Palaeozoic microbes in chert

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Microbes grown in swamp water can be found preserved in chert. The freshwater cyanophyte Croftalania [1] forms filaments and tufts with sizes of about 1mm: Figs.1,2. (See also Rhynie Chert News 56.)
C2CroftalaniaFig.1: Tufts of the cyanophyte Croftalania in the Devonian Rhynie chert. Width of the filaments about 3Ám [1]. Image width 2mm.

Fig.2: Tufts of the cyanophyte Croftalania: surface apparently shaped by grazing creatures. Image width 2mm.

The peaks in Fig.2 do not look like mere tufts of cyanophyte filaments in water. Their smooth surface seems to have been shaped at a stage when the filaments were fixed in gel and grazing creatures were still moving around in the water before all became gel and finally chert. 
Distinctly different from the tuft-like growth of Croftalania is the growth mode producing stacks of thin sheets (Figs.3-6). Often they are nearly plane or slightly curved, also broken. The layered stacks roughly resemble wood fragments cut lengthwise but their occasionally seen delaminating ends and their connection to randomly shaped flakes or lumps in the water of the swamp suggest a microbial origin (Fig.3). (See  Permian Chert News 43.)
The crack in Fig.4 shows that the sheet stack had been stiffenend by early silicification, broken, then fused with the surroundings and hardened by continuing silicification.
Layered deposites of mineral matter mediated by microbes are well known as stromatolites [2] formed in shallow salty water or in freshwater [3].
cracked microbial layer stack stack
Fig.3: Stack of microbial sheets in Permian red chert, delaminating while soft and merging with irregular-shaped microbial lumps on the left. Image width 5.5mm.

Fig.4 (far right): Stack of microbial sheets in Permian red chert, broken while the surrounding swamp matter was still fluid. Image width 5.5mm.
microbial formations


Fig.5: Microbes in Rhynie chert arranged as large flat sheets with emergences therefrom, dark level fills from fluid suspensions. Image width 17mm.


Emergences may arise from large sheets of microbial origin (Fig.5). The little black pools with horizontal surface in between are former troughs where a heavy watery suspension involving dark microbes (?) had accumulated and separated itself from the water above before all turned into gel, then into chalcedony.

Fig.6 (below): Red Permian chert: laminated layer with tunnels eaten by elusive creatures; silica precipitates, probably induced by microbes, including emergences and clouds above. Image width 3.5mm.

Apparently the emergences in Fig.6 are closely related to the laminated red substrate below, which is a stack of microbial sheets as in Figs.3,4.
microbes in red chert
The crack in Fig.4 runs across a stack of microbial sheets but not farther, which indicates that the surrounding swamp matter was still fluid. 
Hence, early silicification making the stack solid must have been caused by the microbes. Thus it can be assumed that in other cases, too, microbes do not only passively get trapped in silica gel but promote silicification.
T
he microbes involved in the formation of the structures shown in Figs.3-6 have not been identified here.
.
H.-J. Weiss   2023

[1]  M. Krings, H. Kerp, H. Hass, T.N. Taylor, N. Dotzler:
      A filamentous cyanobacterium showing structured colonial growth
        from the Early Devonian Rhynie chert.
      Rev. Palaeobot. Palyn. 146(2007), 265-276.
[2]  T.N. Taylor, E.L. Taylor, M. Krings: Paleobotany, Elsevier 2009, 115-117.
[3]  P. Freytet, E. Verrecchia: Freshwater organisms that build stromatolites: A synopsis ...  Sedimentology (1998), 45, 535-563.
Scolecopteris pinnule cross-section, Sardinia Permian Chert News 45
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