Calcified Filaments Research Articles

Speleothems from Mawsmai and Krem Phyllut caves, Meghalaya, India: some evidences on biogenic activities

The Mawsmai cave and Krem Phyllut caves, East Khasi hills, Meghalaya, India has so far not yet attracted the attention of geomicrobiologists. Observations and hypotheses on the possible influence of identified microorganisms for speleothem formations in Meghalaya are reported for the first time. XRD studies identified calcite in speleothems and gypsum in cave wall deposits as the dominant minerals. SEM-EDAX showed interesting microfabric features showing strong resemblance with fossilised bacteria, calcified filaments, needle calcite and numerous nano scale calcite crystals, highly weathered and disintegrated crystals of calcite, that point towards a significant microbial influence in its genesis. Thin section petrography showed laminated stromatolitic features. The microorganisms identified by conventional isolation and further evaluation of isolates by molecular techniques include Bacillus cereus, Bacillus mycoides, Bacillus licheniformis, Micrococcus luteus, and Actinomycetes. Microscopic observations also showed unidentifiable cocci and four unidentifiable strains of CaSO4 (gypsum) precipitating bacteria. Experimental studies confirmed that these bacteria are able to precipitate calcium minerals (calcite, gypsum, minor amounts of dolomite) in the laboratory. These results allow us to postulate that species like these may contribute to active biogenic influence in the cave formations at Meghalaya.

Microcodium: An extensive review and a proposed non-rhizogenic biologically induced origin for its formation

Microcodium has been previously described as a mainly Cenozoic calcification pattern ascribed to various organisms. A review of the available literature and our data reveal two peaks in Microcodium abundance; the Moscovian–early Permian and the latest Cretaceous–Paleogene. A detailed analysis of late Paleozoic and Cenozoic examples leads to the following new conclusions. Typical Microcodium-forming unilayered ‘corn-cob’ aggregates of elongated grains and thick multilayered (palisade) replacing structures cannot be linked to smaller-grained intracellular root calcifications, as became widely accepted after the work of Klappa [Klappa, C.F., 1979. Calcified filaments in Quaternary calcretes: organo-mineral interactions in the subaerial vadose environment. J. Sediment. Petrol. 49, 955–968.] Typical Microcodium is recognized from the early Carboniferous (with doubtful Devonian reports) to Quaternary as a biologically induced mineralization formed via dissolution/precipitation processes in various aerobic Ca-rich soil and subsoil terrestrial environments. Morphology and δ 13C signatures of Microcodium suggest that neither plants, algae, or roots and root-associated mycorrhiza regulate the formation of these fossil structures. Non-recrystallized Microcodium grains basically consist of slender (1.5–4 μm) curved radiating monocrystalline prisms with occasionally preserved hyphae-like morphology. Thin (0.5–3 μm) hypha-like canals can also be observed. These supposed hyphae may belong to actinobacteria. However, thin fungal mycelia cannot be excluded. We propose a model of Microcodium formation involving a mycelial saprotrophic organism responsible for substrate corrosion and associated bacteria capable of consuming acidic metabolites and CaCO 3 reprecipitation into the Microcodium structures.

Microbial carbonates in Holocene beachrocks, Shuiweiling, Luhuitou Peninsula, Hainan Island

Two research methods, geomicrobiology and carbonate sedimentology, were adopted to study the microbial carbonates in the beachrocks, Shuiweiling, Luhuitou, Sanya City, Hainan Island. The results indicate that microbial carbonates occur in Shuiweiling beachrocks and cements related to microbial activitives are common. Microbial carbonates were mainly developed on the surfaces of coral skeletons and within the coral (fragments and shivers) skeletal cavities, and rarely grew in interspaces formed by coral and other biological detritus. The growth morphologies of microbial carbonates are various, including stromatolitic lamination, thin crusts, dome-like, and microbialites. Microbial carbonates commonly trap fine-sized bioclasts, contain tubular filaments, and show laminated growth striation. Microbial carbonates within the coral skeletal cavities are generally stromatolitic cone-shaped, with a random growth direction along the bearing of trend of cavities, showing distinct growth laminae and recognizable growth phases. It is a special growth pattern of crypt microbial carbonates. Microbial cements occur mainly in shape of needle cement, calcified filaments, microborings, and calcified framboidal spheres. The needle cement is the most common type. These microbial cements indicate that microbial activities and microbial carbonates played an important role in beachrock formation in which carbonates were dominant components and the climate of tropical ocean monsoon was influential. Such microbial carbonates and microbial cementation have not been reported here from beachrocks along the northern shorelines of the South China Sea. The results support a clear microbial origin for certain carbonate cements in beachrocks, confirm that microbial carbonates are a significant contributor to carbonate sedimentation, and widen the knowledge of beachrocks from a new field.

Genesis of calcic and petrocalcic horizons from Coimbatore, Tamil Nadu: Micromorphology and geochemical studies

Understanding the development of calcic and petrocalcic horizons is important for palaeoclimate reconstruction. The micromorphology and geochemistry of Quaternary calcic and petrocalcic horizons formed in the area around Coimbatore, Tamil Nadu are discussed. The calcic horizons represent the Bk horizons that occur as thick complex profiles (∼300 cm thick) in the foothill regions while the laminar petrocalcic horizons representing the K horizon (80–100 cm thick) formed on hard rock in the topographic low-lying area. Calcic horizons are represented by powdery, nodular calcrete nodules and root casts with 95.2–64.5% of CaCO 3. Laminar petrocalcic horizons are compact with well rounded coalesced nodules and contain 56–64% CaCO 3. Micromorphological study of the calcic horizons show the occurrence of alveolar septal structures, calcified filaments, coated grains, spherulites, calcified root cells and calcispheres that indicate biogenic origins, mainly induced by plant root related microbial activity. The calcic nodules within the calcic horizons consist of quartz sand grains cemented by finely crystalline, grain-coating, often glaebular and pore-filling micrite. This development has taken place in phases of soil formation, erosion and reworking. The inter-relationships between these processes have caused variations in the phases of accretion of soil profiles developed in the foothill region. Microfabrics of laminar petrocalcic horizons with detritus hosts show replacement, corrosion, displacement and shrinkage indicating that the laminar petrocalcic horizon formed under relatively semi arid conditions. These characteristics indicate that the laminar petrocalcic horizons formed in a moderately near-surface environment with relatively high rates of evaporation and ground water action and are polygenetic in origin.

Weathering and biokarst development on marine terraces, northwest Morocco

AbstractWeathering features are described from an arid coastal area in northern Morocco which are indicative of invasive chasmolithic and endolithic microbial communities. Active weathering of marine terraces and karst development is associated with endolithic and epilithic algae responsible for boring, disintegration of matrix and mineral fabrics, solution and biomineralization that undermines the marine carbonate platform. Evidence of a range of biological weathering agents remains preserved, specifically calcified filaments and sporangial material. An abundance of filaments and spores representative of a consortium of algae, fungi, cyanobacteria and lichens is associated with the most denuded outcrops. The array of microbes contributes to the formation of the stromatolites to depths of 0·5 m within the limestone substrate. The preservation of stromatolites is supported by calcification of spores and filaments, with trapping and binding of carbonate held in suspension by vadose waters. The pervasive weathering on this sector of coastline has important practical implications for coastal planning and development authorities in Morocco. Copyright © 2003 John Wiley & Sons, Ltd.

Calcrete development in mediterranean colluvial carbonate systems from SE Spain

Holocene colluvial deposits from the Prebetic Zone (Betic Cordillera, S Spain) contain calcretes separating two colluvial units. Calcrete profiles contain three horizons: cemented gravels, chalky and massive-laminar. Scanning electron microscopy study revealed the presence of calcified filaments, rounded peloids and coated grains which indicates that calcrete formation was controlled by the microbial activity related with plant roots. X-ray powder diffraction data showed the presence of smectite and hematite and the absence of sepiolite–palygorskite, suggesting a semi-arid climate. The main factors controlling the features of the different calcrete horizons are topography (lateral continuity), inputs of colluvial deposits (cemented gravels and chalky calcrete) and water availability (massive-laminar calcrete).

Effect of deproteination on bone mineral morphology: implications for biomaterials and aging

Bone mineral morphology is altered by processing and this is rarely considered when preparing bone as a bioimplant material. To examine the degree of transformation, a commercial, coarsely particulate bone mineral biomaterial produced by prolonged deproteination, defatting, dehydration, and heating (donor material) was compared with similar particles of human bone (recipient material) prepared optimally by low-temperature milling. The two powders were freeze-substituted and embedded without thawing in Lowicryl K4M before sectioning for transmission electron microscopy (TEM) (other aliquots were processed by traditional TEM methods). To maximize resolution, electron micrographs were image-enhanced by digitization and printed as negatives using a Polaroid Sprint Scan 45. In addition to their morphology, the particles were examined for antigenicity ( specific by reference to fluorescein isothiocyanate [FITC]-conjugated fibronectin, and nonspecific by reference to general FITC-conjugated immunoglobulins). Results showed that the optimally prepared human bone fragments stained discretely for fibronectin with negligible background autofluorescence. In contrast, the bioimplant fragments stained extensively with this and any other FITC-conjugated antibody and, unlike fresh bone, it also autofluoresced a uniform yellow. This difference was also expressed structurally and, although the bioimplant mineral consisted of rhomboidal plates up to 200 nm across and 10 nm thick, the optimally prepared bone mineral was composed of numerous clusters of 5-nm-wide sinuous calcified filaments of variable density and indeterminate length (which became straight needles 50 nm long and 5 nm thick following traditional chemical TEM fixation/staining). It was concluded that the inorganic phase of bone is both morphologically and immunologically transmutable and that, in biomaterials, the transformation is apparently so great that a broad indigenous antigenicity is unmasked, increasing the likelihood of resorption or rejection. This marked change may also provide preliminary insight into a more modest natural aging phenomenon with the localized lateral fusion of calcified filaments into less flexible, more immunologically reactive fenestrated plates.

Evidence for Microbial Involvement in Pool Finger Precipitation, Hidden Cave, New Mexico

Although speleothems are usually considered inorganic precipitates, recent work has demonstrated hitherto unsuspected biogenic influence in some twilight areas. We have expanded this notion to the dark zone, examining pool fingers from Hidden Cave, New Mexico, to test for possible bacterial involvement. The pool fingers in Hidden Cave are pendant speleothems that formed subaqueously in paleo-pools. They are 1 to 4 cm in diameter and 5 to 50 cm long. A knobby, irregular external shape is underlaid by a layered interior on two scales, a 0.5 to 1.0 cm alternation between dense and porous layers and a mm-scale alternation between dark micritic calcite and clear dogtooth spar. The micrite is similar to microbialites identified in modern and ancient carbonates. Fossil bacteria were found in all layers. These include (1) calcified filaments 1 w m in diameter and 5–50 w m long and (2) micro-rods 0.1 w m by 1–2 w m. Most filaments are curved rods with a smooth surface but rare examples display a diamond crosshatch surface. The micro-rods occur as isolated crystals to dense meshes. We interpret the micro-rods as calcified bacilliform bacteria and the filaments as calcified filamentous bacteria. Carbon isotopic data are slightly more negative (by - 0.5 to - 1.0% in micritic layers than in dogtooth spar layers, suggesting a greater microbial influence in the micritic layers. Based on these similarities to known microbialites (e.g., petrographic fabrics, the presence of fossil bacteria, and the suggestive carbon isotopic data), we conclude that microbial activity was an intimate part of pool finger formation in Hidden Cave. The significance of such involvement goes beyond speleological contexts to wider questions of identification of biosignatures in rocks on earth and beyond.

Microbially induced cementation of carbonate sands: are micritic meniscus cements good indicators of vadose diagenesis?

Characteristic fabrics such as micrite envelopes, calcified filaments and micritic grain‐to‐grain bridges are observed in a modern subtidal firmground (Wood Cay, Bahamas) and in a variety of firm‐ and hardgrounds of Lower Cretaceous and Upper Jurassic platform carbonates (Swiss and French Jura Mountains). Their similarity to microbial fabrics described in grapestones and in intertidal to continental vadose environments suggests that microbial activity played an important role in the initial stabilization and cementation of carbonate sands. ‘Meniscus‐type cements’ (to distinguish them from vadose meniscus cements), which clearly formed in subtidal environments, are related to filament calcification, trapping of percolating micrite and microbially induced carbonate formation. Such meniscus‐type cements are commonly micritic, but meniscus‐shaped precipitation of fibrous aragonite or sparitic calcite around organic filaments is also observed. Therefore, an interpretation of vadose early diagenesis should not be based on meniscus cements alone. Similarly, subtidally formed filamentous structures can strongly resemble alveolar septal structures and be interpreted incorrectly as related to subaerial exposure.

Cryopreservation and image enhancement of juvenile and adult dentine mineral.

The inorganic component of bone and related hard tissues is generally described as sheets of uniform needle- and plate-like crystals. However, cryofixation has become the method of choice for ultrastructural studies of bone mineral when ladder-like arrangements of filaments contained within deformable microspheres about 1 microm in diameter are apparently the prime structural feature and are consistent with the optical image. The same methodology has now been applied to mature human dentine in caries-free juvenile and adult teeth. These were fixed, sliced, stained for mineral and examined optically or were snap frozen, fragmented under liquid nitrogen, freeze-substituted with methanol or acetone and embedded without thawing in Lowicryl K4M for electron microscopy. Others were processed by traditional transmission electron microscopy methods. To obtain maximum resolution, the electron micrographs were photographically printed as negatives and image-enhanced by digitisation using a Polaroid Sprint Scan 45 and laser printer. In both optical and cryopreparations of juvenile and adult dentine, mineral microspheres up to 1 microm in diameter, were present in the dentinal tubules and peritubular dentine. Within these objects, the mineral was primarily in the form of sinuous electron dense filaments, 5 nm thick, which had a characteristic periodicity. In these preparations needle-like and plate-like structures were rare. In contrast, after traditional transmission electron microscopy preparation although similar filamentous structures remained, the mineral more generally had the familiar form of needles measuring approximately 50 nm in the long axis. The cryopreserved calcified filaments were apparently particularly densely distributed in the intertubular dentine where their parallel ladder-like arrays often formed highly orientated struts and stays. It was concluded that early dentine mineral has the form of filamentous microspheres and as in bone (and other calcifying tissues and cells) has no specific association with collagen. It was also concluded that these structures compact and deform with maturity into a sub-structural framework which may relate to powerful biomechanical forces transmitted through the tissue. Needle- or plate-like mineral is probably rare in vivo in dentine, only becoming commonplace after extensive chemical processing.

Calcified Fungal Filaments in the Petrocalcic Horizon of Eutrochrepts in Beauce, France

AbstractCalcified filaments could be responsible for the induration of petrocalcic horizons, although they are absent or very rare in petrocalcic horizons with a porosity filled by needle‐fiber and micro‐rod calcite. The aim of this study was to show that calcified filaments were also numerous in these petrocalcic horizons, thus ensuring their high cohesion. Samples were collected in petrocalcic horizons of Eutrochrepts in Beauce, France. They were studied by scanning electron microscopy, and chemical analyses were performed using an energy dispersion spectrometer microprobe. Observations of fragments in the secondary electron mode showed a close packing of needle‐fiber calcite with micro‐rod calcite without any visible calcified filament. In the backscattered electron mode, observations showed numerous rounded voids, 0.5 to 1.0 µm in diameter within the packing of microrods. These voids and their walls exhibited similar morphological characteristics to those of the calcified filaments described earlier and interpreted as the remains of fungal hyphae. Chemical analysis of the rounded voids' walls, needle‐fiber calcite, and micro‐rod calcite revealed the presence of C and Ca as main elements and secondarily of Al and Si, but their respective compositions were not significantly different (P = 0.05). The C/Ca atomic ratio indicated that the analyses did not avoid resin, and that they cannot be used for discussion of calcite or calcium oxalate presence.

Calcified microspheres as biological entities and their isolation from bone.

Calcified microspheres, about 1 microm in diameter, appear at sites of bone formation where they invest the collagenous matrix, become confluent and disappear. Evidence that the particle boundaries are not lost with compaction but merely deformed is supported in section by the granular histochemical staining of the inorganic phase for bone salt, lipid, fibronectin and acid phosphatase in osteomalacic, acid-etched and normal human bone. Their persistence as discrete objects is confirmed by the application of methods for their isolation from the collagenous matrix of immature mouse calvarium and mature bovine femur. Five methods have been used to extract them and include (i) biochemical, (ii) chemical, (iii) mechanical, (iv) pyrogenous and (v) biological separation. Under the optical microscope, all isolates consisted of similar discrete objects and bridged assemblies, whose birefringence varied with treatment. After decalcification, their organic 'ghosts' remained. Each isolated microsphere had a complex substructure of clusters of non-collagenous calcified filaments surrounding a less dense centre. The filaments were 5 nm in diameter with a 5 nm periodicity and regular fine interfilamentous connections. It is concluded that the microspheres are independent, complex, pervasive and central to the containment (i.e. packaging) of calcium phosphate in bone. Their extraction will enable further analysis.

The Genesis and Time Distribution of Two Distinctive Proterozoic Stromatolite Microstructures

Stromatolites are commonly viewed as sedimentary proxies for microbial communities. In consequence, secular variation in stromatolite form has been attributed to evolutionary change in mat organisms and/or their interactions with global environments. This interpretation requires that one be able to identify features of stromatolite macrostructure or microstructure that are under direct biological influence and use them to document the sedimentological or petrological consequences of microbial evolution. Well-preserved stromatolites in Proterozoic carbonates of the Siberian Platform contain two distinctive types of microstructure that enable us to address such issues. Preserved microstructures in Baicalia lacera and Tungussia confusa from lower Neoproterozoic (lower Upper Riphean) platform carbonates of several widely separated regions clearly reflect the biology of underlying mat communities; mm-scale laminae of densely interwoven calcified filaments alternate with filament-poor microspar. Comparable carbonate microstructures are known from a number of other Neoproterozoic stromatolites, but are as yet unreported from older successions. Filamentous cyanobacteria formed mats throughout the Proterozoic Eon; thus, the temporal distribution of this microstructure appears principally to reflect secular and environmental variations in carbonate cementation and diagenesis. Omachtenia omachtensis is the characteristic stromatolite of Mesoproterozoic (Lower and, occasionally, Middle Riphean) successions of the Siberian Platform and elsewhere. Its distinctive microstructure consists of mechanically deposited event laminae separated by thin organic films that served as nucleation sites for micritic and, less commonly, fibro-radiate carbonate precipitates. Microbial mats may have stabilized sediments between events, but there is little evidence that mats played an active role in the trapping and binding or precipitation of laminae. Thus, physico-chemical factors must also be responsible for the distribution of these stromatolites in time and space. Analysis of these two microstructures suggests that, in general, secular trends in stromatolite microstructure may encrypt important information about environmental change through the Proterozoic Eon. Microbial evolution may also play a role in determining the stratigraphic distributions of particular Proterozoic stromatolites, but this remains to be demonstrated. Evolution may be most important in driving the progressive environmental restriction of stromatolite-forming microbial communities through time.

Fan-surface dynamics and biogenic calcrete development: Interactions during ultimate phases of fan evolution in the semiarid SE Spain (Murcia)

Pleistocene alluvial fan surfaces of the Campo de Cartagena–Mar Menor Basin (Murcia, SE Spain) are capped by thick mature calcretes. Calcrete profiles consist mainly of six different horizons: prismatic, chalky, nodular, massive, laminar and coated-gravels. Petrographic study of the calcretes has shown the occurrence of features such as alveolar septal structures, calcified filaments, coated grains, spherulites, calcified root cells and calcispheres that indicate the biogenic origin of the calcretes, mainly induced by plant root related microbial activity. The calcretes studied were formed initially in the soil and represented the K horizon. Development of the calcrete profiles took place in six main stages and was driven by multiple phases of soil formation, erosion and reworking. The relationships between these processes caused the formation of different calcrete profiles in proximal and distal fan areas. In the distal areas, which are controlled by limited distal fan aggradation, episodic sediment input, buried previously developed calcretes and generated new space for calcrete growth by plants growing in the overlying unconsolidated materials. This allowed the renewal of calcrete formation and it led to the development of complex composite profiles which are thicker than in proximal areas, where surface stabilisation and/or dissection enabled calcrete reworking and brecciation. These processes of erosion, sedimentation, reworking and renewed calcrete formation initiated by vegetation were repeated through time. They explain the complex macro- and microstructures of these calcretes and indicate that calcrete development, even reaching mature stages, can start before the fan surface is completely abandoned, but it requires episodic sedimentation. Eventually, distal fan aggradation and continuous calcrete development throughout the entire fan surface, led to the ultimate fan surface induration, controlling subsequent landscape evolution. So, fan surface calcretes cannot be envisaged as simple top-surface carbonate accumulations, but as complex feedback systems in which pedogenic, biogenic and sedimentary processes interact in response to the evolving fan-surface dynamics during the terminal phases of fan development in semiarid environments.

Coral reefs, Sinai, Egypt: A historical perspective

This note complements and supplements the historical perspective of a paper in which Strasser and Strohmenger (1997) related diagenesis to tectonics, sea-level, and climate. Johannes Walther (1888) studied a fossil reef at Ras Muhammed at 6 meters below sea level and suggested that it had subsided. W.F. Hume (1906) described raised coral reefs along the western shore of the Gulf of Aqaba. He suggested a Pleistocene or post-Pleistocene movement of the shoreline of the Gulf. Hume cited Walther's observations of the reef at 6 meters below sea-level as evidence for subsidence. I discovered a fossil reef (Friedman 1966) which gave a radiocarbon age of 477±140 years B.P. This reef reflects either Recent tectonic movement of the Gulf's shoreline or lowering of sea level. Like modern reefs, this fossil reef is composed of aragonite and high-magnesian calcite with subordinate low-magnesian calcite sporadically present (Fig. 1). In modern Red Sea reefs peloids of high-magnesian calcite in cryptocrystalline high-magnesian calcite cement are interpreted as calcified algal filaments.

Microfacies associations in Asbian carbonates: an example from the Urswick Limestone Formation of the southern Lake District, northern England

Abstract Semi-quantitative analysis of allochems from the Urswick Limestone Formation (Asbian) of the southern Lake District area of northern England has revealed a distinctive cyclicity of the microfacies. Cycle-top grainstone microfacies contain an algal flora comprising Koninckopora, Anatolipora and Polymorphocodium , with Girvanella filmaents and Ortonella lumps. Other allochems include intraclasts, large peloids and thick-shelled bivalves and gastropods. The middles of cycles are mostly packstones and micro-grainstones and contain allochems dominated by small peloids and the algae Kamaeena, Kamaenella and Epistacheoides , with the microproblematicum Ungdarella and relatively high abundances of micritic-walled formainifera such as endothyrids. Cycle bases contain a diverse algal assemblage including Coelosporella and Stacheoides , with other allochems represented by trilobites, ostracodes, Saccamminopsis , foraminifera such as Archaediscidae, the base late Asbian guide Howchinia , the base early Asbian guides Gigasbia gigas and Vissariotaxis , bivalves, small gastropods, bryozoans, sponge spicules and bored grains. Other allochems are found throughout most cycles, decreasing only in the very shallowest (intertidal) facies, or have an irregular distribution, and include brachiopod debris, crinoid ossicles and coral fragments. There are significant variations in allochem distribution according to palaeogeography. Close to the shelf margin there are higher abundances in the cycle top grainstones of the algae Koninckopora and Anatolipora , and also of the calcified filaments Girvanella and Ortonella , with thick-shelled gastropods, intraclasts and coarse peloids. At cycle bases, echinoderm arm plates and bryozoans are particularly abundant in packstone-wackestone textures. Platform interior facies are differentiated into a diverse open-marine type, with a high total abundance of bioclasts in the cycle base pack-wackestones including trilobites, Coelosporella, Stacheoides, Kamaena and bored grains, grainstones are dominated by small peloids, Kamaenella and Ungdarella ), and a more restricted cycle type, in which total bioclast abundances are low.

Pedogenesis of siliceous calcretes at Yucca Mountain, Nevada

Siliceous calcretes in faults near Yucca Mountain, Nevada, have been interpreted by some observers as evidence of seismically-triggered eruptions of deep water. Such an origin could have important consequences if Yucca Mountain is developed as an unsaturated site for disposal of high-level nuclear waste. At odds with this hypothesis are the absence of features that should be present at fault-fed springs (e.g., fissure-ridge mounds with microterraces) and the preservation within root casts of delicate pedogenic microfossils such as calcified filaments and needle-fiber calcites. Chemical evidence of pedogenic origin is found in Fe and Sc enrichments that require prior weathering of detritus in the calcretes, as opposed to direct incorporation of adjacent unweathered bedrock. Carbonate and silica abundances and accumulation rates are well within the scope of pedogenic processes. Sources of carbonate are eolian, whereas silica is derived in part by dissolution of local volcanic glasses but also by extensive dissolution of unstable silica minerals (tridymite and cristobalite) that are abundant in the local tuffs. The siliceous calcretes at Yucca Mountain are pedogenic and provide no evidence in support of conjectured spring activity.

Three species of Hydrolithon (Rhodophyta, Corallinaceae): Hydrolithon onkodes (Heydrich) Penrose and Woelkerling, Hydrolithon superficiale sp. nov., and H. samoënse (Foslie) comb. nov. from South Africa

Three species of Hydrolithon are reported from South Africa. Hydrolithon onkodes occurs on the southernmost coral reefs in the western Indian Ocean at Sodwana Bay, Natal. Hydrolithon superficiale sp. nov. is described on the basis of a distinctive layer of conceptacles that occur in a superficial layer of lightly calcified filaments with long, thin cells, that are sharply differentiated from the filaments of short cells in the thallus below. This species has been found only in the Sodwana Bay area of Natal Province. Lithophyllum samoënse Foslie is transferred to Hydrolithon on the basis of the anatomy of the tetrasporangial pore. It was also determined that H. samoënse (Foslie) comb. nov. is conspecific with Spongites wildpretti Afonso-Carrillo and Neogoniolithon illitus (Lemoine) Afonso-Carrillo. Hydrolithon samoënse is known to occur throughout South Africa. Drie spesies van Hydrolithon is uit Suid-Afrika gerapporteer. Hydrolithon onkodes kom voor op die mees suidelike koraalriwwe in die westelike Indiese Oseaan by Sodwanabaai, Natal. Hydrolithon superficiale sp. nov. is beskryf, gebaseer op ’n onderskeidende laag van konseptakels wat voorkom in ’n oppervlakkige laag van ligte verkalkte filamente met lang dun selle wat duidelik verskil van die filamente van kort selle in die onderliggende tallus. Hierdie spesie is slegs in die Sodwanabaai gebied van Natal gevind. Lithophyllum samoënse Foslie is oorgedra na Hydrolithon gebaseer op die anatomie van die tatrasporangiale porie. Daar is ook vasgestel dat H. samoënse (Foslie) comb. nov. konspesifiek is met Spongites wildpretii Afonso-Carrillo en Neogoniolithon illitus (Lemoine) Afonso-Carrillo. Hydrolithon samoënse kom regdeur Suid-Afrika voor.

Calcified Microbes in Neoproterozoic Carbonates: Implications for Our Understanding of the Proterozoic/Cambrian Transition

Tidal flat and lagoonal dolostones of the Neoproterozoic Draken Formation, Spitsbergen, exhibit excellent preservation of carbonate fabrics, including heavily calcified microfossils. The crust-forming cyanobacterium Polybessurus is preserved locally by carbonate precipitated on and within sheaths in mildly evaporitic upper intertidal to supratidal environments. In contrast, calcified filaments in columnar stromatolites reflect subtidal precipitation. Filament molds in dolomicrites independently document extremely early lithification. The presence of heavily calcified cyanobacteria in Draken and other Proterozoic carbonates constrains potential explanations for the widespread appearance of calcified microorganisms near the Proterozoic-Cambrian boundary. We propose that the rarity of Proterozoic examples principally reflects the abundance and wide distribution of carbonate crystals precipitated on the sea floor or in the water column. Cyanobacterial sheaths would have competed effectively as sites for carbonate nucleation and growth only where calcitic and/or aragonitic nuclei were absent. In this view, the Proterozoic-Cambrian expansion of calcified microfossils primarily reflects the emergence of skeletons as principal agents of carbonate deposition.

Role of Calcium Oxalate Biomineralization by Fungi in the Formation of Calcretes: A Case Study from Nazareth, Israel

ABSTRACT Biomineralized fungal filaments (often referred to as calcified filaments) are of interest because of their role in the calcium cycle in calcretes. The mineralogy of the crystals associated with these filaments and the evolution of their numerous crystal habits in a calcrete from Nazareth, Israel, are the subject of this study. Removing the Be window from an energy dispersive spectrometry (EDS) microprobe coupled with a scanning electron microscope (SEM) permitted the measurement of the light elements C and O, us well as Ca. Comparison of crystal spectra to those of known standards demonstrated that the great majority of needles associated with the fungal filaments are calcium oxalate. Based on these results, a model of fungal filament mineralization is proposed that describes the e olution of weddellite (polyhydrate calcium oxalate) into whewellite (monohydrate calcium oxalate) in a sequence that includes all of the crystal morphologies observed by SEM, followed by a less well understood diagenetic transformation of monohydrate calcium oxalate into calcium carbonate.