Calcified root cells in Miocene pedogenic carbonates of the Madrid Basin: evidence for the origin of Microcodium b

Abstract

Calcified root cells, forming microspar and pseudospar mosaics of calcite and/or dolomite crystals, constitute a major component of calcretes and dolocretes from the Miocene of the Madrid Basin. The calcified cells occur in massive nodules or fill root tubes in the calcrete-dolocrete profiles. The arrangement of the cells within the mosaics and their internal features, together with the isotopic data, clearly indicate that the crystals formed through the calcification of root cells and not through recrystallization or dolomitization. Calcified root cells formed in a favourable microenvironment caused by biochemical phenomena associated with plant growth. In these examples, the calcification is incomplete as only in the innermost part of the root the cells were totally calcified, whereas in the root cortex only the cell walls were calcified. The distribution of the calcified cells within the roots was controlled by the different ionic environments which prevail within an active root system. In the inner part the ionic conditions were mostly controlled by the cellular activity of the root creating a suitable microenvironment for the biomineralization of the cells. This differs notably from most published examples of calcified root cells in which it is usual for the cortical cells of roots to be completely calcified. The calcified root cells of the Madrid Basin resemble unequivocally the problematic Microcodium (b), which suggests that this type of Microcodium formed through calcification of root cells.