Exploring the real causes of the end-Permian mass extinction

Abstract

Of the 'big five' mass extinction events in the Earth’s history [1], the end-Permian extinction was the largest. Over the past thirty years, the end-Permian extinction has attracted a lot of attention with many causes being suggested for the mass extinction. Suggestions have included asteroid impact, Siberia volcanism eruption, global warming, oceanic anoxia, massive release of methane, decrease of ozone levels, and an increase of atmospheric CO2 levels [2-5]. However, our understanding of the link between these hypotheses and the selective extinction is poor. A new hypothesis by Song and his colleagues [6] has provided a more plausible alternative. Song et al. have uncovered the most probable cause of the end-Permian extinction by examining the different responses of modern aquatic organisms to environmental changes in the laboratory and during oceanographic surveys [6]. They show that the selective extinction is best explained by a combination of lethally warm shallow water and anoxic deep water. Both lethally warm water temperatures and oceanic anoxia could directly lead to mass mortality of marine organisms when they reach the upper tolerance limits (Fig. 1). However, the effects of these two variables vary within marine ecosystems. Generally, lethal warming mainly affects organisms living in photic zone because seawater temperature declines with increasing water depth. In contrast, anoxia develops in the mid-water column (oxygen minimum zone, OMZ) or the deepest waters of restricted seas and is unlikely to have a direct effect in surface water. Song and colleagues [6] found that organisms such as corals and radiolarians, with low upper thermal limits, living in the photic zone became extinct during the end Permian extinction, whereas ostracods, again in photic zone but with the highest upper thermal limits, appeared to be little affected by this major crisis. Small foraminifers, with a high tolerance to hypoxia but low upper thermal limits, survived provisionally in the intermediate water depths, for example in the Meishan section. Only species tolerant to both high temperature and hypoxia, namely the bivalves and gastropods, were able to thrive in large numbers in the aftermath of the extinction. Their findings present a plausible link between the whammy of anoxia/high temperature and selective extinction during the marine crisis. However it is still controversial as to whether the extraordinarily high temperature is a major cause for the largest extinction [7].