HYDROGEN AND OXYGEN ISOTOPES IN AN EARLY EOCENE OXISOL

An Early Eocene (~52 Ma B.P.) Oxisol in the Ione Fm of north central California (paleolatitude ~38°N) contains quartz, pedogenic goethite, and pedogenic kaolinite. Measurements of δD, δ¹⁸O, and chemical compositions of these fine-grained mixtures were used in combination with a kinetically constrained, evolved CO₂ method and mass balance calculations to determine the isotopic compositions of the endmember minerals. This approach was adopted to evaluate the feasibility of circumventing selective dissolution. Endmember δ¹⁸O values suggest a kaolinite-goethite, mineral-pair fractionation factor (α) of 1.0188(±0.0006). Published curves of the temperature dependence of 1000lnα for kaolinite-water (Sheppard and Gilg, 1996) and goethite-water (Yapp, 1990) suggest a kaolinite-goethite “equilibrium” temperature of about 21(±7)°C. This implies a δ¹⁸O value of -9.5 (±0.9) per mil for the ancient ambient water. With an assumption that the isotopic composition of the ancient water would lie on the modern Global Meteoric Water Line, combination of δD and δ¹⁸O values of the goethite yielded a calculated temperature of 25(±7)°C. The δ¹⁸O value of the corresponding water was calculated to have been -9.0(±0.7) per mil. Within experimental error, the temperatures and isotopic compositions determined by the two approaches are congruent. The analytical uncertainties are larger than optimal. However, there is enough apparent consistency in the results to indicate paleoenvironmental significance. The nominal average paleotemperature of the two approaches (~23°C) is higher than the modern average annual surface air temperature (16°C) in that locale. This warmer temperature is consistent with formation of the Oxisol during the “climatic optimum” of the Early Eocene (~52 Ma B.P.). In addition, the inferred δ¹⁸O value of about -9.2 per mil for the ancient water is approximately 2 per mil more negative than modern precipitation in the area. If correct, this could suggest that Early Eocene precipitation was more intense than at present, which is supported by the existence of the ancient laterite. One speculation is that the lower δ¹⁸O value represents higher proportions of rainfall from more northerly excursions of intense tropical storms along the west coast of North America at that time.