Elizabeth Solleiro

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.

The complex genesis of red soils in peninsula de Yucatán, Mexico: Mineralogical, micromorphological and geochemical proxies

The origin of red clayey soils developed on limestones has been largely questioned. We have analyzed thick red soils on Eocene-Pliocene limestones of the Yucatán peninsula (with detail in the Kantunil Kin area). The morphological, geochemical and mineralogical characteristics were interpreted as a record of pedogenesis and geomorphic dynamics during the Quaternary. Sand fraction mineralogy, the Zr/Ti ratio and ternary diagrams of Trace elements (Ti-Y-Zr, La-Th-Sc and Zr-Th-Sc) indicated a mixed origin of the soil parent material; volcanic and granitic/metamorphic components, redeposited by eolian transport. Within the soil matrix, primary minerals were transformed by weathering and caused desilication with the accumulation of kaolinitic clay. Reworked pedofeatures were originated from different soil bodies eroded. In the lower part of the profiles, we described the zone of carbonate leaching front, were the clay translocation in suspensions and posterior coagulation is more probable than the migration of elements in solutions and posterior synthesis.

Late pleistocene paleosol sequences as an instrument for the local paleographic reconstruction of the Kostenki 14 key section (Voronezh oblast) as an example

A sequence of five paleosol units (with seven individual paleosol profiles) buried in the Late Pleistocene (20–40 ka) deposits was studied at the Kostenki 14 (K14) key section in Voronezh oblast with the use of a set of morphological, physicochemical, and instrumental methods. The upper-lying paleosols differed from the lower-lying paleosols in the less pronounced gley features, stronger aggregation of the soil material, more significant accumulation of carbonates, and higher percentage of calcium humates and fulvates. These features attested to the higher aridity of the paleoclimate and the development of the upper-lying paleosols under grassy vegetation. Within the studied paleosol sequence, the most developed profiles were typical of the soils that formed 27–32 ka ago during the Bryansk interstadial. The good aggregation, the presence of features left by the soil fauna activity, the high magnetic susceptibility, and the morphology of the secondary carbonates in the studied paleosols suggest that they were formed under meadow-steppe vegetation in well-drained positions and resembled modern cryoarid soils.

The humus status of modern and buried volcanic soils in Mexico and its role in the paleogeographic interpretation of tephra-paleosol sequences

Chemical and optical characteristics of soil humus have been analyzed as “memory” components of the Pleistocene volcanic paleosols in Mexico. We have studied the A1 horizons of buried Andosols of the Nevado de Toluca series and of the modern Andosols formed under different bioclimatic conditions. Data on the organic matter of buried paleosols suggest that Andosols of the Nevado de Toluca series were formed in humid forest ecosystems. Optical characteristics of the humic acids and data on the molecular-mass distribution of the humus make it possible to assume that these soils were formed under pine forests rather than under fir forests.