Fe(CO 3)OH in goethite from a mid-latitude North American Oxisol: estimate of atmospheric CO 2 concentration in the Early Eocene “climatic optimum”

Abstract

Measured mole fractions ( X) and δ 13C values of the Fe(CO 3)OH component in pedogenic goethite from a mid-latitude Oxisol of Early Eocene age (≈52 Ma B.P.) range from 0.0014 to 0.0064 and −20.1 to −15.4‰, respectively. These values of X imply that concentrations of CO 2 gas in the paleosol were ≈7400 to ≈34,000 ppm. δ 13C and 1/ X are correlated and define a linear, soil-CO 2 diffusive mixing line with a positive slope. Such positive slopes are characteristic of mixing of two isotopically distinct CO 2 endmembers (atmospheric CO 2 and CO 2 from oxidation of soil organic matter). From the intercept of the mixing line, it is calculated that the δ 13C value of organic matter in the ancient soil was ≈−28.0‰. The magnitude of the slope implies an Early Eocene atmospheric CO 2 concentration of ≈2700 ppm. A simple model for forest soils suggests that a “canopy effect” may cause atmospheric CO 2 concentrations deduced from pedogenic minerals to underestimate the actual concentrations of atmospheric CO 2. If a significant forest canopy were present at the time of formation of pedogenic goethite in the Ione Fm, the concentration of 2700 ppm calculated for atmospheric CO 2 could be slightly low, but the underestimate is expected to be < ≈300 ppm (i.e., less than the analytical uncertainty). The relatively high concentration of 2700 ppm inferred for atmospheric CO 2 at ≈52 Ma B.P. would have been coincident with the Early Eocene climatic optimum. This result seems to support the case for an important role for variations of atmospheric CO 2 in the modification of global paleoclimate.