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.     

Stromatolite-dominated microbialites at the Permian–Triassic boundary of the Xikou section on South Qinling Block,China

Permian–Triassic boundary microbialites (PTBMs) are organosedimentary carbonates formed immediately after the end-Permian mass extinction. All those reported PTBMs constrained by convincing conodont biozones are present stratigraphycally not higher than the Hindeodus parvus zone and most of them are dominated by thrombolites. This paper provides the first record of a brief, but spectacular development of stromatolite-dominated PTBMs within the basal Isarcicella isarcica conodont zone of the earliest Triassic from the Xikou section of South Qinling Block that was at the margin of the North China Block during the Permian–Triassic transition and was geographically separated from the major occurrence of post-extinction microbialites in the South China Block. This stromatolite cap overlies a 3.7-m-thick oolitic limestone and is composed of a lower 0.2-m-thick bed and an upper 0.5-m-thick bed, separated by a 0.2-m-thick greyish green siliciclastic mudstone. These two stromatolite beds mainly consist of columnar stromatolites with subordinate domal stromatolites. The intercolumn and interstitial spaces within the stromatolites are filled with oolitic grainstones. At the microscopic scale, laminoid structures in stromatolites comprise wavy, millimetric-domical and tangled laminae. The increased grain and fossil contents and/or bioturbation in the domical and tangled laminae indicate that the formation of these laminae is likely related to an increase in the populations and the disruptions by benthic metazoans, as well as an influx of sediment grains. The δ¹³C_carb values fluctuate between 2‰ and 3‰ in the uppermost Permian strata; a distinct negative shift of 1.9‰ occurs at the topmost oolitic grainstone, just below the lower stromatolite bed, and the lowest value of −0.1‰ is located at the base of the upper stromatolite bed. The stratigraphic succession from stromatolites to thrombolites of the PTBMs may represent a transgressive succession and/or a transient ecosystem recovery immediately after the end-Permian mass extinction. The thrombolites-dominated PTBMs mainly developed in near-equator shallow marine geographic locations, and stromatolite-dominated PTBMs mainly developed at higher latitude settings, which probably indicates that a relatively lower diversity and abundance of marine benthic metazoans existed at higher latitudes after the end-Permian mass extinction.     

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.     

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.     

Early Neoproterozoic well-preserved stromatolites from southern Liaoning,North China: characteristics and paleogeographic implications

We report morphology and microstructure of the stromatolites of the Ganjingzi Formation in southern Liaoning. Sedimentologic and morphologic analyses indicate that the lower stromatolite mounds formed in a transgressive succession, while the stromatolite columns in the more complex upper biostrome changed vertically from dispersed growth to dense clumping. Biostratigraphic analysis shows that the stromatolites in the Ganjingzi Formation are similar to those from coeval strata in the Xuzhou-Huainan Region and in southern Jilin. Comparisons of the morphotype genera of stromatolites and the sedimentary setting between different areas, imply that sea-level was fluctuating in the east of the North China Craton (NCC) during the Ganjingzi interval and that the transgressions were beneficial to stromatolite growth, as indicated by the increased number of stromatolites in the study area.     

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.     

Modern marine stromatolites in the Exuma Cays,Bahamas: Uncommonly common

Summary Modern stromatolites in open marine environments, unknown until recently, are common throughout the Exuma Cays, Bahamas. They occur in three distinct settings: subtidal tidal passes, subtidal sandy embayments and intertidal beaches. These stromatolites have a relief of up to 2.5 m and occur in water depths ranging from intertidal to 10 m. Surfaces near the sediment-water interface are typically colonized by cyanobacterial mats, whereas high relief surfaces are commonly colonized by algal turf and other macroalgae such asBatophora, Acetabularia, andSargassum. The internal structure of the stromatolites is characterized by millimeter-scale lamination defined by differential lithification of agglutinated sediment. In thin section, the lithified laminae appear as micritic horizons with distinct microstructures: they consist of thin micritic crusts (20–40 μm thick) overlying layers of micritized sediment grains (200–1000 μm thick); the micritized grains are cemented at point-contacts and are trucated along a surface of intense microboring. The Exuma stromatolites are built by cyanobacterial-dominated communities. These laminated prokaryotic structures grade to unlayered thrombolites built by eukaryotic algae. The variety of sites, settings and shapes of stromatolites in the Exuma Cays present excellent opportunities for future studies of stromatolite morphogenesis.     

Microaerophilic Fe‐oxidizing micro‐organisms in Middle Jurassic ferruginous stromatolites and the paleoenvironmental context of their formation (Southern Carpathians,Romania)

Ferruginous stromatolites occur associated with Middle Jurassic condensed deposits in several Tethyan and peri‐Tethyan areas. The studied ferruginous stromatolites occurring in the Middle Jurassic condensed deposits of Southern Carpathians (Romania) preserve morphological, geochemical, and mineralogical data that suggest microbial iron oxidation. Based on their macrofabrics and accretion patterns, we classified stromatolites: (1) Ferruginous microstromatolites associated with hardground surfaces and forming the cortex of the macro‐oncoids and (2) Domical ferruginous stromatolites developed within the Ammonitico Rosso‐type succession disposed above the ferruginous microstromatolites (type 1). Petrographic and scanning electron microscope (SEM) examinations reveal that different types of filamentous micro‐organisms were the significant framework builders of the ferruginous stromatolitic laminae. The studied stromatolites yield a large range of δ⁵⁶Fe values, from ?0.75‰ to +0.66‰ with predominantly positive values indicating the prevalence of partial ferrous iron oxidation. The lowest negative δ⁵⁶Fe values (up to ?0.75‰) are present only in domical ferruginous stromatolites samples and point to initial iron mobilization where the Fe(II) was produced by dissimilatory Fe(III) reduction of ferric oxides by Fe(III)‐reducing bacteria. Rare‐earth elements and yttrium (REE + Y) are used to decipher the nature of the seawater during the formation of the ferruginous stromatolites. Cerium anomalies display moderate to small negative values for the ferruginous microstromatolites, indicating weakly oxygenated conditions compatible with slowly reducing environments, in contrast to the domical ferruginous stromatolites that show moderate positive Ce anomalies suggesting that they formed in deeper, anoxic–suboxic waters. The positive Eu anomalies from the studied samples suggest a diffuse hydrothermal input on the seawater during the Middle Jurassic on the sites of ferruginous stromatolite accretion. This study presents the first interpretation of REE + Y in the Middle Jurassic ferruginous stromatolites of Southern Carpathians, Romania.     

THE ROLE OF DIATOMS IN STROMATOLITE GROWTH: TWO EXAMPLES FORM MODERN FRESHWATER SETTINGS1

Some modern laminated find calcified stromatolitic structures are partially or completely formed by eukaryotes. Diatom populations in freshwater environments with elevated ionic concentrations contribute to calcite precipitation, and the formation of distinctive mineral-rich stromatolitic laminae. Two types of stromatolite-forming diatom populations were observed. In the first example, in stromatolites growing on a quarry ledge near Laegerdorf, North Germany, calcite crystals with biogenic imprints form around polysaccharide stalks of the diatom Gomphonema olivaceum var. calcarea (Cleve) Cleve-Euler. These individually precipitated crystals eventually become cemented together in layers, forming rigid, laminated stromatolitic deposits which drape over the quarry ledge. In the second example, in stromatolites forming in a shallow stream near Cuatro Ciénegas, Coahuila, Mexico, diatomaceous laminae also form by the accumulation of carbonate particles in a matrix of diatoms and their extracellular polysaccharide products. These laminae become thick enough to drape over individual stromatolite heads. The diatoms responsible for these deposits are Amphora aff. A. katii Selva, Nitzschia denticula Grun., and six other species. At Cuatro Ciénegas, in addition to the diatomaceous laminae, carbonate-rich cyanobacterial layers, dominated by two cyanobacterial species with different fabrics and porosities, are also present and contribute substantially to the growth of the stromatolites. In both the Laegerdorf and Cuatro Ciénegas examples, entire stromatolites or thick laminations on stromatolites are built by a small number of diatom species which produce copious amounts of extracellular stalk, gel, and sheath material, a propertuy they share with cyanobacterial stromatolite builders.     

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.     

Cyanobacterial construction of hot spring siliceous stromatolites in Yellowstone National Park

Living stromatolites growing in a hot spring in Yellowstone National Park are composed of silica-encrusted cyanobacterial mats. Two cyanobacterial mat types grow on the stromatolite surfaces and are preserved as two distinct lithofacies. One mat is present when the stromatolites are submerged or at the water-atmosphere interface and the other when stromatolites protrude from the hot spring. The lithofacies created by the encrustation of submerged mats constitutes the bulk of the stromatolites, is comprised of silica-encrusted filaments, and is distinctly laminated. To better understand the cyanobacterial membership and community structure differences between the mats, we collected mat samples from each type. Molecular methods revealed that submerged mat cyanobacteria were predominantly one novel phylotype while the exposed mats were predominantly heterocystous phylotypes (Chlorogloeopsis HTF and Fischerella). The cyanobacterium dominating the submerged mat type does not belong in any of the subphylum groups of cyanobacteria recognized by the Ribosomal Database Project and has also been found in association with travertine stromatolites in a Southwest Japan hot spring. Cyanobacterial membership profiles indicate that the heterocystous phylotypes are 'rare biosphere' members of the submerged mats. The heterocystous phylotypes likely emerge when the water level of the hot spring drops. Environmental pressures tied to water level such as sulfide exposure and possibly oxygen tension may inhibit the heterocystous types in submerged mats. These living stromatolites are finely laminated and therefore, in texture, may better represent similarly laminated ancient forms compared with more coarsely laminated living marine examples.     

Lithostratigraphy and carbonate microfacies across the Permian–Triassic boundary near Julfa (NW Iran) and in the Baghuk Mountains (Central Iran)

Permian–Triassic boundary sections in the Julfa (NW Iran) and Abadeh (Central Iran) regions display a succession of three characteristic rock units, (1) the Paratirolites Limestone with the mass extinction horizon at its top, (2) the ‘Boundary Clay’, and (3) the earliest Triassic Elikah Formation with the conodont P–Tr boundary at its base. The carbonate microfacies reveals a facies change, in the sections near Julfa, within the Paratirolites Limestone with an increasing number of intraclasts, Fe–Mn crusts, and biogenic encrustation. A decline in carbonate accumulation occurs towards the top of the unit with a sponge packstone in the sections, and finally resulting in a complete demise of the carbonate factory. The succession of the ‘Boundary Clay’ differs in the two regions; thin horizons of sponge packstone are present in the Julfa region and ‘calcite fans’ of probably inorganic origin in the Abadeh Region. The skeletal carbonate factory of the Late Permian was restored with the deposition of microbial carbonates at the base of the Elikah Formation, where densely laminated bindstone, floatstone with sparry calcite spheres, and oncoid wackestone/floatstone predominate.     

Hematite‐coated microfossils: primary ecological fingerprint or taphonomic oddity of the Paleoproterozoic?

Microfossils belonging to the 1.88‐billion‐year‐old ‘Gunflint‐biota’ are preserved as carbonaceous and hematitic filaments and spheres that are believed to represent ancient chemolithoautotrophic Fe(II) oxidizing bacteria that grew above a chemocline where ferruginous seawater upwelled into shallow, oxygenated waters. This ‘biological’ model posits that hematite formed during burial from dewatering of the precursor ferric oxyhydroxides that encrusted Fe(II)‐oxidizing bacteria. Here, we present an alternate ‘taphonomic’ model in which iron‐rich groundwaters discharged into buried stromatolites; thus, the mineralization reactions are more informative of diagenetic processes than they are for primary marine conditions. We sampled centimeter‐scale columnar stromatolites from both the lower and upper stromatolite horizons of the Biwabik and Gunflint formations, across a range of metamorphic gradients including unaltered to prehnite‐pumpellyite taconite, supergene altered ore, and amphibolite‐pyroxene grade contact‐metamorphic zones. Fossils are rare to very rare and comprise curved filaments that exist in clusters with similar orientations. The filaments from throughout the Biwabik are similar to well‐preserved carbonaceous Gunflintia from Ontario. Spheres of Huroniospora are also found in both formations. Microfossils from the least altered sections are preserved as carbon. Prehnite‐pumpellyite samples are composed of either carbon or hematite (Fe₂O₃). Within the contact aureole, filaments are densely coated by magnetite (Fe₃O₄); the highest grade samples are secondarily oxidized to martite. The consistency in stromatolite microstructure and lithofacies throughout the metamorphic grades suggests they formed under similar environmental conditions. Post‐depositional alteration led to replacement of the carbon by iron oxide. The facies association, filament distribution, and lack of branching and attached spherical cells argue against Gunflintia being a direct analogue to common marine, chemolithoautotrophic Fe(II)‐oxidizing bacteria. Instead, we propose that the presence of hematite‐coated microfossils is a reflection of taphonomic processes and does not necessarily reflect the byproduct of an original microbial ecosystem.     

The role of microbes in reef-building communities of the Cannindah limestone (Mississippian), Monto region, Queensland, Australia

Reefs in the Cannindah Limestone at Old Cannindah Homestead, Monto region, Queensland, are exceptional in Eastern Australian Mississippian (Carboniferous) build-ups because of their largest dimension and differentiated microbial fabrics. Calcimicrobes and microbial carbonates, which represent a marine reefal environment occupied by both corals and sponges, are particularly abundant in the reef framework fabrics compared to other Mississippian build-ups in the world. They contributed significantly to the rigidity of the reefs on a crinoidal bank setting. Metazoans and calcimicrobes coexisted and played different roles in reef construction. Reef-building and cavity-dwelling microbes include Renalcis, Palaeomicrocodium, Girvanella, problematic Aphralysia, Ortonella, Shamovella-like, Rothpletzella-like, Wetheredella-like, and some problematic calcimicrobes, which occur in inter-corallite infillings of fasciculate rugose corals, in thrombolitic textures, in or within deposits between microdigitate stromatolite and laminated microbialites, and in reef cavities. Some reef intervals are entirely formed by Renalcis, Palaeomicrocodium, problematic calcimicrobes, and cement. Girvanella, as an encrusting calcimicrobe, generally bound bioclasts and micrite, or together with cement, formed boundstone. Microbial carbonates, including thrombolites, microencrusters, microdigitate stromatolite, laminated and tabular microbialite, irregular layers of self-encrusting vesicles, and microbial micrite, occur commonly in reef framestone and boundstone. The role of microbes and relevant microbial carbonates in the Cannindah reef limestone highlighted a significant account of microbial facies complexes associated with the Mississippian reefs.     

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.     

Analysis of intergenic spacer region length polymorphisms to investigate the halophilic archaeal diversity of stromatolites and microbial mats

Hamelin Pool in Western Australia is one of the two major sites in the world with active marine stromatolite formation. Surrounded by living smooth and pustular mats, these ancient laminated structures are associated with cyanobacterial communities. Recent studies have identified a wide diversity of bacteria and archaea in this habitat. By understanding and evaluating the microbial diversity of this environment we can obtain insights into the formation of early life on Earth, as stromatolites have been dated in the geological record as far back as 3.5 billion years. Automated ribosomal intergenic spacer analysis (ARISA) patterns were shown to be a useful method to genetically discriminate halophilic archaea within this environment. Patterns of known halophilic archaea are consistent, by replicate analysis, and the halophilic strains isolated from stromatolites have novel intergenic spacer profiles. ARISA–PCR, performed directly on extracted DNA from different sample sites, provided significant insights into the extent of previous unknown diversity of halophilic archaea within this environment. Cloning and sequence analysis of the spacer regions obtained from stromatolites confirmed the novel and broad diversity of halophilic archaea in this environment.     

Permian algae and algal microfacies from unayzah,quassim district,saudi arabia

Summary Late Permian bioclastic calcarenite beds of the middle Khuff Formation were sampled for their algal constituents near the city of Unayzah, Quassim district in central Saudia Arabia. The algal flora includes two species of udoteacean algae (Succodium difficile andSuccodium sp.), the dasycladacean algaMizzia velebitana and two species of red algae (Gymnocodium bellerophontis, Permocalculus plumosus). Other algal floral remains found forming isolated single layers which have generally been named algal microfacies. These include a phylloid microfacies, an oncoid microfacies and algal stromatolites. The Khuff Formation is well-known for its accumulation of non-associated gas and particularly for its oil accumulation in eastern Saudi Arabia and the Arabian Gulf states.     

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.     

THE ROLE OF DIATOMS IN STROMATOLITE GROWTH: TWO EXAMPLES FROM MODERN FRESHWATER SETTINGS1

Some modern laminated and calcified stromatolitic structures are partially or completely formed by eukaryotes. Diatom populations in freshwater environments with elevated ionic concentrations contribute to calcite precipitation, and the formation of distinctive mineral-rich stromatolitic laminae. Two types of stromatolite-forming diatom populations were observed. In the first example, in stromatolies growing on a quarry ledge near Laegerdorf, North Germany, calcite crystals with biogenic imprints form around polysaccharide stalks of the diatom Gomphonema olivaceum var. calcarea (Cleve) Cleve-Euler. These individually precipitated crystals eventunally become cemented together in layers, forming rigid, laminated stromatolitic deposits which drape over the quarry ledge. In the second example, in stromatolites forming in a shallow stream near Cuatro Ciénegas, Coahuila, Mexico, diatomaceous laminae also form by the accumulation of carbonate particles in a matrix of diatoms and their extracellular polysaccharide products. These laminae become thick enough to drape over individual stromatolite heads. The diatoms responsible for these deposits are Amphora aff. A. Katii Selva, Nitzschia denticula Grun., and six other species. At Cuatro Ciénegas, in addition to the diatomaceous laminae, carbonate-rich cyanobacterial layers, dominated by two cyanobacterial species with different fabrics and porosities, are also present and contribute substantially to the growth of the stromatolites. In both the Laegerdorf and Cuatro Ciénegas examples, entire stromatolites or thick laminations on stromatolites are built by a small number of diatom species which produce copious amounts of extracellular stalk, gel, and sheath material, a property they share with cyanobacterial stromatolite builders.