How Long Does Mass Extinction Take?

By figuring out the timing and rate of the world’s most massive extinction 252 million years ago, scientists hope to figure out how such lethal events work

A pair of Ammonite fossils, about 4 inches across, within a limestone bed very close to the Permian-Triassic boundary. Photo: Seth Burgess

About 252 million years ago, cracks in the Earth's crust in Siberia caused vast amounts of lava to spill out and blanket the region. Up to 6,000,000 cubic kilometers of molten material—enough to cover the continental U.S. at one mile depth—oozed over the Earth, and the upwelling magma that fed this eruption cooked rocks just beneath the Earth’s surface. The cooking rocks released carbon dioxide (CO2) into the magma; once the molten rock reached the surface, the CO2 bubbled into the Earth’s atmosphere. This triggered a huge change in climate and the ocean probably acidified.

Or at least that’s what many scientists think caused climate to change all those years ago. Others hypotheses abound, but scientists do agree on one thing: as a result of climate changes, roughly 90 percent of life died in a mass extinction event that was the most destructive in Earth's history.

A team at the Massachusetts Institute of Technology (MIT) is one of several labs around the world trying to understand how mass extinction works, and they’ve focused their efforts on this major extinction event, which marks the end of the Permian period and the beginning of the Triassic period in geologic history. Hypothesizing that the ancient volcanic eruptions in Siberia were the cause of this great die-out—it's the most likely culprit—the MIT team published a study in the Proceedings of the National Academy of the Sciences that suggests the die-out happened a lot faster than previously thought.

To understand how this mass extinction unfolded and what might have caused it, scientists need a precise timeline. Comparing fossil evidence from the end of the Permian to the beginning of the Triassic period within the stratified rock tells scientists that many species were lost as time progressed: trilobites, scorpion-like marine predators called eurypterids, mollusck species, some squid-like ammonites, and corals are just a few examples. Within 20 or 25 centimeters of a sedimentary rock section, "you have a complete change in the look of the fossil record. It goes from everybody’s happy to nobody’s home," says Seth Burgess, a graduate student at MIT and the lead author on the paper.

However, finer detail on what happened during the actual extinction is hard to find—the exact age of rocks that contain fossils from the extinction event is difficult to determine. These rocks, made of limestone, don't contain minerals that geologists can analyze to accurately determine their age.

But a fossil bed in Meishan, China allows scientists to get around this. About 252 million years ago, south China was home to a diverse marine ecosystem and nearby volcanoes. Interspersed between large chunks of limestone fossil rock known to be from the end of the Permian, based on specific fossil formations as well as attempts to date nearby non-limestone rocks, are volcanic ashbeds. These ash beds hold the key to finding out more precisely when animals died out.

The authors' methods went something like this: They first looked for limestone layers that marked the extinction event—those with a high concentration of Permian fossils indicating that species were beginning to die, above which fossils were absent. Then, they identified ash layers that sandwich the extinction event. 

Within the ashbeds, the MIT researchers zeroed in on a mineral called zircon, which crystallizes in magma and contains uranium and leadScientists, including the MIT team, have long attempted to figure out how old the ashbeds were based on these zircons, but previous studies lacked a high degree of accuracy. However, the group at MIT improved they way they collect and analyze these mineral isotopes in the lab—their radiometric dating of the uranium and lead in the zircons show that the extinction event spanned 60,000 years, plus or minus 48,000 years.

So in comparison to Earth's 4.5 billion years in existence, the extinction event lasted a mere blink of an eye in geological time.

How Long Does Mass Extinction Take?
In Meishan, China, darker layers of volcanic ash beds sandwich layers of limestone that contain the fossil evidence of mass extinction. Photo: Seth Burgess

"The fact that [they] can get down to 60,000 years plus or minus 48,000 years for an event 252 million years ago is pretty remarkable," says Doug Erwin, a paleobiologist at Smithsonian's National Museum of Natural History who wrote a commentary in PNAS on the study. 

Knowing the timing to a higher resolution allows scientists to focus on what happened during the event, instead of just getting before and after shots. So hypotheses of what killed organisms in the end of the Permian, whether they focus on higher ocean temperatures, atmospheric temperature increases, or a spike in CO2, will need to be refined depending on where the evidence fits in with the latest extinction timeline.

For example, the MIT team overlayed their timeline with data from previous research on carbon cycle changes. Scientists think that a drop in atmospheric carbon started before the mass extinction and believe that this represented an addition of isotopically light carbon into the ocean, as evidenced by isotopically light carbon seen in ocean sediments from that time.  The addition of carbon into the ocean could have driven a hotter, more acidic ocean environment. Based on the new timeline, this carbon spike happened just 20,000 years before species started dying.

But how do you move from an acidifying ocean to a mass die-out? Scientists think that one of the key ways to kill marine creatures at the end Permian was something called a calcification crisis. Acidifying the ocean reduces the amount of carbonate in the water and makes it harder for organisms with lower metabolisms, such as some mollusk species, to make shells out of calcium carbonate and thus to survive. This, in turn, breaks links on the food chain, making other creatures vulnerable to death.

The shorter time scale also means that organisms would have had less and less time to react and adapt to these changes in climate, atmospheric CO2 and ocean acidity. Failing the ability to adapt, they died.

As other scientists take the timeline and compare it to other data on climate and ocean acidity, they'll get a better idea of the extinction's play by play, and how quickly ecosystems fell apart. And Burgess thinks that the timespan for the extinction may get shorter still, as geological dating methods improve. “Maybe in 10 years from now, we’ll be able to see it to a higher resolution. The extinction might even be more abrupt than that,” says Burgess.

Other mass extinction events have also been narrowed down to short timeframes using similar methods. For example, radiometric dating of volcanic ashbeds in Montana and Haiti located near geological evidence of the asteroid impact that killed the dinosaurs at the end of the Cretaceous period suggests that mass extinction only took about 32,000 years. A similar study of another mass extinction triggered by volcanic eruptions at the end of the Triassic period suggests it lasted less than 5,000 years.

Despite the fact that all of these extinction events were caused by different things, the ecosystem collapse happened very quickly. "Whatever the causes of the extinctions may be, and it looks like there are very different causes for some of them, the biosphere may collapse in very similar ways once it gets beyond a tipping point," says Erwin. 

All this talk of changing climate and acidifying oceans probably sounds familiar. Today, humans are doing a lot of different things to their environment that could have major ramifications in the future—climate change, pollution, habitat reduction, just to name a few. Some scientists even see the end Permian, particularly the loss of ocean species, as a lesson for this century.

So understanding the conditions leading up to, within, and after a mass extinction event may help us to avoid human-induced ecosystem collapses in the future. As Erwin puts it, "you don't want to start a mass extinction, because once a mass extinction begins, the prognosis is pretty grim.”

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