PALEOPEDOLOGY AND STABLE ISOTOPE GEOCHEMISTRY OF LATE TRIASSIC (CARNIAN) PALEOSOLS, DURHAM SUB-BASIN, NORTH CAROLINA: IMPLICATIONS FOR PALEOCLIMATE AND PALEOATMOSPHERIC PCO2

Paleosols occurring in rift-related Upper Triassic (Carnian) deposits of the Durham basin of North Carolina record evidence of changing paleoclimate, with a shift from wetter to drier conditions, and elevated atmospheric levels of carbon dioxide. Paleosols within siltstone interbeds of border fault-fanglomerate facies in the Pekin Fm. (lower Middle Carnian) are characterized by wet (but not hydric) soil indicators, including numerous reduction spots and haloes, abundant micrite-cemented crayfish burrows, and common Scoyenia trace fossils. In the Sanford Fm. (Upper Carnian), paleosols were identified within fluvial and lacustrine sandstone-shale facies, and record deposition in semi-arid to subhumid climates characterized by seasonal soil moisture deficits and high evapotranspiration. Pedogenic carbonate occurring in both paleosols includes micritic rhizolith, nodule and burrow-fill morphologies. The stable carbon isotope compositions of pedogenic carbonate differ significantly between paleosols that occur within the two formations, indicating different controls imposed by the pedogenic and early post-pedogenic environments. The d¹³C values of pedogenic carbonate rhizoliths in the paleosols (-8.2 to -9.2 o/oo PDB) suggest somewhat elevated pCO₂ during the Carnian, 2.5-3 times PAL (d¹³C atm.=-4.5 o/oo; d¹³C SOM=-23.9 o/oo; soil CO₂=5000 ppmV), and support general estimates of pCO₂ based on theoretical long-term mass-balance carbon models.