Recent analog of gypsified microbial laminites and stromatolites in solar salt works and the Miocene gypsum deposits of Saudi Arabia and Egypt

Accumulation of microbial mats and stromatolites dominate in the crystallization ponds of solar salt works west of Alexandria, Egypt. These microbial mats are laminar in the permanent submerged part of the ponds. The microbial mats commonly form sites for growth of gypsum crystals during periods having higher salinity. In the dominant submerged part of the pond, domal stromatolites are common around groundwater seepage holes. In the shallow, intermittent margin of the ponds, the laminated microbial structure forms laterally close-linked hemispheroidal stromatolite type, with unidirectional and multidirectional ripple mark-like morphology on their surface. The microbial laminite and stromatolite types in the modern solar salt works are similar to the organic-rich Miocene gypsum beds of El-Barqan (west Alexandria, Egypt) and Rabigh (north Jeddah, Saudi Arabia). The Miocene organic-rich beds consist of interlayered dark-colored microbial laminae and light-colored gypsum laminae. These beds may have three different variations: regular even lamination, laterally closed-linked hemispheroidal stromatolites, and/or discrete hemispheroidal stromatolites. Petrographic examination of the microbial laminites and stromatolites in the solar salt works and the Miocene gypsum beds indicate that the dark-colored, organic-rich laminae are composed of micritized microbial laminae and/or brown organic filaments. In El-Barqan area, the light-colored gypsum-rich laminae are composed of either gypsum crystal fragments, or lenticular and prismatic gypsum. These gypsum crystals are either entrapped within the microbial filaments or are nucleated at the surface of the microbial laminae to form a radial pattern, whereas in Rabigh area, the light-colored gypsum-rich laminae are composed of secondary porphyrotopic, poikilotopic, or granular gypsum crystals. By comparison of the microbial structure in the Miocene gypsum beds with the recent occurrence of the microbial laminites and stromatolites in the solar salt works, it is demonstrated that the organic-rich Miocene gypsum beds were formed in a very shallow salina with slightly fluctuating brine levels.     

The calcium carbonate saturation state in cyanobacterial mats throughout Earth’s history

Through early lithification, cyanobacterial mats produced vast amounts of CaCO₃ on Precambrian carbonate platforms (before 540 Myr ago). The superposition of lithified cyanobacterial mats forms internally laminated, macroscopic structures known as stromatolites. Similar structures can be important constituents of Phanerozoic carbonate platforms (540 Myr to present). Early lithification in modern marine cyanobacterial mats is thought to be driven by a metabolically-induced increase of the CaCO₃ saturation state (Ω_CaCO3) in the mat. However, it is uncertain which microbial processes produce the Ω_CaCO3 increase and to which extent similar Ω_CaCO3 shifts were possible in Precambrian oceans whose chemistry differed from that of the modern ocean. I developed a numerical model that calculates Ω_CaCO3 in cyanobacterial mats and used it to tackle these questions. The model is first applied to simulate Ω_CaCO3 in modern calcifying cyanobacterial mats forming at Highborne Cay (Bahamas); it shows that while cyanobacterial photosynthesis increases Ω_CaCO3 considerably, sulphate reduction has a small and opposite effect on mat Ω_CaCO3 because it is coupled to H₂S oxidation with O₂ which produces acidity. Numerical experiments show that the magnitude of the Ω_CaCO3 increase is proportional to DIC in DIC-limited waters (DIC < 3-10 mM), is proportional to pH when ambient water DIC is not limiting and always proportional to the concentration of Ca²⁺ in ambient waters. With oceanic Ca²⁺ concentrations greater than a few millimolar, an appreciable increase in Ω_CaCO3 occurs in mats under a wide range of environmental conditions, including those supposed to exist in the oceans of the past 2.8 Gyr. The likely lithological expression is the formation of the microsparitic stromatolite microtexture—indicative of CaCO₃ precipitation within the mats under the control of microbial activity—which is found in carbonate rocks spanning from the Precambrian to recent. The model highlights the potential for an increase in the magnitude of the Ω_CaCO3 shift in cyanobacterial mats throughout Earth’s history produced by a decrease in salinity and temperature of the ocean, a decrease in atmospheric pCO₂ and an increase in solar irradiance. Such a trend would explain how the formation of the microsparitic stromatolite microtexture was possible as the Ω_CaCO3 of the ocean decreased from the Paleoproterozoic to the Phanerozoic.     

Fine structure of the stratified microbial community at Laguna Figueroa, Baja California, Mexico. I. Methods of in situ study of the laminated sediments

The microbial community of the flat laminated sediments in the hypersaline lagoon at Laguna Figueroa has undergone dramatic changes due to prolonged periods of submergence under fresh water. This paper describes the techniques used in the first comprehensive in situ ultrastructural study of a stratified microbial community formerly dominated by Microcoleus sp. and introduces the changes caused by flooding and burial. Flat mat desiccation polygons fixed in 2.5% buffered glutaraldehyde were studied by light and transmission electron microscopy. Typical pre-flood Microcoleus-dominated laminated mat from 1977 was compared to post-flood laminated mat from 1979. The technique faithfully preserves the various laminae with their associated organisms and sediment, Morphological identification of several genera of photosynthetic prokaryotes including cyanobacteria, purple, and green bacteria was made by analysis of cell membrane and wall structure. Amoebic cysts could also be recognized in situ. Bacteria of unknown generic affinity, but with distinctive ultrastructure were discovered. A filamentous purple photosynthetic bacterium was found to be a common component of the 1979 mat and two bacteria within a bacterium associations were seen in sections from the 1977 mat.     

Biogeochemistry of high salinity microbial mats – Part 1: Lipid composition of microbial mats across intertidal flats of Abu Dhabi, United Arab Emirates

As a contribution to a project on the role of aerobic heterotrophic bacteria in the organic matter turnover within cyanobacterial mats, we investigated selected lipid classes of five morphologically different microbial mats from the intertidal area of Abu Dhabi, United Arab Emirates. The mats thrive under extreme environmental conditions as they are exposed to salinity up to 200 and shadowless sunshine for nearly 12 h per day. The total extractable organic matter is characterised by heavy δ¹³C values of −17‰ to −10‰, suggesting CO₂ limitation. The lipids reflect the biomass of the principal mat-building phototrophic and heterotrophic microorganisms. Moderate amounts of hydrocarbons, mainly cyanobacterial n-heptadecane, short chain monomethyl alkanes and diploptene were detected in all the mats. The ubiquitous n-hexadecanoic acid is dominant amongst the free and ester-bound phospholipid fatty acids (PLFAs), whereas the overall acid composition varies due to differences in the extreme environmental conditions. Additionally, the top layers of two of the five mats were analysed separately to compare their lipid compositions with those of the whole mats. As would be expected, the concentrations of cyanobacterial biomarkers exceed those found in the total mats. Noteworthy is the presence of n-heneicosadiene as predominant hydrocarbon in the top layer of the so called gelatinous mat, which thrives under the locally most extreme conditions. As the n-alkadiene was present to a much lesser extent in the intact mat, it may provide a new biomarker for a so far unknown important aerobic organism.     

The preservation potential of some recent stromatolites

Stromatolites are laminated organo-sedimentary structures, generally compared to present day blue-green algal mats. Their morphology, species composition and overall extent are largely governed by the amount of wetting, although other factors such as competition, predation and desiccation, also contribute. The Trucial Coast mats are essentially intertidal. Stromatolite accretion rates in this area are of the order of 0·2 mm p.a. but lamina growth is far from regular. The area is also characterized by the development of evaporites, especially gypsum which proves to be an important agent of mat destruction. The growth of crystals causes disruption within the upper portions of the stromatolite section with the result that none of the upper intertidal mat forms are preserved. Other agencies of destruction include bacterial decay, desiccation and dehydration, and compaction under burial which may depress and deform the original mat relief. Decay results in the almost total loss of cellular contents, only a few empty sheaths and the pigment surviving into the fossil record. Preservation may be effected via (a) burial or (b) lithification. However, few modern algal mat structures bear any resemblance to fossil stromatolite heads with the exception of those from Shark Bay. From this, one might infer that pene-contemporaneous lithification is a prerequisite for their preservation.     

Fine‐grained agglutinated elongate columnar stromatolites: Tieling Formation,ca 1420 Ma,North China

The Mesoproterozoic Tieling Formation, near Jixian, northern China, contains thick beds of vertically branched, laterally elongate, columnar stromatolites. Carbonate mud is the primary component of both the stromatolites and their intervening matrix. Mud abundance is attributed to water column ‘whiting’ precipitation stimulated by cyanobacterial photosynthesis. Neomorphic microspar gives the stromatolites a ‘streaky’ microfabric and small mud flakes are common in the matrix. The columns consist of low‐relief, mainly non‐enveloping, laminae that show erosive truncation and well‐defined repetitive lamination. In plan view, the columns form disjunct elongate ridges <10 cm wide separated by narrow matrix‐filled runnels. The stromatolite surfaces were initially cohesive, rather than rigid, and prone to scour, and are interpreted as current aligned microbial mats that trapped carbonate mud. The pervasive ridge–runnel system suggests scale‐dependent biophysical feedback between: (i) carbonate mud supply; (ii) current duration, strength and direction; and (iii) growth and trapping by prolific mat growth. Together, these factors determined the size, morphology and arrangement of the stromatolite columns and their laminae, as well as their branching patterns, alignment and ridge–runnel spacing. Ridge–runnel surfaces resemble ripple mark patterns, but whether currents were parallel and/or normal to stromatolite alignment remains unclear. The formation and preservation of Tieling columns required plentiful supply of carbonate mud, mat‐building microbes well‐adapted to cope with this abundant sediment, and absence of both significant early lithification and bioturbation. These factors were time limited, and Tieling stromatolites closely resemble coeval examples in the Belt‐Purcell Supergroup of Laurentia. The dynamic interactions between mat growth, currents and sediment supply that determined the shape of Tieling columns contributed to the morphotypical diversity that characterizes mid–late Proterozoic branched stromatolites.     

Algal mat productivity: Comparisons in a salt marsh

This paper documents the role of salt marsh algal mats in the productivity of a southern California tidal wetland. The productivity of the mats, which are composed of filamentous bluegreen and green algae and diatoms, varies both temporally and spatially in relation to tidal inundation and overstory vegetation. The estimates of net primary productivity (NPP) were highest under the canopy ofJaumea carnosa (Less.) Gray (341 g C m^?2 yr^?1) at low elevation. Elsewhere, NPP appeared to be limited by low light (276 g C m^?2 yr^?1 underSpartina foliosa Trin.) and desiccation (185 g C m^?2 yr^?1 underBatis martima L. and 253 g C m^?2 yr^?1 underMonanthochloe littoralis Engelm). Algal NPP was from 0.8 to 1.4 times that of the vascular plant overstory NPP. It is hypothesized that the arid environment of southern California and resulting hypersaline soils reduce vascular plant cover, which leads to high algal productivity.     

Temporal and environmental significance of microbial lamination: Insights from Recent fluvial stromatolites in the River Piedra,Spain

Despite extensive research, the environmental and temporal significance of microbial lamination is still ambiguous because of the complexity of the parameters that control its development. A 13 year monitored record of modern fast‐accreting calcite stromatolites (mean 14 mm year^?1) from artificial substrates installed in rapid flow in the River Piedra (north‐east Spain) allows comparison of the sedimentological attributes of successive six‐month depositional packages with the known climatic, hydrophysical and hydrochemical parameters of the depositional system. The stromatolites are formed of dense, porous and macrocrystalline composite laminae. The dense and porous composite laminae, which are composed of two to eight laminae consisting largely of calcified cyanobacteria, are characterized by: (i) dense composite laminae, up to 15 mm thick, mostly with successive dense laminae and minor alternating dense and porous laminae; and (ii) porous composite laminae, up to 12 mm thick, consisting mainly of porous laminae alternating with thinner dense laminae. Most of the dense composite laminae formed during the warm periods (April to September), whereas most of the porous composite laminae developed in the cool periods (October to March). Each dense and porous composite lamina represents up to or slightly longer than six months. The alternation of these two types of composite laminae parallels seasonal changes in temperature. The dense and porous laminae result from shorter (for example, intraseasonal) variations in temperature, insolation and hydrological conditions. The macrocrystalline laminae, with crystals >100 μm long, occur isolated and grouped into composite laminae up to 1·7 mm thick. Their occurrence suggests the absence or poor development of microbial mats over periods of weeks to several months. Thus, stromatolite lamination can record different‐order, periodic and non‐periodic changes in the magnitude of environmental parameters over a single year. These results hold important implications for the temporal and environmental interpretation of lamination in microbial structures.     

Stromatolites in the Late Ordovician Eureka Quartzite: implications for microbial growth and preservation in siliciclastic settings

Well-preserved siliciclastic domal stromatolites, up to 2 m wide and 1·5 m high, are found in a 10 to 15 m thick interval within the Late Ordovician Eureka Quartzite of Southern Nevada and Eastern California, USA. These stromatolites appear as either isolated features or patchy clusters that contain more than 70% by volume quartz grains; their association with planar, trough and herringbone cross-bedding suggests that they were formed in an upper shoreface environment with high hydraulic energy. In this environment, sand bars or dunes may have provided localized shelter for initial microbial mat colonization. Biostabilization and early lithification of microbial mats effectively prevented erosion during tidal flushing and storm surges, and the prevalence of translucent quartz sand grains permitted light penetration into the sediment, leading to thick microbial mat accretion and the formation of domal stromatolites. Decimetre-scale to metre-scale stromatolite domes may have served as localized shelter and nucleation sites for further microbial mat colonization, forming patchy stromatolite clusters. Enrichment of iron minerals, including pyrite and hematite, within dark internal laminae of the stromatolites indicates anaerobic mineralization of microbial mats. The occurrence of stromatolites in the Eureka Quartzite provides an example of microbial growth in highly stressed, siliciclastic sedimentary environments, in which microbial communities may have been able to create microenvironments promoting early cementation/lithification essential for the growth and preservation of siliciclastic stromatolites.     

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.     

现代叠层石的多样化构成:认识古代叠层石形成的关键和窗口*

自从Kalkowsky在1908年构筑了叠层石的术语之后,叠层石一直是地质学家采用不同方法研究和思考的主题,而且一直被当作证明地球早期生命历史的代表物而得到深入调查。叠层石确实为地球早期生命历史提供了间接而且复杂的证据,所以,现代叠层石确实代表着明显的生物信号而成为研究的焦点。最为引人注目的是,现代叠层石的多样化构成,确实表明了蓝细菌生物席建造了叠层石,而且进一步表明了微生物席转化成叠层石不是一个直接的作用过程。那些反映现代叠层石多样化构成的典型实例包括:(1)南极Untersee地区的湖泊相锥状泥质叠层石;(2)新西兰North群岛被称为煎锅湖的热水湖泊中以及美国黄石国家公园热泉中的硅质叠层石;(3)巴哈马台地、澳大利亚鲨鱼湾以及巴西东南部海湾碳酸盐沉积物构成的叠层石。由于蓝细菌微生物席是否代表了古代叠层石的形态学前体总是存在争议,而且在生命的图像中叠层石一直是一个迷惑的关键片段,因此,现代叠层石的多样化构成,将成为认识古代叠层石形成的关键和窗口。立足于前人的研究成果,追踪和总结现代叠层石的多样化构成,以及它们所代表的沉积作用和微生物新陈代谢活动丰富而复杂的信息,将不但丰富微生物沉积学的研究内容,还将拓宽沉积相分析的基本内容,对深入了解叠层石复杂的沉积学特征和生物学属性具有重要的科学意义。     

Hydroxy acids in Antarctic lake sediments and their geochemical significance

Hydroxy acids in sediments of Lakes Bonney, Fryxell, Joyce and Vanda, and unnamed ponds (B2, NF1, NF2 and L4) as well as in cyanobacterial mats from the McMurdo Sound region of southern Victoria Land in Antarctica have been studied to clarify their features and elucidate their source organisms. Normal and branched (iso and anteiso) 2-hydroxy acids were found in all the samples studied with the predominance of even- and odd-carbon numbers, respectively. The most dominant 2-hydroxy acids in the sediments were mainly short-chain components (<C₂₀). Normal and branched 3-hydroxy acids were detectewith the predominance of even- and odd-carbon numbers, respectively, in total concentrations between 0.48 and 53 μg/g of dry sediment. (ω-1)-Hydroxy acids were all long-chains (C₂₂, C₂₄, C₂₆, C₂₈ and C₃₀). 9,10-Dihydroxyhexadecanoic and/or 9,10-dihydroxyoctadecanoic acids were identified in all the sediments and a cyanobacterial mat. The composition of hydroxy acids differ considerably among the lakes and ponds, suggesting the difference of source organisms. These 2-, 3- and (ω-1)-hydroxy, and 9,10-dihydroxy acids may be derived from cyanobacteria and microalgae, in addition to non-photosynthetic microorganisms. Cyanobacteria and microalgae which are widely distributed in the world, may be important sources of hydroxy acids in the natural environments.     

Organic matter preservation in the carbonate matrix of a recent microbial mat – Is there a ‘mat seal effect’?

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 ¹⁴C_carbonate 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.     

Seasonal temperatures from δ18O in recent Spanish tufa stromatolites: Equilibrium redux!

This study focuses on recent debate over the value of stable isotope‐based environmental proxies recorded in riverine tufa stromatolites. A twelve‐year record (1999 to 2012) of river‐bed tufa stromatolites in the River Piedra (north‐east Spain) was recovered in this study, along with a partly overlapping fifteen‐year record (1994 to 2009) of accumulations in a drainage pipe: both deposits formed in water with near identical physico/chemical parameters. Measured water temperature data and near‐constant δ ¹⁸O_water composition allowed selection of an ‘equilibrium’ palaeotemperature equation that best replicated actual temperatures. This study, as in some previous studies, found that just two published formulas for water temperature calculation from equilibrium calcite δ ¹⁸O compositions were appropriate for the River Piedra, where tufa deposition rates are high, with means between 5·6 mm and 10·8 mm in six months. The δ ¹⁸O_calcite in both the river and the pipe deposits essentially records the full actual seasonal water temperature range. Only the coldest times (water temperature <10°C), when calcite precipitation mass decreased to minimum, are likely to be unrepresented, an effect most noticeable in the pipe where depositional masses are smaller and below sample resolution. While kinetic effects on δ ¹⁸O_calcite‐based calculated water temperature cannot be ruled out, the good fit between measured water temperature and δ ¹⁸O_calcite‐calculated water temperature indicates that temperature is the principal control. Textural and deposition rate variability between the river and pipe settings are caused by differences in flow velocity and illumination. In the river, calcification of growing cyanobacterial mat occurred throughout the year, producing composite dense and porous laminae, whereas in the pipe, discontinuous cyanobacterial growth in winter promoted more abiogenic calcification. High‐resolution δ ¹⁸O_calcite data from synchronous pipe and river laminae show that reversals in water temperature occur within laminae, not at lamina boundaries, a pattern consistent with progressive increase in calcite precipitation rate as cyanobacterial growth re‐established in spring.     

辽宁兴城中元古界常州沟组砂岩中微生物成因沉积构造

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。     

External controls on the distribution,fabrics and mineralization of modern microbial mats in a coastal hypersaline lagoon,Cayo Coco (Cuba)

Active, carbonate‐mineralizing microbial mats flourish in a tropical, highly evaporative, marine‐fed lagoonal network to the south of Cayo Coco Island (Cuba). Hypersaline conditions support the development of a complex sedimentary microbial ecosystem with diverse morphologies, a variable intensity of mineralization and a potential for preservation. In this study, the role of intrinsic (i.e. microbial) and extrinsic (i.e. physicochemical) controls on microbial mat development, mineralization and preservation was investigated. The network consists of lagoons, forming in the interdune depressions of a Pleistocene aeolian substratum; they developed due to a progressive increase in sea‐level since the Holocene. The hydrological budget in the Cayo Coco lagoonal network changes from west to east, increasing the salinity. This change progressively excludes grazers and increases the saturation index of carbonate minerals, favouring the development and mineralization of microbial mats in the easternmost lagoons. Detailed mapping of the easternmost lagoon shows four zones with different flooding regimes. The microbial activity in the mats was recorded using light–dark shifts in conjunction with microelectrode O₂ and HS^? profiles. High rates of O₂ production and consumption, in addition to substantial amounts of exopolymeric substances, are indicative of a potentially strong intrinsic control on mineralization. Seasonal, climate‐driven water fluctuations are key for mat development, mineralization, morphology and distribution. Microbial mats show no mineralization in the permanently submersed zone, and moderate mineralization in zones with alternating immersion and exposure. It is suggested that mineralization is also driven by water‐level fluctuations and evaporation. Mineralized mats are laminated and consist of alternating trapping and binding of grains and microbially induced magnesium calcite and dolomite precipitation. The macrofabrics of the mats evolve from early colonizing Flat mats to complex Cerebroid or Terrace structures. The macrofabrics are influenced by the hydrodynamic regime: wind‐driven waves inducing relief terraces in windward areas and flat morphologies on the leeward side of the lagoon. Other external drivers include: (i) storm events that either promote (for example, by bioclasts covering) or prevent (for example, by causing erosion) microbial mat preservation; and (ii) subsurface degassing, through mangrove roots and desiccation cracks covered by Flat mats (i.e. forming Hemispheroids and Cerebroidal structures). These findings provide in‐depth insights into understanding fossil microbialite morphologies that formed in lagoonal settings.     

微生物席沉积学:一个年轻的沉积学分支

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...     

Dissimilatory sulfate reduction in hypersaline coastal pans: Activity across a salinity gradient

The impact of salinity on the metabolic activity of sulfate-reducing bacteria in five highly saline to hypersaline coastal pans was studied using a radioactive tracer technique. We recorded sulfate reduction at in situ porewater salinities of up to 422. Furthermore, enumeration of sulfate reduction rates in whole core incubations conducted under in situ conditions suggested a high variability in the activity of sulfate-reducers. Average reduction rates (27-3685 nmol cm⁻³ d⁻¹) varied according to depth, season and site sampled. The highest reduction rates measured in the hypersaline pan were comparable to the highest reported rates from highly productive salt marsh and microbial mat ecosystems. Correspondingly, the depth-integrated rates (integrated to 12 cm) varied from 6 to 241 mmol m⁻² d⁻¹ and were also among the highest ever reported rates. The reduction rates decreased down-core and, surprisingly, were highest in the winter season when the lowest sediment temperatures were encountered.High salt concentrations did not inhibit sulfate reduction rates. Rather, higher rates were measured at pans with higher in situ salinities. In laboratory slurry incubation experiments, sediments from the saltpans were treated with increasing salt concentrations. Regression analysis suggested that the short term response of microbial consortia to up-shock was an increase in sulfate reduction activity up to salinities of 272-311 and 134-244, in hypersaline and highly saline pans, respectively. Beyond these salinities, the cells showed evidence of reduced activities.     

Oceanographic controls on shallow‐water temperate carbonate sedimentation: Spencer Gulf,South Australia

Spencer Gulf is a large (ca 22 000 km²), shallow (<60 m water depth) embayment with active heterozoan carbonate sedimentation. Gulf waters are metahaline (salinities 39 to 47‰) and warm‐temperate (ca 12 to ?28°C) with inverse estuarine circulation. The integrated approach of facies analysis paired with high‐resolution, monthly oceanographic data sets is used to pinpoint controls on sedimentation patterns with more confidence than heretofore possible for temperate systems. Biofragments – mainly bivalves, benthic foraminifera, bryozoans, coralline algae and echinoids – accumulate in five benthic environments: luxuriant seagrass meadows, patchy seagrass sand flats, rhodolith pavements, open gravel/sand plains and muddy seafloors. The biotic diversity of Spencer Gulf is remarkably high, considering the elevated seawater salinities. Echinoids and coralline algae (traditionally considered stenohaline organisms) are ubiquitous. Euphotic zone depth is interpreted as the primary control on environmental distribution, whereas seawater salinity, temperature, hydrodynamics and nutrient availability are viewed as secondary controls. Luxuriant seagrass meadows with carbonate muddy sands dominate brightly lit seafloors where waters have relatively low nutrient concentrations (ca 0 to 1 mg Chl‐a m^?3). Low‐diversity bivalve‐dominated deposits occur in meadows with highest seawater salinities and temperatures (43 to 47‰, up to 28°C). Patchy seagrass sand flats cover less‐illuminated seafloors. Open gravel/sand plains contain coarse bivalve–bryozoan sediments, interpreted as subphotic deposits, in waters with near normal marine salinities and moderate trophic resources (0·5 to 1·6 mg Chl‐a m^?3) to support diverse suspension feeders. Rhodolith pavements (coralline algal gravels) form where seagrass growth is arrested, either because of decreased water clarity due to elevated nutrients and associated phytoplankton growth (0·6 to 2 mg Chl‐a m^?3), or bottom waters that are too energetic for seagrasses (currents up to 2 m sec^?1). Muddy seafloors occur in low‐energy areas below the euphotic zone. The relationships between oceanographic influences and depositional patterns outlined in Spencer Gulf are valuable for environmental interpretations of other recent and ancient (particularly Neogene) high‐salinity and temperate carbonate systems worldwide.     

The microbial community in the layered sediments at Laguna Figueroa,Baja California,Mexico: Does it have Precambrian analogues?

In the hypersaline lagoon at Laguna Figueroa vertically stratified diverse communities of microorganisms thrive. The modern sediments of Baja California at Laguna Figueroa contain cyanobacterial communities and sedimentary structures produced by these blue greens that have already been studied by Horodyski and his colleagues. This paper provides an introduction to the complex microbial communities, primarily those that underlie the laminated Microcoleus mats. They are composed of anaerobic photosynthetic and heterotrophic bacteria.The following genera of cyanobacteria at least are components of these mat communities: Lyngbya, Microcoleus, Entophysalis, Phormidium, Pseudoanabaena, Anabaena and Schizothrix. Among the photosynthetic bacteria several species of Thiocapsa-like microbes formed major surface components of certain mats and scums; rhodospirilli, rhodopseudomonads, chromatis and others were seen.The following nonphotosynthetic bacteria were identified: Nocardia sp., three types of spirilli, two types of Spirochaeta sp., two types of Desulfovibria sp., a new strain of red Beneckea and four distinctive unidentified coccoid and filamentous bacteria. Reasons are given for believing several of the species are new to science and that the microbial diversity is far greater than the approximately twenty species reported here. Eukaryotes are extremely rare. Only one species of animal, a herpachtechoid copepod, was ever seen in the 8-km long microbial communities of the hypersaline basin. Dunaliella salina, a chlorophyte and Aspergillus sydowi, an ascomycetous fungus were the only eukaryotes that were observed to be regular components of mat communities. Ciliates, amoebae (including a chrysarchnion-like microbe) and diatom tests, mostly empty, were the only other eukaryotes observed. Attempts to enrich for eukaryotic microorganisms were not successful whereas attempts to enrich for bacteria, especially anaerobes led to such a profusion of forms that to continue detailed study of them was beyond our means. Unidentified small rods and cocci constituted the largest fraction of individuals in the subsurface community. The microbes isolated from mats are adapted for alternating dry and wet conditions as well as high concentrations of salt and low concentrations of oxygen.