Combining radiocarbon and single-grain optically stimulated luminescence methods to accurately date pre-ceramic irrigation canals, Tucson, Arizona

We present 12 accelerator mass spectrometry (AMS) C-14 and five quartz single-grain optically stimulated luminescence (OSL) ages from three Early Agricultural period irrigation canals recently discovered in the floodplains of the Santa Cruz River and Rillito Creek in Tucson, Arizona. Significant discrepancies in the AMS C-14 ages are associated with fluvial redeposition of charcoal, producing at times, age overestimates of similar to 1000 years. Single-grain OSL ages on canal sediments and application of a minimum age model accurately date the final operation of the canals but have relatively large standard errors-of several hundred years, Based on single-grain-OSL, one-canal-dates 730-310 B.C. and the other two date 560-100 B.C. This corresponds with a period of predominant floodplain aggradation (similar to 500 B.C.-A.D. 1) conducive to water control and irrigation agriculture. Stratigraphic evidence of uncontrolled flooding and rapid sedimentation is present in all three canals and marks the abandonment of two canals. Determining the age of the earliest canals is important for understanding the origins of water management and the transition from foraging to intensive agriculture in the North American Southwest. Due to potential problems with reworked organic material for C-14 dating and partial bleaching (resetting) of luminescence signals in fluvial settings, we recommend a combined AMS C-14 and single-grain OSL approach for dating early canals. 2013 Elsevier Ltd. All rights reserved.