Terrestrial Age Constraints for the Dicynodon Assemblage Zone, South Africa: Implications for the Timing of the End-Permian Mass Extinction

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Terrestrial Age Constraints for the Dicynodon Assemblage Zone, South Africa: Implications for theTiming of the End-Permian Mass ExtinctionRobert A. GastaldoColby CollegeDeep time records provide evidence for changes in biodiversity and ecosystemdisruption and recovery, and act as models for understanding the potential changes nowrecognized affecting Earth. The End Permian Mass Extinction Event is reported to haveimpacted more than 90% of life in the oceans and up to 80% of life on land, with thecollapse occurring simultaneously at 252.2±0.2 million years ago. Recent work of the PIand collaborators demonstrates that the paleontological records from the Karoo Basin,South Africa, on which the idea of sudden collapse on land is based, can not be used toidentify a distinct event. Rather, criteria used to assign an end-Permian age to the biologicalpatterns are inaccurate based on field relationships. The most reliable way to identify thetiming of this event is by using chronometric techniques based on radiometric dating ofsingle crystal zircons from volcanic ash deposits. To date, this has not been possiblebecause it was believed that such deposits didn?t exist near the boundary.A fossilized forest litter, located 70 m below the level where the extinction event isreported, is buried and preserved by volcanic ash deposits in Wapadsberg Pass, EasternCape Province. A pilot study of single crystal zircons analyzed by isotope dilution thermalionization mass spectrometry (ID-TIMS) using the ET535 tracer (spike) solution, distributed bythe EARTHTIME initiative, resulted in a suite of ages indicating a detrital source for thezircons. Ages range from the NeoProterozoic (1038.78 ± 1.42 to 606.76 ± 1.0 Ma [3 grains]) tothe early Mesozoic (252.0 ± 0.5 to 251.8 ± .038 Ma [3 grains]). The project goals include theacquisition of a larger suite of detrital zircon from several stratigraphic horizons inWapadsberg Pass and elsewhere that will be age dated. These will be correlated withchanges in sedimentary environments in the collection sites to place an age constraint onthe biodiversity trends identified in the Karoo Basin. The project tests the hypothesis thatthe ?Mother? of all Mass Extinctions was a phenomenon restricted to the oceans, andchanges in biodiversity trends on land were decoupled from those documented elsewhere.If turnover in continental biodiversity did not happen simultaneously with the patternsidentified in the oceans, a new model for the greatest mass extinction in Earth history willhave to be developed.