The Permian/Triassic extinction event was the greatest extinction event since complex life formed some 540 million years ago. Approximately 96% of all marine life, and 70% of all terrestrial life disappeared.
Contrary to popular belief, there was not just one extinction event. Approximately 270 million years ago there was an extinction event greater than the famous Cretaceous/Tertiary Extinction Event 65.5 million years ago. After another 9 to 10 million years (~260 million years ago) another extinction event occurred, and 9 to 10 million years after that the extinction event that ended the Permian Period.
Yet there is no satisfactory explanation for what caused these events. So I thought I would create a thread and open the discussion as to what might explain these three Permian extinction events.
In order to have an inkling of what was happening during that time we need some data. The atmospheric composition is derived by determining the ratio between different isotopes of the carbon molecule.
Atmospheric Oxygen
The oxygen content of our atmosphere has pretty much leveled out at 21.2% for the last ~40 million years. However, the oxygen content was much lower from the Cambrian to the beginning of the Devonian, ranging from 15% to 18%. Then, gradually, over a period of ~100 million years the oxygen content began increasing. This also corresponds with plants on land beginning to thrive during the Devonian and peaking at the end of the Carboniferous. The oxygen content peaked at the end of the Carboniferous Period at ~35%, and then over a period of ~50 million years dropped to ~15% by the end of the Permian.
There is a ~3% peak increase in the oxygen content during the Triassic, but it does not start to climb again until the Triassic/Jurassic boundary, some 220 million years ago. This also corresponds with the break-up of the super-continent Pangaea.
Atmospheric Carbon Dioxide
The carbon dioxide levels, on the other hand, peaked and then began declining during the Cambrian, and with the exception of a couple of spikes during the Ordovician and Devonian, continued to decline until it reached its lowest level at the end of the Carboniferous. There is also evidence that just before the end of the Carboniferous there was an ice age that lasted into the Permian.
It should also be noted that at the Permian/Triassic boundary, there is no carbon layer in the Karoo, in South Africa, which is where most Permian strata is studied.
Surface Temperature
During the Cambrian and Ordovician the surface temperature was ~24°C, then quite suddenly (less than 10 million years) the temperature dropped by 8°C to 10°C, starting the Silurian Ice Age. Just as suddenly the surface temperature rebounded back to its original Cambrian/Ordovician levels. At the beginning of the Carboniferous the temperature drops by 2°C to 3°C, and then plunges another 6°C to 8°C.
During the Permian, after the Carboniferous/Permian ice age ended, the temperature shoots up again to between 21°C and 22°C around 270 million years ago. Then at the Permian/Triassic boundary, 250 million years ago the temperature spikes again to ~30°C. Some have even placed the mean surface temperature even higher.
Needless to say, it got very warm by the end of the Permian. So warm that some have described equatorial and tropical regions of the planet as a "dead zone." No plant or animal life according to the fossil record in those regions.
Land Mass Distribution
Besides knowing what was going on with the atmosphere and surface temperatures, it would also help to see how the land mass was distributed around the planet at the time.
By the end of the Carboniferous there were two main continents; Gondwanaland and Laurasia. And they are on a collision course. What is now Brazil was located at the south pole. The smaller Eruamerica continent was at the equator, surrounded by shallow seas but still connected the rest of Gondwanaland (Africa, South America, Madagascar, India, Australia, and Antarctica).
Gondwanaland collides further with the Euramerica continent, and closes the gap with Laurasia even more. This also corresponds to when the Carboniferous/Permian ice age ends and surface temperature reach 22°C to 23°C.
During the beginning of the Triassic the super-continent of Pangaea is fully formed. Although it will not stay that way for long. About 20 million years later, at the Triassic/Jurassic boundary, Pangaea breaks up.
Proposed Explanations
There has been a lot of debate as to what caused the Permian/Triassic extinction event.
Some have blamed the volcanic activity from the Siberian Traps, which started to erupt as Pangaea was being created some 248 million years ago. However, that does not fit our observations of volcanic activity. First and foremost, in absolutely every case volcanic activity lowers the surface temperature of the planet temporarily. In no case has volcanic activity ever been observed increasing surface temperatures. Second, volcanic activity typically puts up copious amounts of sulfur dioxide, carbon dioxide, methane, water vapor, and other gases into the atmosphere. Yet the geological record shows only a gradual increase in carbon dioxide over a 50 million year period. Also, the Permian extinction events start 32 million years before, and finish 2 to 3 million years before the Siberian Traps began erupting.
Then there are some who believe the Permian/Triassic extinction event was the result of a meteorite impact. Yet they are not finding an impact crater, or any iridium, shocked-quartz, or micro-diamonds in the rock strata from that period.
While others have blamed anoxic oceans and the mixing of the seas as the cause of Permian/Triassic extinction event. Personally, I tend to favor this explanation because it best fits the data and what was happening to the planet at the time. However, it is also flawed because it does not explain why 70% of the terrestrial life died at the same time. Additionally, a recent study using molybdenum isotope ratios instead of carbon isotope ratios has determined that the northwest continental shelf of Pangea underwent mass extinction under oxic conditions, and therefore anoxia could not be the cause of the Permian/Triassic extinction event.
So we are pretty much back to square one: We still do not know what caused the greatest extinction event in the history of life on Earth.
Does anyone want to try to tie all the data together to come up with a plausible explanation?
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References:
- Benchley, Peter J & DAT Harper (1998), Palaeoecology: Ecosystems, Environments and Evolution. Chapman & Hall, ISBN-10: 0412434504, ISBN-13: 978-0412434501.
- Ward, Peter D (2002), Rivers in Time, Columbia University Press, ISBN-10: 0231118635, ISBN-13: 978-0231118637
- Rapid Eruption of the Siberian Traps Flood Basalts at the Permo-Triassic Boundary
- Molybdenum isotopic evidence for oxic marine conditions during the latest Permian extinction