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Microbial signatures from speleothems: A petrographic and scanning electron microscopy study of coralloids from the Koněprusy Caves (the Bohemian Karst, Czech Republic)
Abstract
Exotic carbonate – siliceous coralloid speleothems of the Koněprusy Caves, which consist of dominant aggregates of feather‐like, radial fibrous, dogtooth and gothic‐arch calcite crystals, contain a diversity of petrified fossil microbes entombed in siliceous parts of the speleothems. Although a complete diagenetic continuum of SiO2 polymorphs, ranging from opal‐A, through opal‐CT, to moganite and crystalline quartz, was identified to form thin irregular laminae and the infills of secondary pores throughout the speleothems, the microbes have been preserved only in opal aggregates concentrated in younger growth zones of the speleothems. The identified biomorphic bodies included ovoid and spheroidal forms, interpreted as coccoid microbes, and tubular, bent and elongated forms believed to represent putative silicified filamentous microbes of unknown taxonomic affinity. Other biomorphic microstructures resembling biofilms, and plastic deformations and binary fissions of individual coccoid microbes have also been recognized. The silicified microbes, most of which have been heavily encrusted in situ, still reveal the presence of organic carbon and other biogenic elements detectable beneath their opal coats. Micro‐cavities beneath and around coccoid microbes, interpreted as micro‐borings, suggest that the microbes were able to remove the opal substrate by chemical etching. The morphology and mineralization styles of the microbes, the age of which was estimated to be in the range of 10⁰ to 10² ka, exhibit similarities to silicified microbes from present‐day siliceous hot‐spring geysers and travertines. In contrast to the siliceous parts of the coralloids, the calcite crystals forming the matrix of the speleothems, do not contain any calcified microfossils. However, deeply etched calcite crystals, spiky calcite sub‐crystals, needle‐fibre calcite and sparmicrite grains indicate that the processes of calcite dissolution–precipitation mediated by the microbes may have also affected the speleothem exteriors.
... Some domes and passages of the caves were decorated with a specific type of carbonatesiliceous speleothem known as "Koněprusy rosettes" that probably represent a testament to an early hypogene stage of cave development [32,33]. These represent cauliflower or bush-like branching, 10-20 cm long coralloids composed of fully crystalline calcite aggregates or grape-like pisolitic calcite units that grow at various angles to the limestone wallrock. ...
... NaCl equiv.), ascending from underlying lower Palaeozoic sedimentary strata, were instrumental in the dissolution of the caves [27,33,67]. Moreover, the petrographic characteristics of the Koněprusy coralloids, particularly the feather-like and acicular calcite aggregates suggestive of replacement of an aragonite precursor, the dogtooth spar and the gothic-arch calcite crystals, and the silica interlayers suggest that the speleothems originated during the final, vanishing stage of the hypogene karstification when the fluids penetrating the cave had probably been still lukewarm and slightly saline [32,67]. ...
... The early petrographic and microthermometric fluid inclusion research on the "Koněprusy rosettes" has already indicated the presence of low-salinity (<3 wt.% NaCl equiv.) NaCl and CaCl 2 solutions, possibly with a minor admixture of hydrocarbons and siliceous pseudomorphs after evaporite minerals that were preserved in the inner growth zones of the coralloids [32,33]. In contrast, our present microthermometric study, concentrating specifically on gothic-arch calcite crystals from the speleothems' exterior, revealed only the presence of freshwater inclusions of very low salinity (close to 0 wt.% ...
Gothic arch calcite, a specific crystallographic variety of calcite known from some hot springs and tufa streams, has been newly recognized in the Koněprusy Caves. The gothic-arch calcite occurs on the exteriors of exotic coralloid speleothems where it coexists with scalenohedral (dogtooth) spar crystals. The crystals exhibit microscopic ultrastructural features including deeply eroded topography, etch pits, and spiky and ribbon calcite crystallites, pointing to its extensive natural etching. Many gothic-arch calcites originated as late-stage, secondary overgrowths on older, etched dogtooth calcite crystals. Its characteristic outward curvature resulted from the recrystallization of etching-liberated fine carbonate grains and newly formed needle-fiber calcite laths, which were accumulated and bound on the faces and at the bases of corroded crystals. These intimately coexisting destructive and constructive processes of carbonate crystal corrosion and growth were probably mediated by bacteria, fungi, or other microorganisms. Fluid inclusions embedded in calcite crystals point to a vadose setting and temperatures below ~50 °C. This, combined with the wider geological context, indicates that the gothic arch calcite crystals originated only during the late Pleistocene to Holocene epochs, when the cave, initially eroded by hypogene fluids in the deeper subsurface, was uplifted to the subaerial setting and exposed to the meteoric waters seeping from the topographic surface. The radiocarbon analysis shows that gothic-arch calcite crystals are generally older than ~55,000 years, but the surface layers of some crystals still reveal a weak 14C activity, suggesting that microbiologically mediated alterations of the speleothems may have been occurring locally until now.
... Some vibrational bands linked to calcite were also reported by FTIR and Raman ( Figures S3B and S4B and Table S1). Carbonate-siliceous coralloid speleothems, with alternating layers showing different mineralogical composition, were also reported in lava tubes (Ló pez-Martínez et al., 2016;Miller et al., 2014), limestone caves (Suchý et al., 2021) and granite cavities (Vidal-Romaní et al., 2010). This mineralogical intercalation during speleothem growth may occur because of changes on surface environmental conditions or land use, or to internal factors within the cave, such as changes in pH, in the chemical composition of the dripping water or adaption works of the cave to tourism accessibility (Miller et al., 2016. ...
... Each spherical bioform measured 3-4 mm in diameter and linked together to form large linear aggregates, some of them resembling binary fission ( Figure 5B). Similar features were reported by Suchý et al. (2021) in carbonated-siliceous coralloids from Kon eprusy caves system (Czech Republic). These bioforms were found embedded in a slimy matrix of EPS, which can provide nucleation sites for the precipitation of minerals, such as NFC (Cañ averas et al., 2006;Cuezva et al., 2012;Miller et al., 2012;Riquelme et al., 2015). ...
The network of lava tubes is one of the most unexploited natural wonders of the Galapagos Islands. Here, we provide the first morphological, mineralogical, and biogeochemical assessment of speleothems from volcanic caves of the Galapagos to understand their structure, composition, and origin, as well as to identify organic molecules preserved in speleothems. Mineralogical analyses revealed that moonmilk and coralloid speleothems from Bellavista and Royal Palm Caves were composed of calcite, opal-A and minor amounts of clay minerals. Extracellular polymeric substances, fossilized bacteria, silica microspheres and cell imprints on siliceous minerals evidenced microbe-mineral interactions and biologically-mediated silica precipitation. Alternating depositional layers between siliceous and carbonate minerals and the detection of biomarkers of surface vegetation and anthropogenic stressors indicated environmental and anthropogenic changes (agriculture, human waste, cave visits) on these unique underground resources. Stable isotope analysis and Py-GC/MS were key to robustly identify biomarkers, allowing for implementation of future protection policies.
... Therefore, they concluded that the probability that the metabolism of the colonising bacteria led to significant carbonate dissolution was very low. Suchy et al. (2020) identified perforations with similar appearance to Type 1 micro-perforations observed in the Nerja Cave in opal deposits preserved in speleothems of the Konӗprusy Caves (Czech Republic), attributing them to the presence of microbes that acted as nucleation sites and/or templates for opal precipitation. Pedley and Rogerson (2010), in a mesocosm experiment, also identified perforations on calcite crystals which were occupied during their development by EPS (extracellular polymeric substances) strands and frequently by small coccoid bacteria. ...
Following the declaration of the COVID-19 pandemic, the Spanish Government restricted non-essential movements of all citizens and closed all public spaces, such as the Nerja Cave, until May 31, 2020. This particular condition of the closure of the cave provided a unique opportunity to study the micro-climate conditions and carbonate precipitation in this tourist cave without the presence of visitors. Our results show the significant effect of visitors on the air isotopic signature of the cave and on the genesis of the extensive dissolution features affecting the carbonate crystals formed in the tourist sector of the cave, alerting us to the possible corrosion of the speleothems located there. The movement of visitors within the cave also favours the mobilisation of aerial fungi and bacterial spores and their subsequent sedimentation simultaneously with the abiotic precipitation of carbonates from the drip water. The traces of these biotic elements could be the origin of the micro-perforations previously described in the carbonate crystals formed in the tourist galleries of the cave, but they are subsequently enlarged due to abiotic dissolution of the carbonates through these weaker zones.
... Many authors have described the presence of these biosignatures for several speleothems [15][16][17][18] . Our previous investigations described already the presence of these imprints within "Grotta Grande dei Cervi" black deposits 12,19 . This cave is one of the main caves present in the karst system of Pietrasecca together with "Ovito di Pietrasecca". ...
This study revealed how Bacteria and Archaea communities and their metabolic functions differed between two groups of black deposits identified in gorge and cave environments. Scanning electron microscopy coupled with energy dispersive spectroscopy was used to analyse the presence of microbial biosignatures and the elemental composition of samples. Metabarcoding of the V3–V4 regions of 16S rRNA was used to investigate Bacteria and Archaea communities. Based on 16S rRNA sequencing results, PICRUSt software was used to predict metagenome functions. Micrographs showed that samples presented microbial biosignatures and microanalyses highlighted Mn concretions and layers on Al-Si surfaces. The 16S rRNA metabarcoding alpha-diversity metrics showed similar Simpson's and Shannon indices and different values of the Chao-1 index. The amplicon sequence variants (ASVs) analysis at the different taxonomic levels showed a diverse genera composition. However, the communities of all samples shared the presence of uncultured ASVs belonging to the Gemmatales family (Phylogenesis: Gemmataceae; Planctomycetes; Planctomycetota; Bacteria). The predicted metagenome functions analysis revealed diverse metabolic profiles of the Cave and Gorge groups. Genes coding for essential Mn metabolism were present in all samples. Overall, the findings on structure, microbiota, and predicted metagenome functions showed a similar microbial contribution to epigean and hypogean black deposits Mn metabolism.
Minerals are the fundamental record of abiotic processes over time, while biominerals are one of the most common records of life due to their easy preservation and abundance. However, distinguishing between biominerals and abiotic minerals is challenging due to the superimposi-tion and repetition of geologic processes and the interference of ubiquitous and diverse life on Earth's surface and crust. Mineral dubiofossils, being potential outcomes of both abi-otic and biotic environments, emerge as valuable entities that can contribute significantly to the understanding of this issue, facilitating the testing and refinement of biogenicity criteria.
Minerals are the fundamental record of abiotic processes over time, while biominerals, are one of the most common records of life due to their easy preservation and abundance. However, distinguishing between biominerals and abiotic minerals is challenging because the record is defined by the superimposition and repetition of geologic processes and the interference of ubiquitous and diverse life on Earth's surface and crust. Mineral dubiofossils, located on the threshold between abiotic and biotic, can help resolve this issue. The aim of this contribution is to decipher the origin and history of branched mineralized structures that were previously considered mineral dubiofossils. While this material has different forms and refers to biological aspects, it is difficult to associate it with any known Fossil Group due to the overlapping geological processes that occur in the Rio do Sul Formation (Pennsylvanian of the Paraná Basin), very close to the contact from a sill of the Serra Geral Group – Lower Cretaceous with a proven thermal effect. The samples were described using a protocol that evaluated: 1) morphology, texture, and structure; 2) relationship with the matrix; 3) composition and 4) context, assessing indigeneity and syngenicity, and comparing them with abiotic and biotic products. Despite conducting an extensive comparison with abiotic minerals, as well as controlled, induced, and influenced biominerals, no more probable hypothesis was found, excluding the possibility of it being a controlled biomineral due to its patternless diversity of forms and the purely thermometamorphic origin due to the branched elongated form. The occurrence of these structures suggests a complex history: a syndepositional or eodiagenetic origin of some carbonate or sulfate (gypsum, ikaite, dolomite, calcite, siderite), which may be linked to the presence of microbial mats, could have served as a template for mineralization and possibly mediated mineral growth. Mesodiagenesis could have also modified the occurrence, but the main agent responsible for its formation was the Cretaceous intrusion, which dissolved and replaced the initial mineral and precipitated calcite, resulting in the dubiofossil. Throughout these steps, physical-chemical and biological reactions, aided by the intrinsic characteristics of the matrix, amount of organic matter, and distance from contact with the intrusive body, may have increased the morphological complexity. This material illustrates how dubiofossils can be the result of a complex history and overlapping geological processes. It also highlights the difficulty in differentiating biominerals from abiotic minerals due to the scarcity of biogenicity arguments.
Palisade fabric is a ubiquitous texture of silica sinter found in low temperature (<40°C) regimes of hot spring environments, and it is formed when populations of filamentous microorganisms act as templates for silica polymerization. Although it is known that postdepositional processes such as biological degradation and dewatering can strongly affect preservation of these fabrics, the impact of extreme aridity has so far not been studied in detail. Here, we report a detailed analysis of recently silicified palisade fabrics from a geyser in El Tatio, Chile, tracing the progressive degradation of microorganisms within the silica matrix. This is complemented by heating experiments of natural sinter samples to assess the role of diagenesis. Sheathed cyanobacteria, identified as Leptolyngbya sp., were found to be incorporated into silica sinter by irregular cycles of wetting, evaporation, and mineral precipitation. Transmission electron microscopy analyses revealed that nanometer-sized silica particles are filling the pore space within individual cyanobacterial sheaths, giving rise to their structural rigidity to sustain a palisade fabric framework. Diagenesis experiments further show that the sheaths of the filaments are preferentially preserved relative to the trichomes, and that the amount of water present within the sinter is an important factor for overall preservation during burial. This study confirms that palisade fabrics are efficiently generated in a highly evaporative geothermal field, and that these biosignatures can be most effectively preserved under dry diagenetic conditions.
The Naica Underground System (NUS) in Northern Mexico comprises a lead, zinc, and silver producing mine and displays the largest gypsum crystals ever found in natural caves. The caves are now closed to the public and mining activities have been suspended for an undefined period since October 2015. Besides its geological, economical, and tourist importance, the bacterial diversity in the NUS has not been fully explored yet. This study surveyed for bacteria present on different mineral substrates (gypsum crystals, iron oxide crusts) and hot spring samples collected before the NUS was inaccessible, using culture-dependent and culture–independent (PCR-DGGE) methods. This study is the first reporting the isolation of microorganisms from Naica. Cluster analysis of DGGE fingerprints revealed slight differences between communities from caves and tunnels and according to their mineral substrate type while communities from solid substrates and water samples appeared to be more distant. Both approaches, culture-dependent and independent, revealed the presence of bacteria from the Firmicutes, Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria in gypsum crystals, iron oxide crusts, and hot springs, respectively. Deinococcus-Thermus and Actinobacteria were only detected by DGGE in hot spring samples. According to 16S rRNA sequencing, heterotrophic bacteria isolated under aerobic conditions were affiliated with Bacillus, Brevibacillus, Paenibacillus, Schlegelella, Cupriavidus, Pseudoxanthomonas, and Lysobacter. Most of the isolates and sequences retrieved by DGGE were related to organisms previously detected in other extreme subsurface environments. Some of the isolates were able to precipitate calcium carbonate and precipitate Fe(III) in solid media but their possible participation in biomineralization processes in situ has still to be investigated. Microbial communities found in the NUS are likely autochthonous with some allochtonous components due to human intervention. Their role in geobiological processes requires further investigation.
Natural mineral formations are a window into important processes leading to carbon storage and mineralized carbonate structures formed through abiotic and biotic processes. In the current study, we made an attempt to undertake a comprehensive approach to characterize the mineralogical, mechanical, and microbial properties of different kinds of speleothems from karstic caves; with an aim to understand the bio-geo-chemical processes in speleothem structures and their impact on nanomechanical properties. We also investigated the biomineralization abilities of speleothem surface associated microbial communities in vitro. Mineralogical profiling using techniques such as X-ray powder Diffraction (XRD) and Tescan Integrated Mineral Analyzer (TIMA) demonstrated that calcite was the dominant mineral in the majority of speleothems with Energy Dispersive X-ray Analysis (EDS) indicating a few variations in the elemental components. Differing proportions of polymorphs of calcium carbonate such as aragonite and vaterite were also recorded. Significant variations in trace metal content were recorded through Inductively Coupled Plasma Mass Spectrometer (ICP-MS). Scanning Electron Microscopy (SEM) analysis revealed differences in morphological features of the crystals which varied from triangular prismatic shapes to etched spiky forms. Microbial imprints and associations were seen in a few sections. Analysis of the associated microbial diversity showed significant differences between various speleothems at Phylum level; although Proteobacteria and Actinobacteria were found to be the predominant groups. Genus level microbial associations showed a relationship with the geochemistry, mineralogical composition, and metal content of the speleothems. The assessment of nanomechanical properties measured by Nanoindentation revealed that the speleothems with a dominance of calcite were stronger than the speleothems with mixed calcium carbonate polymorphs and silica content. The in vitro metabolic activity of the microbial communities associated with the surfaces of the speleothems resulted in calcium carbonate crystal precipitation. Firmicutes and Proteobacteria dominated these populations, in contrast to the populations seen in natural systems. The precipitation of calcium carbonate crystals in vitro indicated that microbial metabolic activity may also play an important role in the synthesis and dissociation of biominerals in the natural environment. Our study provides novel evidence of the close relationship between mineralogy, microbial ecology, geochemistry, and nanomechanical properties of natural formations.
Siliceous, opal-A speleothems of the Chimalacatepec lava tube system in central Mexico are characterized here for the first time. Morphologically, they can be classified into cylindrical and planar, and display a wide array of shapes, inner textures, and locations within the lava tube. All speleothems analyzed here are composed of opal-A, and their pores are filled with calcite and monohydrocalcite. Microscopic examination reveals a variety of microbial-looking, silicified filaments and cell casts embedded within the micro lamination of the structures. The abundance of biofilms in the Chimalacatepec lava tube may share similarities with other volcanic caves elsewhere. The direct presence of such bimorphs in the microstructure of the speleothems suggests the biological mediation of these structures. Potential mechanisms include nucleation and sorption of silica on extracellular polymeric substances in the biofilms that, along with the SiO2 saturation in the water film and evaporative effects, result in the formation of a speleothem. That said, the presence of microbes in these and other cave systems, or their inevitable interactions with the mineral phase of the speleothems, should not be surprising. In view of this, these structures can be most accurately described as biospeleothems. This study contributes to our understanding of the diversity of such structures in these types of cave systems and our ability to recognize the presence of microbes in these.
Caves in the Guadalupe Mountains offer intriguing examples of possible past or present geomicrobiological interactions within features such as corrosion residues, pool fingers, webulites, u-loops, and moonmilk. Scanning electron microscopy, transmission electron microscopy, molecular biology techniques, enrichment cultures, bulk chemistry, and X-ray diffraction techniques have revealed the presence of iron- and manganese-oxidizing bacteria in corrosion residues, which supports the hypothesis that these organisms utilize reduced iron and manganese from the limestone, leaving behind oxidized iron and manganese. Metabolically active populations of bacteria are also found in "punk rock" beneath the corrosion residues. Microscopic examination of pool fingers demonstrates that microorganisms can be inadvertently caught and buried in pool fingers, or can be more active participants in their formation. Enrichment cultures of moonmilk demonstrate the presence of a variety of microorganisms. Humans can have a deleterious impact on microbial communities in Guadalupe caves.
The Karst Waters Institute Breakthroughs in Karst Geomicrobiology and Redox Geochemistry conference in 1994 was a watershed event in the history of cave geomicrobiology studies within the US. Since that time, studies of cave geomicrobiology have accelerated in number, complexity of techniques used, and depth of the results obtained. The field has moved from being sparse and largely descriptive in nature, to rich in experimental studies yielding fresh insights into the nature of microbe-mineral interactions in caves. To provide insight into the changing nature of cave geomicrobiology we have divided our review into research occurring before and after the Breakthroughs conference, and concentrated on secondary cave deposits: sulfur (sulfidic systems), iron and manganese (ferromanganese, a.k.a. corrosion residue deposits), nitrate (a.k.a. saltpeter), and carbonate compounds (speleothems and moonmilk deposits). The debate concerning the origin of saltpeter remains unresolved; progress has been made on identifying the roles of bacteria in sulfur cave ecosystems, including cavern enlargement through biogenic sulfuric acid; new evidence provides a model for the action of bacteria in forming some moonmilk deposits; combined geochemical and molecular phylogenetic studies suggest that some ferromanganese deposits are biogenic, the result of redox reactions; and evidence is accumulating that points to an active role for microorganisms in carbonate precipitation in speleothems.
OPEN ACCESS ARTICLE AT: http://www.nature.com/articles/srep15525
Helictites—an enigmatic type of mineral structure occurring in some caves—differ from classical speleothems as they develop with orientations that defy gravity. While theories for helictite formation have been forwarded, their genesis remains equivocal. Here, we show that a remarkable suite of helictites occurring in Asperge Cave (France) are formed by biologically-mediated processes, rather than abiotic processes as had hitherto been proposed. Morphological and petro-physical properties are inconsistent with mineral precipitation under purely physico-chemical control. Instead, microanalysis and molecular-biological investigation reveals the presence of a prokaryotic biofilm intimately associated with the mineral structures. We propose that microbially-influenced mineralization proceeds within a gliding biofilm which serves as a nucleation site for CaCO3, and where chemotaxis influences the trajectory of mineral growth, determining the macroscopic morphology of the speleothems. The influence of biofilms may explain the occurrence of similar speleothems in other caves worldwide, and sheds light on novel biomineralization processes.
Analysis of water and associated carbonate precipitates from a small, warm-spring travertine system in SW Colorado, USA, provide an example of the: i) great variability of the geochemical parameters within these dynamic systems, and ii) significance of the microenvironment in controlling mineralogy and morphology of carbonate precipitates. -from Authors
These fresh-water springs are very rich in H2S. Probably as a result of this, the carbonate sediments are largely constructed by bacteria instead of algae. Even the chemically-precipitated forms of calcite show a variety of bizarre characteristics, some of them probably due to the interaction of S with the calcite crystal lattice. Among these are thick crusts of radial fibrous calcite crystals up to a meter long, that show probable daily lamination due to the diurnal activity of photosynthetic bacteria and have depositional rates of as much as one meter per year.-from Authors
Deep phreatic shafts and travertine-capped sinkholes characterize Sistema Zacatón, an isolated karst area in northeastern Mexico. At a depth of at least 329 m, El Zacatón is the deepest known underwater pit in the world. Hypogenic karst development related to volcanism is proposed to have formed El Zacatón and is thought to have diminished since the late Quaternary peak activity. The resulting geomorphic overprint of Zacatón displays features similar to hydrothermal groundwater systems throughout the world. Other karst areas in northeastern Mexico are known for deep pits and high-flow springs rising from great depths, but differ from Zacatón in the speleogenetic processes that developed the caves. Sótano de Las Golondrinas (378 m), 200 km to the southwest of Zacatón, is among the deepest air-filled shafts in the world. The Nacimiento del Río Mante, 100 km to the west, is a large artesian spring that extends a minimum of 270 m below the water table. Although these three worldclass karst systems all formed in Cretaceous limestone and are located relatively close together, there are significant differences in lithology, tectonic setting, and geomorphic features. Geochemical, microbiological, and geomorphologic data for Zacatón indicate that cave formation processes are similar to those observed in other volcanically influenced systems.
(Pre)-Cretaceous sediments that fill extensive hydrothermal dissolution cavities in Devonian limestones in the Čertovy schody Quarry contain locally abundant halloysite clays. In most samples halloysite coexists with metahalloysite and kaolinite that form snowy-white and blue-green masses intimately intergrown with manganese oxides.
The determination of kaolinite group minerals was the main purpose of this study. X-ray diffraction method and thermal analyses enabled to differentiate halloyisite from metahalloysite and kaolinite. Tubular morphology of halloysite was observable in electron micrographs.
Field observations indicate that halloysite formed due to in situ selective hydrothermal alteration of clayey matrix of quartz sandstone and/or breccias of unclear original composition that occured within the sedimentary fill. The proposed model for halloysite formation involves alteration by warm (less that 80°C) acid solutions that migrated through the system of tectonic fractures, dissolved limestone host and altered overlying sedimentary members that collapsed into the resulting hydrothermal depressions. Since halloysite partially replaced also Cenomanian sediments, the hydrothermal alteration must have occurred only during the post-Cenomanian time.
Microorganisms have been shown to be important active and passive promoters of redox reactions that influence the precipitation of various minerals, including calcite. Many types of secondary minerals thought to be of purely inorganic origin are currently being reevaluated, and microbial involvement has been demonstrated in the formation of pool fingers, stalactites and stalagmites, cave pisoliths, and moonmilk. We studied the possible involvement of bacteria in the formation of a new type of speleothem from Grave Grubbo Cave, the third-largest gypsum cave in Italy. The speleothem we studied consisted of a large aggregate of calcite tubes having a complex morphology, reflecting its possible organic origin. We isolated an abundant heterotrophic microflora associated with this concretion and identified Bacillus, Burk-holderia, and Pasteurella spp. among the isolates. All of the isolates precipitated CaCO 3 in vitro in the form of calcite. Only one of the isolates solubilized carbonate. The relative abundance of each isolate was found to be directly related to its ability to precipitate CaCO 3 at cave temperature. We suggest that hypogean environments select for microbes exhibiting calcifying activity. Isotopic analysis produced speleothem d 13 C values of about – 5.00%, confirming its organic origin. The lightest carbonates purified from B4M agar plates were produced by the most abundant isolates. SEM analysis of the speleothem showed traces of calcified filamentous bacteria interacting with the substrate. Spherical bioliths predominated among the ones produced in vitro. Within the crystals produced in vitro, we observed bacterial imprints, sometimes in a preferred orientation, suggesting the involvement of a quorum-sensing system in the calcium-carbonate precipitation process.
In a previous paper (Shtober-Zisu et al., 2012) we described millimeter to centimetersized
fluid-free holes within the interiors of stalagmites of widely varying origin. We present
here further observations of this phenomenon, using X-ray tomography, macroscopic and
microscopic observation of sections of twenty-six stalagmites from various sites in North
America and the Caribbean region. We can distinguish three types of cavities in speleothems:
primary μm-sized fluid inclusions; mm to cm sized holes, aligned along the stalagmite growth
axis which are clearly syngenetic; and μm to cm-sized holes away from the growth axis (“offaxis
holes or OAHs”) deeply buried inside their host stalagmites, and cutting primary growth
layers. Neither axial nor off axis holes contain fluid today. Off-axis holes appear to have
been formed by internal corrosion of the calcite host, possibly enhanced by the action of
bacteria which were sustained by permeation of through the body of the stalagmite of water
containing dissolved organic species. A modern stalagmite from Israel is shown to contain
bacteria associated with active hole formation.
Coralloid-type speleothems were recorded on the ceiling of the Ana Heva lava tube in Easter Island (Chile). These speleothems were morphologically, geochemically and mineralogically characterized using a wide variety of microscopy and analytical techniques. They consist dominantly of amorphous Mg silicate and opal-A. Field emission scanning electron microscopy revealed a variety of filamentous and bacillary bacteria on the surface of the Ana Heva coralloid speleothems, including silicified filamentous microorganisms. Among them, intriguing reticulated filaments resemble those filaments documented earlier in limestone caves and lava tubes. The identification of silicified microorganisms on the coralloid speleothems from the Ana Heva lava tube suggests a possible role of these microorganisms in silica deposition.
Tjuv-Antes grotta (Tjuv-Ante’s Cave) located in northern Sweden is a round-abraded sea
cave (‘tunnel cave’), about 30 m in length, formed by rock-water abrasion in a dolerite
dyke in granite gneiss. Abundant speleothems are restricted to the inner, mafic parts of the
cave and absent on granite parts. The speleothems are of two types: cylindrical (coralloid,
popcorn-like), and flowstone (thin crusts). Coralloids correspond to terrestrial stromatolite
speleothems in which layers of light calcite alternate with dark, silica-rich laminae. The dark
laminae are also enriched in carbon and contain incorporated remains of microorganisms.
Two types of microbial communities can be distinguished associated with the speleothems: an
Actinobacteria-like biofilm and a fungal community. Actinobacteria seem to play an important
role in the formation of speleothem while the fungal community acts as both a constructive
and a destructive agent. A modern biofilm dominated by Actinobacteria is present in the
speleothem-free parts of the dolerite and located in cave ceiling cracks. These biofilms may
represent sites of early speleothem formation. Because of its unusual position in between
two types of host rock, Tjuv-Ante’s Cave represents a unique environment in which to study
differences in microbe-rock interactions and speleothem genesis between the granite and
dolerite host rock. Our study shows that the mafic rock is superior
The tufas of the presently shallow Chlupáčova sluj Cave document an unusual history of deeper to shallower hydrologic processes in cave evolution. The wider geological context of the cave, along with fluid inclusion and stable isotope (C and O) analyses of calcite from tectonic veins cutting through its wallrock, are evidence of the origin of the cave in the deeper subsurface, under the influence of ascending 25–70 °C warm, saline (2.0–22.3 wt% eq. NaCl) waters driven by regional hypogene processes. The formation of calcite veins, which occurred at 700–400 ka, was probably coeval with the early stages of cave development. However, two generations of tufa formed in the later Pleistocene during the mature shallow-subsurface stage of cave development when daylight and cold meteoric waters penetrated the cave through ceiling windows and tectonic fractures. The first-generation tufa, which consists of cauliflower-like aggregates of hollow, outward-radiating carbonate tubules covered with isopachous banded calcite cements, is interpreted as ancient bryophyte tufa that was deposited about 620 ± 140–530 ± 110 ka. The presence of microscopic biomorphic fabrics resembling micro-stromatolites, calcareous algal chambers, calcite crystals with cloudy cores and clear sparitic rims, and specific biomarkers identified in tufa extracts suggest that, in addition to mosses, algae and bacteria may have also been instrumental in promoting the deposition of tufa. Tufa stable isotope (C and O) characteristics and fluid inclusion contents point to deposition of this first-generation tufa from fresh water with a minor admixture of higher hydrocarbons at the surface temperature. Post-dating the first-generation tufa, minute grains of exotic minerals (e.g. stibnite, gothic-arch calcite, gypsum, Mn-minerals, opaline silica) and sparry calcite were precipitated in the tufa moulds and between carbonate tubules as cements. These minerals suggest at least one event in which hydrothermal waters entered the cave from below after deposition of the first tufa from cool meteoric waters. In this hydrothermal phase, the cave may have been a hot spring. The second-generation tufa originated during the Riss – Würm Interglacial at 121 ± 2.1–107 ± 2.6 ka, following a period of intense water erosion that partly erased older spelean deposits from cave walls. This tufa consists of highly porous, friable, crudely laminated carbonate sediments with floral and molluscan remains, characteristic of common cold-water karstic tufa. This and the earlier tufa were, in turn, covered by later fine-grained cave sediments, contributing to the preservation of Pleistocene tufa that records an alternation of deep and shallow waters in the cave over at least the last 500,000 years.
Nastanek hidrotermalnih kraških jam je povezan z variskičnimi hidrotermalnimi procesi. Nastali votli prostori so bili zapolnjeni s kristaliničnim kalcitom. Proces je spremljala močna dolomitizacija. Mlajša faza hidrotermalnega zakrasevanja pa ni bila povezana z zapolnjevanjem žil, ampak z globokim kroženjem kraške vode, najbrž v zvezi z “neovulkansko” dejavnostjo v Češkem masivu. To potrjujeta pelod in razpadli vulkanski pepel v kapnikih, ki so nastali po vodilni speleogenetski fazi. Jame v Koněpruskih devonskih kamninah so domnevno nastale v ujetem vodonosniku v freatičnih in batifreatičnih okoliščinah. Termalne okoliščine so se pojavile, ko se je zaradi močne “neovulkanske” dejavnosti povečala paleogeotermična stopnja. Hidrotermalno zakrasevanje je deloma spremenilo obliko jam. Na podlagi velikih kalcitnih kristalov, ki so se odložili v freatićnih in globokih freatičnih okoliščinah, je mogoče sklepati, da je bila največja temperatura 60-700 C. Piezometrični nivo je bil nad apnenci v silikaklastih zgornjekredne platforme, na kar navajajo številne skoraj navpične freatične cevi (“depresije”), zapolnjene s krednimi in terciarnimi sedimenti, ki so zdrsnili vanje, ko se je pritisk vode zmanjšal. Pokovki podobne silificirane “Koněpruske rozete” so lahko nastale zaradi znižanja gladine termalne vode in mešanja s prenikajočo padavinsko vodo. Zunanji del velikih kalcitnih kristalov, ki se je izločal pri temperaturah okoli 400 C, bi lahko kazal na postopno ohlajanje celotnega sklopa.
The origin of hydrothermal karst cavities was connected with the Variscan hydrothermal process. The cavities were formed and filled by crystalline calcite. The process was accompanied by the intensive dolomitisation. Younger phase of hydrothermal karstification was not connected with vein-filling, but with the deep circulation of groundwater, probably associated with neovolcanic activity in the Bohemian Massif. This is supported by pollen grains and decomposed volcanic ash in speleothems which were formed after the major phases of speleogenesis. It is supposed that caves in the Koněprusy Devonian were formed in confined aquifer under phreatic and batyphreatic conditions. Thermal conditions appeared when paleogeothermic gradient was increased due to intensive neovolcanic activity. Hydrothermal karstification partly changed the morphology of caves. The maximum temperatures were stated to 60-70°C from large calcite crystals precipitated under phreatic and deeply phreatic conditions. The piezometric level was situated above limestones in Upper Cretaceous platform siliciclastics as indicated by numerous subvertical phreatic tubes („depressions“) filled with sunkened Cretaceous and Tertiary sediments after the water buyoancy support decreased. Popcorn-like silicified Koněprusy Rosettes can be result of decrease of thermal water level and mixing with infiltrating meteoric waters. Outer zones of large calcite crystals with precipitation temperatures of about 40° C can indicate the gradual cooling of the whole system.
The Buda thermal karst is one of the most characteristic hypogene karst systems of the world developed in Triassic and Eocene limestone and marl in Budapest, on the right side of the Danube, under 300–400 m high hills. Due to the mixing corrosion enhanced by CO2 and perhaps sulfuric acid, the water circulation formed multi-storey cave systems with different morphologies and complicated layout along the tectonic fissures. The arrangement of caves is independent of surface topography. The world-famous medicinal waters of Budapest come to the surface through cave passages. Due to the incision of the Danube and the uplift of the area in the Pleistocene, most passages are presently dry; now they lie 100 m above the Danube, though there are still passages below the water-table. The length of the five largest known caves and about 100 smaller ones reaches 55 km, and it is reasonable to estimate the length of unknown underground passages to be comparable. Caves are characterized by complex morphology, suggesting the formation by rising flow, and by rich carbonate-sulfuric mineral precipitations. Some of them (e.g. barite and calcite veins) are associated with bedrock formation and thus predate the cave forming process. There are minerals that precipitated from the warm water that dissolved the cave (Fe- and Mn-containing minerals formed by bacterial colonies, or cave rafts and folia). Evaporitic precipitations (e.g. popcorns, and frostworks) are remarkable. There are frequent precipitations of gypsum (chandeliers, gypsum flowers, etc.) due to the pyrite content of the marl above the passages. Discovering the concealed and hidden caves since the early twentieth-century was helped by quarries and earthworks followed by systematic and deliberate exploration.
The travertine deposit at Villers-devant-Orval (S. Belgium) formed during the Preboreal till the Boreal. In the first terrace studied in this paper, five lithostratigraphic units were distinguished, based on the presence of gravel, loam, peat and fine, medium grained and coarse travertine. Within the latter, encrusted branches and algal and cyanobacterial filaments are the most important organic components. Vadose diagenesis of the deposit is indicated by the presence of needle-crystals, fiber calcite crystals and dendrites. Meteoric phreatic diagenetic products occur as gothic-arch calcite crystals and euhedral crystals, which cement the pores.
Fossil microbes are generally preserved by authigenic minerals, including silica, apatite, iron minerals, clays, and carbonates. An alternative mode of preservation by entombment in calcite, without replacement, has been identified in carbonate cave pool microbialites that were etched and examined in the scanning electron microscope (SEM). Features identified include filaments, threads, and films that show excess carbon in energy dispersive X-ray (EDX) analyses, suggesting preservation of organic matter. Filaments are single smooth or reticulated strands with curving string-like morphology, often hollow, and with a uniform diameter of 0.5 to 1.0 lm. Threads, in contrast, are variable thickness, from several microns down to 0.1 lm, always solid, and commonly branch. Films are thin (, 1 lm) drapes associated with threads. Filaments are interpreted as microbial filaments, while threads and films are interpreted as preserved extracellular polymeric substance (EPS). In addition, microbial filaments and EPS are only revealed via acid etching, suggesting preservation of organic material by entombment, not by replacement with calcite. To determine whether entombed microbes are a common feature of carbonate microbialites that form in different environmental settings, samples of hot spring travertine, caliche soil, and reef microbialite were examined. Whereas the travertine samples were barren, entombed EPS was found in the caliche soil and the reef microbialite; the latter also contained a few entombed filaments. In addition, entombed microbial material has been reported from carbonate cold seep deposits. Such findings indicate that entombment of microbes and EPS in carbonates is not restricted to cave settings, but is more widespread than previously reported. Possible causes for the lack of preservation in travertines include rapid degradation of microbial material either by sunlight due to photolytic degradation, aerobic microbial degradation, detritivore consumption, or elevated temperatures. Rapid carbonate precipitation is ruled out as, somewhat surprisingly, preservation is better in slower growing cave carbonates than in rapidly growing travertines. Potential long-term preservation of organic material entombed in carbonate has implications for the characterization of fossil microbial communities using molecular biomarkers and the search for life on other planets.
Numerous microfossils were recovered from carbonate speleothems collected in caves of the Guadalupe Mountains, New Mexico. The speleothems consisted of a fragment of rimstone dam and four stalagmites that were dissolved with 5% hydrochloric acid. Treatment with 2% potassium hydroxide removed unwanted organic matter that entrapped many of the fossils. The fossils are remarkably well preserved. Mites are the only arthropods that were recovered intact. Most scales, arthropod fecal pellets, appendage fragments of other arthropods, algae, fungi, pollen, minute fragments of plant material, and hairs of mammals were observed. The speleothems are probably Pleistocene in age. -Authors
Bacteria are ubiquitous life forms and occur throughout most natural environments. Consequently, they have developed a full spectrum of metabolic functions to adapt themselves to a diverse range of external milieux. These adaptations have been so successful that, in a global way, they are responsible for the cycling of many important elements (e.g., C, N, S, P, etc.). In this review we concentrate on their surface interaction with soluble metals and their innate ability to concentrate them and form minerals from metal salts within natural waters, sediments and soils. Since most of the readers of this article will not be bacteriologists, we first give a brief overview of the make-up of bacterial surfaces. We also discuss the formation and composition of microbial biofilms which are instrumental in immobilizing toxic metals.
Thermophilic microbes have long been implicated in the formation of travertine and siliceous sinter. Precipitation of CaCO3 at thermal springs is induced mainly through degassing of CO2. Cyanobacteria and other bacteria can play a role in calcite and aragonite nucleation in warm (20 – 40 °C) and mesothermal (40 – 75 °C) hot springs, and through photosynthesis and other biochemical processes may mediate some mineral precipitation. Many fabrics in warm-spring and mesothermal travertines preserve evidence of microbes. Travertine precipitated at hyperthermal (> 75 °C) spring vents is mainly abiotic and commonly exhibits high-disequilibrium crystal morphologies. Silica precipitation in hyperthermal springs and geysers results mainly from rapid cooling and evaporation. Microbes, however, can play an important role by providing templates for silica nucleation and by controlling development of many sinter and geyserite fabrics.
Recent tectonic activity of the main dislocation within the Palaeozoic core of the Holy Cross Mts. led to formation of a large travertine dome of Holocene age. The main body of the travertine is built up of extremely fast crystallized calcite from highly supersaturated solutions derived from hydrothermal waters circulating through the tectonic dislocation. Many calcite crystals display the remains of calcified bacilliform bacteria rods suggesting an essential part, of the calcite crystallization process was on a bacterial precursor. Successively, after the micrite calcite travertine frame had been formed, almost pure monoclinic manganese oxide (α-MnO2) precipitated filling part of the remaining porosity. The unique characteristics of manganese oxide crystallization also suggest a very fast process of manganese oxidation due to increase in Eh and the activity of abundant fungal species which might be associated with a specific symbiotic bacterium. Specific arrangement of the α-MnO2 crystals into the rosettes seems to be achieved by fast evaporation of upwards migrating solutions from the tectonic zone. The last in this succession is calcite cement filling the pores, mainly in the upper part of the travertine frame. Generally, this was abiologically mediated process of precipitation of calcium carbonate due to rapid degassing of carbon dioxide and occasionally, in the top of the travertine dome, due to direct precipitation from the ground waters.
Silicates such as amorphous silica, quartz, trioctahedral smectite (stevensite), and possibly kerolite have formed with carbonate minerals in carbonate speleothems from caves of the Guadalupe Mountains, New Mexico, U.S.A. The likely sources of Si were the disseminated silicate minerals in situ in the Permian dolostone units in which the caves have developed. Ca- and Mg-carbonate mineral precipitation from thin water-films on speleothems due to evaporation and CO2 loss progressively increased the Si/(Ca+Mg) ratio, and concentrated the initially low percent of aqueous silica seeping into these caves. Amorphous silica precipitated with calcite and aragonite in stalagmites because the formation of these materials was rapid and the stalagmites are too young for the crystallization of quartz. The presence of organic material in the stalagmites hints that the amorphous silica may have an organic-related origin. Quartz and trioctahedral smectite formed with the Mg-carbonate minerals such as dolomite, huntite, and hydromagnesite. The authigenesis of quartz at the low temperature (15-25°C) of these caves, like that of well-ordered dolomite, required extensive periods of time (i.e., ≫1 ka) in a stable environment conducive for silicate precipitation. Kerolite-like silicates, in aragonite crusts, seemed to be intermediate precipitates between amorphous silicates and trioctahedral smectite.
Within the past decade there has been an increasing interest in cave microbiota. Such interest has helped many speleologists both recognize and understand the importance of microbial species in caves, which has led to improvements in cave conservation practices to better conserve these unseen ecosystems. While much information on the metabolic properties and functions of such subterranean ecosystems has been published in the microbial ecology literature, is it sometimes unusable by the non-specialist due to technical "jargon" and unexplained background information. It is the aim of this review to provide such background information and to explain the current technologies available to study cave microbiota. In doing so, it is hoped that this material will make the microbiology literature more accessible to interested non-specialists, and open new areas of inquiry in the study of microbial-mineral interactions.
Scanning electron microscopy for studying microbial communities
Introduction: The contribution of micro-organisms to amorphous silica precipitation in modern geothermal hot-spring environments has been the topic of intense study in the last three to four decades. Here, we present a review on the field and laboratory studies that have specifically addressed bacterial silicification, with a special focus on cyanobacterial silicification. Studies related to the biogenic silicification processes in diatoms, radiolarians and sponges are not discussed, despite the fact that, in the modern oceans (which are undersaturated with respect to silica), the diagenetic ‘ripening’ of such biogenic silica controls the global silica cycle (Dixit et al., 2001). It is well-known that the amorphous silica in these organisms (particularly in size, shape and orientation) is controlled primarily by the templating functions of glycoproteins and polypeptides (e.g. silaffin and silicatein). For information on these issues, we refer the reader to the extensive reviews by Simpson & Volcani (1981), Kröger et al. (1997, 2000), Baeuerlein (2000), Perry & Keeling-Tucker (2000), Hildebrand & Wetherbee (2003) and Perry (2003). In addition, in terrestrial environments, a large pool of amorphous silica is cycled through higher plants (grasses and trees) that are believed to use silicification as a protection mechanism against pathogens and insects. Information on these processes can be found in the papers by Chen & Lewin (1969), Sangster & Hodson (1986) and Perry & Fraser (1991).
Fiber and dendrite calcite crystals generally form from fluids that are supersaturated with respect to calcium carbonate. Although experimental data suggest that both types of crystal can form from the same parent fluid, there are only rare examples where both are present together in nature. Such crystals form through biogenically and abiogenically mediated processes. The common association of these crystals with plant roots or microorganisms suggest that the organisms can produce conditions suitable for growth of the crystals. -from Authors
Monthly monitoring of water chemistry at Octopus Spring, Yellowstone National Park, reveals no changes in the chemical composition over this eight-month study. Differences in silica deposition are attributed to flow dynamics and seasonal air temperatures. Colder air temperatures in winter promote visible accumulation of siliceous sinter probably as a result of decreased solubility. Periodic drying promotes accumulation and subsequent dehydration of the silica. This combination of intermittent supply and periodic dehydration may account for the laminated siliceous deposits observed in the geological record. The processes described herein are important in the siliceous preservation of microbial fossils but the following observations underscore the difficulty in preserving microbial fabrics in the geological record. Pools populated by cyanobacteria are bounded by margins built of silicified microbial mats. Silicification of the pool margins occurs through an evaporative process called wicking; silica-rich water is transported upward through the microbial mats by capillary action, depositing silica at the surface of the mat as water evaporates. The upper surface of such wicking deposits is extremely resistant to brecciation but the underlying support is highly friable and would be difficult to preserve intact. In winter months, temporary isolation of some pools from active flow causes fragile silica deposition around biological and other debris. Film-coated, gas bubbles on the surfaces of microbial mats are observed during all seasons and probably are filled with oxygen released during photosynthesis, although this has not been confirmed. They become coated with silica during winter months presumably because of increased cooling possibly forming thin, discontinuous silica sheets as observed in the geological record.
The siliceous geyserites of Yellowstone National Park (Wyoming, U.S.A.) are morphologically very similar to stromatolites, but are abiogenic. Geyserite is opaline silica deposited non-biogenically within and around hot springs and geysers. The deposits have a variety of shapes, each characteristic of particular environments. The shapes strikingly resemble those of stromatolites; however, the morphogenetic processes are non-biological. Columnar and spicular geyserite forms in subaerial splash zones; stratiform geyserite forms subaqueously; oolitic and pisolitic geyserites form in turbulent water, and may be continually or only intermittently submerged. Geyserite is distinguished from stromatolites by its distinctive distribution around points of water discharge, by microcrosslamination that is common but not ubiquitous, and by its banded lamination with laminae less than about 4μm thick. Spicular and columnar forms could have formed in many types of water-splash zones, such as along shorelines. The word stiriolite is introduced for geyserite-like deposits preserved in the geological record.
The dissolution of the calcite occurred via surface-reaction-controlled kinetic processes that were mediated by the fungi.-from Authors
Coralloid speleothems are commonly distributed in Togawa-Sakaidani-do Cave in Miyazaki Prefecture, Central Kyushu, but their speleological study has not heretofore been achieved. Light and scanning microscopes analyses revealed that coralloid speleothems consist of alternating layers of diatom colonies, detrital minerals and clay. Electron microprobe analysis shows coralloid speleothems to be silicious. This paper asserts that diatom (genus Melosira) is one of the important contributors to siliceous coralloid speleothems in the threshold zone at non-calcareous caves.
Many speleothems can be assigned to one of two morphological groups: massive speleothems, which consist of compact bulks of material, and coralloids, which are domal to digitate in form. Faster growth on protrusions of the substrate occurs in the typical growth layers of coralloids (where those layers are termed “coralloid accretions”), but it is not observed in the typical layers of massive speleothems, which in contrast tend to smoothen the speleothem surface (and can therefore be defined as "smoothing accretions"). The different growth rates on different areas of the substrate are explainable by various mechanisms of CaCO3 deposition (e.g., differential aerosol deposition, differential CO2 and/or H2O loss from a capillary film of solution, deposition in subaqueous environments). To identify the causes of formation of coralloids rather than massive speleothems, this article provides data about δ13C and δ18O at coeval points of both smoothing and coralloid accretions, examining the relationship between isotopic composition and the substrate morphology. In subaerial speleothems, data show an enrichment in heavy isotopes both along the direction of water flow and toward the protrusions. The first effect is due to H2O evaporation and CO2 degassing during a gravity-driven flow of water (gravity stage) and is observed in smoothing accretions; the second effect is due to evaporation and degassing during water movement by capillary action from recesses to prominences (capillary stage) and is observed in subaerial coralloids. Both effects coexist in smoothing accretions interspersed among coralloid ones (intermediate stage). Thus this study supports the origin of subaerial coralloids from dominantly capillary water and disproves their origin by deposition of aerosol from the cave air. On the other hand, subaqueous coralloids seem to form by a differential mass-transfer from a still bulk of water towards different zones of the substrate along diffusion flux vectors of nutrients perpendicular to the iso-depleted surfaces. Finally, this isotopic method has proved useful to investigate the controls on speleothem morphology and to obtain additional insights on the evolution of aqueous solutions inside caves.
The diagenetic grade of a 2.7 km thick sequence of lower Palaeozoic marine sediments in the Tobolka 1 deep exploration borehole was investigated by analysing organic and mineral components and fluid inclusions embedded in vein minerals.
Archean microfossils are notoriously difficult to recognize. Most that have been reported have subsequently proved to be either younger contaminants or abiogenic pseudofossils. To avoid mistaking contaminants for genuine Archean objects, a set of rigorous collecting and preparation procedures should be followed. To avoid mistaking pseudofossils for authentic microfossils, a hierarchical series of recognition criteria can be used. Six classes of microfossil-like objects from a ~3500 m.y. old chert-barite unit in the Warrawoona Group at North Pole, Western Australia, are evaluated using the above technique. -from Author
Volcanic caves have been considered of little mineralogic interest until recent years. As a consequence, very few papers have been printed on this topic in the past. In reality volcanic cavities are a very favorable environment for the development of different minerogenetic processes. Cave minerals actually present in volcanic environments constitute up to 40% of secondary chemical deposits found in all the caves of the world, and 35 of them (corresponding to ~10% of the actually known cave minerals) are restricted to such a environment. In the present paper, the six minerogenetic mechanisms active in the volcanic caves (degassing, solubilization, alteration, karst process, biogenic activity, phase change) are described following the decrease of cave temperature. The genesis of some of the most important secondary chemical deposits is discussed and a tentative list of the most interesting volcanic caves for hosted speleothems is given.