CALCIFIED ROOT-CELLS IN MIOCENE PEDOGENIC CARBONATES OF THE MADRID BASIN - EVIDENCE FOR THE ORIGIN OF MICROCODIUM-B

Calcified root cells, forming microspar and pseudospar mosaics of calc ite and/or dolomite crystals, constitute a major component of calcrete s and dolocretes from the Miocene of the Madrid Basin. The calcified c ells occur in massive nodules or fill root tubes in the calcrete-doloc rete profiles. The arrangement of the cells within the mosaics and the ir internal features. together with the isotopic data. clearly indicat e 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 phenome na 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 wer e calcified. The distribution of the calcified cells within the roots was controlled by the different ionic environments which prevail withi n an active root system. In the inner part the ionic conditions were m ostly controlled by the cellular activity of the root creating a suita ble microenvironment for the biomineralization of the cells. This diff ers notably from most published examples of calcified root cells in wh ich it is usual for the cortical cells of roots to be completely calci fied. The calcified root cells of the Madrid Basin resemble unequivoca lly the problematic Microcodium (b), which suggests that this type of Microcodium formed through calcification of root cells. (C) 1998 Elsev ier Science B.V. All rights reserved.