Microbialites as disaster forms in anachronistic facies following the end-Permian mass extinction: a discussion

Abstract

‘Anachronistic facies’ and ‘disaster forms’ are interpretive terms applied from the early 1990s to sedimentary deposits and biotas in the aftermath of mass extinctions; both terms have been used especially for the deposits formed directly after the end-Permian mass extinction. Microbial carbonates (disaster forms) are abundant in the earliest Triassic and often considered as a return to environmental conditions typical of Neoproterozoic to Cambro-Ordovician times. However, this view does not take into account: (i) the growing evidence that microbialites are stimulated by bicarbonate-supersaturated waters irrespective of mass extinction; (ii) the potential oceanic and climatic effects of the Siberian Traps volcanics; and (iii) the unique global plate-tectonic setting of Pangaea at that time. The configuration of land masses led to near-isolation of Tethys from Panthalassa, with modelled slow circulation and accumulation of anoxic deep water in Tethys. Evidence of catastrophic overturn of the Tethys Ocean reflects instability, possibly driven by climate changes, which released anoxic bicarbonate-rich waters to the surface. Items (ii) and (iii) are features of the Permian–Triassic boundary transition and are not parallels of earlier episodes of Earth history. Taking the argument wider, not all mass extinctions are followed by widespread anachronistic facies and disaster biotas. Therefore, it may be argued that application of anachronism and disaster biota concepts is an oversimplification of mass extinction processes in general, and the Permian–Triassic boundary extinction in particular. Continued use of these terms generates a narrowed view of processes and hinders development of comprehensive interpretations of changes of facies and biotas in mass extinction research.