An isotopic study of soils in chronological sequences of alluvial deposits, Providence Mountains, California

Abstract

The carbon ({sup 14}C and {sup 13}C) 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 {sup 14}C ages of pedogenic carbonate coating on clasts were interpreted in the light of a diffusion-reaction model developed in our recent studies. Model ages of soil formation calculated from the measured {sup 14}C ages of pedogenic carbonate are in correct relative order as determined by geomorphic evidence, and are also consistent with model ages from the measured {sup 14}C ages of soil organic matter. {sup 14}C model ages suggest that the order 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. Stable carbon isotopic composition of the soil carbonate indicates either a slight increase in C{sub 4} or CAM (crassulacean acid metabolism) plants or a decrease inmore » 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. 57 refs., 9 figs., 5 tabs.« less