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

Resumen

Stable carbon isotope
values (δ13C) 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 δ13C 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
13C/12C 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 δ13C 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 δ13C 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 δ13C 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 δ13C 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.