Microbial signatures from speleothems: A petrographic and scanning electron microscopy study of coralloids from the Koněprusy Caves (the Bohemian Karst, Czech Republic)

Abstract

AbstractExotic 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.