Stepwise mass extinctions and impact events: Late Eocene to early Oligocene

Species ranges and relative abundances of dominant planktonic foraminifers of eight late Eocene to early Oligocene deep-sea sections are discussed to determine the nature and magnitude of extinctions and to investigate a possible cause-effect relationship between impact events and mass extinctions. Late Eocene extinctions are neither catastrophic nor mass extinctions, but occur stepwise over a period of about 1-2 million years. Four stepwise extinctions are identified at the middle/late Eocene boundary, the upper Globigerapsis semiinvoluta zone, the G. semiinvoluta/Globorotalia cerroazulensis zone boundary and at the Eocene/Oligocene boundary. Each stepwise extinction event represents a time of accelerated faunal turnover characterized by generally less than 15% species extinct and in itself is not a significant extinction event. Relative species abundance changes at each stepwise extinction event, however, indicate a turnover involving > 60% of the population implying major environmental changes. There microtektite horizons are present in late Eocene sediments; one in the upper G. semiinvoluta zone (38.2 Ma) and two closely spaced layers only a few thousand years apart in the lower part of the Globorotalia cerroazulensis zone (37.2 Ma). Each of the three impact events appears to have had some effect on microplankton communities. However, the overriding factor that led to the stepwise mass extinctions may have been the result of multiple causes as there is no evidence of impacts associated with the step preceding, or the step following the deposition of the presently known microtektite horizons.