首页|Fe(CO3)OH in goethite from a mid-latitude North American Oxisol: Estimate of atmospheric CO2 concentration in the Early Eocene "climatic optimum"
Fe(CO3)OH in goethite from a mid-latitude North American Oxisol: Estimate of atmospheric CO2 concentration in the Early Eocene "climatic optimum"
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Measured mole fractions (X) and delta(13)C values of the Fe(CO3)OH component in pedogenic goethite from a mid-latitude Oxisol of Early Eocene age (similar to52 Ma B.P.) range from 0.0014 to 0.0064 and -20.1 to -15.4parts per thousand, respectively. These values of X imply that concentrations of CO2 gas in the paleosol were similar to7400 to similar to34,000 ppm. delta(13)C and 1/X are correlated and define a linear, soil-CO2 diffusive mixing line with a positive slope. Such positive slopes are characteristic of mixing of two isotopically distinct CO2 endmembers (atmospheric CO2 and CO2 from oxidation of soil organic matter). From the intercept of the mixing line, it is calculated that the delta(13)C value of organic matter in the ancient soil was similar to-28.0parts per thousand. The magnitude of the slope implies an Early Eocene atmospheric CO2 concentration of similar to2700 ppm. A simple model for forest soils suggests that a "canopy effect" may cause atmospheric CO2 concentrations deduced from pedogenic minerals to underestimate the actual concentrations of atmospheric CO2. 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 CO2 could be slightly low, but the underestimate is expected to be < similar to300 ppm (i.e., less than the analytical uncertainty). The relatively high concentration of 2700 ppm inferred for atmospheric CO2 at similar to52 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 CO2 in the modification of global paleoclimate. Copyright (C) 2004 Elsevier Ltd. [References: 57]
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