Abstract

The carbon (14C and 13C) and oxygen isotopic composition of pedogenic carbonate was determined for two soil chronosequences on limestone and granitic alluvium in the Providence Mountains area in the Mojave Desert, California. The measured 14C ages of pedogenic carbonate coating on clasts were interpreted in the light of a diffusion-reaction model developed in our recent studies (Wang et al., 1993a, 1994; Amundson et al., 1994). Several model parameters were fit or constrained by examining the 14C content of a Pleistocene soil collected prior to atomic weapons testing and by examining the 14C content of modern washes in the area. Model ages of soil formation calculated from the measured 14C ages of pedogenic carbonate are in correct relative order as determined by geomorphic evidence, and are also consistent with model ages from the measured 14C ages of soil organic matter. 14C model ages suggest that the older geomorphic surfaces we studied are of late Pleistocene age (ca. 47–17 ka) and the younger surfaces formed during the Holocene (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-stimulated 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 isotopic composition of the soil carbonate indicates either a slight increase in C4 or CAM (crassulacean acid metabolism) plants or a decrease in plant density in this area during the Holocene. Both the carbon and oxygen isotopic composition of soil carbonates suggests that the climate in the eastern Mojave Desert has, in general, become warmer and drier during the Holocene. The presence of multiple Holocene alluvial deposits, coupled with our 14C age data, provide additional insights into the climatic fluctuations during the Holocene. We conclude that 14C dating of pedogenic carbonate laminations is a useful additional tool in Quaternary studies. Further work is needed, however, to reveal the reasons for the discrepancies between this method and other dating techniques.

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