Microcodium Revisited: Root Calcification Products of Terrestrial Plants on Carbonate-Rich Substrates

Abstract

Microcodium is a problematic calcitic microfeature of many calcretes and calcareous paleosols in the Cretaceous and Tertiary continental and marine successions of the peri-Tethyan realm. The main controversy about the Microcodium structures is their origin and possible relation with calcified plant roots. Microcodium and rhizogenic (root-formed) microfabrics were studied in calcrete profiles within the Paleocene shallow-marine carbonate succession in southwestern Slovenia. The prominent laminar calcrete horizons contain abundant calcite aggregates, 150 m to 1 cm in size, with perfectly preserved structural details of plant root tissues. Morphology and structure of these aggregates indicate that they formed through biologically controlled precipitation of calcium carbonate within the root cortical cells. The morphologies intermediate between the typical Microcodium aggregates, composed of a single layer of individual, elongate pyramidal or prismatic crystals of calcite (measuring 100-500 m in length and 20-70 m in width) and calcified roots with multilayer arrangement of isodiametric cells were observed, and this supports previous rhizogenic interpretations of Microcodium structures. Intermediate forms show that the typical Microcodium aggregates formed through morphological transformation of the root tissue by growth of the calcite within the cortical cells, which distorted the cell shape. Calcification of roots and the creation of Microcodium structures can be explained as an effective nutrient-acquiring mechanism used by certain types of terrestrial plants inhabiting nutrient-poor calcareous substrates. The widespread occurrence of Microcodium in almost unaltered shallowmarine limestones indicates that its formation took place during early stages of paleosol development, probably reflecting specific types of vascular plants of a pioneer community that were able to colonize carbonate substrates during the early phases of subaerial exposure.

Micromorphology of Paleosols of the Marília Formation and their Significance in the Paleoenvironmental Evolution of the Bauru Basin, Upper Cretaceous, Southeastern Brazil

“…The microcodium structures consist of cell aggregates composed of individual crystals of calcite (Kosir, 2004). Generally, microcodium is associated with the rhizogenic horizon ( Figure 7c).…”

“…Generally, microcodium is associated with the rhizogenic horizon ( Figure 7c). Plant roots and microorganisms associated with the rhizosphere produce significant accumulations of calcium carbonate near the surface (Kosir, 2004). Microcodium occurs in petrocalcic horizons and calcretes, unlike rhizoliths, which normally occur in soil and sediments not completely cemented by calcium carbonate (Durand et al, 2010).…”

“…Microcodium structures (Figure 7c) are evidence of calcretes of pedogenic origin and indicate biologically controlled precipitation of calcium carbonate. Accumulations of microcodium probably reflect specific types of vascular plants of a pioneer community that had the ability to colonize carbonate substrates during the early phases of subaerial exposure (Kosir, 2004).…”

The mineralogy of paleosols in the Marília Formation and their importance in the environmental evolution of the Maastrichtian of the Bauru Basin in southeastern Brazil

“…Usually microcodium is associated with a rizogenic horizon (Kosir 2004). Calcite pendants (pending calcite) are another striking feature of pedogenic calcretes (Manafi & Poch 2012).…”

Pedogenic calcrete records in southern Transdanubia, Hungary: A brief review with paleoenvironmental and paleogeographic implications

“…Two kinds of Microcodium were defined by Esteban (1972): 'Microcodium a' refers to typical Microcodium, consisting of prismatic calcite crystals; whereas 'Microcodium b' was defined by its smaller grain size and subquadrangular sections of its prisms (atypical Microcodium). Based on a large number of reports, the original interpretation of Microcodium as siphonaceous alga or any other phototrophic microorganism has been rejected; however, the process of Microcodium formation still remains unsolved (Klappa 1978;Alonso-Zarza et al 1998;Košir 2004;Kabanov et al 2008). Microcodium was interpreted as a calcification product of a mycorrhizae-cortical root cell association by Klappa (1978).…”