Microcodium: An extensive review and a proposed non-rhizogenic biologically induced origin for its formation

Microcodium has been previously described as a mainly Cenozoic calcification pattern ascribed to various organisms. A review of the available literature and our data reveal two peaks in Microcodium abundance; the Moscovian–early Permian and the latest Cretaceous–Paleogene. A detailed analysis of late Paleozoic and Cenozoic examples leads to the following new conclusions. Typical Microcodium-forming unilayered ‘corn-cob’ aggregates of elongated grains and thick multilayered (palisade) replacing structures cannot be linked to smaller-grained intracellular root calcifications, as became widely accepted after the work of Klappa [Klappa, C.F., 1979. Calcified filaments in Quaternary calcretes: organo-mineral interactions in the subaerial vadose environment. J. Sediment. Petrol. 49, 955–968.] Typical Microcodium is recognized from the early Carboniferous (with doubtful Devonian reports) to Quaternary as a biologically induced mineralization formed via dissolution/precipitation processes in various aerobic Ca-rich soil and subsoil terrestrial environments. Morphology and δ13C signatures of Microcodium suggest that neither plants, algae, or roots and root-associated mycorrhiza regulate the formation of these fossil structures. Non-recrystallized Microcodium grains basically consist of slender (1.5–4 μm) curved radiating monocrystalline prisms with occasionally preserved hyphae-like morphology. Thin (0.5–3 μm) hypha-like canals can also be observed. These supposed hyphae may belong to actinobacteria. However, thin fungal mycelia cannot be excluded. We propose a model of Microcodium formation involving a mycelial saprotrophic organism responsible for substrate corrosion and associated bacteria capable of consuming acidic metabolites and CaCO3 reprecipitation into the Microcodium structures.