Last January I stood in a ditch in the middle of nowhere in South Africa, and the summer sun beat down as I stared at a heap of grape soda-colored rocks. That’s when the thought came to me: What does a mass extinction taste like? I looked around. Bob Gastaldo, the paleontologist whose research team I had been following through the Karoo for ten days, had his back turned and was busy hitting at rocks with his rock hammer. What the heck. I reached and plucked a quarter-billion-year-old pebble from the planet and popped it into my mouth.
It was hot from the sun. I felt its sharp edges and its rough surfaces with my tongue and lips as it clicked against my teeth. I spat it back onto the ground, where it dried immediately. It did not taste like grape soda — it tasted like nothing. Fitting, I thought, because the mass extinction of life this rock supposedly formed during created just that: a whole lot of nothing, as it caused about 70% of all land life and 96% of all marine life to vanish permanently.
The extinction has a few names. One of them is the Permian-Triassic (PT) mass extinction, for the two geologic periods it separates; another is the “Great Dying,” for all the life-ending change it caused. It is the biggest extinction of the so-called “Big Five” mass extinctions that punctuate the history of life on our planet, and, according to many researchers, one of the best places to see the PT land extinction as it exists in Earth’s many-layered rock record is in the Karoo. Where I stood in that ditch, the extinction horizon — that is, where the rock layers go from Permian to Triassic in age — is supposed to be near the layer made of the not-so-fruity pebbles.
I walked and sat on a pile of dirt and listened for the barks of baboons. I had left my backpack and water above the ditch, and I did not want a troop of baboons waltzing away with it. But I heard nothing. It was quiet, just as I would imagine any graveyard to be, even one as old as this. If some of these rocks were gravestones, you would see names like Diictodon and Daptocephalus — names of land animals that missed the train to the Triassic. Near the ditch, Gastaldo had tripped over the fossil skull of what was probably a Lystrosaurus — a genera with species that died out, and some that survived.
The extinction of so many animals at once, preserved in rock layers believed to correlate to marine extinction rock layers, is how paleontologists like Roger Smith identified a mass extinction here in the first place. “It happened on land and it happened in the sea and it happened at the same time,” Smith said when I visited him where he works at the Iziko South African Museum in Cape Town. Then, as life reset itself, new species appear above the boundary.
Animals went extinct over a relatively long period of time — many thousands of years — and the prime suspect behind their demise is massive volcanism in what is today Siberia, where about 3 million cubic kilometers of lava disgorged onto Earth’s surface over the course of about 60,000 years. The volcanoes let out lava, but also climate changing gases like carbon dioxide that likely warmed the planet. This all set off a cascade of fallout effects that annihilated most life.
The dying in the oceans syncs well with the volcanism’s onset, but it is not clear to experts like Gastaldo, who works at Colby College in Maine, when the land extinction happened. If the two extinctions did not happen at the same time, then some other extinction trigger could be behind the land event. But if the land event happened alongside the marine event, then volcano-made climate change is the most likely perpetrator.
Today, volcanism is no longer pumping so much carbon dioxide into the atmosphere — but humans are, and it’s helping drive what many paleontologists are calling the sixth mass extinction of life, with the long list of victims including things like the once-common golden frog of Panama, and North America’s ash trees. By some accounts, the rate of species loss today has not been seen since the last of the non-avian dinosaurs perished about 66 million years ago.
If one main driver of the PT extinction on land was indeed climate change, then scientists can look to the event as an analogy for how our own homemade sixth extinction might unfold in our near and far futures. But if the trigger was something else, which may be the case if the land extinction did not happen at the same time as the marine extinction, then the analogy, at least for the land story, falls short.
“How well does it rhyme with today?” Gastaldo asked one night in our hotel in the small South African town of Bethulie. That question, and the implications its answer carries, is why he and his team were there exploring the rockscape around Bethulie. We drove through Bethulie each day in two white pickups, past the site of an old concentration camp built by the British during the Boer War, and into the veld. Gastaldo’s team members, like sleuths, set out with their detecting skills — much more fine-tuned than my eat-a-rock technique — in search of clues.
Sandra Kamo’s Ashes
“Look over there,” Sandra Kamo whispered to me.
“What? I don’t see anything.”
“Right there, sitting on that rock.”
“I still don’t see it.”
Kamo pointed, and this time I saw it, and I screamed. Sitting on a rock was a yellow lizard as big as a house cat. It was a rock lizard, and it scurried like lightning toward a wild olive tree after hearing us. (Or, after hearing me.)
I had been following Kamo while she followed rock layers around the extinction horizon, near the grape soda rocks. Her skill as a geochronologist is that, back in her lab at the University of Toronto, she can tell you exactly how old a rock is. That is a powerful skill, because if she can assign absolute ages to the rocks here, then the whole case could be solved, as it would be clear exactly when the land extinction happened. But Kamo can only deduce the age of very specific kinds of rock, and the rock type she needs here — ash from an ancient volcanic eruption — is as rare as a golden frog in Panama. “There was not a lot of volcanic activity in the late Permian,” Kamo said.
But there was some: in 2015, her and the team published a date from one ash bed, and its age was 253.5 million years old — over one million years older than the extinction horizon is supposed to be. So, case-closed, right? Not quite. For a slam dunk, Kamo says she would need to date multiple ash layers from the rock layers around the extinction horizon, because it is hard to say just how much time the rocks between that one ash layer and the extinction horizon actually represent. Did they form continuously, or are there big gaps in time?
The date came from a layer about 60 meters below where researchers like Smith think the mass extinction happened, and so the age could be interpreted a bit differently: “[That date] really helped the cause by showing that this is very close to the Permian-Triassic boundary,” Smith told me during my museum visit. Near where he and I sat, visiting schoolchildren marveled at the fossil jaw of a now-extinct shark.
“It seems like the whole question hinges on the ash,” I said to Kamo.
“On the timing? Yeah — how old are these rocks?”
“And without it, the conversation stays where it is.”
“Yeah, I’d say so.”
John Geissman’s Drill
John Geissman is a geologist from the University of Texas at Dallas, and he makes Swiss cheese out of boulders with a drill powerful enough to drill through rock. “Drilling is the most fun you can have with your clothes on,” he recalled a colleague saying.
Geissman measures the magnetism of the rock samples he drills. He does this because, every so often in Earth’s deep past, the orientation of our magnetic field flips. North becomes south, and south becomes north. There’s a global record of these flips, and Geissman wants to see what the orientations of the magnetism in the rocks here are. The idea: if he can construct a complete record of the flips in the rocks at the field site, he may be able to match it to the global record and see just how old the rocks actually are. And, so far, the pattern seems to match the record of around 253 million years ago — older than the extinction horizon by roughly a million years.
Now the case is closed, right?
Nope, not quite. Smith explains that without firm absolute ages for the rocks — something magnetics cannot give — it is hard to say exactly where the flips fit into the global record.
“So, unless Sandra finds an ash bed…” I said to Geissman.
“That’s right,” said John.
“What if she doesn’t?”
“Well, then we’re limited. But we’re still gunna have a far better dataset than anything else.”
He revved his drill to life, and its din filled the desert quiet.
The Boneyard
Johann Neveling, a paleontologist from South Africa’s Council for Geosciences and a member of Gastaldo’s team, crouched down to inspect a rock with bones sticking out of it. Then he darted off and disappeared into the bush. It was a hot and dry day, and I got a nosebleed as I tried to keep up with Neveling and Gastaldo, who were out chasing fossils.
Above all things, the main clues that there is a mass extinction recorded in these rocks is the fact that animal fossils appear to vanish from these rock layers, and the person who has done more work than anyone to collect and catalogue these fossils is Smith. In a 2014 study he and a colleague defined the land extinction based on the disappearances of myriad fossils from these rock layers.
Using Smith’s GPS coordinates of where those fossils are supposed to be, Gastaldo and Neveling were retracing Smith’s steps so they could see the boundary for themselves. But some fossils, it seemed, were not where their GPS locations say they are. Animals like Daptocephalus, thought to have gone the way of the dodo, seem to persist past Smith’s boundary. “If Permian-aged reptiles are found at the same stratigraphic horizon, and the same position or higher than supposedly Triassic … vertebrates, how can there be an extinction?”
One hitch could be the poor resolution of the GPS data: “On steep slopes, the error in GPS recording … can change the stratigraphic level by up to ten meters up or down from its true position,” Smith says.
Bruce Rubidge, a paleontologist at the University of the Witwatersrand, who is not involved in this research area, has a solution: “Gastaldo and Smith need to get out together and go and talk about it in the veld. That’s what needs to happen,” he says. Because as long as such discrepancies persist, Neveling asserts, extrapolating the extinction story in the Karoo to the scale of the whole planet will remain dubious. “Can we say the things that we do about mass extinctions and climate change in the ancient Karoo that we do? We think not.”
All the while, the ash layers — the key witnesses that could help solve it all, as they would place unquestionable timestamps on these rocks — are MIA. “If the marine and continent biota collapsed simultaneously, or were a little out of phase with each other, we need to know if that model is true,” Gastaldo says. “Because I live on land, I’m concerned about what happens to where I live.”
Almighty Ash
Toward the end of my days with the team we drove to a large rock mound that sat by itself in the middle of a valley. A skinny windmill with only one blade left stood spinning near where we parked the pickups. We hiked to the mound, and at its front, like a hidden opening to a pyramid, was a small dry creek with a familiar sight: grape soda-colored rocks. We were close to the boundary, and the team set out in search of ash.
It seems a strange thing that a chance volcano erupting somewhere around this part of the planet a quarter-billion years ago could be, if researchers find its ashfall, the one clue that reveals the extinction’s timing, and what was likely behind it.
The team worked, and the sky soon became grey. Then the booming of thunder came, like drums rumbling from underground. We hurried to the pickups, and heavy raindrops fell as we drove. I got out to close a cattle gate, and I gazed back as clouds — silver and grey with sudden veins of lightning — came upon us like some kind of encroaching doom.
A version of this story published in Undark on 11 June 2018.