O-18/O-16 and D/H in goethite from a North American Oxisol of the Early Eocene climatic optimum

An Early Eocene Oxisol in the lone Formation of California formed in a coastal continental weathering environment at a paleolatitude of similar to 38 degrees N. The dominant minerals in the Oxisol are goethite, quartz, and kaolinite. Material balance calculations were applied to new measurements of chemical composition, D/H, and O-18/O-16 ratios of Oxisol samples to determine the delta D (-150 +/- 3%) and delta O-18 (-2.4 +/- 0.3%) values of the goethite (alpha-FeOOH). These data, in combination with the global meteoric water line (MWL), yielded an isotopic temperature of 21(+/- 4) degrees C. The nominal value of 21 degrees C contrasts with the modern mean annual temperature (MAT) of 16 degrees C in that area. The warmer temperature is consistent with formation of the goethite during the Early Eocene climatic optimum. The isotopic composition of the goethite and a temperature of 21 degrees C imply ancient water with a delta D value of -61(+/- 4)% and a delta O-18 value of -8.9(+/- 0.5)%. This Early Eocene delta O-18 (or delta D) value is more negative than values in the range of isotopic scatter observed for modern global precipitation at sites with a MAT of 21 degrees C. At times of warm global climates, the location of a near-surface atmospheric isotherm would generally shift relative to its location under modern climatic conditions. A simple Rayleigh-type condensation model indicates that, if one follows the isotherm, the associated scatter in delta D and delta O-18 of precipitation in very warm global climates should shift (for a given isotherm) to more negative values that may be detectable in proxy records. The isotopic results from the goethite of the Early Eocene Oxisol appear to add to evidence in support of this idea. (C) 2008 Elsevier Ltd. All rights reserved.