The Greatest Catastrophe on Earth
Interview with Peter Ward, author of "Gorgon"
|Ward believes that a lowering of atmospheric oxygen caused the P-T extinction, influencing the development of animals (ie. air sacs in birds).
Image Credit: NASA
Peter Ward's newest book, "Gorgon," is part travel journal, part scientific exploration. For over a decade, Ward hunted for fossils in South Africa's Karoo Desert, hoping to figure out what caused the Permian/Triassic extinction. This cataclysmic event 250 million years ago killed off 90 percent of all marine species and 70 percent of vertebrate land species. Some have suggested the P-T extinction was triggered by an asteroid or comet, like the one that killed the dinosaurs 65 million years ago, but Ward comes to a different conclusion.
Ward believes that a lowering of atmospheric oxygen caused the P-T extinction. These low oxygen conditions continued on through the Triassic and most of the Jurassic, influencing the development of animals that evolved during this time. Birds, for instance, developed their unique air-sac respiratory system because of this extremely low oxygen environment.
The reason the atmosphere lost its oxygen, Ward suggests, was because ocean levels dropped, exposing anoxic organic materials to the atmosphere. The newly-exposed materials oxidized, pulling oxygen out of the air, and the iron in these materials rusted, creating the red rock layers that are so distinctive in post-Permian geology. Explosive volcano eruptions from Siberia may have contributed to this loss of oxygen as well, expelling huge amounts of carbon dioxide, carbon monoxide, methane, and other gases into the atmosphere. Whatever happened in the P-T, it happened on a geologically fast time scale, within 50,000 years or less.
In this interview with Astrobiology Magazine, Peter Ward explains how clues in the rocks point to a world that was nearly wiped clean of life.
Astrobiology Magazine (AM): How rare are Permian fossils compared to Triassic and Jurassic fossils? Reading your book, you get the sense that fossilized bones are scattered everywhere, once you know how to look for them.
|This gorgonpsid fossil was discovered in the Karoo region of South Africa.
Image Credit: South African Museum
Peter Ward (PW): There are definitely far fewer later Permian fossils than earlier Permian. To get a sense of how hard it is to find these fossils, while we were hunting for them in South Africa, looking right in the Permian rock layer, we would find maybe one fossil in an 8 hour day of searching.
The Gorgon is an extremely rare fossil - it has only been found in South Africa, except for a few in the Eastern Soviet Union. Collectors would pay any amount of money to get one, but they are literally priceless because they've never been sold. South Africa holds on to all of its fossils. The Soviet Union Gorgons were all sent to Moscow.
AM: You say in your book that while an asteroid impact has been suggested as a cause of the P-T extinction, follow-up studies have not been able to support that claim.
PW: I do not think that asteroid impact was a cause. There is a new paper just out that suggests that explosive volcanism can look like the remains of asteroid impact. The paper, by J. Phipps Morgan, et al., says that explosive volcanic eruptions are sometimes able to generate the shocked quartz, microspherules, and other geologic traces commonly attributed to large extraterrestrial impacts, while also triggering a mass extinction event.
AM: You come to the conclusion that the Permian extinction was caused by lower O2 in the atmosphere, and suggest this was due to the lowering of oceans and the subsequent exposure of organic-rich sediments. What caused the ocean to lower in the first place?
|At the P-T boundary, the rivers go from meandering to braided, indicating a catastrophic loss of land plants.
Image Credit: University of Birmingham, UK
PW: The sea level will drop if the climate cools and the icecaps build up. But in the case of the Permian, we know it was hot. When there's a lot of heat, the ice caps melt, ocean levels rise up and continents are flooded.
But the sea level also will change if there are plate tectonic heat flow changes. The mid-oceanic ridges are basically huge underwater mountain ranges, with huge trenches at the places where the plates spread apart. If there is a lower heat flow, these spreading centers in the middle of oceans reduce in size. The ocean has a constant volume, so if there's a big hunk of rock in the ocean, that's going to make the oceans rise higher. But if the great underwater mountains drop down into the trenches, the sea level will drop too. So with a lower heat flow, the spreading centers get colder, they take up less space, and the ocean water level drops.
AM: You mention that plants cause rivers to meander, while the loss of plants leads to braided rivers. At the P-T boundary, the rivers go from meandering to braided, indicating a catastrophic loss of land plants. Would this plant loss have contributed to the lower O2 of the Triassic, or do you think that plants recovered fairly rapidly?
PW: Even with the catastrophic loss of land plants, the amount of O2 in the atmosphere wouldn't be affected that quickly. Ocean algae, and especially the blue green algae, which is a bacteria, do more than land plants for atmospheric O2. We don't know if there was a comparable extinction in sea plants, however.
|After the Cretaceous extinction that killed the dinosaurs, ferns were the only plants left.
Image Credit: University of Texas
There is good evidence that the Triassic was a quite dead place for a long time. What we find at the Permian extinction is that there are almost no plant fossils. Certainly there are plants that survived, but there was a huge Permian plant extinction that caused whole classes of land plants to disappear. For instance, Glossopteris fauna once covered the Earth, and they disappeared entirely. Its closest modern relative is the ginkgo.
Plant pollen is a better reflection of diversity than fossils, because pollen is extremely tough, and preserves throughout time. Yet during the interval at the end of the Permian, there is no plant pollen - only fungal. So what we see after the Permian is a dead world, with fungus as the last surviving plant life. There may have been ferns, too. Ferns are a great survival species. After the Cretaceous extinction that killed the dinosaurs, ferns were the only plants left. After the Mount St. Helens eruption, ferns were the first plants to recover.
AM: The ancestors of mammals are the cynodonts, which survived the P-T extinction. Why wouldn't mammals have retained the cynodont capacity to exist at lower O2 levels?
PW: Mammals survived only at very low size. There were no large mammals until oxygen went up again.
We don't know about mammals with low O2 capacity, except for those that live at the highest elevations. For instance, the South American alpacas and llamas. They have special respiratory capabilities - they have very big chests and big lungs - and their blood has more hemoglobin. There's no way to tell from the fossil record how much hemoglobin an organism had. We can tell whether they had big chests, though.
|The ancestors of mammals are the cynodonts, which survived the P-T extinction.
Image Credit: BBC, UK
AM: Has a direct link been established between the mammal-like reptile cynodonts and modern day mammals? Or could mammals have evolved independently as an example of convergent evolution?
PW: No, we are definitely part of the surviving stock going back to mammal like reptiles - there are just too many similarities in head and bone anatomy for it to have been convergent evolution.
AM: You say that the air sac system in birds make them very efficient at oxygen acquisition. How does that correspond with the 'canary in the mine shaft' scenario? Wouldn't canaries be more efficient at breathing the available oxygen than the miners?
PW: But it is usually not low oxygen that kills the miners - it is methane, or carbon monoxide. Because birds are so good at extracting gas, these gases kill them even faster than they do us - carbon monoxide, for instance, binds on the oxygen sites.
AM: Are there other animals that evolved during the Triassic that also show evidence of adaptation to low atmospheric O2?
PW: In the oceans there are plenty. Most ammonites are thought to be low oxygen creatures, as were many of the Triassic clams. The oceans were very anoxic at that time, yet zillions of clams lived right at the minimum amount of oxygen. The Triassic is a very boring era - there is just not much diversity.
AM: Why are Permian rocks "green," as you describe them in your book? And why do the rocks go from green to red at the P-T boundary?
|According to Ward, a mmonites are thought to be low oxygen creatures.
Image Credit: UCLA
PW: If you dig down, right now, where you are, you can go very far down and still get oxygenated soil. But when you start digging down in the ocean, you don't go very far before you hit black anoxic soil. That's how it is for soil from the Permian, too. There was so much vegetation, so much organic matter, that oxygen couldn't percolate very far down. When that type of sediment turns to rock, it is a kind of an olive brown color.
When you get soils that have little or no organic matter, on the other hand, you usually find that the rocks are red. That's because the iron and minerals in the soil oxidized, or rusted, when exposed to oxygen. With the loss of plants after the Permian, there was less organic material. The soil became better oxidized as grain size led to more porosity.
We're forming red beds all time, today, in places that have very hot, dry conditions, like the desert. So the red beds of the Triassic indicate desert-like conditions. We move from the plant-rich Permian world, to the hot, plant-poor world of the Triassic.
AM: Why would warm-bloodedness have evolved from the low O2 conditions of the Triassic, as you suggest?
PW: It may not be a reaction to the oxygen, but to temperature. If the world is a desert, there is very little humidity, and it gets very cold at night, even in the hottest desert. I think that the warm-blooded adaptation may have helped mammals and birds survive in an all-desert world.
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