Preservation of Devonian Chemotrophic Filamentous Bacteria in Calcite Veins

Abstract

Filamentous and possible coccoid bacteria are preserved in calcite vein-fills cutting Devonian lacustrine limestones in the Tynet Burn fish bed near Fochabers, N.E. Scotland. Filamentous bacteria grew into open space off the walls of fractures and are concentrated as clumps on some detrital grains. The filaments were coated by iron hydroxide and are now represented by hollow canals 0.3-0.4 mu m diameter enclosed in a sheath of acicular hematite crystals each 2 mu m long by 0.2 mu m broad. Possible coccoid bacteria are represented,by hematite spheres 0.5 to 2 mu m diameter. The bacteria grew in cracks formed by deformation of the sediments after initial compaction, and formation of calcareous concretions. Fossil fish are preserved in red and purple colors and the surrounding matrix displays a halo of oxidation which is considered to have been bacterially mediated. Bacterial invasion of sediments took place during lowstands of the Middle Devonian Orcadian Lake when the sediment was in the vadose zone. A clear association is seen between the bacteria-bearing veins and reddening of the succession.