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. 2015 Nov 1;15(11):998–1029. doi: 10.1089/ast.2015.1374

FIG. 4.

FIG. 4.

Scanning electron microscope (SEM) and light microscope view of carbonaceous features in samples from the 3.33 billion-year-old Josefsdal Chert, Barberton (a–d) and the 3.45 billion-year-old Kitty's Gap Chert, Pilbara (e–h), illustrating their different morphological expressions. (a) Thin section in transmitted light of a packet of thin (∼10 μm), semiparallel, undulating films in primary hydrothermal silica interpreted as fossilized (silicified) anoxygenic phototrophic biofilms. The insert shows details of soft deformation (tearing) of the biofilm due to the force of influx of hydrothermal fluids between the plastically deformable laminae. The excellent preservation of the fine-scale morphological details is due to very rapid solidification and the extremely fine-grained silica matrix. (b) For comparison with the excellent morphological preservation of the phototrophic biofilms in (a), this image shows a transmitted light view of a similar packet of thin, phototrophic laminae that developed on the surface of coarser-grained sediments, which resulted in poorer morphological preservation at the fine scale. (c) SEM view of the surface of one of the phototrophic biofilms, showing a thin, filmy coating with a distinct, striated orientation due to streamlining of the filaments forming the biofilm (black arrows), as also indicated by the small white arrow. Note that different layers of biofilms can be seen in this view (large white arrow). (d) Detail of (c) showing overturning of the filamentous biofilm due to current flow (arrow). (e) SEM view of muscovite that replaced an original volcanic grain. Between the crystalline sheets of the phyllosilicate is a web of mucuslike polymer fibers (arrows). (f) Detrital carbonaceous grain (dark central clot) within silicified volcanic sediments. Note the matrix of small, blocky, microcrystalline quartz crystals. (g) Much of the OM in the Kitty's Gap Chert has an amorphous texture (arrow) and appears to “glue” or cement the fine-grained particulate matrix. (h) Monolayer microcolony of coccoidal, silicified microorganisms on the surface of a volcanic clast (cf., Westall et al., 2006a, 2011a). Note the entrapped mineral particle (white arrow) and apparent two sizes of dividing coccoids.