Initial stages of laminar calcrete formation by roots: examples from the Neogene of central Spain

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.