共查询到20条相似文献，搜索用时 31 毫秒
1.
Pradip Samanta Soumik Mukhopadhyay Anudeb Mondal Subir Sarkar 《Journal of Asian Earth Sciences》2011,40(2):542-549
Ubiquitous microorganisms, especially cyanobacteria preferably grow on the sediment surface thereby producing microbial mats. In the absence of grazers and bioturbators, microbial mat is a unique feature of the Proterozoic. Most of the papers so far published described a wide variety of bed surface microbial mat structures with rare illustrations from sections perpendicular to bedding. Nonetheless, bed surface exposures are relatively rare in rock records. This limitation of bed surface exposures in rock records suggest that a study of microbial mats in bed-across sections is needed. The 60 m thick coastal marine interval of the Sonia Sandstone Formation is bounded between two terrestrial intervals, a transgressive lag at the base and an unconformity at the top, and has been chosen for exploration of microbial mat structures in bed-across sections. A wide variety of microbial mat-induced structures in bed-across sections are preserved within the coastal interval of the Sonia Sandstone. Though many of these structures are similar in some aspects with bed surface structures, some of those presented here are new. The palaeogeographic range of these microbial structures extends from supralittoral to neritic. Diagenetic alterations of microbial mats produce pyrite and those zones are suitable for the preservation of microbial remains. SEM and EDAX analyses show fossil preservation of filamentous microbial remains that confirm the presence of microbial mats within the coastal interval of the Sonia Sandstone. Effects of proliferation of microbial mats in the siliciclastic depositional setting are numerous. The mat-cover on sediment surfaces hinders reworking and/or erosion of the sediments thereby increases the net sedimentation rate. Successive deposition and preservation of thick microbial mat layer under reducing environments should have a great potential for hydrocarbon production and preservation and therefore these Proterozoic formations could be a target for exploration. 相似文献
2.
从席底构造到第五类原生沉积构造:沉积学中具有重要意义的概念 总被引：26，自引：8，他引：18
发育微生物席或微生物膜的沉积底被形象地称为席底,在席底上由于微生物席的发育使沉积物富黏性而抗水流改造,并在沉积面和沉积层内形成一系列奇形怪状的原生沉积构造,如变余波痕、皱饰构造、微生物席碎片、纺锤状或次圆状收缩裂缝等等,这些构造都归为席底构造;与席底相对应,那些不发育微生物席或微生物膜的沉积面则称为混合底,在混合底上单个沉积颗粒受到水流剪切与变形作用的改造而形成诸如正常波痕等原生沉积构造,这些沉积构造则归为混合底构造。席底和席底构造的概念很重要,用来专指微生物沉积作用的产物而区别于混合底和混合底构造。从沉积学概念的演变方面分析,混合底构造就是1964年Pettijohn和Potter所定义的原生沉积构造,他们曾经将其归为4大类并且沿用至今;席底构造也就是微生物形成的(或相关的)原生沉积构造,由于其在形成机理和形态上与Pettijohn和Potter所定义的原生沉积构造存在较大的区别,Noffke 2001年认为其应该具有合适的分类位置,所以就将席底构造归为“第五类原生沉积构造”并纳入Pettijohn和Potter 1964年的原生沉积构造的分类体系之中。从席底构造到第五类原生沉积构造,概念的演变表明了万能的微生物在沉积作用过程中的重要性以及沉积学家对微生物沉积作用认识的深入。这些相关的沉积构造及其所形成的席底,就像叠层石(被当作正生长原生沉积构造)一样在前寒武纪地层之中较为普遍。随着后生动物的大规模出现而造成的寒武纪“农艺革命”(Agronomic revolution)的开始,显生宙的席底及其相关的席底构造多出现在那些不适宜后生动物居住的地方并大多被混合底及其相关的混合底构造所替代;因此,与微生物相关的原生沉积构造,除熟知的叠层石以外,在席底上发育的一系列原生沉积构造,由于其具有特别的指相意义并有助于古环境重塑而被很自然地归到“第五类原生沉积构造”。该类构造由于其在前寒武纪的普遍性和显生宙的局限分布性,特别是在生物大灭绝事件之后的灾难性泛滥,对于研究地球表层环境的演变具有重要意义。所以说,从席底构造到第五类原生沉积构造,是近年来沉积学发展过程中所出现的较为重要的基本概念。根据这一重要概念,对前寒武纪地层中一些常常被认为是遗迹化石、长期以来存在的“臼齿状构造谜”(Molar-tooth structure enigma)等奇形怪状的沉积构造的成因以及从前寒武纪到显生宙地层中均有分布的特殊的纹理化构造等,将会得到一些更加合理的认识;另一方面,关于微生物形成的原生沉积构造的研究,对于了解地球表层生物变化与沉积变化之间的复杂关系将会发现更多的新问题而得到进一步的思考与研究。 相似文献
3.
埃迪卡拉纪以来,微生物与后生动物之间呈此消彼长的关系,但在寒武纪苗岭世却出现了微生物成因沉积构造和后生动物扰动构造短暂共存的现象。笔者在豫西馒头组二段下部识别出包括皱饰构造与微生物席裂构造在内的2种微生物成因沉积构造,对皱饰构造与遗迹化石的共生关系进行了分类,建立了席上足辙迹(Monomorphichnus henanensis)与牧食迹(Jinningichnus badaowanensis)、席下水平漫游迹(Planolites montanus)、席下深层泥岩中生物扰动共3种微生物席与后生动物的共生模式和生态演化模型。上述研究表明,具有特殊环境耐受性的“机会主义”动物在食物来源较为宽松的潮坪环境中与微生物席共存,这种微生物席与后生动物短暂而“和谐”的共存关系不仅延续了埃迪卡拉纪双方的部分共生特征,且在以混合底为主导的显生宙生态环境中得到了进一步变化与发展。 相似文献
4.
臼齿状构造（Molar—tooth structure）是一种主要发育在前寒武纪非叠层石碳酸盐岩中的沉积构造。由于形态多样且缺乏现代的类比物,国内外地质学家曾先后建立多种模式来解释其成因,这些模式大致可以归为两类：地震液化模式和生物成因模式。近年来,一种新的模式：气泡膨胀与迁移模式得到了地质学家的广泛引用。产在天津蓟县剖面高于庄组第三段非叠层石碳酸盐岩沉积序列上部的臼齿状构造,虽然可以应用气泡膨胀与迁移模式来解释其形成机制,但是,考虑到臼齿状构造可能是微生物席封闭沉积面所激发的气泡膨胀与迁移作用的产物,并且气泡可能是由于细粒沉积中微生物提供的有机质的裂解作用的结果,表明了臼齿状构造的形成间接地与微生物生命活动存在关联。因此,可以将臼齿状构造归为席底构造（第五类原生沉积构造）的一种类型。 相似文献
5.
The Tandilia Belt in northeast Argentina includes a Neoproterozoic sequence of sediments (Sierras Bayas Group), in which the Cerro Largo Formation, ca. 750 Ma in age, forms a siliciclastic, shallowing upward succession of subtidal nearshore to tidal flat deposits. Trace fossils Palaeophycus isp. and Didymaulichnus isp. have been described from the upper part of this succession. Specific sedimentary structures consisting of round-crested bulges, arranged in a reticulate pattern, and networks of curved cracks are associated with the trace fossils. These structures are considered to be related to epibenthic microbial mats that once colonized the sediment surface. They reflect stages of mat growth and mat destruction, if compared to analogous structures in modern cyanobacterial mats of peritidal, siliciclastic depositional systems. Also the trace fossils are interpreted as mat-related structures, partly forming components of networks of shrinkage cracks, partly representing the upturned and involute margins of shrinkage cracks or circular openings in desiccating and shrinking, thin microbial mats.
The definition of Didymaulichnus miettensis Young as a Terminal Proterozoic trace fossil is questioned, and it may be considered to interpret the ‘bilobate’ structure as the upturned, opposite margins of microbial shrinkage cracks which have been brought back into contact by compaction after burial. 相似文献
6.
微生物通过自身生命活动可以在硅质碎屑物上形成微生物席.微生物席具有黏结性、柔韧性、抗剥蚀性等特征,与物理营力相互作用可以形成一系列独特的原生沉积构造,即微生物诱发的沉积构造(MISS).形成和保存MISS的基本条件包括经历较低的变质程度、发育于海退—海侵的转折点、出现在有利的“微生物席沉积相”中.微生物席本身很难保存在岩石序列中,所以识别石化MISS具有一定难度.与现代环境中发育的MISS具有相似的几何形态和包含古代微生物席组构是识别石化MISS的有效途径.微生物生长、稳化、障积、捕获和黏结作用可以形成形态各异的MISS,根据这些作用和特征可以将其划分为5大类17小类.对保存在硅质碎屑岩中的微生物诱发的原生沉积构造和现代潮坪中发育的MISS进行综合对比研究,有利于微生物席沉积学的发展,也为进一步理解古代硅质碎屑岩沉积环境提供重要线索. 相似文献
7.
贺兰山地区中元古代微生物席成因构造 总被引：23，自引：2，他引：21
贺兰山中段中元古界黄旗口组石英砂岩中发现丰富的微生物席成因构造(MISS),包括由微生物席生长、破坏和腐烂过程形成的3种类型、9种不同形态的构造;与华北大红峪组发现的同类构造在成因类型与多样性方面具有很强的可对比性.砂岩中发育双向交错层理、冲洗层理、高角度单斜层理系和波痕,泥质粉砂岩夹层中发育波痕与泥裂,表明微生物席主要发育于潮间带上部至潮上带下部环境.MISS构造在华北地台长城系下部砂岩中的广泛存在表明在1.6 Ga前以蓝细菌为主的微生物群在环潮坪碎屑环境也很活跃.可能代表了微生物由海洋向陆地环境发展的过渡阶段.具光合作用功能的制氧蓝细菌的蓬勃发展可能是引发中元古代海洋化学条件发生转变、含氧量增高的重要原因,并为真核生物及宏观藻类的兴起创造了条件.研究表明,黄旗口组与华北大红峪组大致同时,反映了Columbia超大陆裂解期华北地台开始拉伸-张裂、缓慢沉降的构造古地理背景. 相似文献
8.
Peter Sjövall Volker Thiel Sandra Siljeström Christine Heim Tomas Hode Jukka Lausmaa 《Geostandards and Geoanalytical Research》2008,32(3):267-277
Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) is a powerful method for the chemical analysis of solid surfaces. In this paper, the capabilities and limitations of this technique and the potential for its use in geochemical research are outlined. Using ToF‐SIMS, the chemical composition of sample structures down to 10–100 μm can be determined, without the need for pre‐selection or labelling of the analysed substances. In addition, the lateral distribution of organic and inorganic compounds can be mapped in geochemical samples at a resolution in the micrometre range. The capabilities of the technique in geochemistry are illustrated by two examples. In the first example, it is shown that ToF‐SIMS can be used to detect biomarkers in oil samples, making it a promising method for the analysis of biomarkers in fluid inclusions. In the second example, a number of specific lipid biomarkers were identified and mapped on the surface of a microbial mat cryosection surface. Post‐measurement optical microscopy correlated the localisation of the lipids with the presence of methanotrophic archaea in the microbial mat. 相似文献
9.
前寒武纪海侵砂岩中的微生物砂质碎片：以北京南口虎峪剖面大红峪组为例 总被引：5，自引：0，他引：5
碎屑岩中微生物形成的原生沉积构造,由于缺乏直接的微生物证据,其识别常常比较困难。位于北京西北郊的南口虎峪一带,古元古代晚期的大红峪组(1650～1600Ma),海侵体系域砂岩地层与高水位体系域砂泥质白云岩和叠层石白云岩组成了较为有规律的沉积相序特征,成为将大红峪组划分为三个三级层序的主要标志。在大洪峪组的海侵体系域砂岩层中,一种类似于碳酸盐岩中的“内碎屑”的奇特的砂质碎片,代表了前寒武纪海侵砂岩沉积面上曾经发育过微生物席;该砂质碎片与变余波痕和皱饰构造组成系列席底构造,特殊的形态和产出特点成为窥视前寒武纪砂岩中微生物席发育的一种典型标志。因此,南口虎峪剖面大红峪组海侵砂岩中的微生物砂质碎片及其相关的席底构造的发现,有益于今后对类似沉积构造的阐释与研究,特别是那些常常被厘定为前寒武纪古老的后生动物遗迹化石的沉积构造;同时也表明,在前寒武纪地层之中,除了要关注叠层石之类的微生物沉积构造以外,碎屑岩中也存在微生物活动的若干证据即席底构造(第五类原生沉积构造),这些特别的沉积构造对前寒武纪沉积环境重塑具有重要意义。 相似文献
10.
Modern back-barrier tidal flats of Galveston Island, Follets Island, and Matagorda Peninsula of the Texas coast are dominated by mud- to fine sand-sized siliciclastic sediments and prolific microbial mats. These microbial mats modify sediment behavior and result in a variety of microbially induced sedimentary structures (MISS). Common structures include: knobby surfaces, reticulated surfaces, gas domes, mat-cracks, sieve-like surfaces, erosional pockets, wrinkles, and mat chips. In general, mat thicknesses increase from ～ 1 mm in the upper supratidal to ～ 3 cm (maximum) in the lower supratidal and then decrease to ～ 2 mm in the lower intertidal areas. This same wedge-shaped pattern is displayed by detailed measurements of mat thicknesses from the rims into the deeper centers of depressions (pools) on the supratidal flats. Measurements of 175 mat-cracks show that height of the curled edges of the mat-crack polygons increases with increase in mat thickness. Similarly, measurements of 150 gas domes reveal that the size of the gas domes also increases with increasing thickness of the associated mats. Because mat thickness varies with elevation on the tidal flat, curl height of the mat-cracks and size of the gas domes are also related to elevation.Six zones were identified based on the occurrence of MISS within the supratidal (zone-I) to upper subtidal (zone-VI) areas. At the highest elevation, knobby surfaces characterize zone-I whereas zone-II is defined by reticulated surfaces. Along with reticulated surfaces, gas domes and mat-cracks characterize zone-III and zone-IV, respectively. Association of sieve-like surfaces with mat-cracks typifies zone-V whereas mat deformation structures and sieve-like surfaces define zone-VI. Boundaries between the MISS-zones in general are parallel and related to tidal zones. The distribution patterns of the MISS-zones are strongly controlled by local topography of the sediment surface because the degree of inundation is the primary controlling factor for the mat growth and resultant MISS. Therefore, distribution of the microbially induced sedimentary structures in siliciclastics, along with the dimension of the mat-cracks and gas domes, can be potentially helpful in interpretation of topography of paleodepositional surfaces. 相似文献
11.
辽宁兴城中元古界常州沟组砂岩中微生物成因沉积构造 总被引：2，自引：1，他引：1
辽宁兴城夹山中元古界常州沟组潮间带相砂岩层面产出裂隙状构造。这些构造由0.8~3 cm长的纺锤状短裂隙和超过7 cm的长延伸裂隙组成,孤立为主,部分相连。通过与类似的层面裂隙和现代微生物席干缩裂隙的比较分析,将其解释为微生物成因沉积构造分类体系中的收缩裂隙（Shrinkage crack）,并参考席裂多边形（Mat-crack polygon）的术语称其为席裂（Mat-crack）。微生物席在脱水不充分的情况下形成纺锤状席裂,脱水充分的情况下形成席裂多边形。这些构造在常州沟组的出现,表明微生物生态系统在华北克拉通中元古代海侵初期已经开始影响古海岸带的地质作用,这对于恢复燕山裂陷槽盆地的生物—沉积作用和环境演化具有重要意义。 相似文献
12.
燕山东部天津蓟县至河北兴隆一带中元古界长城群细粒碎屑岩中,普遍发育有在层面上表现为纺锤状裂缝的沉积构造,并曾经被解释为后生动物遗迹化石或地震震荡液化脉等。这种沉积构造常与变余波痕、皱饰构造等相伴产出。根据初步观察与研究的结果推断,纺锤状裂缝可能是由于微生物席对沉积面的封闭作用,导致沉积物在成岩作用早期发生脱气与脱水等作用的产物,因此可以将其归入微生物参与形成的原生沉积构造(席底构造)的范畴。纺锤状裂缝与其他的席底构造一样,为前寒武纪的沉积环境重塑提供了一个重要的证据,同时也说明在前寒武纪的地球表层环境中,微生物以其新陈代谢的多样性及巨大的生物量,在沉积纪录中留下了重要的烙印。 相似文献
13.
Enterolithic structures are stratigraphically localized folds in gypsum beds found in certain saline evaporitic sedimentary units in a wide variety of basins. Different models of formation have been proposed, all related to inorganic processes. These models include: diagenetic transformation of gypsum beds producing either displacive growth of crystals or volume changes; mechanical folding caused by compressional stress; and folding produced by slumping. The analysis of three Cenozoic evaporite sequences in Spain reveals that none of the previous models explains their origin and existence. In these outcrops, gypsum enterolithic structures occur in horizontal beds with parallel troughs and crests of the folds. They appear in shoreline facies of lacustrine environments and did not undergo major diagenetic transformations after the primary lithification of the original sediment. Based on these observations, together with the study of a modern analogue in Minorca, Spain, a new model is proposed for the genesis of enterolithic structures. This new model is based on the existence of a microbial mat exposed to brine concentration–dilution cycles and strong wind events. The high wind flow events enhanced folding of the microbial mat that became subaerially exposed and lithified due to subsequent evaporation. Therefore, the presence of enterolithic structures could be used as an indicator of shallow water environmental conditions subject to variations in brine concentration in areas with strong wind flow events. Previous studies of some evaporitic successions should be revisited, taking into account the proposed model, which would imply new depositional environment interpretations. At the same time, the proposed model could explain the existence of Kinneyia‐type structures, also known as wrinkle structures, formed beneath microbial mats in peritidal zones. Moreover, considering enterolithic structures as microbially induced sedimentary structures could be useful as evidence of microbial life in the ancient geological record and on other planets such as Mars. 相似文献
14.
Subir Sarkar Santanu Banerjee Pradip Samanta Silambuchelvan Jeevankumar 《Journal of Earth System Science》2006,115(1):49-60
This paper addresses macroscopic signatures of microbial mat-related structures within the 1.6Ga-old Chorhat Sandstone of
the Semri Group — the basal stratigraphic unit of the Vindhyan succession in Son valley. The Chorhat Sandstone broadly represents
a prograding succession of three depositional facies ranging from shallow shelf to coastal margin with aeolian sandsheet.
The mat-mediated structures were generated because of plastic or brittle deformation of sand, turned cohesive and even thixotropic
because of microbial mat growth. Mat growth also favoured abundant preservation of structures that usually have low preservation
potential. Prolific growth of microbial mat in the subtidal to intertidal zone of the Chorhat sea was facilitated due to lack
of grazing and burrowing activities of organisms in the Precambrian. It further indicates low rate of sedimentation between
the storms, as also attested by frequent superposition of storm-beds, even near the storm wave base. It also reduces erosion
and that, in turn, would imply low sediment concentration in flows leading to development of bedforms that are likely to be
smaller in size and isolated from each other in a single train in contrast to those that form in mat-free sands. 相似文献
15.
北京西郊下苇甸地区下寒武统昌平组底部含燧石角砾白云岩之上发育的一套豹斑灰岩,实为发育在高能环境下的凝块石灰岩,宏观形态上可包括斑状凝块石、带状凝块石和网状凝块石3种,其成因多与厚微生物席相关,且分别具有不同的微观结构。露头实测和镜下观察结果表明凝块石的初始沉积组分受控于沉积大/微环境,且三者的互动决定了凝块石的最终沉积特征。该套沉积成因的凝块石,其白云质凝块不局限于潜穴,且潜穴内部见大量保存完好微生物结构和疑似球状原生白云石,因此这种白云质凝块很可能与微生物白云石化密切相关,并以此区别于局限于生物遗迹位置,且内部充填物为粉细晶白云石的成岩成因的豹斑灰岩。 相似文献
16.
Recently, a paleosurface with microbially-induced sedimentary wrinkle structures that are associated with abundant, well-preserved, iguanodon-type, tridactyl tracks has been documented at the Lotus tracksite near Qijiang (Chongqing, China) in fluvial deposits of the Lower Cretaceous Jiaguan Formation. Two different wrinkle structure types are identified and described from a macroscopic point of view and also by applying microstructure analysis with a scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and other high-resolution instruments, that detected sheath-like and globular organic matter and thus confirm the microbial origin of the observed wrinkle structures. A model for the formation of the two microbial mat induced wrinkle structure types and associated preservation of dinosaur tracks is proposed. Finally, some human footprints were left in comparable modern environments covered with a thin microbial mat, and they are used as a modern analog in order to better understand the track formation and preservation mechanisms of the dinosaur tracks of the Lotus tracksite. 相似文献
17.
It has been suspected for some time that microbial mats probably colonized sediment surfaces in many terrigenous clastic sedimentary environments during the Proterozoic. However, domination of mat morphology by depositional processes, post-depositional compaction, and poor potential for cellular preservation of mat-building organisms make their positive identification a formidable challenge. Within terrigenous clastics of the Mid-Proterozoic Belt Supergroup, a variety of sedimentary structures and textural features have been observed that can be interpreted as the result of microbial colonization of sediment surfaces. Among these are: (a) domal buildups resembling stromatolites in carbonates; (b) cohesive behaviour of laminae during soft-sediment deformation, erosion, and transport; (c) wavy–crinkly character of laminae; (d) bed surfaces with pustular–wrinkled appearance; (e) rippled patches on otherwise smooth surfaces; (f) laminae with mica enrichment and/or randomly oriented micas; (g) irregular, curved–wrinkled impressions on bedding planes; (h) uparched laminae near mud-cracks resembling growth ridges of polygonal stromatolites; and (i) lamina-specific distribution of certain early diagenetic minerals (dolomite, ferroan carbonates, pyrite). Although in none of the described examples can it irrefutably be proven that they are microbial mat deposits, the observed features are consistent with such an interpretation and should be considered indicators of possible microbial mat presence in other Proterozoic sequences. 相似文献
18.
A. Yu. Ivantsov 《Stratigraphy and Geological Correlation》2013,21(3):252-264
Metazoan trace fossils from the Upper Vendian are preserved together with remains of corresponding organisms. The traces belong to “Vendobionta”, representing the Precambrian phylum Proarticulata and to a presumably trochophoran animal Kimberella quadrata. These organisms fed on microbial mats, which preserved fossil traces. Impressions of the mat surface structures, traces, and bodies of animals are preserved in marine terrigenous sediments on the basal surfaces of sandstone beds. Proarticulata grazing traces are represented by groups and chains of impressions left by the ventral side of a body or its central and posterior parts. Kimberella traces are represented by long ridges united into bundles, fans, and chains of fans. All these traces were largely formed mechanically, i.e., by mat scratching with cilia (Proarticulata) or teeth (Kimberella). Proarticulata representatives destroyed only a thin upper layer of the mat, while Kimberella could possibly scratch the mat through its entire thickness or even tear off pieces from it. 相似文献
19.
The extractable lipid composition of four layers of a microbial mat from Hao, French Polynesia, shows differences that reflect both the imprint of their microbial populations and the selective diagenetic transformations of specific microbial compounds.The uppermost layers, principally composed of cyanobacteria and other bacteria, contain lipids typical of such microbes, namely n-heptadecane, heptadecene, hexadecanoic acid, and various sterols. With increasing depth the selective degradation of lower n-alkane homologues occurs; n-alkenes also show enhanced degradation.The predominant sterols in the bottom horizon of the mat are C2, and C30 components, including dinosterol and other 4-methylsterols presumably derived from dinofiagellates. In addition, there is an increase in the proportion of stanols with depth, perhaps arising from preferential degradation of δ5-stenols. 相似文献
20.
《Sedimentology》2018,65(4):1213-1245
In a modern peritidal microbial mat from Qatar, both biomediated carbonates and Mg‐rich clay minerals (palygorskite) were identified. The mat, ca 5 cm thick, shows a clear lamination reflecting different microbial communities. The initial precipitates within the top millimetres of the mat are composed of Ca–Mg–Si–Al–S amorphous nanoparticles (few tens of nanometres) that replace the ultrastructure of extracellular polymeric substances. The extracellular polymeric substances are enriched in the same cations and act as a substrate for mineral nucleation. Successively, crystallites of palygorskite fibres associated with carbonate nanocrystals develop, commonly surrounding bacterial bodies. Micron‐sized crystals of low‐Mg calcite are the most common precipitates, together with subordinate aragonite, very high‐Mg calcite/dolomite and ankerite. Pyrite nanocrystals and framboids are present in the deeper layers of the mat. Calcite crystallites form conical structures, circular to triangular/hexagonal in cross‐section, evolving to crystals with rhombohedral terminations; some crystallite bundles develop into dumb‐bell and stellate forms. Spheroidal organo‐mineral structures are also common within the mat. Nanospheres, a few tens of nanometres in diameter, occur attached to coccoid bacteria and within their cells; these are interpreted as permineralized viruses and could be significant as nuclei for crystallite‐crystal precipitation. Microspheres, 1 to 10 μm in diameter, result from intracellular permineralization within bacteria or the mineralization of the bacteria themselves. Carbonates and clay minerals are commonly aggregated to form peloids, tens of microns in size, surrounded by residual organic matter. Magnesium silicate and carbonate precipitation are likely to have been driven by pH – saturation index – redox changes within the mat, related to microenvironmental chemical changes induced by the microbes – extracellular polymeric substances – viruses and their degradation. 相似文献