Biolithogenesis of Microcodium: elucidation

Abstract

Petrographic studies of Tertiary and Pleistocene caliche from the western Mediterranean show some unusual calcite structures. These structures were designated Microcodium elegans Glück 1912. New data are presented which question earlier interpretations with regard to the origin of this structure. The new discovery of Microcodium in Recent soils extends its stratigraphic range into the Holocene. Retention of fine detail in Recent samples, revealed by light microscopy and SEM, has suggested an origin hitherto unconsidered, calcification of mycorrhizal associations. Ancient and Recent Microcodium fabrics are compared; sufficient preservation of ultrastructure in the Ancient indicates a homologous origin. Environmental, stratigraphic and palaeoecological significance of Microcodium is discussed: correct recognition indicates existence of a palaeosol, and hence is a valuable criterion for recognition of continental conditions, cessation of sedimentation, subaerial exposure, and time‐equivalent horizons. In particular. Microcodium is a characteristic component of caliche in the western Mediterranean. A review of the literature suggests that its presence may have been overlooked or misinterpreted in other parts of the world and, thus, may be more widespread than hitherto suspected. This study, in its embryonic stage of development. illumines the potential importance of biolithogenesis within terrestial carbonates.

Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record

“…The organization of calcifi ed root cells within rhizoliths resembles problematic features knO\vn as Mi crocodium (Fig. 6D) (Klappa, 1978), especially Microcodium (b) of Estebau (1972) or type 3 ofPlaziat (1984) . Experimental studies on recent roots (Jaillard, 1987;Iaillard et aI., 1991) aud examples from the sedimentary record (Alonso-Zarza et aI., 1998b) indi cate that, very probably, this type of Microcodium is fo rmed by the calcifi cation of root structures with or without the infl uence of other microorganisms.…”

Hematite and Calcite Coatings on Fossil Vertebrates

“…3). The radial fibrous structure is common in calcite coatings and is morphologically similar to Microcodium, a biological structure reported from soil carbonates (Klappa 1978;Freytet and Plaziat 1982;Monger et al 1991). The spherical structures bearing radial fabric are 0.1-0.8 mm in diameter and often exist as large aggregates where the fibrous fabric is sometimes obscured by overlap.…”

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

“…During pedogenesis, root respiration produces significant levels of CO 2 (and hence of carbonic acid); additionally, microbial decomposition also releases CO 2 that controls the dissolution and precipitation of pedogenic carbonate (Wright and Tucker 1991;Alonso-Zarza 2003;Brasier 2011). All workers studying Microcodium in relation to the environment of its formation agree on its biogenic, subsurface, non-marine nature (e.g., Klappa 1978;Kabanov et al 2008). Two peaks in Microcodium abundance occur at the Moscovian-Early Permian and latest Cretaceous-Paleogene intervals, which are distinct by low pCO 2 in the atmosphere (Berner 2006;Royer 2006;Kabanov et al 2008).…”

“…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). Alonso-Zarza et al (1998) suggested that 'Microcodium b' formed through calcification of root cells.…”

“…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).…”