STABLE CARBON ISOTOPIC SIGNATURES IN ORGANIC CUMULIC SOILS: A POSSIBLE MARKER FOR ANCIENT MAIZE AGRICULTURE IN THE DESERT SOUTHWEST

The Santa Cruz River in southern Arizona area has a well dated alluvial stratigraphy containing evidence of indigenous agriculture extending back ca. 4100 years. Of archaeological interest is the rate at which maize agriculture was adopted and changes in the intensity of its cultivation through time. As a preliminary study, stable carbon isotopic analysis of bulk organic sediments was conducted on two stratigraphic profiles from a well dated archaeological site located in the Santa Cruz River floodplain near downtown Tucson. These profiles contain Santa Cruz River vertical accretion deposits and organic cumulic (“cienega”) soils that date ca. 4000 cal yr BP to present. Control samples were also collected from the topsoil of modern riparian analogs in the upper Santa Cruz River basin. Results indicate changes in δ¹³C soil organic matter with depth (range: -22.0‰ to -16.9‰) that are consistent with conventional models (based on archaeological and ethnobotanical evidence) of changing late Holocene maize cultivation, i.e., gradual adoption of maize beginning ca. 4000 cal yr BP that peaks ca. 2000-600 cal yr BP and then decreases in response to population collapse and introduction of Old World (C₄) cultigens. This is surprising given the challenges of mixed C₃/C₄ plant communities in the Sonoran Desert and problems of redeposited, detrital organic carbon in alluvial soils. Results presented here suggest that under specific geological and pedological circumstances, δ¹³C analysis of soil organic matter has the potential to aid archaeologists in reconstructing past maize cultivation in desert floodplains of the Southwest.