USE OF AN ARIDITY INDEX FROM PREHISTORIC IRRIGATION CANALS: A TOOL FOR PALEOCLIMATE RECONSTRUCTIONS IN THE ARID SOUTHWEST

The history of irrigation agriculture in the American Southwest is slowly being disclosed, especially along the Santa Cruz River Drainage Basin. Previous studies allowed the identification of the transition from opportunistic to systematic canal operation. The present study offers the first refined reconstruction of a complex irrigation canal network operating during the San Pedro phase of the Early Agricultural period (1200 BC – AD 50). Paleoecological and geochemical data are used in this investigation to recognize the sources of water to the irrigation canals and their potential paleoclimatic impact on agriculture. Stable isotopes (d¹⁸O and d¹³C) and trace element ratios (Mg/Ca and Sr/Ca) from Ilyocypris bradyi, the most common and abundant ostracode species in Las Capas archaeological site, were used for the first time in prehistoric irrigation canals in the Tucson Basin. The combined signatures of Sr/Ca and d¹⁸O provided an aridity index that in turn generated a unique record of water use around 3200 years ago offering a tool for tracking environmental change.