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 occuned 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 !-tm calcite crystals whose anangement indicates that calcification took place in the medulla of the root and probably occuned 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 Vifiegra de Morafia and consists of mucilaginous sheets coated by needle-fibre calcite crystals. The occunence of associated small root casts indicates that formation of the laminae occuned 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 sUliaces. Where the sedimentation rate is lower the laminae tend to amalgamate and thicker laminar calcrete profiles form with little or no interbedded detrital sediment.

Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record

“…The fo rmation of these laminar calcrete horizons has been widely discussed in recent years (Verrecchia et aI., 1995;Wright et aI., 1996;Freytet et aI., 1997;Alonso Zarza, 1999), aud there is a large consensus that they be interpreted as rooteretes (Jones, 1992) or rhizogenic ca1cretes (Wright et aI., 1995) since it is commonly accepted that the main agents responsible for their fo rmation are horizontal root systems (Mack and James, 1992;Wright, 1994). However, cyanophyceae (Vo gt, 1984) or cyanobacteria, (Verrecchia et aI., .. , 1994) and lichens (K1appa, 1979) may also be impor tant in their formation.…”

“…Renewed sedimentation ac counted for the death of the root mats and the develop ment of new laminae on the new surfaces. The result is the occurrence of thin calcrete laminae interbedded within detrital deposits (Alonso-Zarza, 1999). Where sedimentation rates were lower, the laminae tended to amalgamate, and thicker laminae calcrete profiles (Fig.…”

“…Pedogenic carbonate is easily precipitated through the activity of living organisms (Lal and Kimble, 2000). This is shown by the biogenic micro fabrics commonly recognised in calcretes (see above), which are formed in relation to soil microbes such as fungi, bacteria, lichens (see, for example, KJappa, 1979;Verrecchia and Ve rrecchia, 1994), and plants (Semeniuk and Meagher, 1981;Alonso-Zarza, 1999; amongst many others). The faW l a may also contribute to carbonate precipitation in soils, e.g., termites build mOWl ds that contain more Ca + 2 than adjacent soils (Monger and Gallegos, 2000).…”

Calcrete profile development in Quaternary alluvial sequences, southeast Spain: implications for using calcretes as a basis for landform chronologies

“…The laminar horizons are frequently associated with vertical rhizoliths and it is possible that these features represent biogenic laminar horizons (Cohen, 1982Alonso-Zarza, 1999). The massive horizons frequently show micro-fabrics typical of alpha fabric calcretes, i.e.…”

A microscopic approach to the pedogenic formation of palygorskite associated with Quaternary calcretes of the Adana area, southern Turkey

“…The vadose environment is also supported by the presence of the clay coatings as determined by Kapur et al (1987) and Kaplan et al (2013) present within the calcrete nodules and tubes, and the development of palygorskite as sheetlike fibre bundles (Figure 7b) and fibre bridges as meniscus cement between the calcite crystals (Figures 7d, 7e, and 8d). The pedogenic origin of palygorskite is inferred from Strong et al, 1992;Jimenez de Cisneros et al, 1993;Alonso-Zarza and Arenas, 2004;Eren et al, 2004Eren et al, , 2008Shaaban, 2004;Gong et al, 2005;Kaplan et al, 2013), whereas the δ 13 C values reflect a high input of soil-derived CO 2 and are also consistent with a pedogenic origin for the calcrete and associated palygorskite (Cerling, 1984(Cerling, , 1992Boutton, 1991;Wright and Tucker, 1991;Alonso-Zarza, 1999;Eren et al, 2004Eren et al, , 2008Bajnoczi et al, 2006;Kaplan et al, 2013).…”