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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. [ABSTRACT FROM AUTHOR]

The genesis and development of calcretes are discussed by using nuclear and related methods of analyses. In the studied calcrete, smectite is the main clay mineral associated with palygorskite. Chemical elements proportions, particularly V, U, Mn, Cu, Ba and REE, indicate a change to more oxidizing conditions and the contribution of bio-mediated processes in the oxalate-carbonate pathway, also confirmed by the presence of needle-fibre calcite. The proposed pathways leading to the studied calcrete development comprise both abiotic and biogenic genesis, emphasizing the polygenetic character due to different agents and processes and the paleoenvironmental trend towards aridification, with a transition between lacustrine and palustrine environments. [ABSTRACT FROM AUTHOR]

Abstract: Subsurface granitic environments are scarce and poorly investigated. A multi‐disciplinary approach was used to characterize the abundant moonmilk deposits and associated microbial communities coating the granite walls of the 16th Century Paranhos spring water tunnel in Porto city (north‐west Portugal). It is possible that this study is the first record of moonmilk in an urban subsurface granitic environment. The morphology and texture, mineralogical composition, stable isotope composition and microbial diversity of moonmilk deposits have been studied to infer the processes of moonmilk formation. These whitish secondary mineral deposits are composed of very fine needle‐fibre calcite crystals with different morphologies and density. Calcified filaments of fungal hyphae or bacteria were distinguished by field emission scanning electron microscopy. Stable isotope analysis revealed a meteoric origin of the needle‐fibre calcite, with an important contribution of atmospheric CO2, soil respiration and HCO 3 − from weathering of Ca‐bearing minerals. The DNA‐based analyses revealed the presence of micro‐organisms related to urban contamination, including Actinobacteria, mainly represented by Pseudonocardia hispaniensis, Thaumarchaeota and Ascomycota, dominated by Cladosporium. This microbial composition is consistent with groundwater pollution and contamination sources of the overlying urban area, including garages, petrol stations and wastewater pipeline leakage, showing that the Paranhos tunnel is greatly perturbed by anthropogenic activities. Whether the identified micro‐organisms are involved in the formation of the needle‐fibre calcite or not is difficult to demonstrate, but this study evidenced both abiotic and biogenic genesis for the calcite moonmilk in this subsurface granitic environment. [ABSTRACT FROM AUTHOR]