Phosphatic Stromatolites Research Articles

Chemotrophy-based phosphatic microstromatolites from the Mississippian at Drewer, Rhenish Massif, Germany

Abstract The Drewer quarry located in the Rhenish Massif is a well-studied outcrop that comprises Upper Devonian (Famennian) to Lower Carboniferous (Viséan) strata. Within the Drewer deposits two black shale intervals have been described that are linked to two global oceanic anoxic events, the Hangenberg Event and the Lower Alum Shale Event. The black shales associated with the Middle Tournaisian Lower Alum Shale Event contain abundant phosphatic concretions, which were investigated using thin section petrography, powder X-ray diffraction, Fourier-transform infrared spectrometry and scanning electron microscopy. The concretions formed during several growth phases under anoxic and at least episodically sulphidic conditions within the sediment and served as a substrate for subsurface microbial mats that formed phosphatic microstromatolites. The microstromatolites occur either as partially branched columns of up to 600 µm in length attached to the phosphatic concretions or as smaller, bulbous aggregates surrounding the concretions. Element mapping identified the presence of pyrite and other metal sulphides within the phosphatic microstromatolites. The carbon and oxygen stable isotopic composition of phosphate-associated carbonate within the phosphatic microstromatolites suggests that the mat-forming microorganisms were probably anaerobic, chemotrophic microbial communities dwelling in the anoxic environment during the Lower Alum Shale Event. Such interpretation agrees with the deeper-water depositional setting of the Lower Alum Black Shale and its high content of organic matter, suggesting that chemotrophic microbial mats are potent agents of phosphogenesis in general, and of the formation of phosphatic stromatolites in particular.

Chromium isotopes track redox fluctuations in Proterozoic successions of the Chapada Diamantina, São Francisco craton, Brazil

AbstractThe Chapada Diamantina region in the São Francisco craton of eastern Brazil is composed of sedimentary successions containing both Mesoproterozoic and Neoproterozoic carbonate levels, making it a key natural laboratory for understanding the fluctuations of Earth's biogeochemical cycles during its middle age. The ca. 1.4–1.2 Ga Caboclo Formation stromatolites yielded unfractionated δ53Crauth (authigenic) (~−0.54‰ to +0.08‰). Ediacaran cap carbonates and phosphatic stromatolites of the Salitre Formation, on the other hand, yielded fractionated δ53Crauth reaching as high as +0.51‰, suggesting the input of 53Cr-rich Cr(VI), first delivered through meltwater-induced post–snowball Earth fluctuating redox conditions and then through weathering and mobilization under a fully oxygenated environment. The acquired data set highlights the very distinct redox conditions throughout the Proterozoic and reinforces the suggestion that after the Cryogenian global glaciations, Earth's atmosphere and hydrosphere became progressively oxygenated during the Ediacaran-Cambrian transition.

Cover

GeobiologyVolume 19, Issue 1 p. i-i FEATURED COVERFree Access Cover First published: 21 January 2021 https://doi.org/10.1111/gbi.12428AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract The cover image is based on the Original Article Living phosphatic stromatolites in a low-phosphorus environment: Implications for the use of phosphorus as a proxy for phosphate levels in paleo-systems by Steffen H. Büttner et al., https://doi.org/10.1111/gbi.12415 Volume19, Issue1January 2021Pages i-i RelatedInformation

Conserved bacterial genomes from two geographically isolated peritidal stromatolite formations shed light on potential functional guilds.

Stromatolites are complex microbial mats that form lithified layers. Fossilized stromatolites are the oldest evidence of cellular life on Earth, dating back over 3.4 billion years. Modern stromatolites are relatively rare but may provide clues about the function and evolution of their ancient counterparts. In this study, we focus on peritidal stromatolites occurring at Cape Recife and Schoenmakerskop on the southeastern South African coastline, the former being morphologically and structurally similar to fossilized phosphatic stromatolites formations. Using assembled shotgun metagenomic analysis, we obtained 183 genomic bins, of which the most dominant taxa were from the Cyanobacteria phylum. We identified functional gene sets in genomic bins conserved across two geographically isolated stromatolite formations, which included relatively high copy numbers of genes involved in the reduction of nitrates and phosphatic compounds. Additionally, we found little evidence of Archaeal species in these stromatolites, suggesting that they may not play an important role in peritidal stromatolite formations, as proposed for hypersaline formations.

Living phosphatic stromatolites in a low-phosphorus environment: Implications for the use of phosphorus as a proxy for phosphate levels in paleo-systems.

In the geological record, fossil phosphatic stromatolites date back to the Great Oxidation Event in the Paleoproterozoic, but living phosphatic stromatolites have not been described previously. Here, we report on cyanobacterial stromatolites in a supratidal freshwater environment at Cape Recife, South African southern coast, precipitating Ca carbonate alternating with episodes of Ca phosphate deposition. In their structure and composition, the living stromatolites from Cape Recife closely resemble their fossilized analogues, showing phosphatic zonation, microbial casts, tunnel structures and phosphatic crusts of biogenic origin. The microbial communities appear to be also similar to those proposed to have formed fossil phosphatic stromatolites. Phosphatic domains in the material from Cape Recife are spatially and texturally associated with carbonate precipitates, but form distinct entities separated by sharp boundaries. Electron Probe Micro-Analysis shows that Ca/P ratios and the overall chemical compositions of phosphatic precipitates are in the range of octacalcium phosphate, amorphous tricalcium phosphate and apatite. The coincidence in time of the emergence of phosphatic stromatolites in the fossil record with a major episode of atmospheric oxidation led to the assumption that at times of increased oxygen release the underlying increased biological production may have been linked to elevated phosphorus availability. The stromatolites at Cape Recife, however, form in an environment where ambient phosphorus concentrations do not exceed 0.28μM, one to two orders of magnitude below the previously predicted minimum threshold of >5μM for biogenic phosphate precipitation in paleo-systems. Accordingly, we contest the previously proposed suitability of phosphatic stromatolites as a proxy for high ambient phosphate concentrations in supratidal to shallow ocean settings in earth history.

Potential photosynthetic impact on phosphate stromatolite formation after the Marinoan glaciation: Paleoceanographic implications

This study investigated the origin and the depositional age of phosphate stromatolites contained in the Neoproterozoic Salitre Formation, Brazil. The stromatolites exhibited columnar shapes and were intercalated with laminated dolostones. Their depositional environment was interpreted to have been an evaporitic ramp where erosion and reworking by waves prevailed. Laser ablation-inductively coupled plasma-mass spectrometry measurements of the stromatolites yielded a UPb age of 616 ± 32 Ma, suggesting that they were deposited after the Marinoan glaciation. Several types of filamentous structures were recognized in the stromatolites; many of them were “pseudofossil” ambient inclusion trails, but some were possibly microfossils. To consider the general presence of photosynthetic microorganisms on the surface of shallow-water stromatolites, the involvement of microbial photosynthesis in phosphate stromatolite formation was expected. Numerical calculations determined that photosynthesis could have induced phosphate mineral precipitation at the time of deposition if the phosphorus concentration was above ca. 5 μM. To achieve such high concentration at the shallow ocean, globally elevated phosphorus concentration was considered to be the ultimate factor in addition to the local process(es) including upwelling and/or evaporation. Therefore, the phosphate stromatolites in the Salitre Formation are evidence of the development of a phosphorus-rich ocean after the Marinoan glaciation.

Evidence of oxygenic phototrophy in ancient phosphatic stromatolites from the Paleoproterozoic Vindhyan and Aravalli Supergroups, India.

Fossil microbiotas are rare in the early rock record, limiting the type of ecological information extractable from ancient microbialites. In the absence of body fossils, emphasis may instead be given to microbially derived features, such as microbialite growth patterns, microbial mat morphologies, and the presence of fossilized gas bubbles in lithified mats. The metabolic affinity of micro-organisms associated with phosphatization may reveal important clues to the nature and accretion of apatite-rich microbialites. Stromatolites from the 1.6Ga Chitrakoot Formation (Semri Group, Vindhyan Supergroup) in central India contain abundant fossilized bubbles interspersed within fine-grained in situ-precipitated apatite mats with average δ13 Corg indicative of carbon fixation by the Calvin cycle. In addition, the mats hold a synsedimentary fossil biota characteristic of cyanobacterial and rhodophyte morphotypes. Phosphatic oncoid cone-like stromatolites from the Paleoproterozoic Aravalli Supergroup (Jhamarkotra Formation) comprise abundant mineralized bubbles enmeshed within tufted filamentous mat fabrics. Construction of these tufts is considered to be the result of filamentous bacteria gliding within microbial mats, and as fossilized bubbles within pristine mat laminae can be used as a proxy for oxygenic phototrophy, this provides a strong indication for cyanobacterial activity in the Aravalli mounds. We suggest that the activity of oxygenic phototrophs may have been significant for the formation of apatite in both Vindhyan and Aravalli stromatolites, mainly by concentrating phosphate and creating steep diurnal redox gradients within mat pore spaces, promoting apatite precipitation. The presence in the Indian stromatolites of alternating apatite-carbonate lamina may result from local variations in pH and oxygen levels caused by photosynthesis-respiration in the mats. Altogether, this study presents new insights into the ecology of ancient phosphatic stromatolites and warrants further exploration into the role of oxygen-producing biotas in the formation of Paleoproterozoic shallow-basin phosphorites.

Subaerial freshwater phosphatic stromatolites in Deer Cave, Sarawak — A unique geobiological cave formation

A suite of distinctive freshwater subaerial phosphatic stromatolites is developed close to the northeastern entrance of Deer Cave, Gunung Mulu National Park, Sarawak, Borneo, in conditions of very low light but ample supply of nutrients from guano. These stromatolites are not particulate; they are composed of alternating layers of more porous and more dense amorphous hydroxylapatite. This biomineralization occurs as moulds of coccoid (the majority) and filamentous (less abundant) cyanobacteria. Mineralization occurs at a pH of ~ 7.0 in the extracellular sheaths and in micro-domains of varying carbonate content in the surrounding mucus of the biofilm. The most recent surfaces that are not yet strongly mineralized show still-living filamentous, coccoid and rod-shaped forms. Trace element composition shows enrichment in metal ions, especially Mn and Zn. The stromatolites are present as horizontal shelves arranged in series on a steep rock face that is vertically under a guano-laden shelf. The rock face undergoes active dissolution from acidic guano drainage water (e.g., pH of 2.43) and from aggressive rainwater from an overhead discharge. However, the rock surface under the stromatolite is protected while the rest of the cliff face is backcut, creating a hoodoo-like effect. The stromatolites are ~ 15–20 cm deep, ~ 4–7 cm thick, and of variable width, generally ~ 50 cm. Eventually, guano and biological detritus in the descending water film lodge in the lee of the stromatolite lip, causing local acidification and erosion of stromatolite and rock on the underside of the ledge. A dynamic equilibrium is established between upward accretion of the fresh surface and destruction at the base such that the base of the stromatolite does not reflect the date of its inception and the stromatolite climbs up the wall.

Chapter 5 Chemical sediments associated with Neoproterozoic glaciation: iron formation, cap carbonate, barite and phosphorite

Abstract Orthochemical sediments associated with Neoproterozoic glaciation have prominence beyond their volumetric proportions because of the insights they provide on the nature of glaciation and the records they hold of the environment in which they were precipitated. Synglacial Fe formations are mineralogically simple (haematite jaspilite), and their trace element spectra resemble modern seawater, with a weaker hydrothermal signature than Archaean–Palaeoproterozoic Fe formations. Lithofacies associations implicate subglacial meltwater plumes as the agents of Fe(II) oxidation, and temporal oscillations in the plume flux as the cause of alternating Fe- and Mn-oxide deposits. Most if not all Neoproterozoic examples belong to the older Cryogenian (Sturtian) glaciation. Older and younger Cryogenian (Marinoan) cap carbonates are distinct. Only the younger have well-developed transgressive cap dolostones, which were laid down during the rise in global mean sea level resulting from ice-sheet meltdown. Marinoan cap dolostones have a suite of unusual sedimentary structures, indicating abnormal palaeoenvironmental conditions during their deposition. Assuming the meltdown of ice-sheets was rapid, cap dolostones were deposited from surface waters dominated by buoyant glacial meltwater, within and beneath which microbial activity probably catalysed dolomite nucleation. Former aragonite seafloor cement (crystal fans) found in deeper water limestone above Marinoan cap dolostones indicates carbonate oversaturation at depth, implying extreme concentrations of dissolved inorganic carbon. Barite is associated with a number of Marinoan cap dolostones, either as digitate seafloor cement associated with Fe-dolomite at the top of the cap dolostone, or as early diagenetic void-filling cement associated with tepee or tepee-like breccias. Seafloor barite marks a redoxcline in the water column across which euxinic Ba-rich waters upwelled, causing simultaneous barite titration and Fe(III) reduction. Phosphatic stromatolites, shrub-like structures and coated grains are associated with a glacioisostatically induced exposure surface on a cap dolostone in the NE of the West African craton, but this appears to be a singular occurrence of phosphorite formed during a Neoproterozoic deglaciation.

Stratigraphy of Palaeocene phosphate pelagic stromatolites (Prebetic Zone, SE Spain)

The hemipelagic domain of the ancient southern continental margin of Iberia is home to a strongly condensed pelagic succession (6–15 cm thick) characterized by the presence of phosphate stromatolites. This succession, probably generated in the slope of the continental margin, records a period of some 9 Ma, corresponding to the latest Maastrichtian to Late Thanetian interval. A microstratigraphical analysis allows for characterizing and biostratigraphically dating six successive developmental stages in the succession, which outline the main environmental evolution of the depositional setting. The first of them determined the generation of a submarine hardground during the latest Maastrichtian to earliest Danian interval. The other five are represented by five successive microstratigraphical, unconformity-bounded, genetic units, respectively Early–Middle Danian, Late Danian–Early Selandian, intra-Selandian, Late Selandian–Early Thanetian, and Middle–?Late Thanetian in age. The three oldest units are characterized by the accretion of phosphate stromatolites, favoured by very low rates of pelagic sedimentation and by a microbially mediated extra input of phosphate. The two youngest units are dominated by carbonate deposition, which has always taken place at very low rates. Condensed sedimentation was abruptly interrupted at the end of the Palaeocene (?latest Thanetian), when the condensed succession and its hosting substrate were gravitationally slumped and re-deposited at the base of the slope in the form of a mega-debris flow that can be now observed in Sierra de Aixorta (Alicante, SE Spain). The Aixorta pelagic phosphatic stromatolites are among the youngest ever described, and their existence suggests that the oceanographic conditions necessary for their development prevailed during most of the Palaeocene, but disappeared during the Late Selandian, never to return.

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.

The rise and decline of the Ediacaran biota: palaeobiological and stable isotopic evidence from the NW and NE Lesser Himalaya, India

Abstract Sediments of Ediacaran age have recently been recognized in the Krol Belt (NW Lesser Himalaya, India) with the discovery of pre-Ediacaran and Ediacaran biotas, carbon isotopic excursions matching the global curve and evidence for one Neoproterozoic glacial event. Other data include evidence for termination of the Krol algal-stromatolitic carbonate cycle in Late Proterozoic and deposition of phosphorite, phosphatic stromatolites similar to Tommotian taxa, and diversification of Small Shelly Fossils (SSF) of Tommotian/Meischucunian Zone 1 in Lower Tal Formation. Recently, Riphean–Ediacaran organic-walled microfossils, stromatolites and sponge spicules (monaxon) have been recovered. The link between the decrease in microbial-induced carbonate sedimentation and the decline of large Riphean stromatolites is recorded in the inner sedimentary belt (Deoban–Gangolihat). The first appearance of complex organic-walled microfossils and acanthomorphic acritarchs is recorded in the Blaini–Infrakrol sediments along with possible Ediacaran soft-bodied metazoans and metaphytes in the Krol Formation. In India, the Late Proterozoic base occurs at the bottom of a pink, microbial limestone in the Blaini Formation, and the Precambrian–Cambrian boundary occurs somewhere in the Lower Tal Formation. Ediacaran metazoans occur above glacial deposits assigned to the Varangian/Marinoan/Blainian and below the first occurrence of SSF (Meischucunian zone I), in the lowermost Cambrian.

Quaternary phosphorites off the southeast coast of India

Detailed petrological, mineralogical, geochemical and radiogenic (U, Sr, Nd) and stable isotope (C, O, S) studies have been carried out on the Quaternary phosphorites of the continental margin off Chennai, southeast coast of India. These phosphorites are formed as a result of trapping and binding of sediments by microbial mats and are similar to phosphate stratiform stromatolites. Detrital and biogenic constituents enclosed in the phosphorites controlled the major and minor element composition. Except for Sr and U, the concentrations of most of the trace metals are lower than those in average shale and phosphorite. Middle rare earth element (MREE)-enriched patterns are the characteristic feature. The U–Th dating method indicates that the ages of the phosphorites are beyond 300,000 years. 87Sr/ 86Sr ratios of the phosphorites are higher than that of present-day seawater and εNd values are more negative than those of seawater of the Atlantic Ocean. Carbon isotope ratios are within the range expected for the oxic/suboxic zone but sulfur isotope ratios indicate suboxic conditions during phosphorite formation. These results imply that the benthic microbial mats thrived on the shallow shelf during the Quaternary low sea level conditions. Periodic or episodic sedimentation onto the mats led to their death. The bacteria associated with decaying microbial mats utilised phosphorus supplied by continental sources and rapidly precipitated phosphate. The availability of a high percentage of phosphorus in seawater seems to be an important controlling factor for the formation of phosphate stromatolites. The composition of these phosphorites differs from the modern phosphorites in upwelling regions, but are similar to Cambrian apatite stromatolites. These phosphorites provide evidence that the replicates of ancient phosphate stromatolites do exist in the Quaternary.

Genesis of apatite in phosphate stromatolites

Upper Jurassic phosphatic stromatolites associated with condensed pelagic sediments in Alpine-Mediterranean paleomargins mainly consist of microbially precipitated francolite, a low crystallinity carbonate fluorapatite. SEM images of microbial phosphate laminae show highly porous colloform textures formed by dense accumulations of micrometre-size spheroidal, ovoidal and sausage-shaped bodies that in places form strings and resemble bacterial growth structures. TEM observation reveals that these structures are mostly constituted by an arrangement of hexagonal crystallites of calcium phosphate, with sizes ranging between 0.1 and 1 μm in diameter, of the same shape and size as those observed in mature dental enamel. The phosphate crystallites are wrapped in Fe-Al-Si-rich poorly crystalline phases (authigenic Fe-rich smectite plus Fe-Si-Al-rich amorphous phases) that confer them the rounded external morphologies usually observed under SEM. TEM study also reveals the existence of rounded electron-lucent centres of 5–50 nm in diameter within these francolite crystallites, produced by the release of volatile compounds under electron bombardment. HRTEM study of some areas between francolite crystals and clays shows the presence of: (1) pools of extremely small, poorly crystalline calcium phosphate crystals with two ranges of sizes (20–50 nm and 5–15 nm in basal sections), some of them showing modulated contrast interpreted as growth steps; and (2) of an amorphous calcium phosphate. The amorphous phosphate phase constitutes a cloudy film that surrounds crystallites and sometimes forms groups of spherules with sizes ranging between 2–5 nm. Low crystallinity Fe-rich smectite, Fe-Si-Al-rich non-crystalline phases, amorphous calcium phosphate, and francolite appear so closely intergrown with each other that a genetic relation must be inferred for all of them. The observed textural and structural relationships demonstrate that, in the phosphate stromatolites, bacterially-mediated precipitation of an amorphous calcium phosphate precursory of the francolite crystals was associated with that of Fe-Si-Al-rich gels precursory of authigenic smectite. The precipitation of amorphous phases was kinetically favoured, and took place in close relation to bacterial mats and sheaths rich in mucilaginous organic substances, after heterotrophic bacterial degradation of P-rich organic matter. During early diagenesis amorphous phosphate precursors transformed into francolite.

Origin of phosphatic stromatolites in the Upper Cretaceous condensed sequence of the Polish Jura Chain

The Turonian stromatolite-bearing condensed sequence in the Polish Jura Chain (the European epicontinental basin) provides good insight into the environment of formation of Cretaceous phosphatic stromatolites, owing to their purely phosphatic development and negligible post-depositional alteration. The sequence developed as a result of slow pelagic sedimentation and microbial mat phosphatization on a submarine swell surrounded by local basins with non-condensed carbonate deposition. Diagenesis of organic matter and dissolution of biogenic apatite were the major sources of reactive phosphorus for the microbial mat phosphatization. Stromatolite growth occurred due to pulses of amorphous or poorly ordered calcium phosphate precipitation followed by crystallization of carbonate fluorapatite (CFA). The phosphogenic environment left an imprint on the isotopic composition of limestone carbon and lattice-bound carbon and sulphur in CFA, and on the light rare-earth element (LREE) distribution in CFA. The δ13C of the stromatolite-bearing sequence shows a negative excursion (−1 to −3‰), standing in marked contrast to positive carbon values of the surrounding basinal carbonate. Most of the δ34S values of CFA (+20 to +21‰) fit the value range of the coeval seawater sulphate, and the LREE distribution shows a well-defined seawater pattern. This geochemical signature is indicative of intense diagenesis of organic matter at the seafloor, pelagic carbonate dissolution, and prolonged exposure of the deposited phosphate towards the water column. The enhanced deposition and diagenesis of organic phosphorus in the stromatolitic environment reflects elevated levels of the epicontinental basin nutrification related to sea-level rises and the associated oceanographic and geochemical changes.

Quaternary Phosphorites from the Continental Margin Off Chennai, Southeast India: Analogs of Ancient Phosphate Stromatolites

ABSTRACT Pleistocene phosphorites occur on the continental margin off Chennai abundantly in the depth range of 186-293 m. They are associated with outer-shelf glauconites and carbonate skeletals including large shells of molluscs and rhodoliths. These are primary phosphate deposits and are similar to phosphate stratiform stromatolites. This is the first report of phosphate stratiform stromatolites during the Quaternary and modern times, and they provide a Quaternary analog for ancient phosphorites. Thin sections of phosphorites exhibit laminated phosphatized microbial mats consisting of mechanically deposited clastic particles or of phosphatic molds. Clay particles are scattered throughout the matrix. SEM studies reveal that the laminated phosphate matrix is made of thin sheets of tightly packed apatite globules, densely packed intertwined microfilaments, or filament molds. Phosphate with obscured laminae is composed of phosphate tubules. Cell-like structures resembling coccoid cyanobacteria or their botryoidal aggregates are common. Carbonate fluorapatite and low-magnesium and high-magnesium calcites are the major mineral phases. The structural CO2 content of apatite ranges from 4 to 5.5%. Quartz, feldspar, and goethite are accessory minerals. The microbial mats were formed on the outer shelf, most probably during conditions of low sea level in the Quaternary, and microbial processes played a major role in direct phosphatization of these mats. The primary phosphorus source seems to have been the continental supply and degradation of organic matter associated with benthic microbial communities. The phosphate stratiform stromatolites thus formed were subsequently reworked into a shelf-margin depression and resulted in the occurrence of condensed phosphorites.

Bacterially‐mediated authigenesis of clays in phosphate stromatolites

Authigenic clays in close textural relation to carbonate fluorapatite within finely laminated phosphate stromatolites of Upper Jurassic age have been studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and analytical electron microscopy (AEM). Stromatolite laminae consist of hexagonal prisms of francolite (sizes ranging between 0·1 and 1 μm) that are surrounded by poorly crystalline smectite and amorphous Fe–Si–Al oxyhydroxides. Microanalyses show that smectite is Fe rich, with highly variable composition, particularly regarding Fe and Si contents. Smectite has significant beidellitic, montmorillonitic and non‐tronitic substitutions. Although the lack of fringe contrast in some areas adjacent to the smectite packets with 1·0–1·3 nm spacing is due to differences in orientation of layers, textural and analytical data clearly indicate the presence of Fe–Si–Al amorphous phases intimately intergrown with smectite. The occurrence of poorly crystalline smectite and associated amorphous phases within microbially precipitated stromatolite laminae, both as envelopes around, and as pore‐fillings between extremely small calcium phosphate crystals, demonstrates authigenic smectite growth from a precursor Fe–Si–Al amorphous material. This material is formed in close association with a phosphate‐rich precursor. The textural and structural relations, the preservation of chemical precursors of glauconite such as nontronitic montmorillonite, and the presence of Fe–Si–Al amorphous mineral phases, imply crystallization of the observed crystalline phases from synsedimentary (bacterially precipitated) amorphous precursors during early diagenesis in postoxic environments. Carbonate fluorapatite was the first phase to crystallize from the primary gel; smectite and associated amorphous Fe–Si–Al oxyhydroxides were the residual material of the crystallization process. The slow rate of transformation (at low temperatures) from Fe–Si–Al‐rich gels to smectite, explains the textural relations between the poorly crystalline phases and the phosphate crystals, as well as the preservation of amorphous substances in relation to clays. Authigenic smectite represents the first step in glauconitization.

Phosphate stromatolites from condensed cephalopod limestones, Upper Jurassic, Southern Spain

ABSTRACTUpper Jurassic phosphate stromatolites of the Almola Sierra (Southern Spain) encrust macrofossils and hardgrounds, and form oncoids included within pelagic, condensed fossiliferous limestones. Their accretion was determined by bacterially mediated precipitation of phosphate, by the trapping and binding of fine siliciclastics and pelagic biomicrite and by the encrustation of benthonic foraminifera. Phosphorous, trace elements and rare‐earth elements were concentrated from degraded organic matter and seawater by stromatolite‐building communities, which mediated the formation of phosphate‐rich and Fe‐Al‐Si‐rich organic gels under oxic conditions, favouring the precipitation of amorphous mineral precursors (ACP and Fe‐Al‐Si oxyhydroxides). The observed Ce‐enrichment for some stromatolites is explained by oxidative scavenging of Ce4+ from seawater by Fe—Mn oxyhydroxides. The bacterially mediated gels were able to migrate and fill the voids of the stromatolite structure, and later changed to carbonate‐fluorapatite, haematite and poorly crystalline Fe‐rich clays under postoxic conditions. Phosphatization of trapped carbonate particles also occurred. The phosphate stromatolites formed on a sediment‐starved pelagic swell, during periods of no carbonate sedimentation and hardground development. Stromatolite lamination provides evidence for rhythmic alternation between bacterially mediated phosphogenesis, sedimentation and erosion, suggesting episodic changes in the sedimentary environment. Although some of the parameters that controlled phosphate precipitation associated with the stromatolites (local high organic productivity, sediment starvation, moderate depth of deposition and physicochemical conditions) were similar to those found in modern and ancient phosphogenic settings, the palaeogeographical framework and the intensity of sedimentary processes were different to those of the World's major phosphorite deposits.

The Origin and Features of Late Sinian Phosphatic Stromatolites in Fuquan, Guizhou Province

AbstractThe phosphatic stromatolite discussed in this paper is a columnar type, comprised chiefly of collophane and subordinately of microcrystalline apatite. The column is composed of alternations of dark, cryptocrystalline laminae formed by the biological processes of algae and bacteria, and light, microcrystalline laminae formed by chemical precipitation. The intercolumnar filler, showing a concave anti‐stromatolitic structure, is composed of alternations of loose laminae formed by mechanical deposition, and dense, crypto‐, microcrystalline laminae formed by chemical precipitation and algal bacterial organic process.Under the SEM, the column differs clearly from the intercolumnar filler in ultramicro‐fabric. The three basic types of ultramicro‐laminae that can be distinguished in the column are: prismatic apatite laminae of chemical origin, clastic apatite laminae of mechanical origin and branched‐tubular apatite laminae of algal‐bacterial origin, of which the first and second categories constitute light laminae and the third, dark laminae. The dark and light laminae were formed during the alternation of flourishing and dormant periods of algae and bacteria.Under the SEM, the clastic texture is obvious in the loose laminae of the intercolumnar filler. And four types of ultramicro‐apatite assemblages can be distinguished in the dense laminae, i. e., the. framboidal and the tubular apatite of algal‐bacterial origin, the framboidal‐prismatic and the framboidal‐tubular‐prismatic apatite of chemical organic origin. The alternation of the loose and the dense laminae depends upon the changes of the energy in the water.In the stromatolites, no replacement phenomena have been found and there exists abundant prismatic apatite kept in the initial form of upward‐crystallization, and especially, in the forms of algae and bacteria (framboidal, tubular and branched). All these undoubtedly indicate that the stromatolites were originally formed by algae and bacteria.The ultramicro‐fabric features indicate that the collophane is not a simple mineral, but a disorderly arranged aggregate of ultramicro‐granular apatite.

A phosphatic stromatolite (Ilictacf.compositaSidorov) from the Middle Cambrian, northern Australia

Phosphatic stromatolites from the early Middle Cambrian (Ordian) of the Georgina Basin are described and identified as Ilicta cf. composita Sidorov. Based on Opik's interpretation of the early Middle Cambrian, the age of the Georgina Basin specimens is similar to that of the type specimens described from the late Early Cambrian of Eastern Siberia. Phosphatic stromatolites occur at the base of dolomitized and partially silicified bioherms up to 4 m thick. The phosphatic forms overlie impermeable, cemented pavement surfaces which were covered by stratiform stromatolites from which rose columnar forms up to 5 cm in height. The stromatolites were phosphatized by penecontemporaneous diagenetic reactions that took place just below the sediment water interface and above the impermeable substrates.