STABLE-ISOTOPE AND PALEOSOL DATA FROM THE PLIO-PLEISTOCENE FISH CREEK-VALLECITO BASIN, SOUTHERN CALIFORNIA: NEW INSIGHTS INTO REGIONAL TECTONICS AND PALEOCLIMATE

The Peninsular Ranges of southern California create a modern rain shadow in the hyper-arid Salton Trough to the east. In this study we use isotopic analyses and paleosol descriptions in the Fish Creek-Vallecito basin (FCVB) to date creation of the rain shadow by uplift of the Peninsular Ranges. In the FCVB, abundant paleosols are exposed in a thick, tilted stratigraphic section that accumulated in the hanging wall of the West Salton detachment fault. High-resolution magnetostratigraphy allows us to determine the age of paleosol horizons with average uncertainties of +/- 0.06 m.y. Pedogenic carbonate nodules from 48 horizons ranging in age from 3.8 to 0.75 Ma were analyzed for oxygen and carbon isotopic compositions. The data reveal an overall increase in δ¹⁸O values through the studied interval, from -10.6 +/-0.4 permil to -8.5 permil +/- 0.2 permil (VPDB). Carbonate δ¹³C shows large scatter, with average values decreasing from -4.38 +/- 1.92 to -6.9 +/-1.5 permil between 3.8 and 2.5 Ma, followed by slight enrichment to -5.5 +/-1.2 permil by 0.75 Ma. Paleosols are dominantly aridisols and vertisols, with vertisols found only in deposits older than 2.8 Ma.

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).