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Keywords:

  • Permian–Triassic boundary;
  • extinction;
  • confidence interval;
  • pattern of extinction;
  • brachiopods;
  • foraminifers;
  • Iran

Abstract

The Elikah River section in the Alborz Mountains (North Iran) is a fossiliferous, continuous, marine section spanning the Permian–Triassic (P–T) boundary interval. Sixty-eight taxa of brachiopods, foraminifers and algae have been collected bed-by-bed from the topmost 36 m of the Nesen Formation and the basal 2 m of the Elikah Formation, across the P–T boundary that is located in the basal metre of the latter formation. Forty-six of these taxa from 62 samples have been considered for statistical analyses to unravel the pattern of extinction. First, their range extensions have been determined, which should be added to the highest stratigraphic occurrence of each taxon to identify the extinction level within a certain confidence interval (95%). Then the pattern of the extinction has been analysed, plotting the stratigraphic abundance versus last occurrence below the extinction horizon. The resulting pattern is of a sudden extinction occurring 30 cm below the biostratigraphically defined base of the Triassic. The confidence interval (95%) for the mass extinction is 30 cm thick. As predicted by the ‘Signor–Lipps effect’, brachiopod last occurrences underestimate the time of extinction, as their abundance is below 15%. Therefore, also in North Iran located at equatorial palaeolatitudes along the southern shore of the Palaeotethys Ocean, the pattern of the latest Permian mass extinction is abrupt as observed in South China, in peri-Gondwana and in the Southern Alps. One of the most plausible mechanisms of the latest Permian sudden mass extinction could have been the emissions of tholeiitic magmas (Siberian Traps), which could have caused greenhouse gases emission into the atmosphere and thus global warming and ozone depletion, triggering the crisis (Svensen et al. 2008). Copyright © 2010 John Wiley & Sons, Ltd.