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1.
以前的微生物岩识别和分类只根据裸眼可见的中观结构,而对显微镜下的微观结构未予使用。微生物的显微结构有什么基本特征,是否可以用于微生物岩的识别和分类是本文关注的焦点。作者从微生物岩形成机制和实际材料的观察来探讨这个问题。在诱导钙化实验中观察到,蓝藻鞘丝藻Lyngbya的胶鞘表面有碳酸盐矿物形成: 先是在胶鞘表面形成点状碳酸盐颗粒,后来碳酸盐几乎包裹整个丝体,形成一个矿物壳。微生物岩形成的模式推演表明,微生物岩一般都有微生物留下的孔(模孔)和包围模孔的矿物壳2种基本单元。对3个地点的现代微生物碳酸盐岩和1个地点的古代(寒武纪)微生物岩的观察,发现在显微尺度上都具有模孔,此外还可能有矿物壳或胶结物。模孔的形状、大小、排列方式是微生物碳酸盐岩显微结构研究的核心内容,在今后的研究中可以用于微生物岩的识别和分类,以及作为分析形成机制和形成环境的基础。 相似文献
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作为一种缺乏沉积构造的隐晶质块状泥晶所表征的非纹层状微生物碳酸盐岩类型,均一石自从命名之后就很少被描述或自古老的地层记录之中被识别出来,再加上缺乏现代实例,造成了对这种微生物碳酸盐岩类型研究的困难。来自于山东省泗水县圣水峪剖面寒武系苗岭统崮山组中的均一石,以厚层块状、缺乏沉积构造的泥晶灰岩为主,表现为生物层状的微生物礁,与相对较深水的中厚层中至深缓坡相的条带状泥晶灰岩和泥灰岩一起,构成较为典型的潮下型碳酸盐岩米级旋回,集中发育在淹没不整合型三级沉积层序顶部的强迫型海退体系域之中,代表着相对海平面下降阶段的沉积记录。在这些主导着生物层状微生物礁灰岩的均一石中,不均匀地高密度保存着致密缠绕以及较薄的泥晶壁所成型的丝状葛万菌物质体是其最为基本的特征,表明均一石形成于光合作用微生物席复杂的钙化作用。另外,在均一石的致密泥晶组构中,局部发育毫米级至厘米级大小而且形态极端不规则的亮晶显微管状组构。基于其特别的组构特征,将这样的组构解释为具有公共鞘的丝状蓝细菌即念珠菌菌落的钙化作用残余物,其致密泥晶组构可能代表着蓝细菌公共鞘的钙化作用残余物,其中的亮晶显微管则可能为蓝细菌丝状体的钙化作用残余物,而且在这样的组构之中还发育和共生着丝状葛万菌菌落,从而延伸了对蓝细菌菌落钙化作用残余物的认识和解释,并对具这种组构的海绵木乃伊解释提供了另外一种重要的研究镜像。因此,山东省泗水县圣水峪剖面寒武系苗岭统崮山组上部生物层状微生物礁体中的均一石,不仅提供了一个较为典型的寒武纪苗岭世不借助于二氧化碳浓缩作用机制(CCM机制)的蓝细菌钙化作用的实例,而且提供了一个在丝状蓝细菌主导的微生物席之中发育着念珠菌菌落的多重状微生物席的典型实例,从而拓宽了对显生宙最高的大气圈CO2含量以及较高的大气圈O2含量条件下的较为典型的方解石海中与寒武纪后生动物大爆发相平行的蓝细菌繁荣和钙化作用的了解。 相似文献
3.
微生物岩的原始定义指底栖微生物主导形成的沉积体或岩石体。笔者对该概念进行了扩充,认为微生物岩除了包括叠层石、凝块石、纹层石、核形石、均一石之外,还应该包括微生物骨架岩、微生物粘结岩、非钙化浮游或漂浮微生物形成的模铸岩、矿化浮游或漂浮微生物形成的颗粒岩和泥粒岩。P-T界线地层微生物岩的特征是具有由较粗矿物晶体(主要是方解石,其次是白云石)组成的斑点状、树枝状、网状结构的灰岩。这3种结构分别称为斑点状体、树枝状体、网状体,在露头上呈暗色,在薄片中呈浅色,一般由无定形的亮晶充填体和其间的微亮晶组成。亮晶充填体是指无定形的孔洞被不同成岩期形成的矿物充填形成的结构体,因成岩作用各异,造成不同层位、不同地点的亮晶充填体内部的矿物类型和充填顺序存在差异;所有的亮晶充填体都不具有壁,故不是钙化化石。通过形态、大小和生态比较,以及形成演化分析,认为亮晶充填体的前身是漂浮蓝细菌微囊菌,胶鞘是微囊菌形成模铸化石的关键因素。亮晶充填体是表层水漂浮生活的微囊菌沉入海底后,被泥晶沉积物掩埋或者被早期海底胶结物胶结,在泥晶沉积物半固结或固结之后腐烂留下的孔洞被后期成岩作用形成的矿物充填形成的。P-T界线地层微生物岩段顶部遭受成岩作用程度高,树枝状体和网状体中的亮晶充填体的轮廓基本都被破坏,变成微亮晶和亮晶,以前被学者解释为凝块石;但斑点状体、树枝状体、网状体是成岩流体沿着亮晶充填体或其他化石丰富的地方运移形成较粗的晶体而造成的,并不符合凝块石的定义。同时,少数学者把树枝状体本身当成底栖生物,也是没有充分认识树枝状体的矿物组成而做的解释。该微生物岩段含有钙化的小球状化石和同心层状化石,但它们不是组成微生物岩的主体。 相似文献
4.
The Neoarchean Carawine Formation, Hamersley Group, Western Australia is a carbonate ramp that preserves diverse microbial structures which are characteristic of specific depositional environments. These distinctive structures are distributed in five shallow subtidal and two deeper‐water facies in the Oakover area of the Carawine Formation. The shallow subtidal facies are composed of biohermal and bedded stromatolites, centimetre‐scale ridge‐shaped microbialites and wavy‐laminated microbialites. The deeper‐water facies are composed of fenestrate microbialites, planar laminated dolostone and dolostone with rolled‐up microbial laminae. Microbialites in the Carawine Formation lie within a continuum of Archean to Proterozoic microbial facies. Some shallow‐water microbial facies in the Carawine Formation are similar to Proterozoic facies, such as large bioherms internally composed of a variety of stromatolite morphologies. In contrast, fenestrate microbialites grew in quiet subtidal environments and are common in Archean rocks but have not been documented in similar Proterozoic environments. The similarity of shallow‐water facies across the Archean–Proterozoic transition, before and after the oxidation of the atmosphere and surface oceans, indicates that stromatolite growth in shallow subtidal environments was not strongly affected by the chemical changes associated with oxidation of the oceans or by biological responses to those chemical changes. Rather, stromatolite morphology was controlled mostly by the physical environment and the corresponding biological responses to that environment. In contrast, the absence of fenestrate microbialites from Proterozoic deep subtidal environments suggests that the morphology of deep subtidal microbial structures was influenced by chemical or biological changes that occurred in association with oxidation of the surface oceans. 相似文献
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With more than 10 km of total length, Holocene microbialites in Laguna Bacalar, Mexico, belong to the largest freshwater microbialite occurrences. Microbialites include domes, ledges and oncolites. Domal forms can grow to diameters and heights of 3 m. Microbialites are composed of low magnesium calcite which is, to a large extent, precipitated due to the metabolic activity of the cyanobacteria Homeothrix and Leptolyngbya, and associated diatoms. Photosynthesis removes carbon dioxide and triggers carbonate precipitation. Also, an elevated carbonate concentration in lagoon waters, derived from dissolution of Cenozoic limestone in a karst system, supports carbonate precipitation. Trapping and binding of detrital grains is also observed, but is not as common as precipitation. Bacalar microbialites are largely thrombolitic, however, stromatolitic sections occur as well. The bulk of Bacalar microbialites probably formed in the Late Holocene (ca 1 kyr BP until present). According to 14C dating, microbialites accreted 9 to 8 cal kyr BP; however, these ages may be too old as a result of a strong hard water effect. This effect is seen in 14C ages of living bivalve and gastropod mollusc shells from Bacalar Lagoon, which are 8 to 7 cal kyr BP. The modern associated fauna of microbialites is characterized by low diversity and high abundance of the bivalve mollusc Dreissena sp. and the gastropod Pomacea sp. The abundant grazing gastropods presumably hamper modern microbialite formation. A comparison of Bacalar microbialites with other modern microbialite occurrences worldwide shows only a few patterns: sizes, shapes, microbial taxa, mineralogy, type of accretion and settings including water properties of microbialite occurrences exhibit high variability. A trend may be seen in the grazing metazoa, which are rare to absent in the marine and brackish examples, but apparently present in all the freshwater occurrences of microbialites. Also, freshwater examples are usually characterized by elevated concentrations of carbonate and/or calcium ions in the surrounding waters. 相似文献
6.
巨量天然气水合物广泛赋存于大陆坡海底和高纬度永久冻土带沉积物中,是重要的环境影响因素。有些地质历史时期发生的重大地质灾变可能是由天然气水合物分解释放甲烷引起的。持续时间短暂且显著的全球性海相和陆相碳同位素的负漂移、甲烷起源的极负碳酸盐岩碳同位素值(特别是δ13C<-40‰)、碳酸盐岩溶解、指示甲烷浓度异常的生物标志化合物、生物成因重晶石富集、类似冷泉碳酸盐岩发育的特殊沉积组构、海底垮塌及碳酸盐结晶扇(如文石)等是支持天然气水合物分解“假说”的主要证据。但这些证据还存在一定的局限性,该“假说”仍然存在较大的争论,因此需要进一步深入研究。 相似文献
7.
二叠纪末生物大灭绝后,在华南地区碳酸盐岩台地上普遍形成一套微生物岩。微生物岩所代表的是一种以蓝细菌和其他底栖生物为主体的独特的生态系。蓝细菌作为生产者是该生态系的基础,种类单调的小型腹足类和介形虫等广盐性多细胞生物和灾后残存的有孔虫则是该生态系中的主要消费者。根据所处的古地理位置不同,华南微生物岩的分布可分为3个主要地区,即上扬子地区、中下扬子地区及南盘江地区。上述3个地区微生物岩分别产出在生物礁顶、一般碳酸盐岩台地边缘和孤立碳酸盐岩台地环境,且不同地区微生物岩所在剖面的沉积序列也存在差异。根据不同地区微生物岩在宏观沉积构造和碳酸盐岩微相组合上的差异,可进一步划分为层纹状微生物岩、叠层石微生物岩、花斑状微生物岩、树枝状微生物岩及穹窿状微生物岩。不同微相类型的微生物岩在沉积水深、蓝细菌化石、底栖群落组合上仍存在差异,其中树枝状和穹窿状微生物岩沉积时所处水深最小,花斑状微生物岩和层纹状微生物岩沉积时所处水深可能相对较大。对不同地区微生物岩的研究为认识二叠纪末全球事件期间海洋环境的多样性和演化过程提供了重要的信息。 相似文献
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微生物碳酸盐岩是当前沉积学的研究热点之一,但关于古近系湖相微生物碳酸盐岩的报道还非常少。为探讨山东平邑盆地古近系官庄组中段湖相微生物碳酸盐岩的形成机理,笔者进行了野外实测以及室内偏光显微镜和扫描电镜观察。研究结果认为：（1）研究区微生物碳酸盐岩主要有核形石、叠层石和凝块石;（2）在核形石、叠层石和凝块石内部均发现了球状微生物化石、席状体和凝块状泥晶等微生物成因的显微组构,表明了它们形成过程中共同的生长、粘结和钙化等微生物作用;（3）微生物作用对环境变化比较敏感,水体较小、变化频繁的湖泊环境,对微生物碳酸盐岩结构和形态的发育有很大影响;核形石、叠层石和凝块石的形成环境大致呈现出水动力依次减弱、陆源碎屑物质依次减少、水深依次加大的规律;（4）提出了研究区微生物碳酸盐岩的成因模式,认为这3种微生物碳酸盐岩正是以微生物活动为共同基础,在不同的环境条件下形成的。 相似文献
10.
Development of microbial carbonates in the Lower Cretaceous Codó Formation (north‐east Brazil): Implications for interpretation of microbialite facies associations and palaeoenvironmental conditions 
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下载免费PDF全文 Anelize M. Bahniuk Sylvia Anjos Almério B. França Nilo Matsuda John Eiler Judith A. Mckenzie Crisogono Vasconcelos 《Sedimentology》2015,62(1):155-181
The study of microbial carbonates has acquired new significance with the recognition that they retain valuable information related to biomineralization processes associated with microbial activity throughout geological time. Additionally, microbialites have a demonstrated economic potential to serve as excellent hydrocarbon reservoirs. The Lower Cretaceous Codó Formation, located in the Parnaiba Basin of north‐east Brazil, comprises a unique stratigraphic sequence of up to 20 m thick, well‐preserved carbonate microbialites. Deposited in a continental basin during the initial break up and separation of South America from Africa in the Early Cretaceous, this lacustrine carbonate sequence provides an excellent example to investigate the palaeoenvironmental conditions controlling microbialite facies development. Based on macroscopic and microscopic observations of outcrop and drill core samples, four microbialite facies (stromatolite, lamina, massive and spherulite) were defined and distinguished by textures and microbial fossil content. Changes in facies type are related to alternating palaeo‐water depths, as reflected by 87Sr/86Sr cycles resulting from fluctuations in the sources of meteoric water. Clumped isotope measurements of stromatolitic fabrics yield precipitation palaeo‐temperatures with an average value of 35°C. The δ18O values of bulk carbonate (?6·8 to ?1·5‰ Vienna Pee Dee Belemnite) imply precipitation from water with calculated δ18O values between ?1·6‰ and 1·8‰ Vienna Standard Mean Ocean Water, reflecting precipitation from variably modified meteoric waters. The δ13C values of bulk carbonate (?15·5 to ?7·2‰ Vienna Pee Dee Belemnite) indicate a significant input of carbon derived from aerobic or anaerobic respiration of organic matter. Combined, the data indicate that the evolution of the Codó Formation occurred in a closed lacustrine palaeoenvironment with alternating episodes of contracting and expanding lake levels, which led to the development of specific microbialite facies associations. The results provide new insights into palaeoenvironmental settings, biogenicity and early diagenetic processes involved in the formation of ancient carbonate microbialites and, by extension, improve the knowledge of the reservoir geology of correlative units in deep waters offshore Brazil. 相似文献
11.
微生物碳酸盐岩油气储层研究现状与展望 总被引：10，自引：5，他引：5
微生物碳酸盐岩是一种重要的油气储集岩。在中国古老深埋碳酸盐岩地层中它们是主要的岩石类型。微生物碳酸盐岩通常可划分为叠层石、凝块石、树枝石、均一石和核形石等五种类型。储层孔隙系统与微生物岩的沉积结构和构造密切相关,窗格孔（洞）和格架孔（洞）是主要储集空间类型。全球中新元古界至中生界都发现了以微生物碳酸盐岩为储层的油气田,资源潜力巨大。 微生物岩储层的岩石类型、沉积构造、相序结构、沉积模式和储层有利相带尚有许多不明晰之处,是今后进一步深化研究的重点方向。 相似文献
12.
Microbially mediated carbonate precipitation in a hypersaline lake,Big Pond (Eleuthera,Bahamas) 总被引：1，自引：0，他引：1
CHRISTINA GLUNK CHRISTOPHE DUPRAZ OLIVIER BRAISSANT KIMBERLEY L. GALLAGHER ERIC P. VERRECCHIA PIETER T. VISSCHER 《Sedimentology》2011,58(3):720-736
Microbial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability; this can result in the precipitation of carbonate minerals and thus the formation of microbialites. Here, the microbial metabolic activity, the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake, Big Pond, Bahamas, were determined to identify the driving forces in microbialite formation. A conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented. Geochemical modelling indicated an oversaturation with respect to carbonates (including calcite, aragonite and dolomite), but these minerals were never observed to precipitate at the mat–water interface. This failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium. A layer of high Mg‐calcite was present 4 to 6 mm below the surface of the mat, just beneath the horizons of maximum photosynthesis and aerobic respiration. This carbonate layer was associated with the zone of maximum sulphate reduction. It is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface (i.e. the cyanobacterial layer) of the mat bind calcium. Both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances (each process accounting for approximately half of the total consumption) and low molecular weight organic carbon, liberating calcium and producing inorganic carbon. The combination of these geochemical changes can increase the carbonate saturation index, which may result in carbonate precipitation. In conclusion, the formation and degradation of extracellular polymeric substances, as well as sulphate reduction, may play a pivotal role in the formation of microbialites both in marine and hypersaline environments. 相似文献
13.
针对现有微生物碳酸盐岩分类不够系统、岩石类型及组合的环境意义不明确、岩相古地理重建缺乏微生物碳酸盐岩沉积模式指导等科学问题,通过塔里木盆地、四川盆地和鄂尔多斯盆地2个元古宇剖面和3个显生宇剖面详细的岩类学和岩石组合序列研究,取得3项成果与认识:①建立了构造尺度和形态特征相结合的系统的微生物碳酸盐岩分类方案。②明确了微生物碳酸盐岩岩石类型及组合的环境意义。风暴浪基面之下远端以欠补偿黑色泥岩和硅质岩沉积为主,近端的下斜坡以具丘状结构的纹理石碳酸盐岩和灰泥丘建造为主;风暴浪基面和正常浪基面之间的上斜坡以小-中型泡沫状、团块状凝块石碳酸盐岩和小-中型柱状、锥状、穹窿状叠层石碳酸盐岩建造为主;正常浪基面与平均低潮线之间以大-中型波状叠层石碳酸盐岩建造为主;平均低潮线之上的潮坪环境以大-中型层(席)状微生物碳酸盐岩、丘状微生物碳酸盐岩建造为主。③建立了缓坡沉积体系和镶边沉积体系微生物碳酸盐岩沉积模式。这些认识对微生物碳酸盐岩岩相古地理重建和储层分布预测具重要的指导意义。 相似文献
14.
Processes of carbonate precipitation in modern microbial mats 总被引：20，自引：0，他引：20
Christophe Dupraz R. Pamela Reid Olivier Braissant Alan W. Decho R. Sean Norman Pieter T. Visscher 《Earth》2009,96(3):141-162
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. 相似文献
15.
We present the first study of micro‐crustaceans (ostracods) associated with microbial crusts in the aftermath of the most devastating extinction, the end‐Permian extinction (EPE). These post‐extinction microbialites dominated shallow shelf marine environments and were traditionally considered as devoid of any associated fauna. We present a micro‐palaeontological analysis of a large record from microbial and non‐microbial settings following the EPE. This dataset documents the proliferation of ostracods strictly associated with microbialites. Based on the diet of extant ostracods and uniformitarianism, we propose that the abundant microbes in the mats served as an unlimited food supply. Photosynthetic cyanobacteria may also have locally provided oxygen under low oxygen conditions interpreted by others for the microbialites. Microbialites provided a specialised environment that may have acted as refuge for ostracods in the immediate aftermath of the EPE. The surviving faunas may have been progenitors for the starting of the latter radiation. 相似文献
16.
Currently, sedimentologists focus on the challenging issue of microbial carbonates, which are regarded as "one of the sedimentary rocks most difficult to study", having complicated sedimentary fabric. Their characteristic features closely related to microbial activity, distributed over a long period of geological time, and formed in diversified sedimentary environments. The main research concentrations are the calcified microbial mats and biofilms in geological records as the products of lithification and diagenesis. Starting from the origin, this paper systematically reviewed and explained the processes dwelling within two types of microbial communities, the thinner biofilm and the thicker microbial mat, which enabled them to convert into microbial carbonates through biomineralization and lithification. This study proposed that the existence of multiple microbial mats was another important cause for the diversification and complexity of microbial carbonates in addition to its complex depositional process. Moreover, the sedimentary characteristics and classification of different types of microbial carbonates were reviewed, exemplifying the Cambrian microbial carbonates in the North China Platform. These microbial carbonates are suggested to be placed under "bindstone" after Embry and Kloven, which can be further divided into 5 types, stromatolites, thrombolites, oncolites, laminites and leiolites. Dendrolite is not categorized as a separate class, instead attributed to thrombolites. The microbial carbonates may possess good source rock potential because of the enriched organic content, and may also serve as hydrocarbon reservoirs because of certain microbial textures and fabrics leading to significant porosity and permeability. Because of their biomineralization processes related to microbial activity, the microbial carbonates are not only an important window to understand the evolution of the earth's surface environment, but also capable of forming large-scale reservoirs, and their scientific and economic values are self-evident. 相似文献
17.
Mohsen Layeb Moez Ben Fadhel Yosra Layeb-Tounsi Mohamed Ben Youssef 《Arabian Journal of Geosciences》2014,7(8):3349-3363
From northern Tunisia, small-scale well-preserved microbialites, contemporaneous to the global oceanic anoxic event 2 (OAE 2) are first reported on the southern Tethyan Margin. These microbialites are encased within the pelagic organic-rich black shales of the Bahloul Formation (Cenomanian–Turonian transition). Biostratigraphic, petrographic, and geochemical investigations carried out to constrain their biogenicity and genesis character led to consider them as thrombolites and stromatolites occurring in lenticular bioherms/biostromes and columnar bodies co-relatable to the global ‘filament event’ of the authors, close to the base of the Watinoceras ammonite zone. Abundant clotted micrite, cyanobacterial filaments, and algal tissues point to the key process of microbial carbonate precipitation and to a major role played by microbes in the stabilisation and subsequent lithification, which in turn favoured the preservation of the original structure of the microbialites. These microbially induced carbonate formations are considered as favoured by chemosynthetic fauna of bivalve molluscs and lithistid sponges which were able to host symbiotic microbial communities. The latter contributed to the precipitation of authigenic calcite and non-carbonate minerals (e.g. pyrite) fuelled by microbial activity under sulphate-reducing conditions. The carbonate body onset is considered to be initiated by seafloor instability due to syndepositional fault acting that induced the appraisal of uplifted tilted blocks within oxygenated waters but near the anoxic water masses. Generated depressions allowed the development of chemosynthetic-based communities. Deep faults related to Triassic salt domes acted as conducts for hydrocarbon and salt brine expulsion to the seafloor and the microbialite growth was enhanced by an abrupt uprising sea level under warmer conditions. 相似文献
18.
寒武纪初期不仅发生了宏体生物大爆发,而且也出现了地质历史时期少见的蓝细菌鞘体大规模钙化事件。埃迪卡拉纪—寒武纪之交海水化学性质的转变对真核生物的演化起到了重要作用,但是这种转变对微生物岩发育特征以及蓝细菌钙化事件的产生有无影响,目前尚不明确。鉴于此,对华南上扬子北缘汉南—米仓山地区上埃迪卡拉统—寒武系第二统多个典型微生物岩发育剖面进行了系统野外调查和室内岩石学分析,结果表明:上埃迪卡拉统灯影组叠层石广泛发育,以平铺状、缓波状特征为主,而凝块石既可以呈补丁状分布于叠层石纹层间,又可以呈细小的凝絮状、粘结状特征构成厚层岩层;寒武系第二统仙女洞组叠层石丰度显著降低,以高大、坚硬的丘状隆起为特点,包括单独的凝块石丘,以及微生物与古杯的联合建丘。虽然寒武系第二统微生物岩的层状结构和凝块结构与埃迪卡拉系相比并无太大差异,但是寒武系微生物岩内部保存有大量的钙化微生物化石,已识别出附枝菌(Epiphyton)、肾形菌(Renalcis)和葛万菌(Girvanella)等多种类型。在收集、整理前人有关微生物岩特征和发育资料的基础上,本次研究初步整理出华南寒武系第二统微生物岩的时空分布特点,发现寒武纪第二世第三期是钙化微生物大量发育的一个时期,在随后的第四期达到一个小的高峰。对于此次蓝细菌钙化作用幕的启动机制,除前人提出的海水高钙离子浓度和蓝细菌体内二氧化碳浓缩机制等认识外,寒武纪早期海水性质的转变(方解石质原生矿物受成岩改造程度较低)、适度的陆源碎屑输入(黏土组分保护作用)也有利于钙化微生物结构的保存,应引起重视。 相似文献
19.
Microbial deposits in the aftermath of the end‐Permian mass extinction: A diverging case from the Mineral Mountains (Utah,USA) 下载免费PDF全文
下载免费PDF全文 Emmanuelle Vennin Nicolas Olivier Arnaud Brayard Ivan Bour Christophe Thomazo Gilles Escarguel Emmanuel Fara Kevin G. Bylund James F. Jenks Daniel A. Stephen Richard Hofmann 《Sedimentology》2015,62(3):753-792
The Lower Triassic Mineral Mountains area (Utah, USA) preserves diversified Smithian and Spathian reefs and bioaccumulations that contain fenestral‐microbialites and various benthic and pelagic organisms. Ecological and environmental changes during the Early Triassic are commonly assumed to be associated with numerous perturbations (productivity changes, acidifica‐tion, redox changes, hypercapnia, eustatism and temperature changes) post‐dating the Permian–Triassic mass extinction. New data acquired in the Mineral Mountains sediments provide evidence to decipher the relationships between depositional environments and the growth and distribution of microbial structures. These data also help to understand better the controlling factors acting upon sedimentation and community turnovers through the Smithian–early Spathian. The studied section records a large‐scale depositional sequence during the Dienerian(?)–Spathian interval. During the transgression, depositional environments evolved from a coastal bay with continental deposits to intertidal fenestral–microbial limestones, shallow subtidal marine sponge–microbial reefs to deep subtidal mud‐dominated limestones. Storm‐induced deposits, microbialite–sponge reefs and shallow subtidal deposits indicate the regression. Three microbialite associations occur in ascending order: (i) a red beds microbialite association deposited in low‐energy hypersaline supratidal conditions where microbialites consist of microbial mats and poorly preserved microbially induced sedimentary structure; (ii) a Smithian microbialite association formed in moderate to high‐energy, tidal conditions where microbialites include stromatolites and associated carbonate grains (oncoids, ooids and peloids); and (iii) a Spathian microbialite association developed in low‐energy offshore conditions that is preserved as multiple decimetre thick isolated domes and coalescent domes. Data indicate that the morphologies of the three microbialite associations are controlled primarily by accommodation, hydrodynamics, bathymetry and grain supply. This study suggests that microbial constructions are controlled by changes between trapping and binding versus precipitation processes in variable hydrodynamic conditions. Due to the presence of numerous metazoans associated with microbialites throughout the Smithian increase in accommodation and Spathian decrease in accommodation, the commonly assumed anachronistic character of the Early Triassic microbialites and the traditional view of prolonged deleterious conditions during the Early Triassic time interval is questioned. 相似文献
20.
Ó. Cabestrero M. E. Sanz-Montero L. Arregui S. Serrano P. T. Visscher 《Aquatic Geochemistry》2018,24(1):79-105
Interactions of the microbial mat community with the sedimentary environment were evaluated in two shallow, ephemeral lakes with markedly different hydrochemistry and mineralogy. The characterization of growing and decaying microbial mats by light microscopy observations and fluorescence in situ hybridization was complemented with biogeochemical and mineralogical measurements. The lakes studied were Eras and Altillo Chica, both located in Central Spain and representing poly-extreme environments. Lake Eras is a highly alkaline, brackish to saline lake containing a high concentration of chloride, and in which the carbonate concentration exceeds the sulfate concentration. The presence of magnesium is crucial for the precipitation of hydromagnesite in microbialites of this lake. Altillo Chica is a mesosaline to hypersaline playa lake with high concentrations of sulfate and chloride, favoring the formation of gypsum microbialites. Differences in the microbial community composition and mineralogy of the microbialites between the two lakes were primarily controlled by alkalinity and salinity. Lake Eras was dominated by the cyanobacterial genus Oscillatoria, as well as Alphaproteobacteria, Gammaproteobacteria and Firmicutes. When the mat decayed, Alphaproteobacteria and Deltaproteobacteria increased and became the dominant heterotrophs, as opposed to Firmicutes. In contrast, Deltaproteobacteria was the most abundant group in Lake Altillo Chica, where desiccation led to mats decay during evaporite formation. In addition to Deltaproteobacteria, Cyanobacteria, Actinobacteria, Alphaproteobacteria and Gammaproteobacteria were found in Altillo Chica, mostly during microbial mats growth. At both sites, microbial mats favored the precipitation of sulfate and carbonate minerals. The precipitation of carbonate is higher in the soda lake due to a stronger alkalinity engine and probably a higher degradation rate of exopolymeric substances. Our findings clarify the distribution patterns of microbial community composition in ephemeral lakes at the levels of whole communities, which were subjected to environmental conditions similar to those that may have existed during early Earth. 相似文献