FE(CO3)OH IN GOETHITE AS A PALEOENVIRONMENTAL INDICATOR: AN EXAMPLE FROM A MIDDLE EOCENE PALEOSOL

The mole fraction (X) and d¹³C values of the Fe(CO₃)OH component in solid solution in goethites from an oxisol in the Middle Eocene Ione Fm in northern California were determined from incremental, isothermal vacuum dehydration spectra. The values of X range from 0.00145 to 0.0037, while the corresponding d¹³C values range from –15.4‰ to –19.0‰ in samples measured to date. These values of X suggest that the ambient soil CO₂ concentrations at the time of goethite crystallization ranged from approximately 8000 ppmV to 20,000 ppmV. Soil CO₂ concentrations of this magnitude could indicate that soil productivity was lower and/or that soil CO₂ diffusion coefficients were higher than in many goethite-rich soils from presumably wet environments. If goethite (a-FeOOH) forms in the presence of soil CO₂ that represents diffusive mixing of two isotopically distinct endmembers (atmospheric CO₂ and CO₂ from in situ oxidation of soil organic matter), the Fe(CO₃)OH component in solid solution in the goethite can be a source of information on the partial pressure of CO₂ in the Earth’s ancient atmosphere. A plot of d¹³C vs. 1/X defines a coherent linear array with a positive slope, which suggests that pedogenic goethite in the Ione Fm. formed in such a two-component mixing system. The intercept of the data array suggests that the d¹³C value of the organic matter undergoing oxidation at the time of goethite crystallization was –28‰. This comparatively negative organic matter d¹³C value is similar to values measured for organic matter in modern tropical laterites. The slope of the apparent soil CO₂ mixing array indicates that the concentration of atmospheric CO₂ in the Middle Eocene was about 2800 (± 500) ppmV. A high concentration of atmospheric CO₂ and the presence of an oxisol at ~38°N in the Middle Eocene are consistent with other indications of warmer temperatures and relatively wet conditions at high northern latitudes at that time.