An old controversy has finally been laid to rest. Stromatolites, cauliflower-shaped carbonate rocks that are found in abundance in geological formations, including ones that are several billion years old, show definite evidence of ancient biological activity. This has just been shown by researchers at the Institut de Physique du Globe in Paris (CNRS/Université Paris 7), thanks to detailed analysis of 2.7 billion-year-old rocks from cores drilled in Australia. Previously, the oldest known traces of mineralization caused by micro-organisms were only 350 million years old.
Until now, most scientists agreed that fossil stromatolites were connected with the activity of photosynthesizing micro-organisms, as is the case in present-day environments(1). This hypothesis was based on morphological criteria, which appeared to suffice for recent fossil rocks (several million years old) formed in environments similar to today’s. However, this was not the case for older stromatolites (2.5 to 4 billion years old), which developed under very different conditions and for which researchers have proposed (and tested numerically) models of mineral growth in the presence of water.
The researchers collected unaltered fossil stromatolites as part of a drilling project (the Pilbara Drilling Project(2)), funded by CNRS’s National Institute of Earth and Astronomical Sciences and the Institut de Physique du Globe, in the Tumbiana formation in Australia. By using a very high resolution characterization method (of around 10 nanometers), they were able to discover and analyze small globules of organic matter containing nanocrystals of aragonite. It is known that present-day bacteria cause the precipitation of aragonite, a very unstable polymorph of calcium carbonate which rapidly changes into calcite once the micro-organisms die. Thanks to these findings, researchers now have irrefutable proof of microbial mediation in the formation of ancient stromatolites.
1) for instance, at Shark Bay, Australia
2) see the article Drilling the Outback (backstory), in the same issue of the journal Nature Géoscience, which describes the drilling expedition
Journal reference: Microbially influenced formation of 2,724-million-year-old stromatolites, Kevin Lepot, Karim Benzerara, Gordon E. Brown Jr and Pascal Philippot, Nature Geosciences, online publication 28 January 2008.
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