Publication: Initial stages of laminar calcrete formation by roots:
examples from the Neogene of central Spain
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Publication Date
1999
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Elsevier
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Abstract
Thin carbonate laminae formed by root activity are common within Miocene detrital deposits of the Duero and Madrid
basins. The laminae are about 3 cm thick, extending several metres laterally, and displace the original detrital sediment,
which ranges in grain size from fine gravel to sandy clay. The thickness, morphology, microstructure and stable isotope
compositions of the laminae indicate that they formed by the activities of roots and associated microorganisms within the
soil. The laminae are similar to those commonly recognised in thick laminar calcrete profiles. Three types of laminae are
recognised. Differences in the micromorphology of the laminae are explained as reflecting the different organisms involved
and whether calcification occurred when the root was alive or decaying. The first type occurs in a profile at Paracuellos
the Jarama, where the laminae consist of a mosaic of 20 mm calcite crystals whose arrangement indicates that calcification
took place in the medulla of the root and probably occurred while the plant was alive. In a second type in the Villacadima
profile, laminae comprise calcified root mats whose formation indicates the interplay of roots and fungi. Calcification of the
cell-walls and intracellular spaces took place in the cortices of the roots and not in the medullas as revealed by the presence
of central pores in every calcified root. A third type of lamina is recognised in profiles at Vin˜egra de Moran˜a and consists
of mucilaginous sheets coated by needle-fibre calcite crystals. The occurrence of associated small root casts indicates
that formation of the laminae occurred while the root was decaying and was also influenced by fungal activity. These
laminae occur within poorly developed soils and their formation was controlled by the relationship between sedimentation,
erosion and soil formation processes. Thus, the occurrence of these laminae interbedded with detrital sediments reflects
environments where sedimentation was relatively low and episodic, so after the detrital sediment input surfaces were stable
and root mats were able to develop. Renewed sedimentation accounted for the death of the root mats and the development
of new ones on the new surfaces. Where the sedimentation rate is lower the laminae tend to amalgamate and thicker laminar
calcrete profiles form with little or no interbedded detrital sediment.