Modern Ca:Mg carbonate stromatolites form in association with the microbial mat in the hypersaline coastal lagoon, Lagoa Vermelha (Brazil). The stromatolites, although showing diversified fabrics characterized by thin or crude lamination and/or thrombolitic clotting, exhibit a pervasive peloidal microfabric. The peloidal texture consists of dark, micritic aggregates of very high-Mg calcite and/or Ca dolomite formed by an iso-oriented assemblage of sub-micron trigonal polyhedrons and organic matter. Limpid acicular crystals of aragonite arranged in spherulites surround these aggregates. Unlike the aragonite crystals, organic matter is present consistently in the dark, micritic carbonate comprising the peloids. This organic matter is observed as submicron flat and filamentous mucus-like structures inside the interspaces of the high-Mg calcite and Ca dolomite crystals and is interpreted as the remains of degraded extracellular polymeric substances. Moreover, many fossilized bacterial cells are associated strictly with both carbonate phases. These cells consist mainly of 0AE2 to 4 lm in diameter, sub-spherical, rod-like and filamentous forms, isolated or in colony-like clusters. The co-existence of fossil extracellular polymeric substances and bacterial bodies, associated with the polyhedrons of Ca:Mg carbonate, implies that the organic matter and microbial metabolism played a fundamental role in the precipitation of the minerals that form the peloids. By contrast, the lack of extracellular polymeric substances in the aragonitic phase indicates an additional precipitation mechanism. The complex processes that induce mineral precipitation in the modern Lagoa Vermelha microbial mat appear to be recorded in the studied lithified stromatolites. Sub-micron polyhedral crystal formation of high-Mg calcite and/or Ca dolomite results from the coalescence of carbonate nanoglobules around degraded organic matter nuclei. Sub-micron polyhedral crystals aggregate to form larger ovoidal crystals that constitute peloids. Subsequent precipitation of aragonitic spherulites around peloids occurs as micro-environmental water conditions around the peloids change.
In a modern peritidal microbial mat from Qatar, both biomediated carbonates and Mg‐rich clay minerals (palygorskite) were identified. The mat, ca 5 cm thick, shows a clear lamination reflecting different microbial communities. The initial precipitates within the top millimetres of the mat are composed of Ca–Mg–Si–Al–S amorphous nanoparticles (few tens of nanometres) that replace the ultrastructure of extracellular polymeric substances. The extracellular polymeric substances are enriched in the same cations and act as a substrate for mineral nucleation. Successively, crystallites of palygorskite fibres associated with carbonate nanocrystals develop, commonly surrounding bacterial bodies. Micron‐sized crystals of low‐Mg calcite are the most common precipitates, together with subordinate aragonite, very high‐Mg calcite/dolomite and ankerite. Pyrite nanocrystals and framboids are present in the deeper layers of the mat. Calcite crystallites form conical structures, circular to triangular/hexagonal in cross‐section, evolving to crystals with rhombohedral terminations; some crystallite bundles develop into dumb‐bell and stellate forms. Spheroidal organo‐mineral structures are also common within the mat. Nanospheres, a few tens of nanometres in diameter, occur attached to coccoid bacteria and within their cells; these are interpreted as permineralized viruses and could be significant as nuclei for crystallite‐crystal precipitation. Microspheres, 1 to 10 μm in diameter, result from intracellular permineralization within bacteria or the mineralization of the bacteria themselves. Carbonates and clay minerals are commonly aggregated to form peloids, tens of microns in size, surrounded by residual organic matter. Magnesium silicate and carbonate precipitation are likely to have been driven by pH – saturation index – redox changes within the mat, related to microenvironmental chemical changes induced by the microbes – extracellular polymeric substances – viruses and their degradation.
In a multi-scale approach to the study of the organic and mineral components in an active barrage-type tufa system of southern Italy, neo-formed deposits, in both natural depositional sites and on inorganic substrates placed in the stream for this study, were observed and compared through one year of monitoring. Dams and lobes representing the basic morpho-facies of the deposits are composed of two depositional facies: vacuolar tufa (a mixture of phytoclastic and framestone tufa) and stromatolitic tufa (phytoherm boundstone tufa). Three petrographic components comprise both facies: micrite and microsparite, often forming peloidal to aphanitc, laminar and dendrolitic fabrics, and sparite, which occurs as isolated to coalescent fan-shaped crystals forming botryoids or continuous crusts. All fabrics occurring in all depositional facies are organized into layers with a more or less well-developed cyclicity, which has its best expression in stromatolitic lamination. The precipitation of all types of calcite (with Mg 1AE0 to 3AE2 mole % and Sr 0AE5 to 0AE8 mole %) takes place more or less constantly during all seasons, in spite of the low saturation state of the water (the saturation index range is 0AE75 to 0AE89) within the active depositional zone; the latter extends for a few hundred microns through the external surface of the deposit. The active depositional zone has a particular micro-morphology composed of porous micro-columns (50 to 150 lm in size), separated by interstitial channels. Mineral precipitation occurs upon both external surfaces and within internal cavities of the micro-columns, while further point sites of precipitation occur suspended within the masses of cyanobacterial tufts. Sub-spherical mineral units, 'nano-spheres' (10 to 20 nm in diameter) are the basic biotic neoprecipitate; they commonly form by replacing non-living degrading organic matter and at point sites along the external surface of living cyanobacterial sheaths. Nano-spheres agglutinate to form first rod-shaped aggregates (100 to 200 nm) which then evolve into triads of fibres or polyhedral structures. Successively, both triads and polyhedral solids coalesce to form larger calcite crystals (mainly tetrahedrons tens of microns in size) that represent the fundamental bricks for the construction of the micro-columns in the active depositional zone. Precipitation is attributed to the presence of a widespread biofilm that occurs in the active depositional zone; this is composed of a heterogeneous community comprising epilithic and endolithic filamentous cyanobacteria, green algae, unicellular prokaryotes, actinobacteria and fungi, with a variable amount of extracellular polymeric substances. No precipitation takes place where the biofilm is absent, indicating that the biological activities of the biofilm are crucial, with its living organisms and non-living organic 553 matter. Basic aggregates of neo-precipitates do not form in association with any one particular type of organic matter substrate, but appear to be related to the seasonal t...
Triassic stromatolitic dolomite from Italy preserves mineralized bacterial remains, one of the fi rst unequivocal identifi cations of such structures in the geological record. They consist of empty spheroids ~1.0 µm diameter resembling coccoid bacteria, and smaller, 150-400 nm, objects interpreted as dwarf bacterial forms, occurring within and between syn-sedimentary dolomite crystals. Moreover, gently folded sheets, 100-200 nm thick and several micrometers long, form a sub-polygonal network reminiscent of EPS (extracellular polymeric substance). Their granular-textured surfaces suggest bacterial degradation of original organic matter. These features confi rm a biological origin for the stromatolites, as in modern microbial mats, and the preserved original geochemical signatures indicate early precipitation of Mg-carbonates induced through microbial sulfate-reducing metabolic activities.
The origin of fine-grained dolomite in peritidal rocks has been the subject of much debate recently and evidence is presented here for a microbial origin of this dolomite type in the Norian Dolomia Principale of northern Calabria (southern Italy). Microbial carbonates there consist of stromatolites, thrombolites, and aphanitic dolomites. High-relief thrombolites and stromatolites characterize sub-tidal facies, and low-relief and planar stromatolites, with local oncoids, typify the inter-supratidal facies. Skeletal remains are very rare in the latter, whereas a relatively rich biota of skeletal cyanophycea, red algae and foraminifera is present in the sub-tidal facies. Some 75% of the succession consists of fabric-preserving dolomite, especially within the microbial facies, whereas the rest is composed of coarse dolomite with little fabric preservation. Three end-members of dolomite replacement fabric are distinguished: type 1 and type 2, fabric retentive, with crystal size <5 and 5-60 lm, respectively; and type 3, fabric destructive, with larger crystals, from 60 to several hundred microns. In addition, there are dolomite cements, precipitated in the central parts of primary cavities during later diagenesis. Microbialite textures in stromatolites are generally composed of thin, dark micritic laminae of type 1 dolomite, alternating with thicker lighter-coloured laminae of the coarser type 2 dolomite. Thrombolites are composed of dark, micritic clotted fabrics with peloids, composed of type 1 dolomite, surrounded by coarser type 2 dolomite. Marine fibrous cement crusts are also present, now composed of type 2 dolomite. Scanning electron microscope observations of the organic-rich micritic laminae and clots of the inter-supratidal microbialites reveal the presence of spherical structures which are interpreted as mineralized bacterial remains. These probably derived from the fossilization of micron-sized coccoid bacteria and spheroidal-ovoidal nanometre-scale dwarf-type bacterial forms. Furthermore, there are traces of degraded organic matter, probably also of bacterial origin. The microbial dolomites were precipitated in a hypersaline environment, most likely through evaporative dolomitization, as suggested by the excess Ca in the dolomites, the small crystal size, and the positive d 18 O values. The occurrence of fossilized bacteria and organic matter in the fabric-preserving dolomite of the microbialites could indicate an involvement of bacteria and organic matter degradation in the precipitation of syn-sedimentary dolomite.
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