Spatial variability of environment change in the Teotihuacan Valley during the Late Quaternary: Paleopedological inferences

The Teotihuacan Valley, Mexico has been the object of considerable interest because of its importance in the context of human history in America. Although, archeological research has been extensively conducted to understand causes of the decline of Teotihuacan culture, no sufficient scientific evidence has been provided. This paper is focused on paleopedological evidence, especially properties that constitute “soil memory,” in order to reconstruct the spatial distribution of environment variability during the Late Pleistocene and Holocene. Three profiles that include modern soils and buried paleosols were studied in different geomorphic positions: uplands (Cerro Gordo site, CG), lower talus slope/transition to valley bottom (Maseca, MA), and the mouth of the Valley, at the ancient border of the former Texcoco lake. This last site is near Tepexpan (TE) where some of the oldest human remains in Mexico were recovered. The oldest paleosol is at CG at 3000 m a.s.l. It is dated at 22,000 yr BP and exhibits a truncated profile with a very well developed Bt horizon, dominated by kaolinite, with phytoliths of C3 plants. All of its features are related to a humid, probably warm environment. It was classified as a Luvisol. This paleosol is buried by another Luvisol, dated at 13,000 yr BP, with morphological characteristics similar to the previous soil, although clay mineralogy consists mostly of halloysite and a higher percentage of C4-plant phytoliths is apparent. In MA the oldest paleosol was not recognized. It is probable that the two paleosols found in CG merge into one at MA, which shows strong redoximorphic features, but high percentages of C4-plant phytoliths and halloysitic clays. At the TE site paleosols are less developed Fluvisols. Modern soil cover also varies in relation to its geomorphic position. In CG it is a Phaeozem, while in MA and TE present day soils have carbonate accumulation. We interpret prevailing conditions during the Last Glacial Maximum in the Teotihuacan valley to be humid, with a tendency towards drying at the end of Pleistocene. Dry conditions prevail in the valley today.