Fluvial processes and paleopedogenesis in the Teotihuacan Valley, México: Responses to late Quaternary environmental changes

A paleosol sequence in the Teotihuacan valley was studied in order to reconstruct landscape evolution during the Late Pleistocene–Holocene, in a cut belonging to a river gully (San Pablo). The sequence is constituted by 4 pedostratigraphic units labeled as SP1–SP4, separated by volcanogenic-alluvial/colluvial sediments. SP4, at the base of the sequence, has a prismatic structure, clay cutans and vertic features. SP3 shows abundant signs of zooturbation (krotovinas) and clay cutans. Both paleosols are interpreted to have formed in stable, humid, forest landscapes, during Marine Isotope Stage 3 (MIS3). However, towards the end of MIS3, these conditions alternated with the periods of contrasting seasonal climate. The overlying SP2 unit is a pedocomplex demonstrating 2 petrocalcic horizons combined with redoximorphic features. The pedocomplex merges laterally with gradually stratified gleyic channel deposits (gravels in the base to fine sand and silt in the top). The age of the lower caliche is 24,000 BP, and thus SP2 formed during MIS2, including the Last Glacial Maximum (LGM). The environmental conditions during LGM are mostly dry with stages of high humidity evidenced by gleyic processes and fluvial activity. At the end of the Pleistocene, the landscape becomes highly unstable, and pedogenesis is replaced by sedimentation. New paleochannels cut through the previously formed paleosols. These channels are infilled by poorly sorted sediments with no gradation. The configuration of all paleochannels is NNE–SSW, nearly at 90° to the modern San Pablo river flow. Geomorphic stability during the early and middle Holocene allowed the development of SP1 in a dry environment. The A horizon of this paleosol has been dated at 2900 BP. It is well developed with strong vertic features. This paleosol is thought to have formed the soil cover when the Classic period Teotihuacan culture flourished in the valley, between the first and third centuries AD. Soils above SP1 are poorly developed and erosion/deposition processes predominate over pedogenesis. Recent landscape instability is due to human activities that intensified erosion processes during the Classic and, especially, the Postclassic and Colonial periods.