A pedogenic goethite record of soil CO 2 variations as a response to soil moisture content

Abstract

The terrestrial carbon cycle and the role of atmospheric CO 2 concentrations in controlling global temperatures can be inferred from the study of ancient soils (paleosols). Soil-formed goethite and calcite have been the primary minerals used as a geochemical proxy for reconstructing atmospheric pCO 2 from ancient terrestrial records. In the case of goethite, optimum sampling strategies for reconstructing pCO 2 focus on the portion of the soil profile that displays steep gradients in both soil CO 2 concentration and δ 13C values of soil CO 2 such that a keeling plot can be developed for a given soil and atmospheric pCO 2 can be calculated from it. We report data from a Carboniferous paleosol that depart from the expected linear trends. The results indicate that pedogenic goethite is sensitive to variations in the isotopic composition of soil CO 2, over a range of timescales, and can record these variations in the carbon isotope composition and mole fraction of Fe(CO 3)OH in solid solution with goethite. We explore possible environmental conditions that can drive these changes as a function of either moisture controlled variations in soil respired CO 2 or in the residence time of carbon in soils. The implications of this result are overestimation of paleoatmospheric pCO 2 from pedogenic goethite.