LATEST CHANGHSINGIAN PALEOSOL TRENDS AND ESTIMATES OF ATMOSPHERIC PCO2 FROM CARBONATE CONCRETIONS AT WAPADSBERG PASS, KAROO BAIN, SOUTH AFRICA

The latest Permian (Changhsingian) stratigraphy in the Karoo Basin, South Africa, consists of paleosols, channel deposits, and channel-fill sequences which record the physical conditions across time and space. The sequence at Wapadsberg Pass, Eastern Cape Province, provides insight into the climate regime under which paleosols formed prior to the vertebrate-defined Permian–Triassic boundary (PTB).

Two stacked Protosols ~70 m below the PTB are identified using the former soil-air interface of each. These are marked by an autochthonous forest-floor litter in which Glossopteris canopy leaves and Trizygia groundcover plants are preserved. Molecular weathering ratios determined from these paleosols indicate immature soil development under water-saturated conditions. Assuming paleosol-matrix concentrations of trace elements are indicative of Permian soil-solution chemistries, high concentrations of several elements may have been growth-stress factors accounting for the small glossopterid leaf size, in contrast to current models implicating climate stress.

Stable-isotope δ¹⁸O and δ¹³C values are presented for micritic and microspar (<20 μm) calcite cements from carbonate nodules collected at 15 horizons through a 90 m stratigraphic interval up to, and including, the vertebrate-defined PTB. These isotopic ratios exhibit dissimilar trends. No clear trend exists in the δ¹⁸O (PDB values range from -14.7‰ to -21.8‰). In contrast, a trend exists in δ¹³C values where carbonate cements almost certainly precipitated under well drained conditions in an interval that is 60 m below the PTB (-5.3 ‰), while δ¹³C values as low as -16.9‰, indicative of water-logged conditions, begin 90 m below and continue up to the PTB. Hence, no evidence is found for the reported pre-boundary trend towards increasing aridity at this locality. The first estimates of the latest Permian atmospheric pCO₂ from paleosols, based on coexisting calcite and organic matter δ¹³C values from paleosols that developed under well-drained conditions, provide a range of values from 900 to 1900, and 500 to 1300 ppmV, respectively, which are significantly lower than the latest Early Permian when terrestrial biome replacement is documented to have occurred.