Analysis of growth directions of columnar stromatolites from Walker Lake, western Nevada

Samples of digitate, branching, columnar stromatolites were collected from the steep sides and near horizontal top of four in situ boulders located on the southwestern side of Walker Lake, Nevada, to test the widely held assumption that stromatolite column formation represents a phototropic response. We would predict that the columns on the steeply dipping sides of the boulder would bend upwards toward the light during growth if phototropism was significant during stromatolite morphogenesis. Angle of growth measurements on >300 stromatolites demonstrate that the stromatolites grew nearly normal to their growth surface, regardless of the inclination of their growth surface. No significant differences in the distribution of growth angles between north-, south-, east-, or west-facing samples were observed, and stromatolite lamina thickness did not systematically vary with position on the boulder. The lack of a strong phototropic response does not rule out a biological origin for the Walker Lake structures, but it does suggest that phototropic growth was not a dominant factor controlling stromatolite morphogenesis in these stromatolites and that column formation cannot be uniquely attributed as a phototropic response in stromatolites. It is interesting to note that the morphology of the stromatolites on the top of the boulder is identical to stromatolites on the steep sides. Stromatolite morphogenetic models that predict branching typically require a vertically directed sedimentary component, a feature that would have likely affected the stromatolites on the tops of the boulders, but not the sides, suggesting that other factors may be important in stromatolite morphogenesis.     

台湾地区上下第三系界线划分的孢粉学证据

通过对台湾中部南投县国姓地区北港溪剖面的孢粉样品分析,结合已有的台湾北部基隆地区万里－大武仑露头剖面的孢粉资料,认为台湾地区上下第三系界线置于炭寮地层与十四股层（南投）或公馆凝灰岩与木山层（基隆）之间较为合理。其孢粉组合特征,反映出古气候由晚渐新世经含桤木粉－松粉孢粉组合为特征的寒冷潮湿的北亚热带型向早中新世以含榆科、栎属孢粉组合为特征的温暖湿润的南亚热带型过渡趋势。由于南海北部大陆架北坡的珠海组     

Stromatolites and basin analysis: an example from the upper proterozoic of northwestern Canada

Stromatolites have been used for inter-basinal biostratigraphic correlation, rock-stratigraphic correlation within individual sedimentary basins and for palaeoecological studies of various kinds. In the northern part of Victoria Island stromatolites are abundant in the uppermost part of the Gelenelg Formation, which is the lowest unit of the upper Proterozoic Shaler Group. Measurable attributes of these stromatolites include elongate mounds, intermound channel fillings, ridges and grooves, elongate collumns and inclined columns. In a widespread stromatolitic bank that forms the upper part of the Glenelg Formation, and also in stromatolites of the overlying Reynolds Point Formation, several of these features show a preferred orientation in a northeasterly direction. Herringbone cross-beds in associated sandy oolitic limestones show a northeast—southwest bimodal-bipolar distribution that is probably related to tidal activity. This similarity of directional features suggests that the stromatolite orientations are also likely to have been tidally influenced. If each stromatolitic bank were deposited diachronously then the northeasterly preferred orientation may be explained as being due to tidal currents active at a migrating shoreline that trended in a northwest-southeast direction. Alternatively, if, in the absence of metazoan competitors, the stromatolite builders contemporaneously occupied a large part of the basin floor, their northeasterly orientation may reflect tidal currents parallel to the length of an elongate embayment of the Precambrian sea, analogous in many ways to the present-day Persian Gulf. Such an interpretation, involving parallelism between coastline and elongate stromatolites, would differ from those of most earlier reports, in which elongate stromatolites have generally been assumed to have been oriented normal to the ancient shoreline.     

Formation of micro‐spherulitic barite in association with organic matter within sulfidized stromatolites of the 3.48 billion‐year‐old Dresser Formation,Pilbara Craton

The shallow marine and subaerial sedimentary and hydrothermal rocks of the ~3.48 billion‐year‐old Dresser Formation are host to some of Earth's oldest stromatolites and microbial remains. This study reports on texturally distinctive, spherulitic barite micro‐mineralization that occur in association with primary, autochthonous organic matter within exceptionally preserved, strongly sulfidized stromatolite samples obtained from drill cores. Spherulitic barite micro‐mineralization within the sulfidized stromatolites generally forms submicron‐scale aggregates that show gradations from hollow to densely crystallized, irregular to partially radiating crystalline interiors. Several barite micro‐spherulites show thin outer shells. Within stromatolites, barite micro‐spherulites are intimately associated with petrographically earliest dolomite and nano‐porous pyrite enriched in organic matter, the latter of which is a possible biosignature assemblage that hosts microbial remains. Barite spherulites are also observed within layered barite in proximity to stromatolite layers, where they are overgrown by compositionally distinct (Sr‐rich), coarsely crystalline barite that may have been sourced from hydrothermal veins at depth. Micro‐spherulitic barite, such as reported here, is not known from hydrothermal systems that exceed the upper temperature limit for life. Rather, barite with near‐identical morphology and micro‐texture is known from zones of high bio‐productivity under low‐temperature conditions in the modern oceans, where microbial activity and/or organic matter of degrading biomass controls the formation of spherulitic aggregates. Hence, the presence of micro‐spherulitic barite in the organic matter‐bearing Dresser Formation sulfidized stromatolites lend further support for a biogenic origin of these unusual, exceptionally well‐preserved, and very ancient microbialites.     

Diazotrophy in Modern Marine Bahamian Stromatolites

N2 fixation (nitrogenase activity), primary production, and diazotrophic community composition of stromatolite mats from Highborne Cay, Exuma, Bahamas, were examined over a 2-year period (1997-1998). The purpose of the study was to characterize the ecophysiology of N2 fixation in modern marine stromatolites. Microbial mats are an integral surface component of these stromatolites and are hypothesized to have a major role in stromatolite formation and growth. The stromatolite mats contained active photosynthetic and diazotrophic assemblages that exhibited temporal separation of nitrogenase activity (NA) and photosynthesis. Maximal NA was detected at night. Seasonal differences in NA and net O2 production were observed. Photosynthetic activity and the availability of reduced organic carbon appear to be the key determinants of NA. Additions of the de novo protein synthesis inhibitor chloramphenicol did not inhibit NA in March 1998, but greatly inhibited NA in August 1998. Partial sequence analysis of the nifH gene indicates that a broad diversity of diazotrophs may be responsible for NA in the stromatolites.     

Geological and trace element evidence for a marine sedimentary environment of deposition and biogenicity of 3.45 Ga stromatolitic carbonates in the Pilbara Craton, and support for a reducing Archaean ocean

Bedded carbonate rocks from the 3.45 Ga Warrawoona Group, Pilbara Craton, contain structures that have been regarded either as the oldest known stromatolites or as abiotic hydrothermal deposits. We present new field and petrological observations and high‐precision REE + Y data from the carbonates in order to test the origin of the deposits. Trace element geochemistry from a number of laminated stromatolitic dolomite samples of the c. 3.40 Ga Strelley Pool Chert conclusively shows that they precipitated from anoxic seawater, probably in a very shallow environment consistent with previous sedimentological observations. Edge‐wise conglomerates in troughs between stromatolites and widespread cross‐stratification provide additional evidence of stromatolite construction, at least partly, from layers of particulate sediment, rather than solely from rigid crusts. Accumulation of particulate sediment on steep stromatolite sides in a high‐energy environment suggests organic binding of the surface. Relative and absolute REE + Y contents are exactly comparable with Late Archaean microbial carbonates of widely agreed biological origin. Ankerite from a unit of bedded ankerite–chert couplets from near the top of the stratigraphically older (3.49 Ga) Dresser Formation, which immediately underlies wrinkly stromatolites with small, broad, low‐amplitude domes, also precipitated from anoxic seawater. The REE + Y data of carbonates from the Strelley Pool Chert and Dresser Formation contrast strongly with those from siderite layers in a jasper–siderite–Fe‐chlorite banded iron‐formation from the base of the Panorama Formation (3.45 Ga), which is clearly hydrothermal in origin. The geochemical results, together with sedimentological data, strongly support: (1) deposition of Dresser Formation and Strelley Pool Chert carbonates from Archaean seawater, in part as particulate carbonate sediment; (2) biogenicity of the stromatolitic carbonates; (3) a reducing Archaean atmosphere; (4) ongoing extensive terrestrial erosion prior to ～3.45 Ga.     

Microbial diversity in modern marine stromatolites, Highborne Cay, Bahamas

Living marine stromatolites at Highborne Cay, Bahamas, are formed by microbial mat communities that facilitate precipitation of calcium carbonate and bind and trap small carbonate sand grains. This process results in a laminated structure similar to the layering observed in ancient stromatolites. In the modern marine system at Highborne Cay, lamination, lithification and stromatolite formation are associated with cycling between three types of microbial communities at the stromatolite surface (Types 1, 2 and 3, which range from a leathery microbial mat to microbially fused sediment). Examination of 923 universal small-subunit rRNA gene sequences from these communities reveals that taxonomic richness increases during transition from Type 1 to Type 3 communities, supporting a previous model that proposed that the three communities represent different stages of mat development. The phylogenetic composition also changes significantly between these community types and these community changes occur in concert with variation in biogeochemical rates. The dominant bacterial groups detected in the stromatolites include Alphaproteobacteria , Planctomycetes , Cyanobacteria and Bacteroidetes . In addition, the stromatolite communities were found to contain novel cyanobacteria that may be uniquely associated with modern marine stromatolites. The implications of these findings are discussed in the context of current models for stromatolite formation.     

Stromatolite branching in the Neoproterozoic of the Centralian Superbasin, Australia: an investigation into sedimentary and microbial control of stromatolite morphology

The extensive and well-preserved Neoproterozoic Acaciella australica Stromatolite assemblage of Australia is ideal for examining the relative roles of microbial and environmental influences on stromatolite branching and stromatolite macrostructure across a wide geographical area. Detailed sedimentological analyses indicate that the basal hemispheroidal section of bioherms contains abundant sediment. By contrast, the columnar sections of bioherms are composed almost exclusively of micritic laminae. These micritic laminae display little evidence for environmental, especially sedimentary, control over stromatolite morphology. The change from a hemispheroidal morphology to branching morphology is linked to variations in the relative contributions of sediment and framework growth. The shift to columns appears to be closely linked to a decrease in sediment supply that resulted in a more stable environment in which microbially mediated framework growth began to control stromatolite morphology. Branching in the A. australica assemblage stromatolites appears to be caused by shifting sedimentary and microbial control on stromatolite morphology.     

Biotic and Abiotic Imprints on Mg-Rich Stromatolites: Lessons from Lake Salda,SW Turkey

Abstract

Modern hydrated Mg rich stromatolites are actively growing along the shallow shorelines of Lake Salda (SW Turkey). An integrated approach involving isotopic, mineralogical, microscopic, and organic/geochemical techniques along with culture-independent molecular methods were applied to various lake samples to assess the role of microbial processes on stromatolite formation. This study further explores the biosignature preservation potential of fossil stromatolites by comparing with textures, lipid profiles and isotopic composition of the modern stromatolites. Similar lipid profile and δ¹³C isotope values in active and fossil stromatolites argue that CO₂ cycling delicately balanced between photosynthetic and heterotrophic (aerobic) activity as in the active ones may have regulated stromatolite formation in the lake. A decrease in the exopolymeric substances (EPS) profile of the mat and concurrent hydromagnesite precipitation imply a critical role for EPS in the formation of stromatolite. Consistently, a discrete, discontinuous lamination and clotted micropeloidal textures with cyanobacterial remnants in the fossil stromatolites likely refer to partial degradation of EPS, creating local nucleation sites and allowing precipitation of hydrated Mg minerals and provide a link to the active microbial mat in the modern stromatolites. Our results for the first time provide strong evidence for close coupling of cyanobacterial photosynthesis and aerobic heterotrophic respiration on hydromagnesite textures involved in the stromatolite formation of Lake Salda. The creation of photosynthesis induced high-pH conditions combined with a change in the amount and properties of the EPS and the repetition of these processes over time seems to be a possible pathway for stromatolite growth in the lake. Understanding these microbial symbioses and their mineralized records may provide new insights on the formation mechanism of Mg-rich carbonates not only for terrestrial geological records but also for planetary bodies like Mars, where hydrated Mg-carbonate deposits have been identified in possible paleolake deposits at Jezero crater, the landing site of the NASA Mars 2020 rover.     

A likely role for anoxygenic photosynthetic microbes in the formation of ancient stromatolites

Although cyanobacteria are the dominant primary producers in modern stromatolites and other microbialites, the oldest stromatolites pre-date geochemical evidence for oxygenic photosynthesis and cyanobacteria in the rock record. As a step towards the development of laboratory models of stromatolite growth, we tested the potential of a metabolically ancient anoxygenic photosynthetic bacterium to build stromatolites. This organism, Rhodopseudomonas palustris, stimulates the precipitation of calcite in solutions already highly saturated with respect to calcium carbonate, and greatly facilitates the incorporation of carbonate grains into proto-lamina (i.e. crusts). The appreciable stimulation of the growth of proto-lamina by a nonfilamentous anoxygenic microbe suggests that similar microbes may have played a greater role in the formation of Archean stromatolites than previously assumed.     

Lithostratigraphic analysis of a new stromatolite–thrombolite reef from across the rise of atmospheric oxygen in the Paleoproterozoic Turee Creek Group,Western Australia

This study describes a previously undocumented dolomitic stromatolite–thrombolite reef complex deposited within the upper part (Kazput Formation) of the c. 2.4–2.3 Ga Turee Creek Group, Western Australia, across the rise of atmospheric oxygen. Confused by some as representing a faulted slice of the younger c. 1.8 Ga Duck Creek Dolomite, this study describes the setting and lithostratigraphy of the 350‐m‐thick complex and shows how it differs from its near neighbour. The Kazput reef complex is preserved along 15 km of continuous exposure on the east limb of a faulted, north‐west‐plunging syncline and consists of 5 recognisable facies associations (A–E), which form two part regressions and one transgression. The oldest facies association (A) is characterised by thinly bedded dololutite–dolarenite, with local domical stromatolites. Association B consists of interbedded columnar and stratiform stromatolites deposited under relatively shallow‐water conditions. Association C comprises tightly packed columnar and club‐shaped stromatolites deposited under continuously deepening conditions. Clotted (thrombolite‐like) microbialite, in units up to 40 m thick, dominates Association D, whereas Association E contains bedded dololutite and dolarenite, and some thinly bedded ironstone, shale and black chert units. Carbon and oxygen isotope stratigraphy reveals a narrow range in both δ¹³C_carb values, from ?0.22 to 0.97‰ (VPDB: average = 0.68‰), and δ¹⁸O values, from ?14.8 to ?10.3‰ (VPDB), within the range of elevated fluid temperatures, likely reflecting some isotopic exchange. The Kazput Formation stromatolite–thrombolite reef complex contains features of younger Paleoproterozoic carbonate reefs, yet is 300–500 Ma older than previously described Proterozoic examples worldwide. Significantly, the microbial fabrics are clearly distinct from Archean stromatolitic marine carbonate reefs by way of containing the first appearance of clotted microbialite and large columnar stromatolites with complex branching arrangements. Such structures denote a more complex morphological expression of growth than previously recorded in the geological record and may link to the rise of atmospheric oxygen.     

中国叠层石研究的历史和现状

中国前寒武纪地层分布广泛。在20世纪70-80年代,中国学者对晚前寒武纪的叠层石进行了系统的研究,描述了类型众多的叠层石属种,并把叠层石组合应用于地层的划分和对比。近年来,为了揭示叠层石的形态发生,中国学者正在探讨硅质叠层石的生物组构模式和叠层石微层理的成因,以及叠层石中微生物生长、运动和造席过程。     

再论假裸枝叠层石科

概要地介绍了假裸枝科叠层石的特征、分布时限、分类系统、研究方法和形成环境.提出该科叠层石有可能作为我国中元古代乃至蓟县系叠层石标志的意见.     

Composition and Structure of Microbial Communities from Stromatolites of Hamelin Pool in Shark Bay, Western Australia

Stromatolites, organosedimentary structures formed by microbial activity, are found throughout the geological record and are important markers of biological history. More conspicuous in the past, stromatolites occur today in a few shallow marine environments, including Hamelin Pool in Shark Bay, Western Australia. Hamelin Pool stromatolites often have been considered contemporary analogs to ancient stromatolites, yet little is known about the microbial communities that build them. We used DNA-based molecular phylogenetic methods that do not require cultivation to study the microbial diversity of an irregular stromatolite and of the surface and interior of a domal stromatolite. To identify the constituents of the stromatolite communities, small subunit rRNA genes were amplified by PCR from community genomic DNA with universal primers, cloned, sequenced, and compared to known rRNA genes. The communities were highly diverse and novel. The average sequence identity of Hamelin Pool sequences compared to the >200,000 known rRNA sequences was only ~92%. Clone libraries were ~90% bacterial and ~10% archaeal, and eucaryotic rRNA genes were not detected in the libraries. The most abundant sequences were representative of novel proteobacteria (~28%), planctomycetes (~17%), and actinobacteria (~14%). Sequences representative of cyanobacteria, long considered to dominate these communities, comprised <5% of clones. Approximately 10% of the sequences were most closely related to those of α-proteobacterial anoxygenic phototrophs. These results provide a framework for understanding the kinds of organisms that build contemporary stromatolites, their ecology, and their relevance to stromatolites preserved in the geological record.     

Composition and structure of microbial communities from stromatolites of Hamelin Pool in Shark Bay, Western Australia

Stromatolites, organosedimentary structures formed by microbial activity, are found throughout the geological record and are important markers of biological history. More conspicuous in the past, stromatolites occur today in a few shallow marine environments, including Hamelin Pool in Shark Bay, Western Australia. Hamelin Pool stromatolites often have been considered contemporary analogs to ancient stromatolites, yet little is known about the microbial communities that build them. We used DNA-based molecular phylogenetic methods that do not require cultivation to study the microbial diversity of an irregular stromatolite and of the surface and interior of a domal stromatolite. To identify the constituents of the stromatolite communities, small subunit rRNA genes were amplified by PCR from community genomic DNA with universal primers, cloned, sequenced, and compared to known rRNA genes. The communities were highly diverse and novel. The average sequence identity of Hamelin Pool sequences compared to the >200,000 known rRNA sequences was only approximately 92%. Clone libraries were approximately 90% bacterial and approximately 10% archaeal, and eucaryotic rRNA genes were not detected in the libraries. The most abundant sequences were representative of novel proteobacteria (approximately 28%), planctomycetes ( approximately 17%), and actinobacteria (approximately 14%). Sequences representative of cyanobacteria, long considered to dominate these communities, comprised <5% of clones. Approximately 10% of the sequences were most closely related to those of alpha-proteobacterial anoxygenic phototrophs. These results provide a framework for understanding the kinds of organisms that build contemporary stromatolites, their ecology, and their relevance to stromatolites preserved in the geological record.     

Grain trapping by filamentous cyanobacterial and algal mats: implications for stromatolite microfabrics through time

Archean and Proterozoic stromatolites are sparry or fine‐grained and finely laminated; coarse‐grained stromatolites, such as many found in modern marine systems, do not appear until quite late in the fossil record. The cause of this textural change and its relevance to understanding the evolutionary history of stromatolites is unclear. Cyanobacteria are typically considered the dominant stromatolite builders through time, but studies demonstrating the trapping and binding abilities of cyanobacterial mats are limited. With this in mind, we conducted experiments to test the grain trapping and binding capabilities of filamentous cyanobacterial mats and trapping in larger filamentous algal mats in order to better understand grain size trends in stromatolites. Mats were cut into squares, inclined in saltwater tanks at angles from 0 to 75° (approximating the angle of lamina in typical stromatolites), and grains of various sizes (fine sand, coarse sand, and fine pebbles) were delivered to their surface. Trapping of grains by the cyanobacterial mats depended strongly on (i) how far filaments protruded from the sediment surface, (ii) grain size, and (iii) the mat's incline angle. The cyanobacterial mats were much more effective at trapping fine grains beyond the abiotic slide angle than larger grains. In addition, the cyanobacterial mats actively bound grains of all sizes over time. In contrast, the much larger algal mats trapped medium and coarse grains at all angles. Our experiments suggest that (i) the presence of detrital grains beyond the abiotic slide angle can be considered a biosignature in ancient stromatolites where biogenicity is in question, and, (ii) where coarse grains are present within stromatolite laminae at angles beyond the abiotic angle of slide (e.g., most modern marine stromatolites), typical cyanobacterial‐type mats are probably not solely responsible for the construction, giving insight into the evolution of stromatolite microfabrics through time.     

Isolation and biochemical characterization of extracellular polymeric secretions (EPS) from modern soft marine stromatolites (Bahamas) and its inhibitory effect on CaCO3 precipitation

Bahamian soft marine stromatolites consist of cyanobacterial biofilms and carbonate sand grains (ooids) embedded in their extracellular polymeric secretions (EPS). EPS were isolated from natural marine stromatolites and the laboratory cultured stromatolite forming cyanobacterium isolate Schizothix sp. Laboratory investigations were conducted to examine biochemical characteristics and the role of EPS in the inhibition of CaCO3 precipitation. EPS consisted of acid polysaccharides and proteins. SDS-PAGE and amino acid analysis suggested that EPS from both soft marine stromatolite and Schizothrix sp. mat contained small proteins (38 kD and 45 kD) enriched in aspartic acid and glutamic acid. Also, immuno blotting suggested that natural EPS contain high molecular weight acid polysaccharide (500 k) which may represent cross-linked products of laboratory cultured Schizothrix sp. acid polysaccharide (300 k). EPS from both soft marine stromatolite and laboratory cultured Schizothrix sp. inhibited CaCO3 precipitation in vitro, as determined using pH drift assays examining pH decrease which occur in response to CaCO3 precipitation. PH drift assays of enzymatically and chemically modified EPS isolated from soft marine stromatolite and laboratory cultured Schizothrix sp. indicated that both uronic acids and protein fractions may be involved in the inhibition of CaCO3 precipitation.     

Late Paleozoic stromatolites: new insights from the Lower Permian of Kansas

Carboniferous to Permian marine stromatolites are widely dispersed across the Pangaean margins and embayments and are typified by the ‘Ottonosia-grade stromatolite’ (designated herein). This stromatolite type consists of a well-laminated oncoid or domical stromatoid that developed into branching, laminated columns in the upper reaches. To develop a model for the global pattern, we investigated Lower Permian stromatolites from Kansas (Howe Limestone Member, Red Eagle Limestone). Stromatoids from the Lyon County locality typify the Ottonosia-grade stromatolites. The laminae are sharp throughout the stromatoid and are defined by an increase in cornuspirid foraminfera and algal filaments. The upper zone of the stromatoid is composed of well-laminated branching and brecciated columns (‘pseudo-thrombolitic’). Coeval stromatolites from a new exposure at the Tuttle Creek Dam spillway possess a more massive mesostructure. These stromatolites are composed of a turbinate stromatoid or oncoid base and an overlying domical stromatoid, and are rimmed by smaller meandering columns. Only the basal stromatoid, oncoids, and upper columns are well laminated. In both localities, the microbial-constructing ecosystem is dominated by cornuspirids and calcifying filamentous algae (?Girvanella). The mesostructural differences of the stromatolites are due to different environments of formation. The Tuttle Creek stromatolites formed in a shallow-subtidal to intertidal open marine setting. The coeval Lyon County stromatolites formed in a semi-restricted, marginal marine environment such as a lagoon or supratidal zone. Based on this information and independent sedimentological data, we conclude that lagoonal or supratidal zones were common features in the late Paleozoic intracratonal zones of the Pangaean supercontinent and account for Ottonosia-grade stromatolites occurring in the Laurentian mid-continent, the Zechstein Basin, Japan, Brazil, and Tunisia.     

Paleoenvironmental distribution of microfossils and stromatolites in the Upper Proterozoic Backlundtoppen Formation, Spitsbergen

The Upper Proterozoic (ca. 700-800 Ma old) Backlundtoppen Formation, northeastern Spitsbergen, preserves an abundant and varied record of ancient microbial life. Five distinctive microfossil assemblages occur in five equally distinct sedimentary settings; differences among the assemblages appear to reflect original ecological heterogeneity, although taphonomic circumstance may contribute to some distinctions. Microfossil assemblages occur in: oncolites, oolites, and pisolites; stratiform stromatolites and associated intraclastic rudites; partially silicified micrites; and siltites interbedded with quartz arenites. Individual assemblages contain one to eight differentiable taxa; a minimum of 17 distinct populations is present in the formation as a whole. Additional microbial community diversity an be inferred from the presence of domal, columnar, pseudocolumnar, and coniform stromatolites, none of which contains microfossils. On the basis of macrostructure, four stromatolite types appear to be present, but only three distinct mat-building communities can be inferred from microstructural features. Eohyella elongata n. sp., a euendolithic cyanobacterium found in silicified pisolites, is described as new.     

Growth of modern branched columnar stromatolites in Lake Joyce,Antarctica

Modern decimeter‐scale columnar stromatolites from Lake Joyce, Antarctica, show a change in branching pattern during a period of lake level rise. Branching patterns correspond to a change in cyanobacterial community composition as preserved in authigenic calcite crystals. The transition in stromatolite morphology is preserved by mineralized layers that contain microfossils and cylindrical molds of cyanobacterial filaments. The molds are composed of two populations with different diameters. Large diameter molds (>2.8 μm) are abundant in calcite forming the oldest stromatolite layers, but are absent from younger layers. In contrast, <2.3 μm diameter molds are common in all stromatolites layers. Loss of large diameter molds corresponds to the transition from smooth‐sided stromatolitic columns to branched and irregular columns. Mold diameters are similar to trichome diameters of the four most abundant living cyanobacteria morphotypes in Lake Joyce: Phormidium autumnale morphotypes have trichome diameters >3.5   μm, whereas Leptolyngbya antarctica, L. fragilis, and Pseudanabaena frigida morphotypes have diameters <2.3   μm. P. autumnale morphotypes were only common in mats at <12 m depth. Mats containing abundant P. autumnale morphotypes were smooth, whereas mats with few P. autumnale morphotypes contained small peaks and protruding bundles of filaments, suggesting that the absence of P. autumnale morphotypes allowed small‐scale topography to develop on mats. Comparisons of living filaments and mold diameters suggest that P. autumnale morphotypes were present early in stromatolite growth, but disappeared from the community through time. We hypothesize that the mat‐smoothing behavior of P. autumnale morphotypes inhibited nucleation of stromatolite branches. When P. autumnale morphotypes were excluded from the community, potentially reflecting a rise in lake level, short‐wavelength roughness provided nuclei for stromatolite branches. This growth history provides a conceptual model for initiation of branched stromatolite growth resulting from a change in microbial community composition.