Jorge Enrique Gama Castro

RADIOCARBON DATES FROM SOIL PROFILES IN THE TEOTIHUACÁN VALLEY, MEXICO: INDICATORS OF GEOMORPHOLOGICAL PROCESSES

Radiocarbon dates largely obtained from bulk soil samples in 24 soil profiles in the Teotihuac·n Valley, Mex- ico, are reported insofar as they represent a first step towards developing a sequence of soil formation, erosion, vegetation change, and human impact during the Holocene. Limitations of 14C dating in the area are considered, particularly the absence of charcoal in sediments and poor preservation of pollen. A broad temporal scheme is proposed to guide future research in which 4 periods are defined: ~5000-2000 BP (relative stability with short, intermittent episodes of erosion); ~2000-1500 BP (erosion-sedimentation, deforestation, and intensive agriculture); ~1500-1000 BP (relative stability, depopulation, and partial recovery of the landscape); and ~1000-500 BP (erosion-sedimentation, deforestation, and intensive agriculture). INTRODUCTION Between 1992 and 1999, 24 soil profiles were excavated in the Teotihuac·n Valley, State of Mexico (Figure 1), in order to study the formation of soils, erosion, and human impact associated with prehispanic settlement, resource extraction, and production in the region. The region, located approximately 50 km NE of Mexico City, is best known for the archaeological site of Teotihuac·n, the earliest city of its size and density in the Americas, occupied between approximately AD 1-650. Although the prehispanic urban center of Teotihuac·n represents a significant focus for our research, our initial goal was to establish a broad outline of the sequence of landscape transformation throughout the Holocene, based on the analysis of sediments and associated plant remains, including macrobotanical remains, pollen, and phytoliths (McClung de Tapia et al. 2003). In the Teotihuac·n Valley, only very limited paleoenvironmental research had been undertaken prior to our investigation, including geological surveys reported by Mooser (1968) and Barba (1995), a palynological study by Kovar (1970), and an analysis of Aztec and Colonial period landscape change in the Texcoco region, which included a portion of the southern extreme of the Teotihuac·n Valley (Cordova 1997). Only Cordovas research incorporated radiocarbon determinations of organic materials recovered from profiles. Consequently, our analysis represents the first stage of an attempt to develop a sequence of regional landscape dynamics in the area during the Holocene. Paleoenvironmental studies of lake sediments reported from other sectors of the Basin of Mexico indicate relatively dry conditions during the Late Glacial continuing into the Early Holocene (Loz- ano-Garcia and Ortega-Guerrero 1998; Caballero et al. 1999). Palynological studies of cores from lakes Texcoco and Chalco (Lozano-Garcia et al. 1993; Lozano Garcia and Ortega-Guerrero 1998) S and E of the Teotihuac·n Valley, respectively, and Lake Tecocomulco in the NE (Caballero et al. 1999), consistently reveal evidence for human impact during the Middle-Late Holocene that obscures possible indicators of episodes of climatic-induced vegetation change or other evidence for climatic variability that could have affected human settlements in prehispanic times.

History of pedogenesis and geomorphic processes in the Valley of Teotihuacán, Mexico: Micromorphological evidences from a soil catena .

Este trabajo proporciona nuevas evidencias sobre la historia edafologica y geomorfologica en el Valle de Teotihuacan, Mexico. El paisaje del suelo en este lugar esta formado por un Luvisol y un “suelo negro”, cuyos analisis micromorfologicos permitieron: (a) distinguir entre procesos in situ y procesos heredados, y (b) establecer las relaciones espaciales de erosion/sedimentacion a traves de una catena de tres perfiles de suelo. La seleccion de los suelos se realizo a partir de fotointerpretacion y posterior verificacion de una toposecuencia que mostraba una distribucion desde ladera media hasta piedemonte coluvial. Las muestras de estos perfiles fueron caracterizadas de manera fisico-quimica, incluyendo la observacion micromorfologica de laminas delgadas con microscopio petrografico. Los resultados obtenidos se resumen en las observaciones geomorfologicas, la descripcion y el analisis de los perfiles de suelo de la catena Zacatlan, y los resultados micromorfologicos. El conjunto permitio interpretar y reconocer tanto los procesos edafogeneticos actuales como los heredados debidos al transporte por erosion de suelo en epocas pasadas. En el “suelo negro” los procesos clave son el desarrollo de propiedades verticas, la humificacion y la acumulacion de carbonatos secundarios; la micromorfologia tambien revelo fragmentos de arcilla iluviada y nodulos de hierro redepositados. En el Luvisol domina la iluviacion de arcilla asociada con procesos redoximorficos. En los horizontes verticos se observo la presencia de fragmentos de pomez fuertemente meteorizados coexistiendo con fragmentos de roca y minerales con debil meteorizacion. Los edaforrasgos del “suelo negro” difieren fuertemente de los del Luvisol, lo cual permitio reconocer rasgos heredados del Luvisol en los horizontes del “suelo negro”. Por lo tanto, las observaciones micromorfologicas nos permiten proponer que el “suelo negro” probablemente incorporo materiales del Luvisol, los cuales fueron anteriormente coluviados. La presunta fase de erosion puede corresponder con una aridizacion climatica.