Presentation
Preliminary Pliocene-Pleistocene Stable-Isotope and Paleosol Data From the Fish Creek- Vallecito Basin, Southern California: Insights Into Paleoclimate From Pedogenic Carbonate
American Geophysical Union, Fall Meeting 2008
(2008)
Abstract
In this study we use detailed measurements and isotopic analyses of paleosols in the Fish Creek-Vallecito basin (FCVB), southern California, to interpret changes in Pliocene-Pleistocene paleoclimate in the area. The FCVB currently lies in a hyperarid rain shadow (MAP = 15-17 cm) formed by the Peninsular Ranges. The timing of Peninsular Range uplift is not known, although recent work suggests it could have occurred as recently as early Pleistocene (Mueller et al., 2006). 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. New high-resolution magnetostratigraphic dating allows us to determine the age of paleosol horizons to within an average of ∓ 0.06 m.y.. Pedogenic carbonate nodules from 23 horizons ranging in age from 3.7 to 1.0 Ma, spanning a thickness of 2.5 km, were analyzed for oxygen and carbon isotopic compositions on a Gasbench and MAT 253 mass spectrometer. The data reveal an increase in carbonate δ18O values at about 2.5-3.0 Ma, from -10.5 ∓ 0.1 ‰ to -9.2 ‰ ∓ 0.2 ‰ (VPDB). Pedogenic carbonate δ13C values vary between -10.4 ‰ and -3.8 ‰ (VPDB) with no apparent trend. A total of forty-nine paleosols were described in the study interval. Most paleosols have shallow carbonate (Bk) horizons and thin, poorly-developed A horizons. Our finding of an increase in δ18O corresponds broadly to a previous study of fossil horse teeth (Brogenski, 2001), which recorded a 2 ‰ increase in δ18O in meteoric water at about 2 Ma. Preliminary recalculation of Brogenski's fossil site ages suggests that the change in δ18O occurred earlier than previously reported, around 2.4 Ma. The increase in δ18O at 2.5-3.0 Ma coincides with a global climate change caused by the onset of northern hemisphere glaciation, and may reflect (1) an increase in enriched Pacific Ocean-derived storms and decrease in the concentration of isotopically depleted monsoonal sources, (2) a change in the source of atmospheric water vapor within the Pacific Ocean, or (3) an increase in soil water evaporation driven by an increase in local temperature or of summer precipitation. The observed increase in δ18O is opposite of the change that would be produced by the onset of a rain shadow in the FCVB. This suggests that uplift of the Peninsular Ranges occurred before 3.7 Ma or after 1 Ma, or perhaps took place in two stages before 3.7 Ma and after 1 Ma. We interpret the majority of paleosols as Aridisols that formed under arid to semi-arid conditions. Measurements of depth to the soil carbonate (Bk) horizon show an average decompacted depth to Bk of 19.7 ∓ 1 cm, which corresponds to a mean annual precipitation of approximately 25 cm (Retallack, 2005). This is similar to modern annual rainfall in coastal San Diego and is 8-10 cm more than in the present-day FCVB. While there is considerable scatter in depth-to-Bk measurements, clear trends are not apparent, suggesting that aridity was the dominant climate condition in the basin between 3.7 and 1.0 Ma. Other indicators of climate change in the area, including rise and fall of lake levels and inferences from faunal assemblages, may reflect external factors (i.e. fluvial inflow) rather than local climate conditions.
Keywords
- Stable isotope geochemistry,
- Paleoclimatology,
- Tectonics and climatic interactions
Disciplines
- Geomorphology and
- Geology
Publication Date
Fall 2008
Citation Information
T. C. Peryam, Rebecca J. Dorsey, Ilya N. Bindeman, Bernard A. Housen, et al.. "Preliminary Pliocene-Pleistocene Stable-Isotope and Paleosol Data From the Fish Creek- Vallecito Basin, Southern California: Insights Into Paleoclimate From Pedogenic Carbonate" American Geophysical Union, Fall Meeting 2008 (2008) Available at: http://works.bepress.com/bernard_housen/69/