Phanerozoic Mass Extinctions and Indian Stratigraphic Records

Abstract

The paper discusses important changes in faunal and floral diversity during the Phanerozoic Eon. These are represented by the ‘big five’ mass extinctions, viz. end-Ordovician, Frasnian-Famennian boundary of the Devonian Period, Permian-Triassic boundary, Triassic-Jurassic boundary, and the Cretaceous-Palaeogene boundary. Major biological and geological events and geochemical anomalies associated with these mass extinction boundaries and potential causes for the extinctions such as bolide impact, volcanism, sea level changes, ocean anoxia, and methane hydrate release are discussed in detail. Though the causes for these mass extinction events are still being debated, wider acceptance for glaciation-related climatic changes at the end of the Ordovician and climatic perturbation with ocean anoxia at the end of Devonian exists. The remaining three mass extinctions, i.e., at the end of Permian, end of Triassic, and end of Cretaceous coincide with volcanic eruptions of large igneous provinces. The end Cretaceous extinction also coincides with the impact of an asteroid at Chicxulub in Mexico. In recent years, the eruption of Siberian Traps of Russia and related climatic perturbations were considered as the major cause for the Permian-Triassic boundary extinction. Likewise, the eruption of the Central Atlantic Magmatic Province was linked to the Triassic-Jurassic boundary mass extinction. The Deccan volcanism and bolide impact at Chicxulub, may have equally contributed to the demise of a large number of species at the end of Cretaceous. The Indian stratigraphic record for these mass extinction events is poorly documented for the end-Ordovician, end-Devonian, and end-Triassic events. The best studied section for the Permian-Triassic boundary is the Guryul Ravine section in Jammu and Kashmir where the transition from the Permian to Triassic is well preserved and the boundary is precisely delineated based on conodonts, negative δ¹³C isotope excursion, geochemical signatures, and change in depositional environments. Although extensive work has been carried out on the Cretaceous-Palaeogene boundary sections in the Deccan volcanic province, no precisely delineated boundary clay horizon has been demarcated so far. All the outcrop sections of the Deccan intertrappean sections preserve either Maastrichtian part of the Cretaceous or Early Palaeocene P1a or P1b zones. The subsurface ONGC well sections also reveal K/Pg boundary transition but no precise boundary layer. However, the marine K/Pg boundary section of Um Sohryngkew River preserves the K/Pg boundary layer as attested by the faunal changes and iridium anomaly.

There are millions of different species of animals and plants on earth—possibly as many as forty million. But somewhere between five and fifty billion species have existed at one time or another. Thus, only about one in a thousand species is still alive—a truly lousy survival record: 99.9% failure!

David Raup