ELSEVIER Sedimentary Geology 126 (1999) 193–204 Microbial origin for pedogenic micrite associated with a carbonate paleosol (Champagne, France) C. Loisy a , E.P. Verrecchia b,L , P. Dufour c a Universite´ de Reims Champagne–Ardennes, De´partement des Sciences de la Terre, 2 Esplanade Roland Garros, 51686 Reims, France b Universite´ de Bourgogne, UMR 5561 CNRS, 6 Blvd. Gabriel, 21000 Dijon, France c Universite´ de Bourgogne, Laboratoire de Re´activite´ des Solides, 21004 Dijon Cedex 4, France Received 20 March 1998; accepted 3 March 1999 Abstract Micro-rods have been observed in indurated carbonate horizons (30 cm thick) that overlie a periglacial chalk formation, in contact with the present-day soil (Champagne, France). They are numerous in the upper part of the hardened layers. Variations in micro-rod morphologies are related to progressive biomineralization of organic matter, transforming purely organic rods into calcite. Mineralized rods undergo diagenesis and their arrangement evolves from a random mesh fabric to recrystallized micritic platelets to microsparite. 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 CaCO 3 content of the primary matrix and infill its microporosity. Their diagenetic evolution into microsparite contributes to soil layer induration.  1999 Elsevier Science B.V. All rights reserved. Keywords: microbial carbonate; bacteria; calcrete; diagenesis; Holocene; Champagne 1. Introduction Many needle microcrystal shapes have been de- scribed in carbonate accumulations. A morphologi- cal synthesis (Verrecchia and Verrecchia, 1994) dis- tinguishes two groups of needle-fiber calcite (NFC): monocrystalline rods and polycrystalline chains. The first group includes: (1) smooth, single micro-rods M (width D 0.5 μm, length D 2 μm); (2) smooth paired rods MA (width D 0.5–2 μm, length < 100 μm); and (3) serrated-edged paired rods MB (width L Corresponding author. E-mail: eric.verrecchia@satie.u-bourgogne.fr D 2–20 μm, length D 30–1000 μm). The morphol- ogy and crystallography of MA and MB needle-fiber calcite forms are now well known and their origin is attributed to fungal biomineralization (Callot et al., 1985; Verrecchia and Dumont, 1996). The present investigation is part of a comprehensive study of the micro-rod group (M), whose origin is not as well understood. The term ‘micro-rods’ is retained by de- fault from the English terms ‘microfibers’ (Calvet, 1982), ‘rods’ (Phillips and Self, 1987) and French term ‘baˆtonnets de calcite’ (Pouget and Rambaud, 1980; Riche et al., 1982; Verge`s et al., 1982). As for the polycrystalline chains, they are considered to be purely physico-chemical features. 0037-0738/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII:S0037-0738(99)00040-8