STABLE-ISOTOPE AND PALEOSOL DATA FROM THE PLIO-PLEISTOCENE FISH CREEK-VALLECITO BASIN, SOUTHERN CALIFORNIA: NEW INSIGHTS INTO REGIONAL TECTONICS AND PALEOCLIMATE
The observed increase in δ18O in the FCVB is opposite of the change that would be produced by the onset of a rain shadow in the FCVB, suggesting that uplift of the Peninsular Ranges occurred before 3.8 Ma. The increase in δ18O may signify long-term aridification and enrichment of soil waters through evaporation, or a change in the source of Pacific Ocean water vapor. The early decrease in δ13C records an increase in C3 plants at the expense of C4 grasses. This finding suggests that summer precipitation, which C4 grasses require to thrive, decreased in the study area from 3.8 to 2.5 Ma, possibly due to a weakening of the summer monsoon. Absence of vertisols in post-2.8 Ma deposits supports this hypothesis, as a weaker monsoon pattern would weaken the cycle of seasonal water-logging that is required for vertisol formation. Long-term aridification as suggested by δ18O data is also consistent with a weakening monsoon, and both phenomena may be related to Plio-Pleistocene global climate change. Our findings correspond broadly with a previous isotopic study of fossil horse teeth from the FCVB (Brogenski, 2001; Spero et al., 2003).