Biofilms, microbial mats and microbe-particle interactions: electron microscope observations from diatomaceous sediments

Sediments and diatoms from the mudflats of the Bay of Bourgneuf in western France were examined in an electron microscope study of biofilms and microbial mats. The sediments were kept in an aquarium for study and a diatom culture was made of the benthic diatoms. The sediment biofilm was composed of exopolymeric substances (EPS), incorporated clay particles and, rarely, bacteria. This film coated all particles at the sediment-water interface. Its surface morphology reflected its composition and internal structure. Thin films were smooth, whilst a lumpy structure or incorporated fibrils produced either a mammillated or ropy surface, and clays in the structure gave rise to a flaky morphology. At shallow depths in the sediment column (0.5 cm) the biofilm was already degraded. The biofilm coating degraded diatom frustules in the benthic diatom culture consisted of EPS and bacteria and presented a ragged appearance. Microbial mats occurred on the surface of the fresh littoral sediments as well as those in the aquarium, and on the wall of the aquarium. The mat on the surface of the aquarium sediments had an open structure with webs of fibrils and bacteria in the pore space. It formed in a relatively quiet environment. Pore space was more limited in the mat from the surface of the fresh littoral sediments, in which direct contact between biofilm coated particles was common. In the exposed environment of the aquarium wall there was a thick, resistant coating of EPS. In addition to binding particles together, the presence of mats and biofilms in sediments affects sediment physical properties such as porosity and permeability, the flux of dissolved substances in pore waters and the dissolution of particles and can, therefore, influence early diagenesis. Mats and biofilms seem to be more readily preserved in the geological record than the micro-organisms, such as bacteria, which produce them. Their identification in the sedimentary record would greatly aid interpretation of sediment genesis and evaluation of the microbial role in sediment formation.     

Carbonate precipitation in the alkaline lake Specchio di Venere (Pantelleria Island,Italy) and the possible role of microbial mats

Alkaline lakes like the hydrothermally affected lake Specchio di Venere (Pantelleria Island, Central Mediterranean) are typical geological settings harbouring calcified microbial mats. The present work is focused on the discrimination between biotic and abiotic processes driving carbonate precipitation in this lake, using hydrochemical, mineralogical and isotopic data. Hydrochemical analyses demonstrate that the lake is nearly 10−fold supersaturated with regard to aragonite and seasonally reaches hydromagnesite supersaturation. Microscopic observations depict organosedimentary laminated structures consisting of microbial communities and aragonitic precipitates, which are rather disseminated in pores than directly linked to microorganisms. Oxygen isotopic data indicate that authigenic carbonate crystallisation from evaporating water is the dominant precipitation process, further suggested by the absence of textural evidence of diagenetic processes. Conversely, the observed δ¹³C values reflect an influence of extracellular polymeric substances (EPS) on carbon fractionation during the precipitation process, due to the selective sequestration of ¹²C in the biomass. The above considerations suggest that at lake Specchio di Venere the carbonate precipitation is mainly of inorganic nature, but a minor role played by biologically influenced processes in microbial mats is not excluded.     

Microbe–mineral interactions: early carbonate precipitation in a hypersaline lake (Eleuthera Island, Bahamas)

Microbialites (benthic microbial carbonate deposits) were discovered in a hypersaline alkaline lake on Eleuthera Island (Bahamas). From the edge towards the centre of the lake, four main zones of precipitation could be distinguished: (1) millimetre‐sized clumps of Mg‐calcite on a thin microbial mat; (2) thicker and continuous carbonate crusts with columnar morphologies; (3) isolated patches of carbonate crust separated by a dark non‐calcified gelatinous mat; and (4) a dark microbial mat without precipitation. In thin section, the precipitate displayed a micropeloidal structure characterized by micritic micropeloids (strong autofluorescence) surrounded by microspar and spar cement (no fluorescence). Observations using scanning electron microscopy (SEM) equipped with a cryotransfer system indicate that micrite nucleation is initiated within a polymer biofilm that embeds microbial communities. These extracellular polymeric substances (EPS) are progressively replaced with high‐Mg calcite. Discontinuous EPS calcification generates a micropeloidal structure of the micrite, possibly resulting from the presence of clusters of coccoid or remnants of filamentous bacteria. At high magnification, the microstructure of the initial precipitate consists of 200–500 nm spheres. No precipitation is observed in or on the sheaths of cyanobacteria, and only a negligible amount of precipitation is directly associated with the well‐organized and active filamentous cyanobacteria (in deeper layers of the mat), indicating that carbonate precipitation is not associated with CO₂ uptake during photosynthesis. Instead, the precipitation occurs at the uppermost layer of the mat, which is composed of EPS, empty filamentous bacteria and coccoids (Gloeocapsa spp.). Two‐dimensional mapping of sulphate reduction shows high activity in close association with the carbonate precipitate at the top of the microbial mat. In combination, these findings suggest that net precipitation of calcium carbonate results from a temporal and spatial decoupling of the various microbial metabolic processes responsible for CaCO₃ precipitation and dissolution. Theoretically, partial degradation of EPS by aerobic heterotrophs or UV fuels sulphate‐reducing activity, which increases alkalinity in microdomains, inducing CaCO₃ precipitation. This degradation could also be responsible for EPS decarboxylation, which eliminates Ca²⁺‐binding capacity of the EPS and releases Ca²⁺ ions that were originally bound by carboxyl groups. At the end of these processes, the EPS biofilm is calcified and exhibits a micritic micropeloidal structure. The EPS‐free precipitate subsequently serves as a substrate for physico‐chemical precipitation of spar cement from the alkaline water of the lake. The micropeloidal structure has an intimate mixture of micrite and microspar comparable to microstructures of some fossil microbialites.     

光合作用生物膜诱发的放射鲕粒:以江苏徐州贾旺剖面寒武系苗岭统张夏组为例*

几年来针对巴哈马现代文石鲕粒的持续性研究表明,微生物和细胞外聚合物质(EPS)在鲕粒的形成和发育中起着关键而重要的作用,从而产生了一个重要的认识,即: 鲕粒可以看作是“纹层状的有机沉积构造”并遵循着微生物岩体系的一些形成特征。但是,鲕粒30亿年的发育历史、多样化的产出环境、特征性的矿物构成和各种各样的沉积组构,确实赋予了鲕粒生长和形成机理的复杂性和神秘性,因为鲕粒在何处而且如何形成、以及鲕粒究竟记录着何种生物与非生物过程的许多问题还存在剧烈争论。来自于江苏徐州贾旺剖面苗岭统张夏组上部鲕粒滩相灰岩,由较为典型的方解石放射鲕粒所组成,表现出放射状、放射—同心状和泥晶质的沉积组构,而且在鲕粒核心、鲕粒皮层以及在鲕粒间的不规则团块或凝块的暗色泥晶质构成中高密度地保存着精美的葛万菌(Girvanella)化石,进一步表明了这些暗色泥晶构成代表着较为特征的光合作用生物膜,从而提供了一个苗岭世方解石海中放射鲕粒形成较为直接的微生物证据,以及与光合作用生物膜之间复杂的成因联系,因为葛万菌是相对较为肯定地类比于近代钙化织线菌(Plectonema)的丝状蓝细菌化石,尽管还可类比于现代的伪枝菌(Scytonema)。虽然形成放射状鲕粒皮层的放射纤维状方解石的沉淀作用确实不能解释为直接的微生物沉淀作用的结果,但是,这些放射鲕粒确实表现出光合作用生物膜诱发、滋养并促进了放射纤维状方解石皮层增生作用的重要证据,为拓宽“鲕粒谜”的阐释提供了一个较为重要的典型实例,而且还成为寒武纪苗岭世方解石海与后生动物辐射相耦合的蓝细菌繁荣的重要证据。     

Tepee-shaped agglutinated microbialites: an example from a Famennian carbonate platform on the Lennard Shelf, northern Canning Basin, Western Australia

Distinctive, metre‐scale antiformal structures are well developed in a Famennian carbonate platform in the Chedda Cliffs area of the Lennard Shelf reef complexes. The structures are distinguished by chevron‐shaped crests and thickened cores and contain abundant non‐skeletal allochems (ooids/pisoids, peloids and intraclasts) of silt to pebble size and variably developed laminations and fenestrae. The internal morphology and pervasive occurrence of fenestral clotted and wavy laminated fabrics suggest that these structures are microbial mounds composed of agglutinated stromatolites and thrombolites. Microbial fabrics most probably originated through sediment trapping and binding by microbial mats with early lithification involving microbial calcification and cementation of trapped sediment. The facies and stratigraphic context of the mounds support a shallow subtidal, transitional backreef to reef‐flat setting; however, alone these mounds do not provide unequivocal environmental information. Other large antiformal structures in Famennian platforms on the Lennard Shelf, previously described as tepee structures, show morphological similarities to the Chedda Cliffs mounds, which suggests that these other structures may also be microbial mounds. The presence of microbial mounds in platform successions further highlights the importance of microbial communities in the Lennard Shelf reef complexes.     

Microscopic calcite dendrites in cold-water tufa: implications for nucleation of micrite and cement

Dendritic calcite forms in an active cold-water tufa system in association with extracellular polymeric substances (EPS) that discontinuously coat bryophytes and cyanobacteria. Dendrites consist of 100–200 nm thick calcite fibres that form 3D lattice-like domains. In each dendrite domain, fibres have three structurally equal orientations, which correspond in disposition to radii from the centre of a calcite unit cell to the convex triple face junctions on its surface. Fibres do not form in the orientation of the c-axis. The external form of each dendrite has the shape of half of a shortened octahedron, with an upper triangular surface parallel to the substrate. Dendrite nucleation takes place on or in microbial EPS, whether microbial cells are present or not, and is probably effected by attraction of Ca²⁺ cations to negatively charged EPS, together with CO₂-degassing and concomitant pH increase of supersaturated spring water in stream splash zones. Ensuing dendrite growth is abiogenic and controlled by diffusion. Dendrite c-axes are perpendicular to the substrate, probably because the negative charge of EPS forces the orientation of Ca²⁺ and CO planes within the developing dendrite crystal to be parallel to the EPS film surface. Dendrites are eventually filled and overgrown by solid, syntaxial calcite, which gradually and completely obliterates the dendrites as more familiar calcite crystal forms develop. No trace of the dendritic nucleus remains in the rock record. Calcite crystal nucleation may take place by this mechanism in many marine and meteoric settings, given that microbial EPS is now assumed to be virtually ubiquitous in these environments. This phenomenon could contribute to the development of familiar fabrics such as marine micrite cement and fibrous calcite cement, radial ooids, peloids, ‘abiogenic’ stromatolites, sea floor precipitates, microbialites, tufa, travertine, speleothems, and some meteoric cements. It may also contribute to the substrate-normal orientation of c-axes of common cement fabrics.     

北京西山下苇甸剖面寒武系徐庄组鲕粒研究

对北京西山下苇甸剖面寒武系徐庄组鲕粒灰岩进行了观察和研究,在其中不仅发现了有核及无核同心-放射状鲕、单(或多)晶鲕、泥晶鲕和复合鲕,而且还发现了暗色颗粒包壳、凝聚颗粒和黄铁矿聚集体等特殊组构。暗色颗粒包壳的发现尚属首次,其可能是丝状蓝细菌菌体的残留物。凝聚颗粒由方解石或白云石构成,可能代表了微生物席内的含氧气泡。生物碎屑上的附着体可能是微生物群落的痕迹,在其内部发现有类似鲕粒的球状颗粒。复合鲕胶结松散,包壳不发育,表明其可能形成于相对稳定的水体环境。研究表明,徐庄组鲕粒可能形成于水动力条件相对较弱的浅海环境中,微生物在鲕粒的形成过程中发挥了较大的作用。     

Microbial biomineralization processes forming modern Ca:Mg carbonate stromatolites

Modern Ca:Mg carbonate stromatolites form in association with the microbial mat in the hypersaline coastal lagoon, Lagoa Vermelha (Brazil). The stromatolites, although showing diversified fabrics characterized by thin or crude lamination and/or thrombolitic clotting, exhibit a pervasive peloidal microfabric. The peloidal texture consists of dark, micritic aggregates of very high‐Mg calcite and/or Ca dolomite formed by an iso‐oriented assemblage of sub‐micron trigonal polyhedrons and organic matter. Limpid acicular crystals of aragonite arranged in spherulites surround these aggregates. Unlike the aragonite crystals, organic matter is present consistently in the dark, micritic carbonate comprising the peloids. This organic matter is observed as sub‐micron flat and filamentous mucus‐like structures inside the interspaces of the high‐Mg calcite and Ca dolomite crystals and is interpreted as the remains of degraded extracellular polymeric substances. Moreover, many fossilized bacterial cells are associated strictly with both carbonate phases. These cells consist mainly of 0·2 to 4 μm in diameter, sub‐spherical, rod‐like and filamentous forms, isolated or in colony‐like clusters. The co‐existence of fossil extracellular polymeric substances and bacterial bodies, associated with the polyhedrons of Ca:Mg carbonate, implies that the organic matter and microbial metabolism played a fundamental role in the precipitation of the minerals that form the peloids. By contrast, the lack of extracellular polymeric substances in the aragonitic phase indicates an additional precipitation mechanism. The complex processes that induce mineral precipitation in the modern Lagoa Vermelha microbial mat appear to be recorded in the studied lithified stromatolites. Sub‐micron polyhedral crystal formation of high‐Mg calcite and/or Ca dolomite results from the coalescence of carbonate nanoglobules around degraded organic matter nuclei. Sub‐micron polyhedral crystals aggregate to form larger ovoidal crystals that constitute peloids. Subsequent precipitation of aragonitic spherulites around peloids occurs as micro‐environmental water conditions around the peloids change.     

Fabrics,facies control and diagenesis of lacustrine ooids and associated grains from the Upper Triassic,southwest England

Petrographic analysis of ooids from the Upper Triassic (Mercia Mudstone Group) of southwest England provides an opportunity to assess in detail the origins, transport pathways and diagenesis of an ancient oolite. The Clevedon Oolite is dolomitized and contains a variety of dissolved ooids (oomoulds) and associated grains. The oomoulds occur in well‐sorted, planar and cross‐stratified grainstones, packstones, sandstones and conglomerates associated with shoreface, intershoal, foreshore, beachrock and littoral strandplain deposits. The ooids grew in suspension in the shoreface zone and developed a radial aragonite microstructure. The ooids grew to 0.80 mm in diameter, after which they fractured or ceased growing. Broken grains deposited on or near mobile shoals were rapidly recoated, while other grains, deposited in less agitated, intershoal and lower foreshore areas, were micritized or microbially bound into grapestone aggregates. Locally peloids, intraclasts, quartz grains and micritized grains from intershoal areas supplied nuclei for ooids on nearby shoals. Grains deposited in foreshore areas were rapidly cemented into beachrock and reworked into conglomerates. Soon after deposition, the ooids were subjected to widespread aragonite dissolution followed by dolomitization. The lack of pre‐dolomitization calcite, together with the abundance of early (pre‐compaction) dolospar cements and fabric‐selective dolomitization of micritic fabrics, suggest aragonite dissolution by dolomitizing fluids. Copyright © 2002 John Wiley & Sons, Ltd.     

New interpretations of Great Salt Lake ooids and of ancient non-skeletal carbonate mineralogy

Earlier interpretations of textural alteration affecting Great Salt Lake ooids have greatly influenced concepts of ooid diagenesis. Scanning electron microscope study shows, however, that the coarse radial aragonite rays are depositional, that no recrystallization of pellet cores has occurred, and that Great Salt Lake ooids have not suffered noticeable diagenesis. As suggested by Kahle (1974), radial texture in ancient calcitic ooids is probably mainly original, not diagenetic. Retention of such fine textures has been attributed to organic matter (since found to be equivalent in modern skeletal and non-skeletal grains) or to paramorphic replacement (proposed for non-skeletal grains whose original aragonite mineralogy was only inferred from modern analogs). Pleistocene ooids known to have been aragonite alter like aragonite shells to coarse neomorphic calcite, often with aragonite relics. The striking uniformity of that coarse texture in neomorphic calcite replacing known skeletal aragonites throughout the geologic record has been noted for over 100 years. In contrast, Mississippian ooids retain fine texture as do calcite layers of coexisting gastropods, but unlike the strongly altered aragonite layers of these same gastropods. Therefore, inferences of original aragonitic mineralogy of ancient non-skeletal carbonate grains (including muds) which are now calcite but retain fine texture appear unwarranted, as do assumptions of differential diagenetic behaviour of ancient aragonitic skeletal and non-skeletal grains. Accordingly, modern depositional environments of marine ooids and carbonate muds must be rejected as chemically unrepresentative of comparable ancient environments. It is inferred that ancient non-skeletal carbonates were originally predominantly or exclusively calcite because of an earlier lower oceanic Mg/Ca ratio (<2/1) which altered progressively to values favouring aragonite (modern Mg/Ca value = 5/1). Major influencing factors are: selective removal of calcium by planktonic foraminifers and coccolithophorids since Jurassic-Cretaceous time and by abundant younger, Mg-poor aragonite skeletons and an erratic trend toward decreasing dolomite formation (decreasing removal of oceanic Mg). The change to aragonite dominance over calcite for non-skeletal carbonates was probably during early to middle Cenozoic time.     

鲕粒成因研究的新进展

鲕粒的成因一直是一个谜一样的沉积学难题。Brehm等在2006年的实验室研究的结果表明,将鲕粒的形成可以与叠层石进行类比,是一个特殊的球状微生物席的产物,从而将鲕粒归为微生物成因。最近,来自于巴哈马现代鲕粒的研究,Duguid等在2010年认为,鲕粒形成与微生物活动不存在一个直接的关系,重新强调了鲕粒形成的化学过程,即...     

广西隆安都结剖面下石炭统都安组上部鲕粒类型及其地质意义*

鲕粒是一类特殊的沉积颗粒,为古气候和古海洋环境的重要指示器。为了深入认识此类特殊颗粒的成因机制、形成环境及地质意义,对广西隆安地区都结剖面下石炭统都安组上部含鲕粒地层开展了古生物学、沉积学和岩相学研究。研究区共识别出5种主要的鲕粒类型: 放射状纹层鲕粒(O1)、规则同心放射状纹层鲕粒(O2)、不规则同心放射状纹层鲕粒(O3)、泥晶鲕粒(O4-A和O4-B)和复合鲕粒(O5)。各类鲕粒的显微组构和沉积环境指示其具有不同的形成过程,其中水动力条件影响和控制着鲕粒的发育和分布情况。研究区含鲕粒地层形成于维宪期末—谢尔普霍夫期,恰好对应早石炭世晚期冰川作用的开始。受冰川作用影响,全球海平面频繁波动,研究区地处低纬度地区并以浅滩和潮坪沉积环境为主,为鲕粒的形成提供了适宜的水体条件,即温暖、动荡的浅水环境。此外,含鲕粒岩层内广泛发育钙质微生物和微生物席,说明微生物活动在研究区较为常见,可能与鲕粒的形成过程具有一定的关联。     

广西隆安都结剖面下石炭统都安组上部鲕粒类型及其地质意义

鲕粒是一类特殊的沉积颗粒,为古气候和古海洋环境的重要指示器。为了深入认识此类特殊颗粒的成因机制、形成环境及地质意义,对广西隆安地区都结剖面下石炭统都安组上部含鲕粒地层开展了古生物学、沉积学和岩相学研究。研究区共识别出5种主要的鲕粒类型:放射状纹层鲕粒(O1)、规则同心放射状纹层鲕粒(O2)、不规则同心放射状纹层鲕粒(O3)、泥晶鲕粒(O4-A和O4-B)和复合鲕粒(O5)。各类鲕粒的显微组构和沉积环境指示其具有不同的形成过程,其中水动力条件影响和控制着鲕粒的发育和分布情况。研究区含鲕粒地层形成于维宪期末-谢尔普霍夫期,恰好对应早石炭世晚期冰川作用的开始。受冰川作用影响,全球海平面频繁波动,研究区地处低纬度地区并以浅滩和潮坪沉积环境为主,为鲕粒的形成提供了适宜的水体条件,即温暖、动荡的浅水环境。此外,含鲕粒岩层内广泛发育钙质微生物和微生物席,说明微生物活动在研究区较为常见,可能与鲕粒的形成过程具有一定的关联。     

鲕粒原生矿物识别及对海水化学成分变化的指示意义

鲕粒是碳酸盐沉积过程中一类非常特殊的颗粒类型, 为研究当时的沉积背景、水动力条件、气候环境, 甚至储层特征提供了重要线索。然而, 鲕粒的矿物组成及控制因素问题, 长期受到忽视。组成鲕粒的原生矿物类型在地质历史时期呈周期性变化, 在显生宙表现为三个以文石和高镁方解石占主导的时期以及两个以低镁方解石占主导的时期, 这也被称作“文石海”和“方解石海”时期。原生矿物的组成, 制约着鲕粒的纹层结构、保存程度以及成岩特征, 还蕴含着海水化学成分变化的线索。鲕粒原生矿物识别主要依据:①原生纹层结构;②保存程度;③微量元素浓度, 尤其是Sr-Mg的浓度。文石质鲕粒受文石不稳定性的影响, 原生结构保存程度较差;一般保存有典型的文石残余纹层结构(例如砖砌结构、溶解变形结构以及偏心结构等);在封闭成岩环境下原生矿物为文石质的鲕粒Sr浓度往往大于2 000 ppm;纹层结构主要为切线状(占主导)和放射状。方解石质鲕粒包括低镁方解石和高镁方解石两种类型:低镁方解石为稳定矿物, 原生结构一般保存良好。尽管高镁方解石也为亚稳定矿物, 但成岩转换后的保存程度好于文石。两者Sr含量一般均低于1 000 ppm, Mg含量一般在0~20 mol % MgCO₃(两者以4 mol % MgCO₃为界)。高镁方解石受成岩作用影响, 在纹层中往往保留有微粒白云石包裹体;海相地层中保存的方解石质鲕粒为放射状或同心-放射状结构。另外还存在一类由两种矿物共同构成的双矿物鲕粒, 可以通过分析两类纹层在结构和保存特征上的差异进行区分。鲕粒原生矿物成分随时间的波动变化受到海水化学条件, 尤其是Mg/Ca比值, 大气二氧化碳分压以及碳酸盐饱和度的控制。Mg/Ca比值的波动决定着鲕粒原生矿物类型的长期变化规律。一些突发性事件可能会扰动(区域)短时间尺度下鲕粒原生矿物的组成, 造成鲕粒原生矿物的转换。通过研究碳酸盐鲕粒原生矿物特征以及控制因素进而了解海水的化学特征, 是独立于古生物学和地球化学分析之外的一种较为可靠的沉积学方法。     

Biomediated Precipitation of Calcium Carbonate in a Slightly Acidic Hot Spring, Yunnan Province

A slightly acidic hot spring named "Female Tower"(t=73.5°C, pH=6.64) is located in the Jifei Geothermal Field, Yunnan Province, southwestern China. The precipitates in the hot spring are composed of large amounts of calcite, aragonite and sulfur. Scanning electron microscopy(SEM) analyses reveal that the microbial mats were formed from various coccoid or rod-shaped filamentous microbes. Transmission electron microscopy(TEM) shows that the intracellular sulfur granules are commonly associated with these microbes. A culture-independent molecular phylogenetic analysis demonstrates that the majority of the bacteria in the spring are sulfur-oxidizing bacteria. In the spring water, H_2S concentration is up to 60 ppm, while SO_4~(2-) concentration is only about 10 ppm. We speculate that H_2S might derive from sulfur-oxidizing bacteria in this hot spring water, leading to the intracellular formation of sulfur granules. Meanwhile, this reaction increased the p H in the micronscale microdomains, which fosters the precipitation of calcium carbonate in the microbial mats. The results of this study indicate that the sulfur-oxidizing bacteria might play an important role in calcium carbonate precipitation in slightly acidic hot spring environments.     

Processes of carbonate precipitation in modern microbial mats

Microbial mats are ecosystems that arguably greatly affected the conditions of the biosphere on Earth through geological time. These laminated organosedimentary systems, which date back to > 3.4 Ga bp, are characterized by high metabolic rates, and coupled to this, rapid cycling of major elements on very small (mm-µm) scales. The activity of the mat communities has changed Earth's redox conditions (i.e. oxidation state) through oxygen and hydrogen production. Interpretation of fossil microbial mats and their potential role in alteration of the Earth's geochemical environment is challenging because these mats are generally not well preserved.Preservation of microbial mats in the fossil record can be enhanced through carbonate precipitation, resulting in the formation of lithified mats, or microbialites. Several types of microbially-mediated mineralization can be distinguished, including biologically-induced and biologically influenced mineralization. Biologically-induced mineralization results from the interaction between biological activity and the environment. Biologically-influenced mineralization is defined as passive mineralization of organic matter (biogenic or abiogenic in origin), whose properties influence crystal morphology and composition. We propose to use the term organomineralization sensu lato as an umbrella term encompassing biologically influenced and biologically induced mineralization. Key components of organomineralization sensu lato are the “alkalinity” engine (microbial metabolism and environmental conditions impacting the calcium carbonate saturation index) and an organic matrix comprised of extracellular polymeric substances (EPS), which may provide a template for carbonate nucleation. Here we review the specific role of microbes and the EPS matrix in various mineralization processes and discuss examples of modern aquatic (freshwater, marine and hypersaline) and terrestrial microbialites.     

Bathymetry and sediments on the carbonate platform off western India: Significance of Halimeda bioherms in carbonate sedimentation

Bathymetry across the carbonate platform off western India indicated small-size pinnacles and their lateral coalescence into 2 -6-m high mounds landward, and linear elongated carbonate ridges and troughs, mounds and banks up to a height of 20-m seaward of the platform. Seismic data indicated that these mounds were transparent with no rigid internal structure and can be defined as bioherms. The sediments were abundantly aragonite faecal pellets, Halimeda grains and ooids and their radiocarbon ages ranged from 11 to 7.5 ka BP. It appears that the growth of Halimeda bioherms on the platform was facilitated by intense upwelling during the early Holocene. The terrigenous sediments brought by rivers were deposited in the inner shelf and have not affected the growth of bioherms. It is estimated that the platform comprises at least 1.85 Gt of mass CaCO\(_3\) accumulated during the early Holocene and comparable to those on the Great Barrier Reef. Halimeda bioherms produce abundant carbonate sediments and their growth period represents a geological carbonate sink and release of high CO\(_2\) to the atmosphere. Detailed shallow seismic studies and sediment cores are needed to quantify the exact mass content of CaCO\(_3\) and model climate change during the early Holocene.     

云南罗平中三叠统关岭组沉积环境演化及化石埋藏

罗平生物群是继二叠纪末全球生物大灭绝后海洋生态系统全面复苏的典型代表.为了更全面地认识罗平生物群产出的沉积环境,通过露头观察、微相分析等方法对云南省罗平县宜石打剖面中三叠统关岭组二段进行了研究,共识别出开阔台地、局限台地（泻湖）、潮坪、台盆、台内滩5种沉积相类型.台内滩的颗粒类型以似球粒、包粒为主,其次为少量鲕粒、生物碎屑,颗粒之间主要为亮晶胶结,局部含少量灰泥,滩体规模较小,指示中等水能量环境,且由下向上具多期发育特征.综合研究表明,中三叠世安尼期Pelsonian亚期早期发生了全球性的海平面上升,导致沉积环境水深迅速增加.研究区沉积相从潮坪相迅速转变为台内盆地相,其相对低能缺氧的环境有利于罗平生物群化石的保存.      

Origin of Cretaceous phosphorites from the onshore of Tamil Nadu,India

Cretaceous phosphorites from the onshore of Tamil Nadu have been investigated for their origin and compared with those in the offshore. Cretaceous phosphorites occur as light brown to yellowish brown or white nodules in Karai Shale of the Uttatur Group in the onshore Cauvery basin. Nodules exhibit phosphatic nucleus encrusted by a chalky shell of carbonate. The nucleus of the nodules consists of light and dark coloured laminae, phosphate peloids/coated grains and detrital particles interspersed between the laminae. Scanning electron microscope (SEM) studies reveal trapping and binding activity of microbial filaments. A mat structure with linearly arranged microbial filaments and hollow, cell-based coccoid cyanobacterial mat are present. Nodules contain abundant carbonate fluorapatite, followed by minor calcite, quartz and feldspar. The P₂O₅ content of the phosphorites ranges from 18 to 26%. The CaO/P₂O₅, Sr and F contents are higher than that of pure carbonate fluorapatite. Concentrations of Si, Al, K, Fe, and Ti are low. We suggest that the nuclei of the nodules represent phosphate clasts related to phosphate stromatolites formed at intertidal conditions. At high energy levels the microbial mats were disintegrated into phosphate clasts, coated with carbonate and then reworked into Karai Shale. On the other hand, Quaternary phosphorites occur as irregular to rounded, grey coloured phosphate clasts at water depths between 180 and 320m on the continental shelf of Tamil Nadu. They exhibit grain-supported texture. Despite Quaternary in age, they also resemble phosphate stromatolites of intertidal origin and reworked as phosphate clasts onto the shelf margin depressions. Benthic microbial mats probably supplied high phosphorus to the sediments. Availability of excess phosphorus seems to be a pre-requisite for the formation of phosphate stromatolites.     

四川广元中二叠统栖霞组似球粒灰岩微相特征及沉积学意义

四川广元上寺中二叠统栖霞组中段发育碳酸盐岩浅滩相.露头观察、镜下分析得出,其颗粒类型中,似球粒、包粒十分丰富,占较大比重.研究层段识别出粪球粒、巴哈马似球粒、微生物似球粒三种类型.其中以巴哈马似球粒最为常见,它是由生物碎屑的泥晶化导致的.基于颗粒类型、沉积组构的详细观察和微相分析,得出研究层段总体沉积环境为似球粒为主的...