What mechanistic causes were involved in the largest mass extinction that wiped out more than 95 % of marine species 252 million years ago? Collaborative efforts of two teams led respectively by Hugo Bucher (UZH, Paleontology) and Urs Schaltegger (UniGE, Geochronology) question the traditional view of the Permian-Triassic boundary mass extinction, which is customarily seen as the consequence of lethally hot climate. The vast majority of the community agrees that the biggest mass extinction was synchronous with the initial and explosive phase of the Siberian Traps, a large igneous province that outpoured an estimated volume from 3 to 5 x10⁶ km³ of magmatic products. 

On continental shelves, the mass extinction is always within a hiatus of unknown duration that straddles the Permian-Triassic boundary, world-wide. This generalized gap resulting from a global regression has been a long standing obstacle for the understanding of the extinction. Complete records were only recently discovered in rarely preserved deep marine sections, where the duration of the extinction (ca.1 x10⁵ years) was first established.

The Zurich-Geneva team set out to investigate a Permian-Triassic basin in present-day South China, where shallow marine shelves are separated by deep troughs. Two successions of stratigraphically closely spaced volcanic ash layers interbedded with fossil-rich marine rocks, one from a deep water trough and one from an adjacent shallow water platform were dated by means of U-Pb high-resolution ID-TIMS ages. Chemical fingerprinting of accessory minerals also allowed some individual ash layers to be correlated between the two settings, thus providing invaluable time lines for high-resolution correlation. The age and duration of the hiatus in the shallow water platform was precisely quantified for the first time and was demonstrated to be identical with the age and duration of the mass extinction as documented in deep water settings.

Hence, perfect synchronicity between the mass extinction and the world-wide sea level drop down was newly established with an enhanced temporal resolution of 30’000 years. The only known mechanism able to produce such a geologically short-lived global regression is an ice age, through the storage of ice on continents. This result is at striking variance with the mantra of a peaking greenhouse climate as a cause for the extinction. It also implies that the emersion of continental shelves, where most of the marine diversity is harbored, was among the leading kill mechanisms. The authors also hypothesized that this ice age was caused by stratospheric injection of large masses of H2S and SO2 degassed from early eruptive phase of the Siberian Traps, thus leading to the formation of S-rich aerosols that dimmed the solar energy reaching the Earth’s surface. Another consequence of this model is the intense acidification of both oceans and continents, thus providing a putative additional very efficient kill mechanism. Geochemical and sedimentological evidence supporting acidification is still tenuous, but the Zurich team is now targeting the record of terrestrial plants with the hope of finding more conclusive evidence. Ironically enough, it clearly emerges that global warming does not have the exclusivity of generating global ecological catastrophes and that the deep time record holds exciting challenges for the understanding of Earth System.

Björn Baresel, Hugo Bucher, Borhan Bagherpour, Morgane Brosse, Kuang Guodun & Urs Schaltegger; Timing of global regression and microbial bloom linked with the Permian-Triassic boundary mass extinction: implications for driving mechanisms, www.nature.com/articles/srep43630 

Spectacular domal stromatolites (Cyanophyceae) THRIVED immediately after the Permian-Triassic boundary mass extinction (Wangmo, southern GuizhouProvince, PR China). The work published in Scientific Reports established that such microbial limestone proliferated on shallow water platforms during the next 13 +/- 57 kyr after the extinction crisis. Mass occurrences of microbial limestone only occurred when sea >> level rose again, after an ice age whose duration of 90 +/- 38 kyr is newly established and which was concomitant with the most recently obtained age for the biggest mass extinction, some 251,958 +/- 0.018 Ma ago.

University of Zurich
Prof. Hugo Bucher
Paleontological Institute and Museum
Karl Schmid-Str. 4
8001 Zurich
 

Phone: +41 44 634 23 44
Email: Hugo Bucher