Plio-Pleistocene climate change and timing of Peninsular Ranges uplift in southern California: Evidence from paleosols and stable isotopes in the Fish Creek–Vallecito basin

Abundant Plio-Pleistocene paleosols in the Fish Creek–Vallecito basin record the effects of late Neogene global climate change on inland southern California. Analysis of 82 paleosol horizons ranging in age from 3.9 to 0.7 Ma reveals two primary populations: vertic paleosols with abundant mud cracks and calcic paleosols with pronounced calcite accumulation zones. Vertic paleosols are dominant in older deposits but disappear from the stratigraphic record by 2.8 Ma, after which point calcic paleosols are nearly the only paleosol type present. Pedogenic carbonate nodules from 47 horizons were analyzed for oxygen and carbon isotopic compositions. In late Pliocene time carbonate δ13C values become depleted, from an average of − 5.1‰ between 3.9 and 3.5 Ma to − 7.3‰ (VPDB) between 2.75 and 2.25 Ma, recording an increase in C3 plants at the expense of C4 grasses. This finding suggests that summer precipitation decreased in the study area from 3.8 to 2.5 Ma, possibly due to a weakening of the summer monsoon. The absence of vertic paleosols in post-2.8 Ma deposits supports this hypothesis. The increase in abundance of calcic paleosols through time suggests a long-term increase in aridity. A gradual enrichment of about 2‰ in δ18O values from 3.9 to 0.7 Ma also likely resulted from increased aridification and evaporative enrichment of soil water. Our findings suggest that global cooling in late Pliocene time resulted in increased aridity in southern California due to decreased monsoonal activity and the end of persistent El Niño conditions in the Pacific Ocean. The observed enrichment in δ18O in the Fish Creek–Vallecito basin is opposite the change that would be produced by the onset of a rain shadow, suggesting that significant uplift of the Peninsular Ranges occurred before 3.8 Ma.