Clues to the Great Dying

Volcanoes and acid oceans appear to have triggered the biggest extinction of all time
Oct 27, 2015 — 7:15 am EST

Here’s an artist’s idea of what the intense volcanic activity in ancient Siberia might have looked like. Many scientists suggest that effects of this volcanism, which lasted some 60,000 years,  may have triggered the extinction of most living species.

José-Luis Olivares/MIT

Some 252 million years ago, life on Earth nearly ended. In a geologic flash, at least 70 percent of Earth’s land species vanished along with 96 percent of all species in the oceans.

This was a mass extinction. The biggest ever, it far exceeded the one that snuffed out the dinosaurs some 186 million years later. That one knocked out only 50 percent of Earth’s species. The earlier catastrophe is now known as the Permo-Triassic mass extinction because it occurred as the Permian Period gave way to the Triassic.

Its impacts were so massive that scientists now refer to that time as the Great Dying.

Details of this period are written in stone. Literally. Details have been emerging from fossils in rocks that developed at this time. Many organisms suddenly disappeared from the fossil record. Marine arthropods called trilobites, for instance, are common in rocks from the Permian. By the time Triassic age rocks developed, all signs of these critters had vanished.

What caused the Great Dying has long been a mystery. Many scientists suspected that a massive volcanic eruption had something to do with it. Occurring in what is a part of Russia now known as Siberia, this eruption lasted millennia. But there was little hard evidence linking that eruption to the extinctions. So the biggest wipe-out in the history of life remained a cold case.

Until now.

Scientists have new evidence placing blame solidly on the Siberian eruptions. They triggered deadly conditions on both land and in the seas. And new analyses of them could offer important lessons about how to interpret changes occurring on Earth today.

Life interrupted

During the Permian, nearly all of Earth's landmasses had clumped into one mega-continent. It’s name: Pangaea (Pan-JEE-uh). Earth had one main ocean and one smaller sea. Pangaea’s enormous size altered the planet’s climate. Most places became warm and dry.

This Permian world teemed with life. On land, insects buzzed and crawled, including primitive dragonflies and cockroaches. Big, plant-eating reptiles and amphibians grazed its forests. The oceans were ruled by fish. Coral reefs thrived. Trilobites scuttled along the seafloor.

Then, at the end of this lush period, volcanoes began to erupt. And erupt. And then erupt even more — spewing what would seem a never-ending cascade of lava over a period of 60,000 years. The most massive in Earth’s history, the eruptions poured out enough lava to cover the entire continental United States to a depth of nearly 1.6 kilometers (a mile)!

Very big volcanic eruptions can mess with life in several ways. First, they shoot clouds of ash high into the atmosphere. That can block out a lot of sunlight. The eruptions also may add heat to the oceans and atmosphere. This would warm surface temperatures across the globe, along the way altering weather patterns. Eruptions pump out gases, especially carbon dioxide. As a greenhouse gas, carbon dioxide keeps heat from escaping away into space. This too warms the climate. The oceans, too, will absorb plenty of carbon, leaving the water ever more acidic.

The huge eruption of the Siberian volcanoes and the Permo-Triassic mass extinction occurred at just about the same time. Coincidence … or not? That has been a big — and lingering — question.

To find out if the Siberian volcanoes could have caused the Great Dying required knowing very precisely when each event occurred. Enter geochronologist Seth Burgess. He knew that if those eruptions occurred just after the extinction, for example, they could not be blamed.

Geochronologists (JEE-oh-kron-ol-uh-jizts) specialize in determining how old a rock is. As a PhD student at the Massachusetts Institute of Technology (MIT), he led a team that analyzed rocks from China. These had formed during the late Permian. These scientists wanted to know what ancients rocks could tell them about the timing of those Siberian volcanoes.

The Chinese rocks contained layers of limestone. Such sedimentary rocks are created in chronological order, with the youngest at the top. In the lowest limestone layers, the scientists found fossils of animals that lived in the Permian. But those fossils were missing in more recent Permian layers. So the Great Dying had to have occurred somewhere in between.

By looking at two constituents of the limestone — carbon and zircon — Burgess and his team were able to come up with a timeline of the Siberian eruptions. The fossils, they found, had disappeared within an interval of 60,000 years. In the last 10,000 years, though, there was a sudden pulse of volcanically derived carbon.

Concludes Burgess: “It looked like business as usual until all of a sudden, everything changed.” His team published its findings in the February 10, 2014, Proceedings of the National Academy of Sciences.

Acidity on the rise

But how did that carbon change the world? One group of scientists was curious about why ocean pH changed as it had during those 60,000 years. They especially wanted to know if something unusual had happened to the pH during those last 10,000 years. Might it be linked to the extra carbon that Burgess and his team had found?  

To explore the mystery, Matthew Clarkson led a research team that travelled to the United Arab Emirates. It’s a tiny country in the Middle East. A geochemist, Clarkson was at the University of Edinburgh in Scotland when he conducted that work in 2014. His team found limestone rocks that once were at the bottom of an ancient Permian ocean. Those rocks provide a record of ocean conditions back then.

To learn how pH had changed, the group tracked differences in the rocks’ amounts of two forms of boron, an element that came from the seawater of that Permian ocean.

“Boron is found in seawater in two different forms, boric acid and borate,” explains Clarkson. “The amount of each in the ocean is dependent on the pH of the water.” By determining the ratio — or relative amounts — of borate and boric acid in the limestone, researchers can calculate what would have been the pH of ocean water when the rock was forming.

Clarkson’s team plotted the ratio of the two forms of boron in the limestone layers. Sure enough, they could see a pattern in the pH values. Just at the Permian-Triassic boundary, there was a fast, dramatic drop in pH. That meant the water had become more acidic.

“We see a change in boron,” Clarkson says, “that corresponds to about 0.6 pH units.” His team compared those data to Burgess’s precise carbon ages from Chinese rocks of the same age. They discovered that this big shift towards more acidic oceans occurred during the same 10,000 years as the shift in amounts of volcanic carbon.

Knockout punch

But that wasn’t the end of it. The first 50,000 years of eruptions in Siberia would have released enough carbon dioxide to have substantially warmed the planet’s surface. Many land animals might not have been able to easily adapt to the evolving conditions. In addition, species losses would have stressed ecosystems far and wide, concludes Clarkson, now at the University of Otago in Dunedin, New Zealand. And reduced oxygen from those 50,000 years of eruptions might have further weakened many species, he says.

From his data and Burgess’s, it appears that a second and devastating phase of eruptions happened in the last 10,000 years. The final phase of those eruptions, says Burgess, delivered more carbon dioxide than would normal ones.

“Something added more carbon dioxide,” he says. One possibility: The Siberian volcanoes began erupting through a layer of limestone. That eruption likely would have been so hot that the limestone melted. This would have released the carbon it contained, adding to the gases being spewed directly by the eruptions.

The extra boost of carbon dioxide seems to have provided the final, knock-out punch for ocean creatures, Clarkson suspects.

Much of that carbon dioxide dissolved into the oceans’ water. That would have caused a sharp and rapid drop in pH. The more acidic water would have dissolved the calcium-based mineral that makes up sea shells. It also would have made it harder for creatures to build new shells. The process of using calcium to create shells is called calcification.

“Some of the organisms that rely on calcification to create shells would have suffered particularly badly,” says Clarkson. Fossils show that the creatures that went extinct in the highest numbers were the ones that depended the most on shells, he notes. In those last 10,000 years, many species that had survived the first phase of volcanic eruptions were finally done in.

The boron data “provide the most direct look yet at how ocean pH may have changed during the Permian-Triassic boundary interval,” notes Jonathan Payne. He is a geologist at Stanford University in California. An expert in how ancient life and the environment are connected, he was not involved in the study.

There is also an indirect link to extinctions on land, he argues. The same increase in carbon dioxide that caused the ocean to become more acidic might also have sparked major ecological damage on land. A global warming and changes in rainfall or snows would likely have wiped out many remaining land-based organisms, Payne suspects.

Catastrophe repeated?

After the Great Dying, Earth slowly came back to life. After all, there were a few survivors. They became the ancestors of many new plants and animals, including the dinosaurs. The Great Dying cleared the way for some seriously successful newcomers.

Among them would be people. But to remain a successful species, people should heed lessons from that ancient period, the scientists now argue. After all, don't  climate change and ocean acidification sound eerily familiar? They are happening again, right now.

Just as during the Permo-Triassic extinction, today’s changes are caused by extra carbon dioxide in the atmosphere. Only this time, much of the carbon dioxide is coming not from Siberian volcanoes but from people burning carbon-rich fossil fuels. 

Clarkson says that in the early 1800s, Earth’s oceans had a pH of 8.2. Since then, human activities have pumped lots of carbon dioxide into the atmosphere. Ocean pH has already fallen to 8.1. “It’s projected to fall another 0.3 to 0.4 pH units by the end of this century. That’s 0.3 to 0.4 in 300 years,” he says. That’s enough to cause serious damage to animals that depend on building shells, and to predators that eat those animals.

At that rate, by the year 2400, the oceans may become acidified by the same amount seen in the Great Dying — when nearly every living species on Earth went extinct.

Correction (10/28/15): An earlier mention of placoderms has been removed to reflect the general consensus that this fish had likely gone extinct prior to the Permian period.



Word Find (click here to enlarge for printing)

Power Words

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acidic  An adjective for materials that contain acid. These materials often are capable of eating away at some minerals such as carbonate, or preventing their formation in the first place.

acidification  A process that lowers the pH of a solution. When carbon dioxide dissolves in water, it triggers chemical reactions that create carbonic acid. 

amphibians  A group of animals that includes frogs, salamanders and caecilians. Amphibians have backbones and can breathe through their skin. Unlike reptiles, birds and mammals, unborn or unhatched amphibians do not develop in a special protective sac called an amniotic sac.

arthropod  Any of numerous invertebrate animals of the phylum Arthropoda, including the insects, crustaceans, arachnids and myriapods, that are characterized by an exoskeleton made of a hard material called chitin and a segmented body to which jointed appendages are attached in pairs.

atmosphere   The envelope of gases surrounding Earth or another planet.

boron   The chemical element having the atomic number 5. Its scientific symbol is B.

calcium  A chemical element which is common in minerals of the Earth’s crust. It is also found in bone mineral and teeth, and can play a role in the movement of certain substances into and out of cells.

carbon  The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

carbon dioxide  A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter (including fossil fuels like oil or gas) is burned. Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

chronological  An adjective that refers to something that occurs in an order that advances with time, from earlier to later.

climate   The weather conditions prevailing in an area in general or over a long period.

climate change  Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

continent  (in geology) The huge land masses that sit upon tectonic plates. In modern times, there are six geologic continents: North America, South America, Eurasia, Africa, Australia and Antarctic.

coral  Marine animals that often produce a hard and stony exoskeleton and tend to live on the exoskeletons of dead corals, called reefs.

Devonian Period    A span of geologic time that ran from roughly 416 million years ago to 360 million years ago. Earth’s land masses collected into two supercontinents: Gondwana and Euramerica. It’s when some of the earliest plants emerged, initially with no leaves or roots. By the end of this period, ferns and seed plants had evolved. Some shellfish and trilobites shared the oceans with various fishes. And many wingless arthropods (ancestors to spiders and insects) colonized the land.

dinosaur  A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs. Their descendants eventually split into two lines. They are distinguished by their hips. The lizard-hipped line became saurichians, such as two-footed theropods like T. rex and the lumbering four-footed Apatosaurus (once known as brontosaurus). A second line of so-called bird-hipped, or ornithischian dinosaurs, led to a widely differing group of animals that included the stegosaurs and duckbilled dinosaurs.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create for that organism or process. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature, humidity and placement of components in some electronics system or product.

evolve  (adj. evolving) To change gradually over generations, or a long period of time. In living organisms, the evolution usually involves random changes to genes that will then be passed along to an individual’s offspring. These can lead to new traits, such as altered coloration, new susceptibility to disease or protection from it, or different shaped features (such as legs, antennae, toes or internal organs). Nonliving things may also be described as evolving if they change over time. For instance, the miniaturization of computers is sometimes described as these devices evolving to smaller, more complex devices.

extinction  The permanent loss of a species, family or larger group of organisms.

fossil  Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.

fossil fuels  Any fuel — such as coal, petroleum (crude oil) or natural gas —  that has developed in the Earth over millions of years from the decayed remains of bacteria, plant or animals.

geochronology The study of the age of rocks. Geochronologists often use known rates of radioactive decay of elements in rocks to determine the age of the rock.

geology  The study of Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists. Planetary geology is the science of studying the same things about other planets.

global warming  The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

Great Dying    A period of time roughly 252 million years ago when at least 70 percent of all land species and 96 percent of ocean species went extinct. Ot was the greatest mass extinction in Earth’s history. These species fell victim to changing climate and ecological changes.

greenhouse gas   A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

insect  A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

lava  Molten rock that comes up from the mantle, through Earth’s crust, and out of a volcano. 

limestone    A natural rock formed by the accumulation of calcium carbonate over time, then compressed under great pressure. Most of the starting calcium carbonate came from the shells of sea animals after they died. However, that chemical also can settle out of water, especially after carbon dioxide is removed (by plants, for instance).  

marine  Having to do with the ocean world or environment.

mass extinctions  Any of several periods in the distant geological past when many — if not most — of the larger animals on Earth disappeared forever. One that occurred as the Permian period gave way to the Triassic, sometimes called the Great Dying, led to the loss of most fish species. Our planet has experienced five known mass extinctions. In each case, an estimated 75 percent of the world’s major species died off in a short time period, typically defined as 2 million years or less. 

millennia    (singular: millennium) Thousands of years.

Pangaea  The supercontinent that existed from about 300 to 200 million years ago and was composed of all of the major continents seen today, squished together.

Permian  A time in the distant geologic past, about 250 million to 300 million years ago. Many reptiles rose to prominence on land; these were not yet dinosaurs. Many large invertebrates ruled the oceans during this period. But most would die off at the end of the Permian, as it gave way to a new geologic period known as the Triassic.

pH A measure of a solution’s acidity. A pH of 7 is perfectly neutral. Acids have a pH lower than 7; the farther from 7, the stronger the acid. Alkaline solutions, called bases, have a pH higher than 7; again, the farther above 7, the stronger the base.

placoderm    An extinct type of armored fish which lived from the Silurian to Devonian periods. Some reached 9 meters (30 feet) in length.

predator (adjective: predatory) A creature that preys on other animals for most or all of its food.

ratio  The relationship between two numbers or amounts.

reptile    Cold-blooded vertebrate animals, whose skin is covered with scales or horny plates. Snakes, turtles, lizards and alligators are all reptiles.

sedimentary rock A type of rock that forms from accumulated material that has been eroded from other rocks.

Silurian Period    A span of geologic time that ran from roughly 443.7 to 416.0 million years ago. During this period, Earth underwent important changes, starting with a melting of large glaciers. This caused a rise in the levels of the major seas. Earth’s climate also began to stabilize. Life across the planet responded to these changes. For instance, some fish began inhabiting freshwater and the first fish with jaws evolved. Coral reefs emerged for the first time. And fossils from this period show the first strong evidence of land-based life, including plants and relatives of spiders and centipedes.

species  A group of similar organisms capable of producing offspring that can survive and reproduce.

Triassic Period  A time in the distant geologic past, about 200 million to 250 million years ago. It’s best known as the period during which dinosaurs first emerged.

trilobite   An extinct group of arthropods that were related to modern-day insects.

volcano  A place on Earth’s crust that opens, allowing magma and gases to spew out from underground reservoirs of molten material. The magma rises through a system of pipes or channels, sometimes spending time in chambers where it bubbles with gas and undergoes chemical transformations. This plumbing system can become more complex over time. This can result in a change, over time, to the chemical composition of the lava as well. The surface around a volcano’s opening can grow into a mound or cone shape as successive eruptions send more lava onto the surface, where it cools into hard rock.

zircon  A gemstone that contains traces of the radioactive element uranium. It develops as a crystal that forms as magma (from deep inside Earth) begins to cool. Some of the oldest minerals surviving on the planet are crystals of zircon that are at least 4.2 billion years old. For perspective, Earth is only 4.56 billion years old.


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M.O. Clarkson et al. Ocean acidification and the Permo-Triassic mass extinction. Science. Vol. 348, April 10, 2015, p. 229. doi: 10.1126/science.aaa0193.

S. Burgess et al. High-precision timeline for Earth’s most severe extinction. Proceedings of the National Academy of Sciences. Vol. 111, February 10, 2014, p. 3316. doi: 10.1073/pnas.1317692111.

Further Reading

A. Yeager, “Microbes Indicted in ancient mass extinction,” Science News Online. March 31, 2014.

S. Ornes. “Explainer: Ocean acidification.” Science News for Students.  December 5, 2012.

S. Ornes. “Sea changes.” Science News for Students.  April 7, 2011.

A. Biskup. “Global warming and the greenhouse effect.Science News for Students. May 7, 2010.

Questions for "Clues to the Great Dying"