Microbial reefs in the Devonian swamps at Rhynie
deutsche Version
microbial cloud in Rhynie chert
Microbes are abundantly present in the Rhynie chert but often not recognized as such. They may appear as mm- to cm-size
bulging clouds, apparently structureless, and may remain unnoticed unless more conspicuous, as those in Rhynie Chert News 68.
In the present sample there are microbial clouds or blobs clearly separated from their yellowish surroundings, as in Fig.1.

Fig.1: Raw outside of a sample from Rhynie with bulging top of a chert layer and yellow sediment above. Width of the image 17mm.

The bulging cloud in Fig.1 at the top of the chert layer, like everything else now seen in the chert, had grown or formed in water. Judging from the distinct boundary, the cloud had not been a loose assembly of microbes when the yellow sediment settled. There is no indication of deformation under load, hence one can assume that the dark cloud had been solid in the swamp water before. This is not surprising since differential rates of silicification of objects in the swamp is a well-known fact.
Possibly the microbes kept themselves together in a big cloud or blob by some kind of organic gel between them, which triggered early silicification while the surrounding water remained fluid. Apparently a subseqent muddy flow did not affekt the craggy shapes, and the mineral debris had not entered but accumulated above (Figs.1-3), which invokes the idea of buried reefs.
The nature of some details seen here and occasionally with similar microbial formations in other samples, as irregular black inclusions of various size and shape as in Fig.2, remains obscure.
Possibly the microbial clouds could have offered suitable places for other organisms to thrive: An assembly of pale spheres is seen in one cloud of this sample: Fig.3. The latter probably belong to some fungus. Black spheres of similar size are clearly seen in Rhynie Chert News 68.

Fig.2 (left):  Microbial formations like reefs: not deformed by the silt deposited later. Cut face, same sample as Fig.1 but other position. Width of the image 11mm.  
cliff-like microbial cloudspheres in microbial cloud
Fig.3: Middle of Fig.2 enlarged such that pale spheres of sizes up to about 50Ám become visible.
Width of the image 1.3mm.

H.-J. Weiss       2017




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