Morphogenesis and quantification of the pore space in a tephra-palaeosol sequence in Tlaxcala, central Mexico

Tepetates are indurated subsoil horizons developed in tephra-derived materials in various parts of the Trans-Mexican Volcanic Belt. The term ‘tepetate’ includes fragipans, duripans, pedosediments and saprolites, derived from vitric rhyolitic tuff, Pleistocene volcanic ashes or pyroclastic flows. All soils with tepetates are at high risk of erosion and so they have been intensively studied for decades. The tepetates are common in Tlaxcala State of central Mexico, being formed under Ustic Isomesic soil climate. The aim of this investigation was to characterise the pore space of fragipan-type tepetates and the role of clay components in their formation. We studied porosity of tepetate from a tephra-palaeosol sequence in the north of Tlaxcala State, in undisturbed soil samples. Observations of pore space were made in 2D and 3D by analysing microscope images of thin sections and cross-sectional images from a computed tomography scanner. In the thin sections we also identified and described clay illuvial pedofeatures. Micromorphological observations showed two main pore types. Small rounded pores had a homogeneous distribution and were probably formed before the clay illuviation process that took place in a palaeosol formed on the tepetate material. The distribution pattern of the small pores in the studied tepetate was similar to that in the fragic horizon, which was probably formed by a hydro-consolidation process. Large crack-pores were formed during the palaeosol formation. Later these large pores were filled by illuvial clay coatings and so we conclude that each tepetate was part of the set of Bt horizons in the palaeosols of Luvisol type.