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1.
P. Yu. Petrov 《Lithology and Mineral Resources》2016,51(4):290-309
Structures and textures of the peloidal wackestones, as well as size, shape, and composition of peloidal grains, from the Mesoproterozoic (Middle Riphean) Sukhaya Tunguska carbonate platform in the Turukhansk Uplift (Siberia) are considered. It is shown that these grains formed in the course of diagenesis were closely associated with the microsparitic replacement and the formation of molar tooth (MT) structures. Diagenetic transformations of rocks were related to the activity of anaerobic microbial communities inside the buried carbonate silt layers. The microbial activity during diagenesis was governed by the carbonate sediment composition and conservation mechanism of the high-molecular organic matter of primary producers therein, since this organic matter was the nutritious substrate for the primary anaerobe communities. 相似文献
2.
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
3.
Mukund Sharma 《Journal of Earth System Science》2006,115(1):67-98
Mesoproterozoic (∼ 1600 Ma old) Salkhan Limestone (Semri Group) of the Vindhyan Supergroup, exposed in Rohtas district of
Bihar, India, preserves an abundant and varied ancient microbial assemblage. These microfossils are recorded in three distinctly
occurring cherts viz., bedded chert, stromatolitic chert and cherty stromatolites. 27 morphoforms belonging to 14 genera and
21 species have been recognized. Six unnamed forms are also described.
The microbial assemblage, almost exclusively composed of the remnants of cyanobacteria, is dominated by entophysalidacean
members and short trichomes and can be termed as ‘typical Mesoproterozoic microbiotas’. The assemblage includes characteristic
mat-forming scytonematacean and entophysalidacean cyanobacteria.Eoentophysalis is the dominant organism in the assemblage. Ellipsoidal akinetes of nostocalean cyanobacteria(Archaeollipsoides) and spherical unicells also occur; both are distinct from mat forming assemblage, allochthonous and possibly planktic. Co-occurrence
of the microbiotas and precipitates is related to the depositional environment of the Mesoproterozoic tidal flats with high
carbonate saturation. 相似文献
4.
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
5.
Microfossils from the Upper Sinian (Late Neoproterozoic) Doushantuo Formation in Changyang, Western Hubei, China 总被引：12，自引：0，他引：12
YIN Chongyu 《Continental Dynamics》1999,(2)
l.IntroductionIn1978,YinLeimingandLiZaipingreportedonProterozoicmicroplansinsouthwestChina.Alt-houghmostwerefoundinmacerationofsiliclasticsandcarbonates,thinsectionsoftheDoushantuochertfromtheTianzhushandistrictnearChangyang(samples#TianR.l9and#TianR.29,inL.YinandLi,l978)containedtwolargespinosespeciesfAs-lerocaPsoidessinensis(upto325Umindiameter)andTianzhushaniasPinosa(45opmindiameter).Subsequentst-UdiesoftheDoushantuoFormationindicatethatthemicrofossilsintheDoushantuochertandphosph… 相似文献
6.
P. Yu. Petrov 《Stratigraphy and Geological Correlation》2011,19(3):247-267
A new way of formation of the problematic Molar Tooth (MT) structures, which, along with stromatolites, could be considered
as a “visiting card” of the Riphean, was examined on the example of carbonate sediments of the Riphean Sukhaya Tunguska Formation
from the Turukhansk Uplift, Siberia. These structures were formed due to the consequent replacement of oozy constituent of
carbonate sediments with calcitic microsparite in the course of diagenesis with substantial shift of the dynamic equilibrium
toward the solution in the sediment—porous solution—microsparite system. An excess of soluble magnesium significantly hampering
precipitation of crystal cores of the future calcitic microsparite could be one of the possible causes of the shift. It is
suggested that magnesium mole fraction of the porous solutions was related to the early dissolution of the metastable high-Mg
calcite of the silty sediment brought from the areas of active stromatolite formation. The facies occurrence of the MT-structures
indicates that they occurred in descending trails of the most productive zones of carbonate platforms tapering toward the
basin. The synchronism of stratigraphic trends of the MT-structures and stromatolites in the Precambrian resulted from the
evolution of carbonate-productive microbe-mineral systems which were the immediate suppliers of fine-grained carbonate sediment
into external zones of carbonate platforms. 相似文献
7.
Marta B?K Lucyna NATKANIEC-NOWAK Beata NAGLIK Krzysztof B?K Pawe? DULEMBA 《《地质学报》英文版》2017,91(1):39-50
An association of organic-walled microfossils consisting of filamentous cyanobacteria, algal coenobia and acanthomorphic acritarch have been documented from non-calcareous claystones and mudstones of the Pepper Mountains Shale Formation(PMSF), located in its stratotype area in the Pepper Mountains, which are part of the Holy Cross Mountains in Poland. These sediments represent the oldest strata of the ?ysogóry Unit, deposited on the edge of the East European Craton(Baltica). Non-branched, ribbon-like and thread-like cyanobacteria trichomes exhibit morphological similarities to families Nostocaceae and Oscillatoriaceae. Cells assembled in rounded to irregular clusters of monospecific agglomerations represent multicellular algal coenobia, attributed to the family Scenedesmaceae. The co-occurrence of acritarchs belonging to species as Eliasum llaniscum, Cristallinium ovillense and Estiastra minima indicates that the studied material corresponds to the lower Middle Cambrian. Deposition of the PMSF took place in shallow marine environment, influenced by periodical freshwater inputs. The varying degree of coloration of organic-walled microfossils is interpreted in this study as factor indication of possible different source of their derivation. Dark brown walls of cells assembled in algal coenobia might have sustained previous humification in humid, terrestrial environments, which preceded their river transport into the sea together with nutrients, causing occasional blooms of cyanobacteria in the coastal environment and the final deposition of both groups of organisms in marine deposits. 相似文献
8.
<正>Microbial mats are ecosystems that can control or induce the precipitation of calcium(Ca) carbonate on Earth through geological time.In the present study,we report on a novel accumulation of Ca,together with iron(Fe),in a microbial mat collected from a slight acidic hot spring(pH=5.9) in south China.Combining an array of approaches,including environmental scanning electron microscopy,X-ray microanalysis,transmission electron microscopy,and selected area electron diffraction,we provide ultrastructral evidence for amorphous acicular aggregates containing Ca and Fe associated with cyanobacteria precipitating in the microbial mats.Cyanobacterial photosynthesis and exopolymeric organic matrixes are considered to be responsible for the precipitation of Ca.These amorphous acicular aggregates might imply the early stage of calcification occurring in microbial mats.Ca and Fe coprecipitation indicates another potential important way of inorganic element precipitation in hot spring microbial mats.Our results provide insight into the possible mechanism of cyanobacterial calcification and microfossil preservation in slight acidic hot spring environments. 相似文献
9.
河北承德路通沟剖面芙蓉统凤山组中部发育厚层块状叠层石生物丘,构成一个淹没不整合型层序的强迫型海退体系域,指示这些叠层石形成于中高能浅海环境。该生物丘宏观上主要由柱状叠层石组成,叠层石内部纹层较粗糙,在构成叠层石的致密泥晶和微亮晶组构中,还见到球粒、底栖鲕粒及凝聚颗粒等多种生物成因颗粒类型,代表着复杂的微生物活动特征,以此而区别于前寒武纪的叠层石。更为重要的是,叠层石生物丘中的致密泥晶基质中发育一些“石松藻(Lithocodium)”状的钙化蓝细菌菌落残余物,以及一些丝状钙化蓝细菌化石,指示了形成叠层石的微生物席为蓝细菌所主导的微生物席。因此,凤山组叠层石生物丘内复杂而特殊的碳酸盐岩沉积组构为研究叠层石形成过程中复杂的微生物代谢活动所产生的钙化作用机制提供了一个宝贵的地质实例。 相似文献
10.
Studies of modern cyanobacterial mats and biofilms show that they can precipitate minerals as a consequence of metabolic and degradational activities paired with ambient hydrochemical conditions. This study looked at modern microbial mats forming giant, tower‐like, groundwater‐fed, calcareous microbialites in the world's largest, highly alkaline lake; Van Gölü (Lake Van), East Turkey. Results show that microbial systems play a role not only in carbonate precipitation but also in the formation of siliceous mineral phases. Transmitted light microscopy, scanning electron microscopy and spectral observations revealed that, within the extracellular polymeric substances excreted by the mats abundant minute aragonite grains precipitated first in vivo. These minute grains were quickly succeeded and/or supplemented in the dead biomass of the cyanobacterial mat by authigenic Al–Mg–Fe siliceous phases. Silicon dioxide is available in large concentrations in the highly alkaline water of Lake Van. Divalent cations (Ca and Mg) are delivered to the microbialites mostly by groundwater springs. The precipitation of the fine‐grained siliceous phases is probably mediated by bacteria degrading the cyanobacterial biomass and complexing the excessive cations with their extracellular polymeric envelopes. The bacteria serve as nucleation centres for the subsequent precipitation of siliceous mineral phases. Generally, the biphasic (calcareous and siliceous) mineralization – characterizing Lake Van microbialites – is controlled by their interior highly dynamic hydrogeochemical situation. There, the dramatically different alkaline lake water and the Ca–Mg‐charged groundwater mix at various rates. The early diagenetic replacement of the in vivo aragonite by authigenic siliceous phases significantly increases the fossilization potential of the mat‐forming cyanobacteria. Lake Van and its giant microbialite tufa towers act as a model explaining the transformation of early diagenetic mineral phases observed in many modern and ancient carbonate marine deposits, particularly those influenced by diffusion of silica‐enriched and metal‐enriched pore waters from below the water–sediment interface. 相似文献
11.
微生物席沉积学:一个年轻的沉积学分支 总被引：6，自引：0，他引：6
现代实例和岩石记录的研究表明,微生物席是一个特别的微生物群落,这个特殊的微生物群落就像一个复杂的食物网一样,群落中的每一个组成成员紧密相互依赖,从而构成了地球上形成最早、延续时间最长的生态系.微生物席在沉积岩中留下了丰富而且复杂的记录,在碳酸盐岩中最为典型的产物就是叠层石,在碎屑岩中最具有代表性的产物就是"微生物诱发的... 相似文献
12.
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
13.
V N Sergeev A H Knoll S P Kolosova P N Kolosov 《Stratigraphy and Geological Correlation》1994,2(1):19-33
Silicified shallow-water marine carbonate deposits of the Proterozoic Debengda Formation (the Olenek Uplift, northeastern Siberia) contain well preserved microfossils. One or two distinct assemblages consists only of filamentous Siphonophycus microfossils, which are presumably the extracellular sheaths of hormogonium cyanobacteria. The other is dominated by coccoidal microfossils, first by the entophysalidacean cyanobacterium Eoentophysalis. The coccoidal assemblage was recognized in the layered carbonate precipitate structures of a superficially stromatolite appearance. Despite its simple composition, the microfossil assemblage supports the generally accepted Mesoproterozoic (middle Riphean) age of the Debengda Formation. This conclusion corresponds to the available data on isotopic geochronology, and to the composition of columnar stromatolites from the Dehengda Formation. Both the structural features and carbon isotopic composition of its rocks are comparable to those of rocks of known Mesoproterozoic age, but differ from the characteristics of definitely Neoproterozoic deposits. 相似文献
14.
V. Purnachandra Rao Pratima M. Kessarkar R. Nagendra E. V. S. S. K. Babu 《Journal of Earth System Science》2007,116(6):525-536
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. 相似文献
15.
Christopher T. Perry 《Sedimentology》1999,46(1):33-45
Micrite envelopes are a common feature in carbonate sediments and are typically associated with the micrite filling of borings produced by microendolithic organisms. These are referred to as 'destructive micrite envelopes' and have long been recognized as reflecting an important early diagenetic process. Recent analysis of sediments collected from back-reef environments at Discovery Bay, north Jamaica, however, has demonstrated 'envelope' formation on the surfaces of carbonate grains, clearly distinct from the micrite filling of microborings. Such constructive envelopes occur almost exclusively in sediments from grass-bed environments and are always intimately associated with 'biofilms' comprising abundant mucilage, cyanobacteria, bacteria and diatoms. It is suggested that these envelopes represent a product of both biologically mediated micrite precipitation (occurring within the biofilm mucilage and around the biofilm components, i.e. cyanobacteria and diatoms) and associated trapping of carbonate mud and fine-grained sediment. Their recognition only within grass-bed sediments may enable their use as a diagnostic feature of grass-bed environments or vegetation-stabilized substrates in the rock record. 相似文献
16.
A study on the predominant sources of organic matter and the main diagenetic processes in two different cyanobacterial mats from evaporite-controlled environments was performed. Fatty acids, hydrocarbons, alcohols, ketones and aldehydes were anlyzed in selected millimetre and submillimetre core sections. The changes in lipid composition were evaluated by comparison with the vertical distributions of the populations observed by optic microscopy and with the lipid patterns of enrichment cultures of species such as cyanobacteria, diatoms, purple bacteria, sulphate-reducers and methanogens obtained from the mats.The cyanobacteria Phormidium valderianum and Microcoleus chthonoplastes are the predominant primary producers, and occur almost as monocultures in the respective top layers. However, these mat-forming organisms only leave minor features in the solvent-extractable lipid sedimentary record. The predominant fatty acid distributions parallel the composition observed in the enrichment cultures of purple bacteria and appear mixed with acids characteristic of heterotrophic eubacteria such as sulphate-reducers. The concentrations of these lipids are, however, 5–10 times lower than the cyanobacterial acids from the top layer. De novo heterotrophic eubacterial synthesis is also observed in cases such as the highly branched isoprenoid eicosenes, the major hydrocarbon in the deep layers (>2 mm) of the Phormidium mat. Other major diagenetic changes involve dehydration and hydrogenation. These two processes take place concurrently under anoxic conditions and have been observed among the sterols and the isoprenoid alcohols. Significant amounts of 5ß(H)-stanols were observed in the more reducing sections where molecular indicators of methanogenic bacteria were also found. 相似文献
17.
Marine pisolites from Upper Proterozoic carbonates of East Greenland and Spitsbergen 总被引：1，自引：0，他引：1
Upper Proterozoic carbonate successions from central East Greenland (the Limestone-Dolomite ‘Series’ of the Eleonore Bay Group) and Svalbard (the Backlundtoppen Formation of the Akademikerbreen Group, Spitsbergen, and the Upper Russö Formation of the Raoldtoppen Group, Nordaustlandet) contain thick sequences dominated by pisolites. These rocks were generated in shallow marine enviroments, and the pisoids are essentially oversized ooids. A marine environment is supported by the thickness and lateral extent of the carbonates; by a sedimentary association of pisolites with stromatolites, flake-conglomerates, calcarenites, calcilutites, microphytolites, and ooids similar to that found in numerous other Proterozoic carbonate successions; by sedimentary structures, including cross-beds and megaripples that characterize the pisolitic beds; and by microfossils of endolithic cyanobacteria that are specifically comparable to microorganisms that inhabit modern marine ooids of the Bahama Banks. Petrographic features and strontium abundances suggest that the pisoids were originally aragonitic, but neomorphism, silicification, calcitization, and dolomitization have extensively modified original mineralogies and fabrics. The East Greenland and Svalbard pisolitic carbonates reflect similar depositional environments and diagenetic histories, reinforcing previous bio-, litho-, and chemostratigraphic interpretations that the two sequences accumulated contiguously in a coastal zone of pisoid genesis which extended for at least 600, and probably 1000 or more, kilometres. 相似文献
18.
G. GERDES K. DUNAJTSCHIK-PIEWAK H. RIEGE A. G. TAHER W. E. KRUMBEIN H.-E. REINECK† 《Sedimentology》1994,41(6):1273-1294
Non-skeletal carbonate particles in microbial mats were studied using thin sections and scanning electron microscopy. The microbial mats form biolaminated units (so-called potential stromatolites) in salterns. This study emphasizes the coexistence of different particle forms and makes a genetic connection between the heterogeneity of the organic substrate built by bacteria and diatoms and their extracellular polymeric substances (EPS). Whereas allochthonous particles are scarce, Lanzarote microbial mats provide various autochthonous surfaces for the attachment of cells and EPS, including sheaths and capsules of cyanobacteria, frustules of diatoms, metabolic products such as gas bubbles, liquid globules and faecal pellets, as well as the carbonate precipitates themselves. Morphologically different carbonate precipitates are: (i) calcified organic clumps (peloids), (ii) particles composed of concentric aragonite and biofilm laminae (ooids and oncoids), (hi) isolated particles floating in gel-supported mats and coated by rims of fibrous cement (cortoids), (iv) particles bound by cryptocrystalline matrices or cement, resulting in aggregate grains and (v) lobate cement which fills out spaces and pores and fixes the particles. Peloids are suggested to represent faecal pellets although microbial systems also generate cell clumps by non-faecal processes. Ooid and oncoid constructions clearly record alternating processes of biofilm accumulation and aragonite encrustation. Further characteristic features of carbonate particles generated within a microbial mat are: (i) an irregular distribution ranging from isolated particles floating within the gel-like matrix to closely packed particles, (ii) the amalgamation of different particle types (e.g. peloids and ooids) in aggregate grains, (iii) the heterogeneous nature of nuclei comprising bacterial clumps, intraclasts, individual cells, cell colonies and bubbles, (iv) the enrichment of remains, casts and imprints of cells within precipitates and (v) deformation (e.g. truncated cortices) of particles. 相似文献
19.
微生物席成因构造(microbially induced sedimentary structures, MISS)是由微生物与沉积物相互作用形成的生物-沉积构造, 可作为早期微生物群活动的重要标识, 但对其指示古环境的研究尚显不足.华北地台南部中-新元古代汝阳群(Pt2)和洛峪群(Pt3)以近岸浅水陆源碎屑沉积为主, 其中发育大量MISS, 包括多向波痕、微生物席稳化波痕、微生物席碎片、微生物席平滑波痕、多种微生物席脱水形成的砂裂及不规则网状生长脊等.研究表明, 在潮坪环境中MISS最为发育.地势差异对水动力、基底暴露、水分补给和沉积条件等环境因素具有显著控制作用, 影响微生物席的发育与结构, 因此, MISS的形态组合特征能够反映沉积微相变化.研究发现, 在潮上带以微生物席脱水形成的砂裂构造为主, 潮间带上部以破坏-改造型构造为主, 潮间带下部-潮下带一般少见原位MISS, 但可见再沉积微生物席碎片.据潮间带下部至潮上带上部MISS产出类型及其形态组合分析, 识别了4个MISS形态组合带, MISS形态组合由潮间带下部至潮上带上部的变化反映了古地形由低到高的明显变化. 相似文献
20.
Phototrophic mats (microbial mats with a phototrophic top layer) are complex systems in terms of microbial diversity, biogeochemical cycles and organic matter (OM) turnover. It has been proposed that these mats were a predominant life form in Proterozoic shallow water settings, prior to the emergence of bioturbating organisms in the Ediacaran–Cambrian transition. For most of the Precambrian, microbial mats were not only quantitative important carbon fixing systems, but also influenced the transfer and transformation of OM before it entered the geosphere. The profound alteration of compound inventories during transit through microbial mats, implying substantial consequences for OM preservation in the Proterozoic, was recently proposed as a “mat-seal effect” [Pawlowska et al. (2012) Geology 41, 103–106]. To obtain a better understanding of the early diagenetic fate of primary produced OM in microbial mats, we studied a recent calcifying mat from a hypersaline lake in Kiritimati, which showed in the deeper mat layers a maximum 14Ccarbonate age of ∼1500 years. We particularly focused on OM entrapped in the carbonate matrix, because of the better potential of such biomineral-encapsulated OM to reach the geosphere before degradation (and remineralization). Our data indicate that selective preservation is important in phototrophic mats. While a diagenetic transformation of lipid fatty acids (FAs) was evident, their fatty acyl-derived hydrocarbon moieties were not introduced into protokerogen, which was instead mainly comprised of cyanobacterial and/or algal biomacromolecules. Our data support the proposed major impact of the “mat-seal effect” on OM turnover and preservation; i.e. the suppression of biosignatures derived from the upper mat layers, while signals of heterotrophic microbes thriving in deeper mat layers become preferentially preserved (e.g. high hopane/sterane ratios). This mechanism may have broad consequences for the interpretation of biomarkers from Proterozoic shelf environments, because biosignatures of phototrophic mat dwellers as well as planktonic signals may have become heavily biased by the production and turnover of OM in microbial mat systems. 相似文献