Can the end-Permian mass extinction be attributed to a single, catastrophic event?
Robert Broughton (UK)
The end Permian mass extinction occurred 251mya and marked the end of the Palaeozoic era. The loss of life is currently estimated to consist of 95% of the marine fauna and around 70 to 77% of the known terrestrial fauna (where the fossil record is inevitably less complete). This article will provide an overview of the many events and processes that played a part and a discussion whether they can all be attributed to a single, root cause.
At this time, the landmass was united into the single, super-continent of Pangea, surrounded by warm shallow seas with abundant reef systems. This extensive reef fauna supported a variety of suspension feeders (for example, crinoids, rugose and tabulate corals, and so on), which were the most heavily hit by the extinction event, with all the known corals dying out. Modern scleractinian corals only appeared in the Triassic and there is a considerable gap in the coral fossil record at this time. Other reef inhabitants, such as the last phillipsid trilobites also became extinct. All these creatures were sessile or relatively immobile inhabitants of the reefs that occupied a relatively narrow zone on the continental shelf. This habitat must have been destroyed almost globally by a number of factors, but importantly, the single shelf margin around Pangea meant there was no other shallow reef environment for the fauna to migrate to.
The single continent of Pangea was always doomed to split apart. Rock is a great insulator of heat and the thick mass of continental crust allowed heat to build up in the mantle. Eventually, this upwelling of heat would have had a ‘blow torch’ affect and the continent rifted apart. Today, this process can be seen along East Africa’s Rift Valley. Two hundred and fifty one million years ago, it was Pangea’s turn. It is suggested that a hotspot under present-day Siberia caused the initial fracturing, which released huge quantities of fluid, flood-basalt lavas. Because all landmasses were united into a single continent, there were no controls or limits on the radiating fracture systems that would propagate outwards from the initial epicentre. The resulting eruption lasted about one million years and three million cubic kilometres (about 1,860,000 cubic miles) of basalt flooded the Earth’s surface.
As much of the continent’s interior was desert, this increased pressure on available inhabitable areas. The vast amounts of volatile gasses and dust produced would have had a serious impact on plant life and the world’s oceans. It is thought that acid rain and surface runoff from the continent altered PH levels in shallow continental seas by turning them more acidic. For reef building organisms that relied on the secretion of calcium, this severely increased the solubility of calcium in water. As a result, it became more difficult to secrete a calcite shell and, for species such a corals and brachiopods, to construct their supporting skeletons and retain shell material. This may have been a factor in the tendency for ‘dwarfism’ that occurred among shelly marine faunas at this time – a smaller surface area is less effected by acidic PH and easier to retain.
In spite of many species adapting to the conditions, it is clear from the chemical composition of P-T rocks that a major faunal collapse occurred. By measuring the ratios of normal Carbon 12 to Carbon 14 (a carbon isotope linked to photosynthesis), it has been demonstrated that a major die-off of marine phytoplankton occurred. This would have had a twofold effect on both marine and terrestrial environments. Firstly, upper waters in shallow tropical seas would have become mildly anoxic (as well as already being acidic). The lack of photosynthesis would have stopped or slowed the rate at which CO2 is fixed into sea water (the oceanic ‘Carbon sink’) – these factors combined to decimate the shelly fauna and others such as the phillipsid trilobites.
As plankton, then as now, were a key basal component of the food chain, it is likely this was a causal factor in pelagic extinctions, for example, the free swimming goniatites. Secondly, phytoplankton contribute a significant proportion (by both mass and output) of photosynthesising plants and their loss resulted in elevated CO2 levels in addition to what the Siberian basalt traps were contributing – there remained very few CO2 controls and runaway global warming was inevitable.
While CO2 emissions from the Siberian traps caused initial global warming, their effects, by themselves, should not have caused a global catastrophe. It is hypothesised that average global temperatures increased by around 5°C, whereas a rise in the magnitude of 10°C is thought to be the minimum needed to cause such rapid adverse affects.
So what made up this missing contribution? It is thought that this small rise in temperature allowed frozen methane gas hydrates on the sea floor to be released and further accelerate the process. However, this global warming event was short lived and preceded the major periods of marine and terrestrial extinctions. With fewer carbonate producing and photosynthesising sea creatures, the CO2 emissions within the atmosphere resulted in major cloud formations that limited sunlight penetration. While not the sole cause (more on that later), the inevitable result was an ice age of global proportions with glacial sediments having been detected in sub-Saharan Africa, India and China. Closer to home, they are known from the North Sea as well as, paradoxically, Siberia.
On the single super continent of Pangea, such an event had a severe impact on the terrestrial fauna (for example, a dramatic decline in rynchodont reptiles – the ‘pigs’ of the Permian) because it placed increased pressure on remaining habitats, bearing in mind that large portions of the interior still remained an arid desert.
All these events are related to the rifting of the pangean supercontinent and it could be thought of as the single underlying root cause. However, there are two other, astrological factors that have been proposed. In Antarctica, elevated levels of the rare-earth elements such as iridium have been detected. Such elevations are usually taken as a fingerprint marker for meteorite impacts. However, to date, no impact site has been located and the levels involved are not as highly elevated as those at the infamous K-T boundary event 65mya. The possible effects of such an impact are still being debated.
The other astrological theory connects periods of climatic change with changes in the eccentricity of the Earth’s orbit. Known as ‘orbital forcing’, these Milankovitch cycles (named after the Siberian engineer who first proposed them) occur at definite intervals. In fact, there are three separate factors involving our orbital path around the sun:
- The degree of axial tilt.
- The rate of spin.
- The axis tilt or obliquity.
It is the last that is thought to have had the major effect on the P-T ice age, coinciding as it did with the Siberian Traps event.
The Earth’s axis varies between 22.1 to 22.6 degrees of tilt and this markedly affects the intensity of solar radiation received. At the time of the P-T mass extinction, tilting away from the sun increased the surface area per wattage received, thereby having a cooling effect. Coupled with an eccentric orbit taking us further from the sun, the result was a brief but intense ice-age in the order of a few tens of thousands of years.
No fauna escaped the P-T event unscathed. Over the duration of two million years, the fauna and ecology of our planet changed dramatically as a result of extraordinary co-incidences and a series of delicate, inter-dependant relationships that control our climate. The final rifting and break-up of Pangea happened to coincide with a series of Milankovitch cycle events and other factors, including the fact that Pangea, in spite of its size, had a limited habitable surface area for reef marine and terrestrial faunas. Perhaps, the biggest single ecological event was the collapse of the marine food chain from the bottom up. However, all these events are largely interrelated cause and effect relationships that serve to underlie just how fragile – and globally important – our biosphere is.
Perhaps, the only winners in the P-T event were the mobile and arid-adaptable reptiles – the dinosaurs – although they would have to wait another 30Ma or so before the climate stabilised and their radiation really began at the end of the Triassic.