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| Titel |
Mineral artefacts mimicking microfossils in Archean rocks |
| VerfasserIn |
K. Lepot, P. Philippot, K. Benzerara |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250025551
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| Zusammenfassung |
| Because prokaryotes populating the early Earth were structurally and morphologically very
simple, it is difficult to obtain taxonomic information from microfossils, and even more
problematic, to distinguish true fossils from abiotic objects. For example, many
self-assembly processes associated with the precipitation of nanoscale minerals in
the presence of organic compounds generate cell-like structures. Based on high
resolution microscopy observations on natural samples, we describe three types of
features common to Archean rocks and suggest that they represent microfossil-like
artefacts.
Using Scanning Electron Microscopy we have observed carbon-free silica inclusions in
carbonate sediments that are very similar in size and shape (rods and spheres) to
microorganisms. The common distribution of organic carbon at grain boundaries in those
rocks indicate that such cell-like minerals, when coated by secondarily-migrated
carbonaceous mater, could easily be mistaken for microfossils.
The organisation and the micro- to nano-structure of bacteriomorphs might be even more
confusing. We have observed chains of spheres that match in size and arrangement with some
coccoid bacteria such as streptococci. Transmission Electron Microscopy (TEM) observation
of Focused Ion Beam (FIB) sections cut through these spheres shows that they are composed
of TiO2nanocrystallites partly rimmed or linked by nanoscale chlorite films. This
assemblage creates smooth cell-like structures at the micron-scale. However, the absence
of organic carbon in those structures as well as the observation of many similar
TiO2 chains of spheres dispersed in volcanic glass shards argue against a biologic
origin.
Ambient inclusions trails also generate filamentous structures that can be mistaken for
microfossils. (Knoll and Barghoorn, 1974) suggested that such pseudofossils could have
formed by the displacement of a crystal (e.g. pyrite) in its mineral matrix owing to pressure
solution processes linked to gas evolution of associated organic material. We have found
garnet filaments within agate of the Maddina Formation (2.7Ga) basalt. TEM analysis of FIB
sections cut trough these filaments and Raman mappings of carbonaceous matter confirm the
role of organic matter displacement in the formation of those microfossil-like structures.
Thus, the existence of such organo-mineral features shows the importance of correlating
microbial morphologies with the texture and distribution of organic matter within (to
support an indigenous origin) or around (to identify migrations) cell-like micro to
nano-structures.
Knoll, A. H. and Barghoorn, E. S., 1974. Ambient Pyrite in Precambrian Chert: New
Evidence and a Theory. Proceedings of the National Academy of Sciences (US) 71,
2329-2331. |
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