracks. The microbial inter-clot pores are the main reservoir space, formed by trapping and binding of marls by benthic microbial communities. These pores were partially filled with evaporites because of the arid climate, which were subsequently dissolved (mainly gypsum) in the syn-depositional period, thus greatly improving the quality of reservoirs. Although some pores were occluded by multi-stage cements during the burial stage, major pores were well preserved own to the early dolomitization, rapid burial of the Leikoupo Formation, and early charging of hydrocarbon. The early dolomitization enhanced the anti-compaction ability of microbial carbonates during the burial stage. Rapid burial of the Leikoupo succession slowed down early cementation, and it also accelerated the maturation and expulsion process source rock to promote early charging of hydrocarbon in pores, which created a closed system, inhibiting strong burial cementation.

Oxygen isotope composition of waters recorded in carbonates in strong clumped and oxygen isotopic disequilibrium

Abstract. Paleoenvironmental reconstructions, which are mainly retrieved from oxygen isotope (δ18O) and clumped isotope (Δ47) compositions of carbonate minerals, are compromised when carbonate precipitation occurs in isotopic disequilibrium. To date, knowledge of these common isotopic disequilibria, known as vital effects in biogenic carbonates, remains limited, and the potential information recorded by δ18O and Δ47 offsets from isotopic equilibrium values is largely overlooked. Additionally, in carbonates formed in isotopic equilibrium, the use of the carbonate δ18O signature as a paleothermometer relies on our knowledge of the paleowaters' δ18O value, which is often assumed. Here, we report the largest Δ47 offsets observed to date (as much as −0.270 ‰), measured on microbial carbonates that are strongly linked to carbonate δ18O offsets (−25 ‰) from equilibrium. These offsets are likely both related to the microorganism metabolic activity and yield identical erroneous temperature reconstructions. Unexpectedly, we show that the δ18O value of the water in which carbonates precipitated, as well as the water–carbonate δ18O fractionation dependence on temperature at equilibrium, can be retrieved from these paired δ18O and Δ47 disequilibrium values measured in carbonates. The possibility to retrieve the δ18O value of paleowaters, sediments' interstitial waters or organisms' body water at the carbonate precipitation loci, even from carbonates formed in isotopic disequilibrium, opens long-awaited research avenues for both paleoenvironmental reconstructions and biomineralization studies.

Effect of simulated acid rain on the stability of calcium carbonate immobilized by microbial carbonate precipitation

The stability of carbonate products resulting from microbially induced carbonate precipitation (MICP) under acid rain is under question. The present study investigated the stability of CaCO3 precipitated by MICP in soil under simulated acid rain (SAR). Soils were treated continuously for two months with four SAR pH levels: 3.5, 4.5, 5.5, and 7.0. During SAR, biostimulation using nutrient broth containing urea and calcium chloride was adopted to ensure CaCO3 precipitation. At the end of treatments, soil samples from top and bottom layers were analyzed for bacterial diversity by Illumina MiSeq sequencing, Fourier transform infrared (FTIR) spectroscopy for identification of chemical functional groups related to calcite precipitation, and X-ray diffraction (XRD) for identification of the main crystalline phases. The analysis identified several ureolytic bacteria mainly from Arthrobacter and Sporosarcina genera in SAR-treated soils accelerated with biostimulation, and urease quantities of greater than 300 mg NH4+ per kg soil at all pH levels. The precipitation of CaCO3 was pronounced and its stability was maintained even when the pH was as low as 3.5. The results obtained in this study are helpful to the scientific community to ensure the immobilization of heavy metals with microbial carbonate precipitation in soil under acid rain.

The Record of Environmental and Microbial Signatures in Ancient Microbialites: The Terminal Carbonate Complex from the Neogene Basins of Southeastern Spain

The Messinian microbialites of the Terminal Carbonate Complex (TCC) from the Neogene basins of southeastern Spain show both diversified morphologies and an excellent preservation of primary microbial microstructures. Their stratigraphic architecture, fabric (micro-, meso-, and macro-fabric), and mineralogical composition were investigated in eight localities from three sedimentary basins of southeastern Spain: The Sorbas and Bajo Segura basins and the Agua Amarga depression. Two recurrent microbialite associations were distinguished. Laterally linked low relief stromatolites predominated in Microbialite Association 1 (MA1), which probably formed in low energy lagoons or lakes with fluctuating normal marine to hypersaline water. The microfabrics of MA1 reflected the predominance of microbially induced/influenced precipitation of carbonates and locally (Ca)-Mg-Al silicates. Microbialite Association 2 (MA2) developed in high energy wave and tidal influenced foreshore to shoreface, in normal marine to hypersaline water. High-relief buildups surrounded by mobile sediment (e.g., ooids or pellets) dominated in this environment. MA2 microbialites showed a significant proportion of thrombolitic mesofabric. Grain-rich microfabrics indicated that trapping and binding played a significant role in their accretion, together with microbially induced/influenced carbonate precipitation. The stratigraphic distribution of MA1 and MA2 was strongly influenced by water level changes, the morphology and nature of the substratum, and exposure to waves. MA1 favorably developed in protected areas during third to fourth order early transgression and regression phases. MA2 mostly formed during the late transgressions and early regressions in high energy coastal areas, often corresponding to fossil coral reefs. Platform scale syn-sedimentary gypsum deformation and dissolution enhanced microbial carbonate production, microbialites being thicker and more extended in zones of maximum deformation/dissolution. Microbial microstructures (e.g., microbial peloids) and microfossils were preserved in the microbialites. Dolomite microspheres and filaments showed many morphological similarities with some of the cyanobacteria observed in modern open marine and hypersaline microbialites. Dolomite potentially replaced a metastable carbonate phase during early diagenesis, possibly in close relationship with extracellular polymeric substances (EPS) degradation. Double-layered microspheres locally showed an inner coating made of (Ca)-Mg-Al silicates and carbonates. This mineral coating could have formed around coccoid cyanobacteria and indicated an elevated pH in the upper part of the microbial mats and a potential dissolution of diatoms as a source of silica. Massive primary dolomite production in TCC microbialites may have resulted from enhanced sulfate reduction possibly linked to the dissolving gypsum that would have provided large amounts of sulfate-rich brines to microbial mats. Our results open new perspectives for the interpretation of ancient microbialites associated with major evaporite deposits, from microbe to carbonate platform scales.

Development of Middle Cambrian leiolitic bioherms dominated by calcified microbes: A case study of the Xinji Section (North China Platform)

Among the varieties of microbial carbonate rocks, leiolites are rare in ancient rock records and have only few examples in the modern environment, which makes them difficult to study. This study documents the sedimentary environment, characteristics and composition of microbial carbonates, dominated by calcified microbes, which developed in the Cambrian Miaolingian Xuzhuang Formation on the North China Platform. The upper part of the formation in the Xinji section of Lushan City, Henan Province is bounded by a drowning-unconformity surface of a third-order depositional sequence. Beside the development of an ooid shoal in response to sea-level fall, a bioherm dominated by leiolite also developed in the upper part of the Xuzhuang Formation. These leiolites represent special sedimentary phenomena due to their occurrence with high-energy ooid shoals. Microscopically, they are characterized by abundant calcified cyanobacteria fossils, such as dichotomous-branching Epiphyton and filamentous cyanobacteria, thus showing the diversity and complexity of the cyanobacterial microbial mats that predominate the leolitic bioherm. Four groups are confirmed for Epiphyton based on the morphological characteristics of calcified filaments. The filamentous cyanobacteria can be divided into Girvanella and Subtifloria , and their possible forming mechanisms are discussed. The macroscopic and microscopic characteristics of the current leiolites provide important research clues and typical examples for understanding the origin of the Miaolingian leiolitic bioherm and its paleoenvironmental significance. • The Cambrian leiolitic bioherm developed in a high-energy environment during a drop in sea-level on the North China Platform. • The present study delineates the chambered, shrub-like Epiphyton and filamentous cyanobacteria (Girvanella and Subtifloria) • The microbial calcification occurred prior to early diagenesis, thus opposing the possibility of Epiphyton as a “diagenetic fossil”.

A report on the IAS field trip “Microbial carbonate reservoirs architecture:From depositional facies to seismic scale geometriesin the Triassic of the Dolomites (Italy)”

A report on the IAS field trip “Microbial carbonate reservoirs architecture:From depositional facies to seismic scale geometriesin the Triassic of the Dolomites (Italy)”

Climatic signals in lacustrine deposits of the Upper Yacoraite Formation, Western Argentina: Evidence from clay minerals, analcime, dolomite and fibrous calcite

AbstractAuthigenic clays are an important control on reservoir quality in lacustrine carbonates but remain challenging to predict. Lacustrine depositional systems respond to climatic variations in rainfall, surface runoff and groundwater input, and evaporation, and result in rapid and frequent changes in lake volume; this is expressed through changing water depth and shoreline position. In the upper portion of the Early Palaeocene Yacoraite Formation of the Salta Basin in Argentina, extensive lacustrine deposits were deposited during the sag phase of rifting. Prior high‐resolution stratigraphic studies have suggested that climatic factors control microbial carbonate sequences within a ‘balanced fill’ lake, with variation in the lake level having a major influence on facies association changes. This study characterizes the evolution of facies and mineralogy within the Yacoraite Formation, focusing on the distribution of clay minerals, making a link between the high, medium and low‐frequency sequence stratigraphic cycles. The low‐frequency transgressive hemicycle of the upper portion of the Yacoraite Formation is comprised of abundant siliciclastic facies, suggesting a wetter period. Microbialites occurring in this interval are coarse‐grained and agglutinated. Detrital clay minerals such as illite and chlorite and associated siliciclastic sediments were input to the lake during high‐frequency transgressive periods. During high‐frequency regressive hemicycles, sedimentation was dominated by carbonate facies with Ca‐rich dolomite and the authigenic clays are comprised of chlorite/smectite mixed‐layers. By contrast, the low frequency regressive hemicycle records fine‐grained agglutinated microbialite with horizons of fibrous calcite, more stoichiometric dolomite, barite and authigenic magnesian smectite. This indicates elevated ion concentrations in the lake under intense evaporation during an arid period. Understanding the conditions that are favourable for formation and preservation of authigenic clays within the lacustrine environment can improve understanding of reservoir quality in comparable economically important deposits.

Drowning of microbial mounds on the slopes of the Latemar platform (middle Triassic)

Two microbial mound-shaped carbonate bodies buried below the slope deposits of the middle Triassic Latemar platform (Dolomites, Italy) have been studied. The two sedimentary bodies, after having reached different stages of evolution, drowned and were covered by the slopes of the Latemar carbonate platform. The estimation of the depth and time at which they were located when they were buried made it possible to infer the average rates of relative sea-level rise to which they were subjected, revealing that these latter exceeded the growth rate of the main Latemar buildup. Given the estimated rates of sea-level rise, the two satellite bodies reached a critical depth at which microbial carbonate production stops, or it is significantly reduced, before being buried. As extensional tectonics was active in the area of the Dolomites during the Anisian, subsidence is the more likely cause of sea level rise. It is therefore hypothesized that the cause of the ultimate drowning of the “mounds” were subsidence rates exceeding their aggradation potential and inducing a progressive deepening that brought them below the lower depth threshold of microbial carbonate production.

Diverse thrombolites from the c. 2.4 Ga Turee Creek Group, Western Australia

Observations from a stromatolite-thrombolite reef complex of the c. 2.4 Ga Turee Creek Group, Western Australia, indicate diversity in stromatolite and microfossil morphology, in addition to the appearance of clotted (thrombolite-like) microbialites (Barlow et al., 2016; Barlow and Van Kranendonk, 2018). Here, we document the diverse morphologies and textures of clotted microbialites from the Turee Creek Group reef complex that occur interbedded and interlayered with stromatolites and sediments in the supratidal to subtidal facies. The clotted microbialites consist of four types with distinct gross morphology, mesostructure, and microtextures, including: 1) an aggregate of isolated, millimetre-scale organic-rich clots that accumulated following storm events and were cemented by coarsely crystalline radiating carbonate; 2) domal bioherms composed of a framework of non-laminated branching columns; 3) stratiform clotted microbialites consisting of fine-grained clots that form both an interconnected framework surrounded by coarsely crystalline cement, or comprise a poorly-defined mixture of fine- and coarsely crystalline textural components; and 4) thrombolites that comprise lobate bioherms with non-layered internal textures. Variations in thrombolite texture and morphology are likely driven by a combination of environmental and biological factors, as is the case in modern microbial carbonate environments, such as Shark Bay and the Bahamas. These findings show that the Turee Creek Group clotted microbialites are significantly more diverse than first recognised, adding to previously documented expressions of microbialite complexity from this microbialite reef complex.

Mathematical modelling and simulation of microbial carbonate precipitation: the urea hydrolysis reaction

Microbial carbonate precipitation is expected to play a major role in next-generation soil improvement technologies. To date, research into this method has relied on experimental and/or observational approaches. However, in order to understand the long-term effects of microbial carbonate precipitation and apply these to real environments, it is necessary to employ a predictive approach to determine the future state of the soil when using this soil improvement method. In this work, a mathematical model and numerical simulations based on the reaction–diffusion system for the microbial urea hydrolysis reaction are proposed. These techniques may be used to provide the spatiotemporal precipitation patterns of carbonates between soil particles and the particle surfaces. The simulation results revealed that the characteristic precipitation patterns depend on the diffusion of carbonates caused by the microbial urea hydrolysis reaction, and there was a significant shift in the amount of carbonate from a dissipated state into an equilibrium state.

Desert soil sequesters atmospheric CO2 by microbial mineral formation

Microbial carbonate formation in soil may play a critical role in sequestering atmospheric CO2, specifically in drylands with sparse vegetation and harsh environments. However, few studies have examined the mechanism of CO2 fixation by soil microbes. Here, we investigated the sign of soil inorganic carbon (SIC) formed by microbial populations recovered from soil in the Mu Us Desert and identified the carbon source of this process by tracking the isotope profile of atmospheric CO2 in the SIC pool. In addition, we used high-throughput sequencing to predict the function of soil microbial genes involved in metabolic pathways associated with the formation of carbonaceous minerals, and characterised these minerals using scanning electron microscopy, energy-dispersive X-ray, and X-ray diffraction spectrometry. We found that atmospheric CO2 was incorporated into SIC via microbial processes at a rate of approximately 52.13 μg C kg−1 d−1 during a 2.5-month labelling period under 13CO2, and that microbial genes predicted to function in metabolic pathways for carbonaceous mineral formation were widespread among desert soil microbes. Moreover, the composition of microbial populations, along with the predicted metabolic pathways for microbial mineral formation, correlated significantly with rate of microbial CO2 incorporation into SIC. Importantly, we observed the formation of carbonaceous minerals such as calcite in desert soil samples inoculated with indigenous microbes. To our knowledge, we demonstrated for the first time that desert soil microbes are directly involved in fixing atmospheric CO2 via biomineralisation. Our findings provide empirical evidence for soil microbe-driven sequestration of atmospheric carbon and further reveal the potential capacity of desert soil for carbon sequestration.

Synchronized changes in shallow water carbonate production during the Carnian Pluvial Episode (Late Triassic) throughout Tethys

Abstract Quantitative petrologic analysis on carbonates was carried out on stratigraphic sections encompassing the Carnian Pluvial Episode from northwestern Sichuan Basin, South China and eastern Southern Alps. The Carnian Pluvial Episode, or CPE, is a period of climate change that occurred between the early Carnian and the beginning of the late Carnian (Late Triassic) and coincides with multiple, sharp negative excursions in the record of δ13C that are thought to be evidence of perturbations of the global carbon cycle. During the CPE, relevant environmental modifications and biological turnovers occurred in the marine realm. In particular, microbial carbonate platforms, that were dominant in northwestern Sichuan Basin and throughout Tethys, underwent widespread demise and the microbial component in shelf carbonate sediments sharply decreased and was replaced by ooid- and skeletal grains. Our results show a Tethys-wide coincidence between this change in the carbonate factory. Most notably, both in eastern and western Tethys microbial carbonate production recovered in the late Tuvalian, and the timing of recovery was not influenced by differences of basin evolution and geodynamic setting that characterize such distant domains.

Diagenetic Processes and Reservoir Heterogeneity in Salt-Encased Microbial Carbonate Reservoirs (Late Neoproterozoic, Oman)

A common problem in dolomite reservoirs is the heterogeneous distribution of porosity-reducing diagenetic phases. The intrasalt carbonates of the Ediacaran-Early Cambrian Ara Group in the South Oman Salt Basin represent a self-sourcing petroleum system. Depositional facies and carbonate/evaporite platform architecture are well understood, but original reservoir properties have been modified by diagenesis. Some of the carbonate reservoirs failed to produce hydrocarbons at acceptable rates, which triggered this study. The extent of primary porosity reduction by diagenetic phases was quantified using point counting. To visualize the distribution of diagenetic phases on a field scale, we constructed 2D interpolation diagenesis maps to identify patterns in cementation. The relative timing of diagenetic events was constrained based on thin-section observations and stable isotope analyses. Near-surface diagenesis is dominated by reflux-related processes, leading to porosity inversion in initial highly porous facies and a patchy distribution of early cements. This strong diagenetic overprint of primary and early diagenetic porosity by reflux-related cements leads to a reduction of stratigraphic and facies control on porosity. Calcite was identified as a burial-related cement phase that leads to an almost complete loss of intercrystalline porosity and permeability. Bitumen is an important pore-occluding phase and time marker of the deep-burial realm. The stratigraphic position of the dolomite reservoirs embedded at the base of a salt diapir had a strong impact on its diagenetic development. The salt isolated the dolomites from external fluids, leading to a closed system diagenesis and the buildup of near lithostatic fluid pressures. In combination, these processes decreased the impact of further burial diagenetic processes. The study highlights that cement distribution in salt-encased carbonate reservoirs is mainly related to early diagenetic processes but can be very heterogeneous on a field scale. Further work is needed to implement these heterogeneities in an integrated numerical reservoir model.

Thrombolitic clots dominated by filamentous cyanobacteria and crusts of radio-fibrous calcite in the Furongian Changshan Formation, North China

As a variety of non-laminated microbial carbonate or calcified microbialite, thrombolites commonly comprise diffusely clotted micrites as their basic structural unit. The current study is aimed at discussing the composition and forming mechanism of thrombolitic clots, which most likely represent the product formed by calcification of extracellular polymeric substances. It provides instances of sophisticated calcification of thrombolite-forming microbial mat in the calcite sea that is expressed by the distinctive composition of thrombolites. Thrombolites in the top part of the Furongian Changshan Formation are chiefly composed of chaotic large and small micritic clots as observed under low microscopic resolution. However, high density preservation of fossils of calcified sheaths of cyanobacteria is observed within most of the clots under higher resolution. Their distribution as groups or colonies suggests that they can be interpreted as Subtifloria of Girvanella group. These microbial components together with the enigmatic Epiphyton reflect fundamental features of microbial growth, and suggest a microbial origin of these clots. The general abundance of these calcified fossils within individual clots offers an instance for the formation of thrombolitic clots from sophisticated calcification of photosynthetic biofilms. It refutes the traditional view that both the thrombolites and the oncoids develop by accretion of microbial mats dominated by coccoid cyanobacteria. Aggregation of several individual clots to form millimetric agglutinated clots shows that their formation is genetically related to sophisticated calcification of extracellular polymeric substances making up multiple biofilms within relatively thick microbial mats. This inference is strengthened by their encrustation by radio-fibrous calcite, which leads to enlargement of the clots. Besides the clots, benthic ooids, pyrite and dolomite also developed in the current thrombolites, which are largely attributed to microbially mediated organomineralization.

Macro-micro features of microbial carbonates affected by volcanism in lower Cretaceous Shipu Group in Zhejiang Province, East China

Based on macro-scopic observations of outcrop, microscopic examination of thin sections, Field Emission Scanning Electron Microscopy(FESEM) imaging analysis and Energy Dispersive X-ray Spectrometry (EDS) analysis, nine units of microbial carbonates has been recognized in Lower Cretaceous Shipu Group and their thicknesses increase gradually upwards as the volcanism strength decreasing. Unit 7 is the thickest association of microbial carbonates-volcanics. These microbial carbonates consist of stromatolites, spherulites and laminated micorbialites with common recrystallization and local dolomization and analcitization. Thanks to intensive silicification, microorganisms have been preserved in crystal lattice so well that the original microstructure even the chamber of microorganism can be observed clearly through Field Emission Scanning Electron Microscopy(FESEM) imaging analysis. Macro structure of microbialites are massive, domal or laminated and micro structure of them are fanshaped, wavy, crenulate or spherical.

Geochemical and Petrographic Analyses of the Cambrian Oncoids of the North China Platform: Implications for Their Paleogeography and Paleoenvironment

After the global extinction event at the end of the Cambrian Epoch 2, widely spread oolitic bank dominated the North China Carbonate Platform during the Cambrian Miaolingian Epoch. To better understand the influence of paleogeographic and paleoenvironmental factors on microbial carbonate particles during this period, carbonate oncoids of the Cambrian Miaolingian Series were selected to reconstruct the paleogeography and paleoenvironment. The study samples were collected from four different sections: Wuhai, Diaoquan, Xiawidian and Sandaogou (from west to east in the North China Platform). Stratigraphic, sedimentological, petrological and geochemical studies including analysis of trace and rare earth elements were carried out to define the stratigraphic location, and morphological and geochemical characteristics of oncoids. Geochemical analysis of oncoids reveals that Er/Nd and Y/Ho values are relatively low, signifying that the formation of oncoids was influenced by terrestrial inputs. Meanwhile, Sr/Cu, Sr/Ba, V/Sc and V/Cr values indicate that the oncoids were developed in a shallow marine environment under oxidizing conditions. The low content of total rare earth elements, low LREE/HREE ratios and LREEs, negative anomalies of Ce/Ce* and Eu/Eu* as well as (La/Sm)N suggest that the oncoids were less influenced by late diagenetic processes. More importantly, morphological differentiations of oncoids in the study area coincide with the changing trend of Y/Ho and Sr/Ba. The results of this study show that oncoids with regular morphology mainly formed at offshore area, while those with irregular shape and preserving rough laminae mostly occurred at nearshore area. From the comparison made between the paleogeographic locations of the study sections, it is proposed that the paleosalinity of marine depositional environment and the transportation distance are the prime controls for morphological differentiation of oncoids.