Lately there is much talk about our climatic future; how hot will the earth get? How far can ecosystems be pushed under rising temperatures? Following on from last week, there prehistoric events that we can analyse to help answer these questions. One such runaway greenhouse event is known as the Permian extinction, which closed the Phanerozoic eon around 252 million years ago.
During the Permian, the earth was very different; there were only two continents, Gondwana and Laurentia, which were beginning to collide, forming a supercontinental mass known as Pangea. This collision resulted in the Appalachian Mountains, a vast chain which would have interrupted global atmospheric circulation considerably. Most of internal Pangea resided along the equator resulting in high temperatures and extensive aridity (as demonstrated by Permian gypsum and evaporite deposits). As a result of this continental configuration and coastal mountain rain shadows, Pangaea was dominated by intense monsoons and equatorial aridity.
The exact cause of the extinction has remained elusive, but it coincides with the eruption of the Siberian Traps. The traps today are nothing but vast basaltic plateaus covering 1.6 million square kilometres at a depth of 2000-3000 metres across north-eastern Russia. However, during the Permian, the Traps were active and erupting, spewing out approximately 2-3 million cubic kilometres of basaltic lava, as well as vast quantities of greenhouse gases and ash, consequently resulting in short term global cooling (by increasing the Earth’s albedo) and long term global warming. The eruptions took place in 3 bursts over no more than a million years, just before, during and after the Permo-Triassic boundary.
The resulting GHG emissions from the eruptions may have warmed the oceans just enough to dislodge deep sea methane clathrates, disrupting oceanic circulation, atmospheric composition and global temperature.
In my opinion, the release of deep sea methane clathrates is a vital topic requiring further study. At present, a significant temperature rise threatens to trigger the thawing of the Siberian permafrost, which contains an immense reservoir of carbon dioxide and methane. The release of these gases would not only contribute to global warming but its effects may mirror the Permian extinction.
A 6° rise in temperature ensued across the western Tethys ocean as indicated by arid soils which dominated as far south as 60° latitude. Marine anoxia is also evident from carbon-rich deep sea sediments abundant in pyrite and the selective survival of low oxygen-tolerant species. The rise in deep sea temperature severely impacted circulation, forcing nutrient-poor water upwards, depriving planktonic algae of phosphorus leading to a collapse of the marine food chain.
Late Permian shallow marine assemblages were dominated by rugose and tabulate corals (the pictured rugose coral is one of my own, it is from the Ordovician of Morocco, but the family existed up until the Permian), bryozoans, crinoids and echinoderms, all of which were mostly replaced by opportunistic taxa of the early Triassic such as Lingula brachiopods and certain bivalves. With the future of lush coral reefs under threat, the difference in marine fossil assemblages between the late Permian and early Triassic may provide a frightening insight to the future of our oceans.
The Permian shares similar levels atmospheric composition, temperature increase, biodiversity loss and ocean acidification to modern times, making it a relatively useful analogue for predicting our future. One major difference was the position of the continents and equatorial aridity (which we do not see today) and strong monsoonal activity.
The Permian extinction is the largest ever known, with an estimated 90% of species perishing. The level of carbon dioxide released into the atmosphere annually by modern society is comparable to that which was released all those years ago during the Permian. Perhaps we should look to the Permian as a warning of how far life can be pushed under an ever-warming climate.
For further reading, Benton’s "When Life Nearly Died" covers the Permian extinction in great detail.
Comentarios