Stable Carbon Isotopes Applied to Vegetation Reconstruction in the Teotihuacan Valley, Mexico

Abstract

Stable carbon isotope values (δ 13 C) from organic material in soils and sediments suggest possible sources of carbon in the organic component and contribute to the development of hypotheses concerning the composition of regional vegetation. This paper explores the application of δ 13 C to the study of soils and sediments in conjunction with the analysis of plant remains in the Teotihuacan Valley, Mexico, located approximately 50 km NE of Mexico City, known for the archaeological site of Teotihuacan, occupied between ca. AD 1-650. The ratios of 13 C/ 12 C from soil organic matter (SOM) provide a complementary approach towards the analysis of past vegetation change in regions characterized by relatively poor preservation of plant macrofossils and pollen. The analysis of phytoliths recovered from soils and sediments offers an additional source of evidence for broad changes in environmental conditions based on changes in the relative proportions of C3 and C4 grasses. In this paper δ 13 C ratios obtained from SOM in profiles in the Teotihuacan Valley are compared with the results of phytolith analysis in the same horizons. The relative proportion of carbon derived from C4-CAMS (Crassulacean Acid Metabolism) plants was calculated. Phytolith analysis focused on the relative proportions of C3 (diagnostic phytoliths of the Pooideae subfamily of Poaceae) and C4 grass taxa (subfamilies Panicoideae, Chloridoideae and Aristodoideae). The δ 13 C ratios obtained from Tlajinga, located in an area of prehispanic irrigation south of the San Lorenzo River in the central Valley, indicate the predominance of C4-CAMS plants while phytoliths from the same horizons reflect a greater proportion of C4 grasses, although proportionally lower than isotopic indicators. The values of δ 13 C in SOM from Otumba, situated in close proximity to a river that also provided water for a prehispanic irrigation system, reflect a slightly lower predominance of C4-CAMS plants, and a slight increase in C4 grass phytoliths, with respect to Tlajinga. San Pablo provided evidence for the greatest predominance of C4-CAMS plants based on δ 13 C values in SOM whereas the phytoliths recovered from the same horizons indicate a higher proportion of C3 grasses. In general, the isotopic signatures reported here indicate changes in relative proportions of C4-CAMS plants, suggesting variability in the response of vegetation to local conditions, with a predominance of C4-CAMS plants in soil organic matter and considerable variability in proportions of C4 grass phytoliths in the contexts analyzed.