Calcified filaments - an example of biological influences in the formation of calcrete in South Australia

Abstract

Scanning electron microscope (SEM) studies of calcareous soils and calcretes from South Australia reveal a fossilized community of soil micro-organisms dominated by filamentous structures preserved in fine detail by calcite. In the various calcrete lithological facies, the filaments form dense mats within channels and voids, and also occur within the matrix where they are intimately associated with micrite. The calcite forming the filaments has a variety of crystal habits: the nature of the microcrystals is specific to each filament but varies significantly between adjacent filaments. In the calcareous soils there are various stages between the primary filaments and the calcite encrusted structures characteristic of the calcretes, suggesting that in vivo biochemical processes dominate the mechanisms of calcification. This hypothesis is supported by the specificity of the habit of calcite microcrystals on each filament. It is suggested that the organisms deposit calcite microcrystals within the mucilaginous sheath or in the cell wall (or both) as a detoxification mechanism in response to their highly calcareous environment. Based on the identification of structures resembling fruiting bodies, at least some of the filaments appear to have been fungal hyphae, which are known to be responsible for stabilizing macroaggregates in soils. Calcified filaments may produce permanently stabilized macroaggregates which provide the locus for further carbonate precipitation, leading to eventual induration of the soil.

Hematite and calcite coatings on fossil vertebrates

“…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. The origin of Microcodium and other fibrous fabrics has been attributed to calcite microcrystal precipitation within fungal or bacterial mucilaginous sheaths, thereby retaining the biogenic growth form (Poncet 1976;Klappa 1983;Phillips et al 1987). The Microcodium-like structures reported here, however, are closely associated with vertebrate remains, not plant roots as suggested by Klappa (1978).…”

What do we really know about early diagenesis of non-marine carbonates?

“…Verrecchia et al (1993) showed that short crystals and spikes surrounding the exterior of the calcified fungal filaments in some Quaternary calcretes of Israel had compositions better compatible with calcium oxalates than with calcite and that most needles and mineral encrusted fungal hyphal walls had intermediate compositions. Verrecchia et al (1993) envisaged that transformation of calcium oxalate to calcite was bacterially mediated, following hypotheses of Cromack et al (1977) and Philips et al (1987). Experiments have since shown that some soil bacteria (including Ralstonia eutropha and Xanthobacter autotrophicus) degrade oxalates, using them as a carbon and energy source, resulting in production of carbonic acid and ultimately carbonate ions that can react with locally sourced calcium to produce calcite or vaterite (Braissant et al, 2002).…”

Estimating rates of calcrete formation and sediment accretion in ancient alluvial deposits

“…Instead the microfabric consists of a dense crystalline mass with floating sediment grains, various fractures and relic nodules. However, many calcretes exhibit striking evidence of biological involvement in carbonate formation (Calvet, 1982;Jones & Kwok-Choi, 1988;Klappa, 1979Klappa, , 1980Phillips, Milnes & Foster, 1987;Semeniuk & Meagher, 1981). They contain such features as rhizocretions, microbial coatings and fungal products such as needle-fibre calcite.…”