RADIOCARBON AGES ON ORGANICS FROM PIEDMONT ALLUVIUM, AJO MOUNTAINS, ARIZONA

Abstract

Alluvium in dry lands is considered difficult to date by radiocarbon methods because of the paucity of organic matter. Although organic materials of sufficient size for conventional ¹⁴C dating are rare, wet sieving of alluvium in the Sonoran Desert yields sufficient organics for ¹⁴C measurements by accelerator mass spectrometry (AMS). Detrital charcoal from two Quaternary fluvial fill terraces on the western side of the Ajo Mountains yielded ¹⁴C ages of 14,880 ± 70 yr B.P. (CAMS-12408) for the Qt 1 terrace and 2490 ± 60 yr B.P. (CAMS-12414) and 2510 ± 60 yr B.P. (CAMS-12415) for the smaller inset Qt 2 terrace. These ¹⁴C ages are consistent with what is known about rates of soil development in the region. The earlier aggradation event appears to be supported by regional and possibly global climate change at about 14,000 ¹⁴C yr B.P. The more recent aggradation event does not appear to be synchronous with periods of frequent paleofloods in the southwest. The offset between ¹⁴C and ³⁶Cl ages for the same terraces provides a general indication of the time taken for the clasts to be transported to their current positions on the terraces. [Key words: soils, organic matter, 14C, 36Cl, Quaternary dating methods, piedmont, geomorphology, Sonoran Desert.]