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- Coney, Louise, et al.
(author)
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Geochemical and mineralogical investigation of the Permian-Triassic boundary in the Continental Realm of the Southern Karoo Basin, South Africa
- 2007
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In: Palaeoworld. - : Elsevier BV. - 1875-5887 .- 1871-174X. ; 16, s. 67-104
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Journal article (peer-reviewed)abstract
- Mineralogical and geochemical studies of the non-marine Permian–Triassic (P–Tr) boundary across two stratigraphically wellconstrained sections (Commando Drift Dam and Wapadsberg, Eastern Cape Province) in the southern Karoo Basin, South Africa, have been undertaken to provide further input on the cause of this mass extinction event, and so has a sedimentological and geochemical evaluation of a third P–Tr boundary section at Injusiti (Kwazulu-Natal) in the eastern Karoo Basin, South Africa. The Commando Drift Dam section has been constrained by previous palaeontological and palaeomagnetic work, with a palaeomagnetic reversal positioned 5.3m above the palaeontological P–Tr boundary. The Wapadsberg section has been constrained palaeontologically. All these P–Tr sections studied here mostly comprise mudstones, together with siltstones, sandstones, and in the southern Karoo Basin, carbonate nodular horizons. A change in colour of the mudstones from green-grey to red-brown takes place at the palaeontologically defined boundary. Variations in the major and trace element abundance profiles are usually limited to the carbonate nodular horizons, besides the overall effects of weathering. Bulk carbon isotopic studies of the Commando Drift Dam section revealed a negative δ13Cbulk excursion (background values of −15 to −20‰, with total extent of excursion to −24.9‰) 2 cm below the palaeontological boundary, followed by a gradual recovery and then another decrease in values leading towards the palaeomagnetic boundary. Above this boundary recovery to less negative δ13Cbulk values (ca. −18‰) occurs. The organic carbon record from the Commando Drift Dam (southern Karoo Basin) and Injusiti (eastern Karoo Basin) oscillates between −26.1 and −28.9‰, which is comparable to previous studies of different sections in the southern Karoo Basin. The magnitude of both the bulk and organic carbon isotopic variation can be interpreted to indicate a number of inputs (due to the fluctuating values) of organic carbon. The carbon isotope data for carbonates in the Injusiti section are different from the results on carbonates from other studies, but more work to expand this dataset is necessary. The palynological studies on the Commando Drift Dam section reveal the presence of a low diversity flora composed principally of bryophytes, lycophytes, and gymnosperms. These forms, including several Late Permian key-species, are traces of the surviving plants enduring after the major extinction-pulse. The presence of fungal palynomorphs and dearth of pollen/spores related to photosynthetic plants some metres above the palaeontological P–Tr boundary demonstrate similarities to the pattern of floral extinction at the Cretaceous–Palaeogene (K–Pg) boundary. The timing of vertebrate extinctions in the Karoo Basin has so far not been determined, but the 252.5 Ma age for a single zircon crystal from Commando Drift Dam dated here gives a maximum constraint on the age of the event bed, which is in agreement with the accepted age of the boundary. No evidence for impact-produced microdeformation features were found in quartz grains from either the Wapadsberg or Commando Drift Dam sections. Also, siderophile element data (including platinum group element (PGE)concentrations) do not support the possible presence of a meteoritic component at the boundary. Thus, a link between impact and P–Tr extinction is not indicated by the results of this study.
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