Detection of ancient maize in lowland Maya soils using stable carbon isotopes: evidence from Caracol, Belize

Maize (a C4 plant) is enriched in the stable isotope 13C relative to most other cultivated and wild plants in Mesoamerica. Hence, a distinctive carbon isotope signal produced by ancient maize crop residues may be preserved in the most stable humic component of the soil organic matter. We extracted humic substances from terraced soils surrounding the ancient Maya center of Caracol, Belize that were likely used for agriculture. Total organic matter was only slightly enriched in 13C relative to a control section whereas both humic acid and humin were significantly enriched in 13C, indicating that maize had been grown in these areas. Studying terraced plots that may have been used to grow maize, we find that the maximum δ13C signal is found near the bedrock–soil interface, while shallower soils near the former level of cultivation exhibit native C3-like δ13C values. We infer that humic matter derived from post-agricultural vegetation has replaced the 13C-enriched humus of the upper levels. Analyses of δ13C in soil humus should enable archaeologists to resolve discrepancies between the size of some Maya settlements and the extent of nearby cultivated areas.