CARBON AND OXYGEN ISOTOPES IN PEDOGENIC GIBBSITE: AN EMERGING RESOURCE FOR UNDERSTANDING SOIL FORMING AND TRANSPORT PROCESSES

The timing of crystallization and subsequent extent of mineral transport during soil formation are two factors that are often difficult to pinpoint in terrains dominated by moderate to severe chemical weathering. We suggest the complex interplay of factors responsible for soil mineralization such as changing respiration rate, parent mineral, biota, and climate, may be recorded by the stable and radiogenic isotopes trapped in the “carbonate” structure of pedogenic gibbsite and goethite. The exact mechanism for inclusion of carbon and oxygen into this carbonate component is poorly understood. Regardless, our observations of stable carbon and oxygen isotopes (and co-variation) in pedogenic gibbsite at various soil horizons in Cecil soils formed in the Southeast US Piedmont appears to reliably record the dynamics of soil CO₂ gas cycling. Dissolution and re-precipitation of gibbsite after incipient formation is suggested by the pattern of oxygen and carbon isotope compositions down profile, where at the top of the B_t horizon signatures are most like current atmosphere and deep in the C horizon signatures are most like the plants above. The gradient between the B_t and C is reflective of atmospheric CO₂ concentration / isotopic composition and the soil CO₂ concentration / isotopic composition at the time of mineralization. This suggests that carbonate component of pedogenic gibbsite may be useful for understanding soil forming conditions, which extends to proxying for paleo-PCO₂ estimates from paleosols (provided additional generations of recrystallization have not occurred).

Estimated exposure ages of the soil based on cosmogenic ¹⁰B and ²⁶Al is useful for establishing the residence time of pedogenic minerals. Most soils are formed over longer time scales than natural climate change cycles and it is important to understand if the climate information is integrated and recorded in soil minerals. Resolution is needed to know if these isotopic signatures are representative of the most recent conditions or a combination of all past conditions. Carbonate occluded in pedogenic minerals offer the chance to determine both the age and environmental conditions of mineral formation and are therefore a powerful tool whose potential is only now beginning to be realized