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

Colby College, Waterville ME

Investigators

Abstract

Terrestrial Age Constraints for the Dicynodon Assemblage Zone, South Africa: Implications for the Timing of the End-Permian Mass Extinction Robert A. Gastaldo Colby College Deep time records provide evidence for changes in biodiversity and ecosystem disruption and recovery, and act as models for understanding the potential changes now recognized affecting Earth. The End Permian Mass Extinction Event is reported to have impacted more than 90% of life in the oceans and up to 80% of life on land, with the collapse occurring simultaneously at 252.2±0.2 million years ago. Recent work of the PI and 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 to identify a distinct event. Rather, criteria used to assign an end-Permian age to the biological patterns are inaccurate based on field relationships. The most reliable way to identify the timing of this event is by using chronometric techniques based on radiometric dating of single crystal zircons from volcanic ash deposits. To date, this has not been possible because 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 is reported, is buried and preserved by volcanic ash deposits in Wapadsberg Pass, Eastern Cape Province. A pilot study of single crystal zircons analyzed by isotope dilution thermal ionization mass spectrometry (ID-TIMS) using the ET535 tracer (spike) solution, distributed by the EARTHTIME initiative, resulted in a suite of ages indicating a detrital source for the zircons. Ages range from the NeoProterozoic (1038.78 ± 1.42 to 606.76 ± 1.0 Ma [3 grains]) to the early Mesozoic (252.0 ± 0.5 to 251.8 ± .038 Ma [3 grains]). The project goals include the acquisition of a larger suite of detrital zircon from several stratigraphic horizons in Wapadsberg Pass and elsewhere that will be age dated. These will be correlated with changes in sedimentary environments in the collection sites to place an age constraint on the biodiversity trends identified in the Karoo Basin. The project tests the hypothesis that the ?Mother? of all Mass Extinctions was a phenomenon restricted to the oceans, and changes in biodiversity trends on land were decoupled from those documented elsewhere. If turnover in continental biodiversity did not happen simultaneously with the patterns identified in the oceans, a new model for the greatest mass extinction in Earth history will have to be developed.