SINGLE AND MULTIGRAIN QUARTZ-LUMINESCENCE DATING OF IRRIGATION-CHANNEL FEATURES IN NEW MEXICO AND ARIZONA

We applied multigrain-quartz (MGQ) and single-grain-quartz (SGQ) single-aliquot-regenerative-dose (SAR) photon-stimulated-luminescence (PSL) sediment-dating procedures to deposits from two contrasting-scale irrigation-channel systems. For each of these systems, ¹⁴C dating was limited to non-existent. Such irrigation features become filled with floodwater and other fluvial deposits after irrigation usage stops. A chronology of last use can be obtained by dating the infill deposits. In the Phoenix (Arizona) area, the Hohokam people built one of the largest and most sophisticated irrigation systems in the prehistoric New World. Some canals were up to 27 m wide and 3 m deep at their head gates, extending as far as 25 km across the low-relief desert of the lower Salt River Valley. Historic canals followed many of the prehistoric alignments. In contrast, irrigation channels and ditches at Santa Fe (New Mexico) were small (1-6 m wide) and filled by high-energy ‘fluvial’ deposits because these features are set into a relatively high-relief terrain. Applied to both ‘historic’ (for age checks) and ‘prehistoric’ deposits in each setting, SGQ PSL provides the most accurate age estimates. In Phoenix SGQ ages range from 75±8 yr (historic) to 1295±51 yr for irrigation-feature deposits. Although not dramatically different, the corresponding MGQ age estimates are believed to be less accurate because of some greater ambiguities in interpretation of the equivalent-dose (D_E) probability-density distribution plots. At Santa Fe an MGQ SAR PSL age of 92±10 yr was obtained for a sample known to be <127 yrs old. An SGQ SAR age of 376±31 yr was obtained for a deposit known to be ≤400-450 yrs old, the time of first settlement by Spanish people. The MGQ age estimates for this oldest deposit depend strongly on aliquot size, and exceed significantly the SGQ result. In both study areas, SGQ PSL dating circumvents the limitations of ¹⁴C dating (lack of datable material, ambiguity in calendar-yr conversion for deposits younger than ~ 600 yr).