AN ISOTOPIC STUDY OF SOILS IN CHRONOLOGICAL SEQUENCES OF ALLUVIAL DEPOSITS, PROVIDENCE-MOUNTAINS, CALIFORNIA

The carbon (C-14 and C-13) and oxygen isotopic composition of pedogeni c carbonate was determined for two soil chronosequences on limestone a nd granitic alluvium in the Providence Mountains area in the Mojave De sert, California, The measured C-14 ages of pedogenic carbonate coatin g on clasts were interpreted in the light of a diffusion-reaction mode l developed in our recent studies (Wang et al., 1993a, 1994; Amundson et al., 1994). Several model parameters were fit or constrained by exa mining the C-14 content of a Pleistocene soil collected prior to atomi c weapons testing and by examining the C-14 content of modern washes i n the area, Model ages of soil formation calculated from the measured C-14 ages of pedogenic carbonate are in correct relative order as dete rmined by geomorphic evidence, and are also consistent with model ages from the measured C-14 ages of soil organic matter, C-14 model ages s uggest that the older geomorphic surfaces we studied are of late Pleis tocene age (ca, 47-17 ka) and the younger surfaces formed during the H olocene (ca, 11-4 ka), These age estimates of the geomorphic surfaces are older than the previously assigned ages based on a combination of soil development, geomorphic relationships, and several infrared stimu lated luminescence dates, but they are within a few thousand years of these other age estimates, Several possibilities for this discrepancy are evaluated, but no conclusive reason was found, Stable carbon isoto pic composition of the soil carbonate indicates either a slight increa se in C-4 or CAM (crassulacean acid metabolism) plants or a decrease i n plant density in this area during the Holocene, Both the carbon and oxygen isotopic composition of soil carbonates suggests that the clima te int the eastern Mojave Desert has, in general, become warmer and dr ier during the Holocene, The presence of multiple Holocene alluvial de posits, coupled with our C-14 age data, provide additional insights in to the climatic fluctuations during the Holocene, We conclude that C-1 4 dating of pedogenic carbonate laminations is a useful additional too l in Quaternary studies, Further work is needed, however, to reveal th e reasons for the discrepancies between this method and other dating t echniques.