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Abstract

Paleosols in rift-related Upper Triassic (Camian-Norian) terrestrial deposits of the Durham sub-basin of North Carolina record evidence of a shift from wetter to drier conditions, and slightly elevated atmospheric levels of carbon dioxide. Paleosols in siltstone interbeds of Lithofacies Association IIa (upper Carnian to lower Norian) are characterized by abundant micrite-cemented crayfish burrows, Scoyenia trace fossils, and numerous reduction spots and haloes indicating wet, but not hydric, soil conditions. Paleosols in fluvial-lacustrine sandstone-shale fades of Lithofacies Association IIb (lower Norian) record deposition in semiarid to subhumid climate characterized by seasonal soil-moisture deficits and high evapotranspiration. Pedogenic carbonate in both paleosols includes micritic rhizolith, nodule and burrow-fill morphologies. The carbon-isotope compositions of pedogenic carbonate differ significantly between paleosols within the two lithofacies, indicating different controls imposed by the pedogenic environments. The δ13C values of pedogenic, micritic rhizoliths in the paleosols suggest somewhat elevated pCO2 during the late Carnian-early Norian, two to three times modern levels, and support general estimates of pCO2 based on similar carbonates in time-equivalent paleosols.

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