Exceptional preservation of Mn-oxidizing microbes in cave stromatolites (El Soplao, Spain)
Many ferromanganese stromatolites of El Soplao Cave (N Spain) are characterized for the exceptional preser-vation and high diversity of microbial fossils, probably representing the best example of microbial preserva-tion described in ferromanganese deposits so far.The El Soplao stromatolites are mai...
| Autores: | , |
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| Tipo de recurso: | artículo |
| Fecha de publicación: | 2012 |
| País: | España |
| Institución: | Consejo Superior de Investigaciones Científicas (CSIC) |
| Repositorio: | DIGITAL.CSIC. Repositorio Institucional del CSIC |
| OAI Identifier: | oai:digital.csic.es:10261/276845 |
| Acceso en línea: | http://hdl.handle.net/10261/276845 https://doi.org/10.1016/j.sedgeo.2012.02.003 |
| Access Level: | acceso abierto |
| Palabra clave: | stromatolites manganese oxides Mn-oxidizing microbes fossil microbes Cueva El Soplao España |
| Sumario: | Many ferromanganese stromatolites of El Soplao Cave (N Spain) are characterized for the exceptional preser-vation and high diversity of microbial fossils, probably representing the best example of microbial preserva-tion described in ferromanganese deposits so far.The El Soplao stromatolites are mainly formed by polymetallic Mn-rich oxides with subordinate and variableamounts of detrital material, and consist of both dendritic and laminar microfacies. In both microfacies, mi-crobial forms are abundant in the relatively pure Mn-oxide rich material, whereas they are scarce in areaswith significant detrital material. Microbial forms are observed either in cross section, completely embeddedin the Mn-oxide-rich matrix, or in three dimensions lining the walls of pores. Based on their morphology, wehave separated the most abundant microbial forms into six main morphotypes and six additional submor-photypes, most of which can be assigned to bacteria. Most morphotypes consist of coccoid, coccobacilus, orfilamentous forms. Therefore they are not diagnostic of any particular bacterial group. However, the ovoidcells of morphotype B show cylindrical polar protuberances typical of prosthecate alpha-Proteobacteria.Onthe basis of characteristic morphological features, three submorphotypes of morphotype B can be assignedto three alpha-Proteobacteriagenera: Hyphomicrobium, Pedomicrobium, and Caulobacter. This ascription issupported by the well known Mn-oxidizing behavior of bothPedomicrobiumandCaulobacter, and by thecommon presence ofHyphomicrobiumin ferromanganese deposits elsewhere.The excellent microbial preservation is partly related to the origin of the ferromanganese oxides, i.e. extracel-lular precipitation induced by microbial metabolism. Other factors contributing to the good microbial preser-vation are the relatively low degree of diagenetic alteration, and the relatively high accretion rates ofstromatolites compared to other ferromanganese deposits. The generally low degree of diagenesis is likelybecause the stromatolites have remained relatively stable and at a rather low temperature since they wereformed at least 1 Ma ago. Still, some stromatolites have suffered diagenetic alteration (partial dissolutionand replacement by calcium carbonates and Fe-rich oxides), obscuring or even obliterating their bioforms.The El Soplao case represents an example of how easily biogenic Mn oxides can be altered, and their bioformsblurred, in a relatively short geological time span in spite of being in a relatively stable, alkaline, and low-temperature setting. A geological implication is that the absence of bioforms in other ferromanganese deposits,including rock varnish and Precambrian iron formations, does not disprove their possible biogenic origin, sincethe high chemical reactivity of biogenic manganese oxides makes them highly vulnerable to diagenesis |
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