Pore space memory of the Tlalpan pedosedimentary sequence as an indicator of paleopedogenesis

Background

Imaging techniques of analysis in undisturbed soil samples are powerful tools for understanding soil properties and functioning. Although micromorphological analysis has always been applied for studying soil genesis, the modern 3D X-ray Computed Tomography (CT) approach is mostly used in soil physical studies related to soil functioning and ecological services.

Aim

In this study, we interpret ancient soil formation of the Late Pleistocene pedosedimentary archive of the Tlalpan sequence in the State of Tlaxcala in Central Mexico, based on soil and sediment porosity distribution in 2D and 3D.

Methods

In order to interpret ancient soil formation based on the porosity distribution, we applied both micromorphological (2D) and CT (3D) analyses in undisturbed samples of each horizon of the Tlalpan sequence.

Results

Our micromorphological observations have shown that pore space arrangement is unique for each soil horizon and that it is predetermined by the dominant pedogenetical processes and their succession, such as bioturbation, clay illuviation, and vertic shrinking/swelling. Most of the channels formed by biogenic agents (roots and mesofauna) subsequently underwent shape deformations and/or refilling. The “accommodating planes” type of voids resulted from the development of features of vertic paleosols that, in some cases, erased the past pedogenic pore space organization. Although biogenic turbation, compaction, and clay illuviation mostly affected the macro- and mesopores, shrinking/swelling processes affected macro- and mesoporosity distribution. The process that was only reflected at the microporosity level is hydroconsolidation.

Conclusions

Despite the common idea that textural pores are more resistant to changes compared to meso- and macropores, most of soil formation processes registered in the Tlalpan sequence, including anthropogenic impact, were identified in macro- and mesopore space. Moreover, the changes that were registered in pore space transformation by 2D and 3D methods of analysis in undisturbed samples are crucial for identifying the sequence of formation processes and, therefore, for paleopedological interpretation. This study shows that the CT is a useful tool to access the soil formation, and pore space memory studied both in 3D and 2D is an important proxy for paleopedological research.