Stable carbon isotope budget of CO 2 in a wet, modern soil as inferred from Fe(CO 3)OH in pedogenic goethite: possible role of calcite dissolution

Abstract

δ 13C values of the Fe(CO 3)OH component in pedogenic goethites from a young soil developed on the Eocene Weches formation in east Texas increase from approximately −13 or −14‰ at depths of 31–64 cm to values around −6 to −4‰ at depths greater than 122 cm. This spatial distribution of δ 13C values suggests that dissolution of precursor fossiliferous marine calcite (still present at deeper levels in the soil) has contributed significantly to the isotopic budget of CO 2 in this soil. A local isotopic material balance was calculated for the soil CO 2 at each sample depth using a calcite δ 13C value of −1‰, an organic matter δ 13C value of −25‰, and the measured δ 13C value of the Fe(CO 3)OH component in each sample. Although there is presently no calcite in the upper 120 cm of the profile, the calculated apparent contribution of CO 2 from a calcite source ranges from 16% to 45% at these shallow depths. Below 120 cm, dissolution of calcite appears to contribute more than 50% of the CO 2 in the soil gas. Similar results might be expected in other wet, goethite-bearing soils that contain relict calcite and thus have not achieved the highly leached characteristics of laterites (such as those in the Amazon basin). Models of the soil CO 2 budget in such systems may need to consider both oxidation of organic matter and dissolution of carbonate minerals as in situ sources of carbon isotope variation in CO 2. The variation with depth of the δ 13C values of the Fe(CO 3)OH component suggests an ongoing process of goethite dissolution and reprecipitation in the active, aerobic soil zone. If so, the extremely low solubility of goethite in oxidizing environments suggests that this dissolution process is probably biologically mediated. Dissolution and reprecipitation of goethite in an aerobic soil would favor the recording of steady-state soil CO 2 δ 13C patterns. Preservation of such information in ancient soils would probably depend upon burial and consequent removal from the biologically active soil zone.