SOIL CO2 VARIATIONS AS A RESPONSE TO SOIL MOISTURE CONTENT RECORDED IN PEDOGENIC GOETHITE

Soil-formed goethite and calcite comprise the minerals used to reconstruct ancient atmospheric pCO₂ from terrestrial records. In the case of goethite, optimum sampling strategies for reconstructing pCO₂ focus on the portion of the soil profile that displays steep gradients in both soil CO₂ concentration and δ¹³C of soil CO₂, such that a linear keeling plot can be developed for a given soil and atmospheric pCO₂ can be calculated from it. Here, we report non-linear keeling plots derived from carbon isotope analysis of a goethite-Fe(CO₃)OH solid solution series sampled from a plinthite, collected from the mid-Pennsylvanian succession of the Paganzo Group, northwestern Argentina. Non-linearity of δ¹³C and 1/CO₂ from the studied plinthite imply hysteresis of the shallow (<20cm) soil CO₂ system. Goethite-ferric carbonate most depleted in ¹³C coincides with the highest mol fraction of Fe(CO₃)OH. The δ¹³C value of this goethite-ferric carbonate solid solution suggests equilibrium fractionation between soil-respired CO₂ and goethite, indicating a departure from the normative diffusion-production system that regulates soil CO₂ concentration and δ¹³C. The consequence of this dynamic soil CO₂ system is that atmospheric pCO₂ estimates from pedogenic goethite may be overestimated if they formed under variable moisture content in the shallow soil above the characteristic production depth. This study further confirms that high-precision incremental dehydration-decarbonation experiments of goethite-ferric carbonate solid solution in conjunction with an analysis of mineralogy and Al-substitution in goethite are necessary to disentangle the effects of seasonally variable soil moisture content, which confound prevailing methods for estimating paleoatmospheric pCO₂ from pedogenic goethite.