Microbial origin for pedogenic micrite associated with a carbonate paleosol (Champagne, France)

Micro-rods have been observed in indurated carbonate horizons (30 cm thick) that overlie a periglacial chalk formation, in contact with the present-da y soil (Champagne, France). They are numerous in the upper part of the hard ened layers. Variations in micro-rod morphologies are related to progressiv e biomineralization of organic matter, transforming purely organic rods int o calcite, Mineralized rods undergo diagenesis and their arrangement evolve s from a random mesh fabric to recrystallized micritic platelets to microsp arite. Two types of organic micro-rods have been observed: bacilliform and thread-like bacteria, Mineralogically, micro-rods are low-magnesian calcite . Crystallographically, their length is parallel to the (104) axis and they grow along the (010) plane. Micro-rods can be associated with needle-fiber calcite. They increase the CaCO3 content of the primary matrix and infill its microporosity. Their diagenetic evolution into microsparite contributes to soil layer induration. (C) 1999 Elsevier Science B.V. All rights reserv ed.