All of us, young and old, belong to an anxious generation. The ever-present threat of nuclear war heightens our perception of risks.
A good example is how, back around the turn of the century, after media reports surfaced that Yellowstone is a supervolcano, most laypeople, including me, pictured the environmental consequences of any Yellowstone eruption as a global nuclear winter.
We can relax a little about that. No scientific law requires supervolcanoes to always have supereruptions.
According to volcanologists, chances for another supereruption at Yellowstone are 1 in 730,000, or 0.000014%.
Nevertheless, our planet is still more likely to experience a supereruption somewhere on its surface than a large meteorite impact. (Self)
We must prepare for life during and after a supereruption. But since one hasn’t happened in recorded history yet (Oppenheimer), how can we know what to expect?
While we laypeople and the media try to get psyched for the volcanic apocalypse with doomsday documentaries and fiction, researchers are using supercomputers to model supereruptions as realistically as possible. (See, for instance, Jones and others; Mastin and others; Oppenheimer, Chapter 14; Sparks, Self, and others; Self)
Scientists have three types of data to work with:
- Evidence of past supereruptions from the geologic record, which may include ice cores as well rock formations and deep sea drilling.
- Well-documented “normal” eruptions, like Mount Pinatubo’s magnitude 6.1 (Oppenheimer) eruption in 1991, in which hypothetical extreme values can be substituted.
- Fossil evidence from living beings near a supereruption. It was signs of lung disease in plant eaters that first clued investigators in on the long-distance lethal effects from one of Yellowstone’s supereruptions. (BBC)
Some model results, based on data from Earth’s last known ignimbrite eruption, which happened at Indonesia’s Toba volcano 74,000 years ago, show that this magnitude 8.8 (Mason and others) cataclysm could have triggered a global chill. (Rampino and Self)
The “volcanic winter” scenario is indeed one possibility . . . but there are others that are much more benign.
In some models, supereruptions stress life on Earth at first, but the effects don’t last very long. The climate recovers in just a few decades, and glaciation doesn’t happen. In fact, the researchers couldn’t even trigger a “volcanic winter” with an eruption 900 times more intense than Pinatubo’s in 1991. (Robock; Robock and others; see Oppenheimer, Chapter 8)
While the scientists try to bring their models into agreement, the rest of us can take comfort in an “experiment” that life itself ran in the past.
Fossils show that a collection of prehistoric wildlife on the northern Great Plains not only survived the Great Ignimbrite Flareup for millions of years but actually prospered while it happened nearby. No one knows how they did it.
An ignimbrite flareup, as we saw last time, means tens of thousands of cubic miles/kilometers of pyroclastic flows blasting up into the stratosphere and surging across the landscape of western North America and northern Mexico, over and over again, for millions of years. (Best and others; Chapin and others; Ferrari and others)
Huge numbers are so mind boggling. Let’s just say that during one peak of activity between 33 and 34 Ma (million years ago), when a supereruption was happening about once every 15,000 years (Oppenheimer), this group of critters experienced five Toba’s – just across state lines – over a span of time that was the same length as the present interval between us and that one Toba ignimbrite eruption back in the Middle Stone Age.
And yet . . .
. . . the faunal composition of central North America was so stable between approximately 37 and 27 [Ma] that it has been recognized as the “White River Chronofauna.” (Van Valkenburgh)
What were these – super-animals? No. There has never been and, hopefully, never will be a super-snail.
The White River Chronofauna
Land snails were one of the signature White River species. So were the sort of primitive-looking critters you might expect to see in North America halfway through the Age of Mammals. Early versions of very ordinary modern animals like gophers and weasels also roamed the White River plain. (Prothero, 2006)
There were even some dogs and “cats,” although they wouldn’t have made good pets.
The dogs weren’t exactly Lassie. Hesperocyon, the earliest true canid, looked more like a modern civet, so that’s actually an African palm civet in the image above. Hesperocyon could climb trees, and it ate plants as well as small vertebrates. (Van Valkenburgh)
No wonder today’s dogs eat anything they can get into their mouths!
Modern bears are omnivores, too, but back in the Eocene and Oligocene, their side of the caniform family hadn’t quite gotten its act together yet. There were a few “bear-dogs,” also known as amphicyonids. These were destined to develop into impressive apex predators, but it didn’t happen during the White River days.
Perhaps the “cats” forced those early caniforms to keep their heads down.
Cat-like nimravines (Bryant) were the most diverse apex predators during much of the Oligocene epoch. (Prothero and Heaton, figure 5)
While we can describe some White River animals as humpless camels (Prothero, 2006) or hippo-like rhinoceroses (Stoffer), nimravines have no living descendants and therefore they only have scientific names.
About the only informal way a time traveler on the White River plain might refer to a nimravine would be “Aagh! That flat-footed housecat has saberteeth!”
Indeed, the nimravine Eusmilus was one of the all-time smallest known sabertooths. (Anton)
Most nimravines were lynx- to lion-sized hypercarnivores, and all but one genus had saberteeth. These animals looked a lot like modern cats, although they had shorter legs and smaller feet. They also tended to walk flat-footed like a bear, not on their toes like a modern cat. (Anton)
Nimravines had retractable claws, and their skulls so closely resembled those of modern cats that early paleontologists called them “paleofelids.” (Anton)
However, closer examination has shown that nimravines probably weren’t the ancestors of modern cats, after all. However, the jury is still out on a later nimravid wave – the barbourofelines – that appeared long after White River days. (Bryant; Werdelin and others)
Paleoartist Mauricio Anton has done some gorgeous reconstructions of these prehistoric predators.
Eusmilus was one of the more extreme White River sabertooths, along with Dinictis, Pogonodon, and Hoplophoneus. Nimravus had much less obvious saberteeth, while upper canines in the cheetah-like Dinaelurus were more like those of modern cats. (Anton)
Some species of Hoplophoneus had extreme bodies, too, weighing in at over 130 pounds (60 kilograms). They were even more powerful than Smilodon (Anton), which is the Ice Age sabertoothed cat that we all know and love.
We wouldn’t feel so affectionate if Smilodon or any sabertoothed nimravid was alive today.
Sabertooths hunted a little differently than what we’re used to seeing. They couldn’t hold onto prey with teeth and claws the way modern lions do – the elongated upper canines would break and then the hunter would starve to death. Instead, they probably used their powerful forelimb, shoulder, and neck muscles to knock prey over and immobilize it flat on the ground before delivering the killing bite. (Anton; Turner and others)
That’s horrible, but relax . . . the last nimravines disappeared around 23 Ma (Van Valkenburgh), and the last known Smilodon cat died in the La Brea tar pits some 13,000 years ago. (Werdelin and others)
Little is known about the daily lives of White River fauna. Nimravines must have terrorized all of the various horses, deer-like ruminants, titanotheres, sheep-like burrowing oreodonts, and other large prey animals (Prothero, 2006) that lived on a vast, soggy floodplain (Stoffer), part of which is now Badlands National Park in South Dakota.
The White river saga probably began late in the Middle Eocene – between roughly 41 to 37 Ma – when nimravines and some types of rodents migrated into North America from Asia. Fellow travelers over the Bering land bridge included amphicyonids, rhinoceroses, and ancestral camels. (Figueirido and others)
Dogs were probably the most prominent North American natives. (Werdelin) Various kinds of horses were there, too, but they had migrated in from the other direction – Europe. (Agusti and Anton)
And so the White River Chronofauna slowly came together. It would eventually include more than 170 species.
When nimravines arrived, North America’s central plains were still densely forested (Retallack), but judging by some of the fossils, there may also have been many open areas, perhaps temporary ones. (Webb)
It’s anyone’s guess what might have caused those ephemeral clearings. Storms and wildfire could do it, and perhaps volcanic ash did, too.
Today, an ash layer just 1 inch/2.5 centimeters thick can take out plants for more than a year. Very deep burial in ash sterilizes the ground, due to oxygen deprivation. Plants might return in years to decades after heavy ash fall, but the new soil will be unstable for several hundred to a few thousand years. (New Zealand Ministry of Agriculture and Forestry, quoted in Oppenheimer, Table 2.1)
Climate change was also opening up the White River forest. From the late Middle Eocene on, temperatures generally were cooler and less rain fell. In the Oligocene, open woodlands appeared, and these eventually developed into wooded savanna lands (Retallack; Stromberg) covered by C3 grasses. (Kidder and Gierlowski-Kordesch)
Any combination of trees and clearings or savanna was ideal country for nimravines. They were ambush hunters. (Werdelin) Trees provided good cover and an escape route, just as they do for modern cats. An interconnected network of meadows or savanna would ensure a steady stream of victims coming into range of the nimravine’s short charge and powerful leap.
In spite of the peril, this mosaic landscape was also a good place for plant eaters. There was probably a wide variety of browse available, and since parts of Asia had been arid in the middle Eocene (Berggren and Prothero), many of the immigrants were already used to life in a more open landscape . (Retallack, 1983; Webb)
Over the next 7-10 million years, various species came and went, but most White River lineages stuck around. (Prothero and Heaton; Van Valkenburgh) That’s what the experts mean by “chronofauna.” No matter when you dropped in, during that multi-million-year time span, you would always see the same general collection of animals.
It’s like there wasn’t a volcanic apocalypse going on just over what would eventually be the state line.
Now, those nimravines were very impressive. The titanotheres were, well, titanic; and herds of burrowing sheep-like little animals are cool anywhere you find them, except next door, but did you notice anything “super” about any of them?
Neither did I. Perhaps they lived in a really stable area?
The White River ecosystem
Today’s White River began flowing through this region relatively recently. Back in the days of the ignimbrite apocalypse, the region was an alluvial plain located just east of the Black Hills and the Rockies. It collected sediment as wind and water eroded the high country. (Stoffer)
Over 3 feet/1 meter of rain fell each year. (Retallack, 1992) Many rivers, all of them unnamed, meandered across the rolling plain, dropping their loads of silt, volcanic ash, and gravel along the way. Most streams and lakes were shallow during the Eocene. At that time, the plain was probably forested, with sedge meadows and scattered lakes and oxbow ponds. (Mullin and Fluegeman)
Later, as we have seen, less rain fell (Retallack, 1992) and the forest opened up into wooded savanna. Yet, as the Oligocene climate got drier and drier, ground water was never far from the surface. (Mullin and Fluegeman)
It was a good place to live . . . in between supereruptions, anyway.
Whenever the sky darkened and volcanic plumes spread overhead, water and land were covered with ash. (Mullin and Fluegeman) That’s all I can say for sure about effects of the Great Ignimbrite Flareup on the White River plain. No one apparently knows how individual supereruptions affected the region.
The problem is that the fossil record just can’t be focused closely enough to show the necessary details.
Thanks to technological breakthroughs and over a century’s worth of painstaking work here by dedicated paleontologists and other earth scientists, we can see events that happened 100,000 years apart. (Prothero and Heaton)
But if you looked at human history in 100,000-year increments, the modern era wouldn’t show up at all. All you would know about human history would be that a new species, Homo sapiens, had just left Africa after emerging there roughly 100,000 years earlier. (Johanson and Wong)
Today’s experts in ancient life can and do work wonders with these 100,000-year slices of data from so long ago, but information at this resolution doesn’t preserve details about how the White River animals survived supereruption.
Another issue is that “soft” features that are vital to understanding White River animal life and evolution – things like DNA, metabolism, feeding and reproductive behavior, birth and death rates, migration patterns, growing seasons, and the many interactions life has with its environment and with other living beings – don’t last for 30 million years or fossilize.
The stability of the White River Chronofauna, and its seeming immunity to nearby swarms of supereruptions, will remain a mystery until we know a lot more about their habitat and how they lived in it.
At first, ash layers didn’t last very long. They were slowly buried and mixed into the soil as rivers brought in more high-country sediment. Today geologists call this mixed formation the Scenic member of the White River rock group. Later on, starting at around 32 Ma with the Poleslide member, volcanic ash became much more common. (Mullin and Fluegeman; Stoffer)
Overall, according to Larson and Evanoff, an estimated 6,000 cubic miles (25,000 cubic kilometers) of ash from the ignimbrite flareup fell on the White River environment between roughly 36 and 29 Ma. (Even more ash fell after that, per Stoffer, but let’s just focus Larson and Evanoff, since their time interval overlaps with the 37 to 27 Ma White River Chronofauna heyday.)
It makes up an estimated 60 percent of the White River rocks today. (Larson and Evanoff)
At first look, that isn’t too surprising, considering the nearby ignimbrite flare. What’s a little strange, to this layperson anyway, is its source region.
Most of the White River air-fall tuffs that Larson and Evanoff sampled in modern-day Wyoming, Nebraska, and Colorado came from explosive volcanism in Utah and Nevada between approximately 36 and 30 Ma (Table 3); only one tuff was from a Colorado volcanic field.
The researchers suggest that high-level prevailing winds carried ash plumes to the White River area. They offer very good reasons why the samples couldn’t have come from ongoing supereruptions in the Southwest, Colorado (except for that one tuff), or from Mexico. (See Larson and Evanoff, pages 8-11.)
What I don’t understand – and I’m a layperson and therefore mostly clueless anyway – is why ash from those other ongoing supereruptions didn’t fall on the White River plain, too.
Here is what I understand.
According to Table 2 in Mason and others, and going by the Larson and Evanoff cutoff dates of 36 to 30 Ma, there were at least eight of Earth’s largest explosive eruptions happening in New Mexico and Trans-Pecos Texas during the same time interval as the White River tuff ash eruptions – and possibly also one supereruption in Colorado (La Jara Canyon, at Platoro), although that is uncertain.
I accept the researcher’s conclusion that none of White River tuffs they sampled came from those eruptions in New Mexico and Texas (and maybe Colorado), which each had a direct-rock-equivalent (DRE) erupted volume anywhere from roughly 450 to 1,200 cubic kilometers. (Mason and others)
But why didn’t any of the ash from those New Mexico and Texas supereruptions make it to the White River plain?
For that matter, why did prevailing winds carry the Great Basin ignimbrite to the White River plain in the first place?
Computer modeling of a much smaller but still “super” 330-cubic-kilometer DRE eruption at Yellowstone (Mastin and others) shows that the umbrella cloud of ash can move upwind and crosswind for almost a thousand miles (1,500 kilometers). This is why Yellowstone supereruptions can affect so much of the continent.
Overall wind patterns over North America during the Oligocene were the same as they are today. (Rea and others) So one can’t help but wonder why ash from the supereruptions of the Great Ignimbrite Flareup also isn’t spread out over a big part the continent, let alone on White River region.
Researchers are undoubtedly hard at work on these and many other questions. Keep the White River fauna and the Great Ignimbrite Flareup in mind when you are looking at science news.
Some very exciting discoveries in paleontology and geology await us in the future, and they may help us set up a workable plan for dealing with the next supereruption, whenever and wherever it happens.
This brief introduction to the White River fauna does provide some good anxiety relief. Life found a way through an event that was far more intense and long-lasting than the Toba supereruption 74,000 years ago. So will we, even if we’re forced to wean ourselves from the Internet briefly.
But have you noticed that there is an elephant in the room? All the time that we’re talking about supereruptions, the climate is changing.
If you look up research papers about the White River fauna, the main question is actually why this collection of animals was so stable while global climate crashed (Prothero and Heaton) and Antarctica began to freeze up for the first time in over 200 million years (Smithsonian), not how they survived the Great Flareup.
Supereruptions and climate
There were lots of supereruptions in the Eocene and Oligocene – many more, in other parts of the world, than those mentioned here. And that was when Earth’s “thermostat” broke (Lyle and others) and the greenhouse world began to ice up.
Was it cause and effect?
We’ll check that out next time.
FRONT PAGE IMAGE: Basaseachic Falls, Sierra Madre Occidental, Chihuahua, Mexico. Image by Err0neous, CC-by-2.5.
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