OPTICALLY STIMULATED LUMINESCENCE DATING ON ANCIENT IRRIGATION CANALS IN SONORA, MEXICO

Earthen irrigation canals have been constructed on the floodplains of the southwest United States beginning almost 4,000 years ago. Irrigation canals and ditches were also constructed in nearby Sonora, Mexico, but little geochronological data exists from canals in that region. These data are critical in understanding the timing of the development of this technology across the greater southwest US/northwest Mexico region. Dynamic floodplain processes can make dating the irrigation canals challenging. Radiocarbon dating on detrital sediments may yield erroneously old dates due to erosion and redeposition of dateable organic materials. Optically Stimulated Luminescence dating (OSL) was conducted directly on canal sediments from the La Playa archaeological site in Sonora. Because canal sediments have been eroded and transported through the canal system before being deposited and buried, the date of the last exposure to light represents canal use.

Four samples were collected from the upper portion of infilled canals to determine the ages of the final sedimentation episodes. Single-grain OSL dating on individual quartz grains can avoid problems associated with incomplete bleaching during transport. Samples were dated following single-aliquot regenerative-dose procedures using single-grain OSL dating of 125-180 μm quartz sand. IRSL was used to check the purity of quartz samples. Equivalent dose was determined using the Central Age Model. Results are reported at the 2σ standard error.

Results from two overlapping canal segments buried under floodplain alluvium were 2.26 ± 0.26 ka and 1.90 ± 0.23 ka, representing two iterations of canal use. Samples were also collected from a primary canal and a connecting secondary canal, dating to 2.31 ± 0.29 ka and 1.90 ± 0.22 ka, respectively. The OSL results are significant because they place the final sedimentation dates of the irrigation canals securely at the latter end of the Early Agricultural period (2100 B.C.- A.D. 50), a period when people first began developing this agricultural technology in the region.