THE TEOTIHUACAN "BLACK" PALEOSOLS: PEDOGENESIS AND EVIDENCE OF USE BY THE ANCIENT INHABITANTS

Teotihuacan was one of the largest settlements of ancient Mexico, known for hosting one of the greatest pre-Hispanic urban civilizations, which flourished in the interval I-VII centuries A.D., reaching an extension of approximately 20 km² and 125,000 inhabitants. Until now, it is accepted that the landscape in the Teotihuacan valley is somewhat similar to the present day. However this assumption does not take into account the large scale modification of the soil mantle by accelerated human-induced erosion-sedimentation processes from the prehispanic and Colonial periods up until the present. In consequence, past soil cover in Teotihuacan valley throughout the Formative and Classic periods (approximately B.C. 400 –A.D. 650) is not well known. During several soil surveys and archaeological work we have recognized that the modern soil mantle shows remarkable differences in comparison to the Teotihuacan one. This research focuses on the “black” paleosol that was identified as the soil cover during the Teotihuacan period. The date obtained from soil organic matter from a buried AE horizon was dated at 2 060 +/- 40 ¹⁴C BP representing its minimum age. The “Black” paleosol is very well developed, has AE, ABtss, BCk horizons. The features of the horizons are the results of different genetics process: ABtss horizon is dark with strong vertic features (angular blocks, stress cutans, hard consistence), indicating seasonal climate, the thin section show illuvial clay in the fractures, representing an increase of the humidity, also this increase is identified by the Fe nodule, and illuvial clay in pale AE horizon, more evidence of this horizon show in the thin section agrocutans, fragments of charcoal and crust fragments indicating that high humidity come from its use in agriculture by the inhabitants of Teotihuacan. BCk horizon has both illuvial clay and on infillings of calcium carbonate, showing two phases of pedogenesis (humid and dry, respectively). This soil has also been identified in both natural landscapes and under Teotihuacan buildings and even inside the fills of the Moon Pyramid, indicating the several uses it has during the time of occupation.