PROTRACTED GLOBAL OCEAN ANOXIA DELAYED BIOTIC RECOVERY FOLLOWING THE END-PERMIAN MASS EXTINCTION

The Early Triassic was a transitional interval between the largest biotic crisis ever – the Late Permian Mass Extinction (LPME) – and the subsequent biotic recovery and establishment of thriving Mesozoic ecosystems. Fossil evidence indicates that it took more than 5 million years for the biosphere to recover from the LPME. Persistent environmental stresses such as hyperwarming and ocean anoxia may have delayed the biotic recovery [1,2,3]. In order to characterize the duration and extent of ocean anoxia after the LPME, we have measured the U isotope composition (δ²³⁸U) – a proxy for global ocean anoxia – in well-preserved marine carbonates across the Late Permian to the Middle Triassic at Zal (Iran). Our data show a sharp shift to lighter δ²³⁸U values at the LPME that persist through the Early Triassic period, which indicates a protracted period of anoxia. Gradual return of δ²³⁸U to pre-LPME values is followed by a second sharp lightward shift at the Early/Middle Triassic boundary. This pattern suggests that a return to oxygenated conditions was punctuated by another major anoxic event, possibly revealing another hammer blow that hindered the recovery of the marine biota. The nature of the protracted global ocean anoxia might be a key and direct factor that caused the delayed biotic recovery from the LPME.

[1] Payne et al. (2004), Science 305, 506-509. [2] Bond and Wignall (2010), Geol. Soc. Amer. Bull. 122,1265-1279. [3] Sun et al. (2012), Science 338, 366–370.