Publication Date:
2014-05-03
Description:
Terrestrial settings preceding the end-Permian crisis are reported to trend toward increasingly dry and arid conditions, resulting in landscape change and a shift in fluvial architectures and regimes. Much of the latest Permian (Changhsingian) stratigraphic record in the Karoo Basin, South Africa, consists of paleosols, which record the physical conditions across time and space. Preboundary sequences at Wapadsberg Pass, Eastern Cape Province, provide insight into the climate regime that influenced paleosol formation at that time. A high-resolution sedimentological and geochemical study of two, stacked aggradational paleosols, in conjunction with stable isotope geochemical characterization of paleosol carbonate-cemented concretions over a 90 m section at this locality, demonstrates that these landscapes were predominantly wetland terrains without a demonstrable trend in increasing drying up to the Permian-Triassic boundary, as defined by vertebrates in the area. Two paleosols examined 70 m below the Permian-Triassic boundary are identified as Protosols, and the former soil-air interface of each is marked by an autochthonous forest-floor litter in which canopy leaves of Glossopteris and groundcover plants of Trizygia are preserved. Molecular weathering ratios (e.g., base loss, clayeyness, chemical index of alteration minus potassium [CIA–K], etc.) determined from these horizons are indicative of immature soil development under water-saturated conditions. Assuming that paleosol-matrix concentrations of trace elements are indicative of Permian soil-solution chemistries, high concentrations of Ni, Cu, Ba, and Cr may have been growth-stress factors that may account for the small glossopterid leaf size in the megafloras, in contrast to current models that implicate stress in response to climate change. Stable isotope 18 O and 13 C 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 Permian-Triassic boundary. These isotopic ratios exhibit dissimilar trends. No clear trend exists in 18 O (Peedee belemnite [PDB] values range from –14.7 to –21.8). In contrast, a trend exists in 13 C values, where carbonate cements almost certainly precipitated under well-drained conditions in an interval that is 60 m below the Permian-Triassic boundary (–5.3), while 13 C values as low as –16.9, indicative of water-logged conditions, begin 90 m below and continue up to the Permian-Triassic boundary. Hence, no evidence is found for a preboundary trend toward increasing aridity at this locality. The first estimates of the latest Permian atmospheric pCO 2 from paleosols, based on coexisting calcite and organic matter 13 C 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.
Print ISSN:
0016-7606
Electronic ISSN:
1943-2674
Topics:
Geosciences
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