December 31. The 6th Extinction

So we’ve made it to the end of our geological year, covering 4.6 billion years of geologic time. 366 episodes, close to 180,000 words and a total of about 25 or 30 hours of programs. I hope you’ve enjoyed it! The podcast will not end, but the structure will change as we go into next year. It won’t be daily any more, sorry to say – there were times when it was really touch and go in terms of me getting the episodes out, but I’m happy to say I managed. Thanks for your interest – that was my main motivation once things got going. 

I’m not certain exactly what sorts of topics next year will bring. I’m not going to try to cover “current events” in any particular way because there are many good blogs and podcasts that do that. I expect I may do a few more that use my own work as a basis, and some posts will probably be based on topics in my other book, What Things Are Made Of. I want to try to do a few interviews with geoscientists working on interesting topics, and that may give it a Montana-centric flavor, but we’ll try to make things pertinent to a wide audience. Since I only covered 366 topics – and they were selected largely based on my own prejudices – there’s certainly plenty more that we can look at. Feel free to submit questions or suggestions, either through the blog or email me at rigibson at earthlink.net. You’ll get a spamblocker message, but I’ll find your email. I’m not going to make any promises, but I am going to shoot for at least one episode every week or ten days next year. 

I also will be assembling the existing podcasts into single recordings. I’m not sure how it will work in terms of file sizes, but I’m hoping that I can make each month of the previous series into one or two packages – without the repetition of the intro music and exit tagline. I’ll try to edit the episodes into those assemblies early in the coming year and make them available in the usual way, through the blog. 

To close the year, I thought I might address what has been called the Sixth Extinction. Of all the mass extinctions in earth history, only five have been really, really devastating. Those are the ones at the end of the Ordovician, in the Late Devonian, the biggest of all at the end of the Permian, the Triassic-Jurassic extinction, and the one that ended the Cretaceous Period and the Mesozoic Era. The case has been made that we are presently in the midst of another mass extinction, the Sixth great one.

There is an entire book, titled The Sixth Extinction: An Unnatural History, by Elizabeth Kolbert. It just came out in 2014, and I recommend it highly. The New York Times Book Review listed it as one of the ten best books of 2014. The book makes the case for a present-day massive die-off of organisms, ranging from bugs and bats to corals and rhinos. We’ve seen in the podcasts in this series that things like climate change certainly affect extinction rates, and there is no question that earth’s climate is changing at high rates right now. There’s also no question that human activities are affecting many of the things that contribute to climate change. It’s happening.

Kolbert integrates our knowledge of past extinctions, such as the disappearance of the ammonites, which you have heard about in this podcast series, as lessons for the present.

We don’t really know how many species of plants and animals exist on earth today, although we have good ball-park estimates. New species, even new large animals, are being found all the time. So it’s hard to say with certainty what kind of extinction rate is in progress, but some estimates say that as far as we can tell, extinction rates today are as much as 1,000 times those that typified most of earth’s history. We might argue about whether what’s happening now is on a scale comparable to the Big Five Extinctions, but at some scale, an extinction event is assuredly in progress.

The big, charismatic animals, like rhinos, elephants, tigers, and whales, that need extensive spaces for their lifestyles, are probably most threatened by human pollution and invasion of habitat, but who knows what’s going on in the insect world or the frog world? Those who study frogs are concerned. Is it possible to help all, or even most, species survive in a human-dominated world? I certainly don’t know. Should we even try? From both the altruistic and selfish points of view, it would probably be advantageous to try. Even human-centric people can’t deny the continual discoveries of beneficial products that come from obscure plants and animals.

For me as a geologist, I sometimes take the long view – the earth does not care. If humans kill off lots of things, including perhaps themselves, earth, and life, will continue. It always has, and until the sun burns out or some incredible catastrophe happens, it always will.

Thanks for joining me on this journey. If you’ve learned a tenth of what I have learned in putting these talks together, you’ve learned a lot! I hope it was fun!

—Richard I. Gibson

LINKS:
Extinction rates

Animals that went extinct in 2014

December 25. Quaternary Megafauna Extinctions

Mammoths, mastodons, saber-toothed cats, American horses and camels, the giant bear, the Irish elk, Glyptodonts, which were giant relatives of the armadillo, the Megatherium, an elephant-sized ground sloth, giant kangaroos, a 7-foot-long beaver, and dozens of other large animals and animal groups, collectively called megafauna, died off during the Quaternary Period of the Cenozoic Era, many of them at about the time of glacial retreat, 10,000 to 12,000 years ago at the end of the Pleistocene Epoch. Some of them were relatively new to planet earth, and some species had been around for millions of years. 

Don’t visualize this extinction as an instant in time, or really even a short period of a few hundred years. It’s true that there seems to be a spike in extinctions right at the time the glacial period was ending, 10 or 12 thousand years ago, but many animals, including for example the giant terror birds of South America, were becoming extinct at about the beginning of the Pleistocene glaciation, 2.5 million years ago.   

Mammoth (left), American mastodon (right) Image by Dantheman9758, used under Creative Commons license. 

And how do we know something was completely extinct? We can only go by the fossil record, and fossils of modern large animals are rare. Even for a particular species, extinction can take thousands of years, winding down to the last mastodon, or passenger pigeon, or whatever. Mammoths got started about five million years ago, in the Pliocene, and were mostly gone at about the end of the glacial period, 12,000 years ago. But small populations survived on isolated Arctic islands such as St. Paul, off Alaska, and Wrangel, off Siberia, as recently as 3700 years ago. Different species in different niches disappeared sooner or later, depending on circumstances. The pygmy mammoth probably survived on the Channel Islands of California until about 11,000 years ago.

So, why did the mammoths, giant sloths, and all the rest die? The first obvious explanation is the dramatically changing climate. Glacial environments certainly constrain food supplies and habitat, and other habitats, even in temperate zones, change. That could account for some of the extinctions early in the Pleistocene, 2 to 2½ million years ago. But why would so many go extinct at the end of the glaciation? Well, change is change, and warmer might not necessarily always be better for all species; forests supplanted grasslands in the north, perhaps denying mammoths their main food sources. It might be a simple matter of inability to adapt to the changes. Once a population becomes small, many things – disease, a big storm, a flood – could have been the final blow that ended a species that may have been in decline for some time.

The second primary idea for the extinction of the Quaternary megafauna is still somewhat controversial, and involves hunting of the animals by predators – specifically, early humans. There’s no question that humans hunted and ate some of these animals – or hunted them to remove them as threats. Homo erectus was killing mammoths a million and a half years ago. The question really is, were humans the primary, or even a major, cause of the extinctions.

There’s pretty good correlation between the arrival and expansion of humans in various areas with the demise of the large mammals. The correlation in time and space alone does not mean the one caused the other. Arguments against this hunting hypothesis include the idea that the small populations of humans would not likely have been enough to eradicate millions of animals. And while the correlations are best in the Americas and Australia, the extinctions take place at about the same time in Eurasia, where these animals had been exposed to human threats for far longer.

The bottom line is that the ultimate cause of extinction of so many large mammals is uncertain. Humans may have had a role, either large or small, and climate factors are likely to have been important, and perhaps even controlling in the extinctions. When you take multiple climatic factors into account, including temperature changes, precipitation patterns, vegetation and habitat changes, maybe that’s enough to explain most of the extinctions, and humans or disease or something else were simply the exclamation point at the end. In such complex systems as the earth, it’s almost impossible to point to any single thing as THE cause, and as usual, the most likely answer is a combination of many factors. We really don’t know.

—Richard I. Gibson

Giant mammals
Mastodon extinction 
Extinctions affected survivors (e.g. coyote) 
Pleistocene beasts (an excellent blog)
Debunking the impact idea 

Image by Dantheman9758, used under Creative Commons license.   

December 3. Recovery from extinction

To begin today I want to mention a new podcast I’ve just found out about, called Evolution Talk. It just began in August 2014 and as the title suggests, it’s all about evolution, including Darwin and other scientists that formulated the theory. It’s produced by Rick Coste on a weekly schedule, and I recommend it to you. It’s on iTunes, and I have a link on my blog to it as well. You can find information at EvolutionTalk.

Today’s topic is the recovery from the extinction at the end of the Cretaceous. Some of the great extinctions in earth history clearly impacted the planet for a long time. The Permian extinction in particular, together with the single-continent nature of Pangaea, seems to have resulted in an early Triassic that was pretty antagonistic for life for a few million years, at least. 

Life seems to have recovered from the end-Cretaceous event relatively quickly, which intuitively could be seen to support the idea of a very specific event, the asteroid impact, as the primary cause. 

One study led by researchers at MIT even suggests that the recovery of oceanic algae and cyanobacteria was underway as soon as just 100 years after the impact. Since those microorganisms are the base of the food chain, their recovery would have paved the way for the expansion of other life as well, but if you look at the recovery in other ways, it may have taken more time, even much more, a typical million years or so. 

The argument has been made that survivors were animals that could forage on dead plant or animal matter – which was presumably in abundance – rather than those that fed on living plants. A study of the interlinked recovery of plants and insects, led by researchers at Penn State, found that plant diversity was very low for 800,000 years after the extinction event, and for 8 million years, generally, plants and insects remained relatively low in diversity. But there were some exceptions where plants and bugs thrived, quite early in the Paleocene, the epoch immediately after the extinction. That kind of makes sense to me, that there might have been refuges, or locations where the devastation was not so intense, that could maintain or re-develop their diversity relatively quickly.

However long the recovery took, it certainly did happen. As the phytoplankton in the oceans recovered, so did the surviving bony fishes. On land in the course of the 10 million years after the extinction, the Paleocene epoch, mammals and birds both diversified and became larger than their Cretaceous ancestors. It would appear that being small was no longer such an advantage, with the big predatory dinosaurs gone. By the end of the Paleocene, 56 million years ago, and into the following Eocene Epoch, some flightless birds were growing to six feet in height. Mammals mostly were still small compared to later in the Cenozoic, but there were several different mammals that reached a meter or more in length. I think it is clear that they were getting bigger, and we’ll talk about some of the mammals and birds of the Cenozoic later this month.

I have links below to reports on the nature of the recovery from the end-Cretaceous extinction.

—Richard I. Gibson

LINKS:
Chaotic recovery?

Fish diversify
Rapid recovery? 
Recovery intervals
Recovery dynamics 
Paleocene mammals 

November 30. It’s the end of the world as we know it….

The end of the Cretaceous – and the end of the Mesozoic Era – were marked by a mass extinction, one of the largest in earth history. Unlike some of the earlier extinctions, we pretty much have a smoking gun for this one: the impact of an asteroid in what is now northwestern Yucatan, Mexico. 

Not that it was an easy sell. Since I was a grown-up geologist in the early 1980s when the team led by Luis Alvarez and his son Walter began to suggest the impact as the cause of the extinction, I remember some of the controversy. It was a radical idea; any kind of catastrophe flew in the face of the long-accepted idea of uniformitarianism, that changes on earth resulted from the application of slow, steady changes over long, long times. I’m really not sure why the idea that catastrophes DID happen was so hard to accept, but it was. The Alvarezes were ridiculed in some professional circles. But accumulating evidence really pretty rapidly, in just a few years, convinced I would say all but a few geoscientists of the validity of the impact as at least a major factor in the extinction. 

Gravity map of Chixulub crater (source: NASA)

The evidence includes a layer around the world right at the top of the Cretaceous, coinciding with the extinction, of iridium, an element that’s abundant in asteroids. Shocked quartz grains are present as well, and result from impacts. Rocks of the right age in Texas and elsewhere in the Gulf of Mexico and Caribbean region look like gigantic tsunami deposits. And finally, magnetic data being analyzed by geophysicist Glen Penfield for oil exploration in Yucatan defined a likely crater on the northwestern coast of the peninsula. It took a while for the age of the Chicxulub impact crater to be determined accurately, but now it is considered to have happened at 66.04 million years ago. It is within 33,000 years of the best estimate for the extinction event, so practically, the two are simultaneous. The Chixulub crater shows up nicely in gravity data too, and even today the cenotes, natural sinkholes scattered across Yucatan, follow the crater margin even though it is deep in the subsurface.

The potential effects of such an impact can be estimated from the size of the crater and the impact angle. Vast quantities of dust, ash, and aerosols likely went into the atmosphere, creating toxicity as well as probably a nuclear winter; it’s been speculated that most of North America burned in an incendiary forest fire across the continent.

The effects resulted in the destruction of about 75% of all species on earth. The most famous victims are the non-avian dinosaurs, all of which perished. All the ammonites died. The giant marine reptiles and the pterosaurs were gone. Pretty much every group, including things like mammals and birds that survived as a class, lost many members. Everything from plankton to plants to snakes and turtles suffered. Surviving birds and mammals did undergo a dramatic diversification and radiation soon after the extinction – a typical reaction, as the survivors adapt and expand into niches vacated by other animals.

While it’s pretty much agreed that the impact was a major factor in the extinction that ended the Mesozoic, there’s still debate about whether it was the only factor. Other things were going on that might have had roles. We’ve talked several times in this series about massive volcanism as a possible factor in extinctions, especially the Siberian flood basalts at the end of the Permian. From about 68 to 66 million years ago in India, similar flood basalts erupted to create the Deccan Traps. Before the asteroid impact was known, the Deccan was usually seen as the gradual, 2-million-year cause of the extinction. There are some declines in diversity among several groups, including some dinosaurs, that may be related to an ongoing effect of the Deccan. The asteroid might have simply been an exclamation point on an already-in-progress extinction. Besides the Deccan volcanics, there were other ongoing climate changes that could have had a role in the general decline and ultimate extinction.

There was also a sharp regression or sea-level drop during the Maastrictian, the last epoch of the Cretaceous. It’s not clear what might have caused that fall in sea level, but other extinctions have been tied pretty clearly to such activity.

So the impact is pretty well accepted as the killer. It’s possible that there were actually multiple impacts. Craters in various parts of the world, from the Ukraine to the North Sea to India are dated to within a few million years of the Cretaceous extinction. There’s a well-known crater in Iowa, the Manson Crater, but it is now dated to about 74 million years ago, too early to have much effect 8 million years later.      

—Richard I. Gibson

The K-T extinction (Richard Cowan) 
Long-term climate stress caused extinction 
Single impact killed dinosaurs

Chixulub gravity map from NASA (public domain)

September 30. End Triassic extinction

It’s the end of the Triassic, and it’s another mass extinction. It was one of the 5 largest extinction events, and some measures make it the second worst, after the event at the end of the Permian. About half the species living at the time disappeared, including a third of the marine genera and many land animals, from amphibians and reptiles to the ancestors of mammals. The conodonts, the little eel-like animals whose teeth are found throughout the geologic record from the Cambrian onward, survived the Permian extinction, but the group was totally eradicated by this one at the end of the Triassic. 

Unlike some mass extinctions, which seem to have spanned quite a period of time, a million years or more, this one appears to have had a sudden onset, and to have lasted for as few as 10,000 years – just an instant in geologic time.

And it’s an event that has no really clear cause. Over the course of these podcasts you’ve heard many causes for extinctions, from voluminous volcanism to glaciation and other climate change, changing sea levels and atmospheric chemistry to meteorite impacts. All those are possibilities for the end-Triassic event, but except for an impact, they are gradual processes, generally, and are challenging to use to explain the suddenness and short duration of the extinction.

There’s no evidence for a large impact crater at this time, and there’s no geochemical evidence such as an iridium layer, either.

Pangaea showing areas of volcanism near the end of the Triassic.
Map by Williamborg.

There was a period of massive volcanic eruptions at this time, just about 201 million years ago. As Pangaea began to break up and the Atlantic Ocean began to form between eastern North America and northwest Africa, the crust broke. You heard about the precursors to that break-up when we talked about the Newark Grabens a few days ago. As the rifting progressed, vast lava flows erupted to produce the Central Atlantic Magmatic Province (CAMP). Today the remnants of those flows and related intrusions are found in Morocco and throughout West Africa, in younger parts of the Newark Supergroup in eastern United States, from Newfoundland to Georgia, as well as Spain and Brazil.

There isn’t any doubt that they are related to the break-up of Pangaea, but there are some issues with the timing. It’s pretty close to the end-Triassic extinction, and might have been a factor. But why the very sudden onset of extinction?  These igneous rocks span about 600,000 years over a million or more years total. One idea is that one specific volcanic event created a tipping point, a volume of CO2 or acidic aerosols that was devastating to life on a global scale. Another is that greenhouse conditions that might have resulted from volcanic eruptions could have warmed the earth so that methane hydrates, methane frozen into water ice within the sea floor, could have melted. If it were released catastrophically, that might account for the suddenness of the extinctions. There are problems with that, too, in that the volcanic rocks closest in age to the extinctions are often separated from the extinction layers by sedimentary rocks indicating many hundreds of thousands of years. In many ways, this all comes down to the accuracy of age dating, which does have error bars for dates 200 million years ago.

I think this extinction is the least understood of all the Big Five extinctions. The Atlantic volcanism seems to be the best candidate as the cause, one way or another, but the jury is still out. One thing is clear, though. While dinosaurs were definitely impacted by the event, it was the dinosaurs that survived that were able to exploit the niches vacated by other forms of life. Tomorrow, when the Jurassic begins, we will really begin to see the Age of Dinosaurs.

* * *

Robert Folk was born September 30, 1925, in Cleveland, Ohio. He is a professor emeritus at the University of Texas where he taught for many years. He’s probably most noted for establishing a detailed classification system for carbonate rocks, but he’s also worked in the diverse fields of archaeological geology and on bacteria in sediments, research that has had implications for the study of structures in Martian rocks.

—Richard I. Gibson

CAMP website

Map by Williamborg, used under Creative Commons license. 

September 9. Recovery from extinction

The extinction at the end of the Permian decimated marine invertebrates, with 90 to 95% of all species disappearing. The ammonites, the shelled cephalopods related to octopuses and squids and the modern chambered nautilus, were almost wiped out completely, but the few survivors – perhaps just a single lineage – adapted and increased in diversity almost explosively during the Triassic, but underwent another decline late in the period.  

Encrinus, Triassic crinoid from Germany. Photo by Ghedoghedo under GFDL. 

Rocks from the first few million years of the Triassic, right after the extinction, contain few marine invertebrate fossils, as you might expect. Some of the groups that had been prominent during the Paleozoic, including brachiopods and crinoids, never really rebounded from the extinction, although both survive to this day in small numbers and without much biodiversity.

Corals and clams seem to have adapted pretty well to the new conditions and their recovery from the extinction was accomplished within about 10 million years or so – but during the early Triassic, there may have been strong variations in the carbon cycle, affecting carbon dioxide in the atmosphere as well as in sea water, and that would have had a big impact on things like corals and clams that secrete calcium carbonate. Link (carbon cycle) Despite this, a new important branch of corals called the scleractinians evolved during the Triassic. They are also called stony corals and they represent the modern reef-building corals we have today. They probably evolved from relatives of the rugose corals that lived in the sea from Ordovician to Permian time, but rugose corals didn’t survive the end Permian extinction.

There are no known coal beds from the early Triassic, so much so that it’s called the global coal gap. Probably the best theory to explain this is the idea that land plants in enough abundance to make coal were severely affected by the Permian extinction, and they did not really recover for at least 6 and more like 20 million years – at least that’s how long it took for coal to appear in the rock record again in much volume. They would have had to evolve adaptations to the new conditions, and that takes time – probably more time for relatively long-lived things like trees than for short-lived ammonites. A further explanation for why it took so long for trees to recover is the suggestion that temperatures were “lethally hot” during the early Triassic, at least in tropical zones, maybe even too hot for trees to survive. The high temperatures may have extended to tropical sea water, with one report suggesting surface sea temperatures of 40°C, 15 degrees higher than today. That could certainly have affected sea life.  I have links below to several technical papers that address these issues if you are interested in more detail.

—Richard I. Gibson

Links:
Coal Gap 

Lethally hot temperatures (Sun et al. 2012) 

Early Triassic ocean life (Bottjer 2012)

Encrinus, Triassic crinoid from Germany. Photo by Ghedoghedo under GFDL. 

August 31. The Great Dying

We’ve reached the end of the Permian Period, and the end of the Paleozoic Era, which we’ve been plodding through since the first of February. This ending is marked by the most devastating extinction event in the entire history of the earth. It’s the end of the world as we knew it, and lots of species don’t feel fine.  

Most estimates say that at least 90%, maybe 95%, of all marine species and 70% of all land animals did not survive this mass extinction. Trilobites, fusulinids, blastoids, rugose corals, and eurypterids were gone for good. Every other marine group was decimated – bryozoa, crinoids, ammonites, brachiopods were all hard hit. On land, most of the large amphibians disappeared, and even the large trees that made forests were gone. Even insects were impacted. And most of the early therapsids, the mammal-like animals including the dimetrodons, also disappeared, although some of their descendents survived.

Source: Wikipedia

Before we look at what may have caused the extinction, let’s look at what happened. In the oceans, water became seriously depleted in oxygen. The anoxia alone would have affected many organisms. We know about this because of abundant organic-rich shales, deposited in conditions where oxygen was not around to decompose the debris. Pyrite, iron sulfide, in those shales suggests a lot of sulfur, which in turn implies acidification of ocean waters. Overall, temperatures rose dramatically at the end of the Permian.

So what caused all that? After the “smoking gun” of a catastrophic impact was identified and finally accepted as the event that put an end to the dinosaurs, at the end of the Cretaceous Period, it became popular to look for impact events to relate to other extinctions. There are some at about the end of the Permian, notably the Bedout feature offshore northwestern Australia. Its age is essentially identical to the end-Permian extinction, but it’s not entirely accepted as an impact structure. Also, there’s geophysical evidence in Wilkes Land, Antarctica, that can be interpreted as a really big crater. It got a lot of attention in the popular press a few years ago as the potential cause of the Permian extinction. But it’s under the ice – we have no rocks from it – and its age is pinned down to only 100 million to 500 million years ago – hardly a close match for the Permian extinction. Also, there’s no conclusive geochemical evidence, such as the famous iridium layer associated with the end-Cretaceous impact, to support a huge Permian impact. I really think that for now, impact hypotheses for this event have no weight.

Much more interesting is the close correlation in time with the vast Siberian basalt flows that we talked about yesterday. Such volcanism could put acid aerosols into the atmosphere – consider Laki, the volcano that erupted in Iceland in 1783, whose aerosols put poisonous fogs as far away as France. Volcanoes also introduce ash into the atmosphere, which might create a brief nuclear winter, a quick cool-down, but they also inject carbon dioxide into the atmosphere, which would soon lead to greenhouse conditions and the warming that we observe. Warmer conditions could have led to melting of methane hydrates in the sea floor, and releasing methane would add to the greenhouse effect.

Earlier this year, the popular press was inundated with headlines like “Bug farts nearly killed us” – a reference to an idea that methane-generating organisms, archaea, microorganisms that were once considered to be bacteria, were the culprits. They are now classified at the same level as the kingdoms of plants and animals. The idea is that these critters evolved a new way of growth, just about at the end of the Permian, that led to exponential reproduction and an associated generation of huge amounts of methane, producing the greenhouse conditions and acidification and anoxic waters that we know were present. There are some aspects of carbon isotope ratios that this idea explains better than the concept of volcanism as the cause of the extinction, but the jury is still out on this, I think.

I think that the Siberian basalt flows probably have the strongest support as the cause for the Permian extinction, but even that isn’t really a smoking gun. At least not yet.

As with so much in geology – and this is what keeps it interesting – it’s not completely certain that this was even one “event.” You’ve heard me refer to mountain-building “events” that lasted tens of millions of years. Geologists tend to play pretty loosely with our time scales, even though we recognize the value of accurate age dating. Often, we just don’t have it. So consequently there’s some debate about the nature of the Permian extinction. There is really no doubt that the final blow, at about 251 million years ago, was very much a “spike” in terms of geologic time. But such a spike could represent most of a million years, or as few as 200,000 years, and some scientists suggest that most of the Great Dying took place within a span as short as 20,000 years – and that really would be essentially instantaneously, geologically speaking. The other view would have the final blow coming as the culmination of a 15-million-year period of climate change that was punctuated by other more short-lived episodes. There isn’t really any doubt that there was another specific event near the end of the Middle Permian, at about 260 million years ago – we mentioned that the other day in connection with the last trilobites – and there seems to be a decent correlation with it and a glacial pulse that lowered sea level. At this point we could almost call that a “conventional” cause for extinction. But was it an isolated event or was it, and the big one at the end, all part of some more general set of changes? We don’t really know, but obviously everything from glacial advances to aerosols from volcanoes to ocean circulation patterns caused by Pangaea’s size and shape and position – all those things could be part of the cause for the greatest extinction the world has ever seen.

As you can imagine there is a vast amount of work that has been done on this topic, so please look at this as just the barest summary of possibilities. 

* * *

On a far smaller scale than the Permian extinction, on August 31, 1886, a strong earthquake struck a very unexpected place – Charleston, South Carolina. The magnitude is estimated at 7.3 – huge by any estimation. It was felt as far away as Boston, where the shaking rang church bells. Almost every building in Charleston was severely damaged or destroyed, and at least 60 people were killed. The area had a long history of low-level seismic activity, just as it does today. But why is there any seismicity? It’s far from plate margins and volcanic zones. It’s really challenging to explain these earthquakes that occur far from areas of active fault motion, but I think the current explanation for the Charleston quake is that there was significant faulting in this area when the Atlantic Ocean opened, and that the 1886 quake was a rejuvenation of that faulting related to the ongoing pull-apart of the Atlantic. Right now in our calendar, we’re at the end of the Permian, and South Carolina is still firmly attached to Africa. It didn’t begin to break apart until the Jurassic, which we’ll get to in October.

Tomorrow, the Triassic Period and the Mesozoic Era begin.

—Richard I. Gibson

References:
Graphic from Wikipedia under GFDL 

Palaeos mass extinctions 

Methane-generating bugs

May 31. End Devonian extinctions

We’ve talked this month about the abundance of life in the Devonian – the spreading of plants across the land, and even the first forests. Both diversity and abundance resulted in oil and natural gas source systems that add up to one of the largest in all of geologic history.  It all came to a rather screeching halt with two major extinction events toward the end of the Devonian. 

The first occurred before the end of the period, at the boundary between the last two subdivisions of Devonian time, the Frasnian and Fammenian stages, at about 374 million years ago. A second event took place at the end of the Devonian, about 359 million years ago.

Together these two events wiped out something like 75% or more of all species – some estimates say as much as 87%. Depending on how you look at it – diversity, abundance, marine, non-marine, the end Devonian extinctions might be second to the end Permian extinction, but in any case, the end of the Devonian is one of the “big five” mass extinctions in earth history.

It’s quite clear that the extinctions were associated closely with anoxic periods in the world’s oceans – times when the oxygen content of the water decreased drastically. That gives us a starting point for trying to determine the causes.

One possibility is climate change. You may recall that western Gondwana was situated over the south pole during much of the Devonian, but there’s not much evidence for glaciation until late in the period. A Late Devonian glacial epoch would have decreased sea level, and as we’ve heard in previous extinctions, that reduces shallow water niches for life as well as cooling the average water temperature. Life that lives in warm, shallow, well-oxygenated water suffers.

Another possibility is that all the life on land – plants including trees, for the most part – would have resulted in a dramatic increase in the volume of nutrients washing from the land into rivers and ultimately into the sea. Enough nutrients could have led to immense algal blooms that could have produced eutrophication – stagnation – in shallow or restricted seas.

Plants on land would have accelerated chemical weathering of rocks as they form soil. By burying organic matter, it would have been a form of carbon sequestration, and reducing carbon dioxide in the atmosphere would have reduced the greenhouse effect and resulted in cooling. That in turn might be tied to the glaciation, or the glaciation might have begun for some other reason such as changes in the earth’s orbit, but it might have been accelerated by the removal of CO2 from the atmosphere by land plants.

There’s some speculation that an impact or impacts from an asteroid or other bolide might have triggered, or at least contributed to, the climatic effects that resulted in extinctions, but even though there are a couple known impacts at about the right time, they really don’t seem to have been big enough to have had the dramatic effects that are observed in the fossil record. Nor would meteoric impacts have necessarily produced the global oceanic anoxia that we know was present and the timing isn’t really right, either.

The anoxic conditions contributed to the organic-rich black shales we’ve talked about this month, so to an extent, the extinctions that led to trapping of organic matter in the rocks helped generate some important oil source rocks.

All the phyla that we have today survived the extinctions in the Late Devonian, but several sub-groups did not. The arthrodires, the huge predatory fish, went extinct, and so did the primitive armored fish, placoderms and ostracoderms. Corals suffered badly and while most groups survived, abundance decreased so that reef-building pretty much ceased and there were no substantial reef ecosystems for the next 100 million years. 

The late Devonian extinctions seem to have clearly come in at least two distinct pulses, each one lasting a million years or so, but as more information has become available, it is also looking like the “events” might have been the culminations of a longer period of crisis for life that spanned as much as 15 million years. To my mind, it’s much easier to explain things like this when they are gradual, since changes like the oceans becoming anoxic and the atmosphere losing carbon dioxide would be expected to take time. With some exceptions, of course. Catastrophes CAN happen. But it seems to me that the end Devonian extinctions probably result from changes that took a lot more time than something like a cosmic impact, and that they came about from multiple, interconnected causes.

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On this day, May 31, 1970, an earthquake in Peru killed more than 67,000 people, perhaps as many as 100,000. It was a subsea quake just off the northern coast of Peru, but there was no tsunami. The death toll was mostly the result of a landslide and avalanche in the mountains that took ice, rock, water, and debris 11 miles down a river valley to bury several towns beneath as much as 80 million cubic meters of material. The quake was related to the subduction of the Nazca Plate – part of the oceanic crust beneath the Pacific Ocean – and the South American continental plate. It was part of the process that continues to build the Andes Mountains. 

—Richard I. Gibson

Link:
British Natural History Museum

April 27. Extinction events

Yesterday I mentioned in passing some relatively small extinction events during the Silurian. There were at least three, which more or less mark the boundaries between the four epochs of the Silurian. The first one, at the boundary between the Llanderovy and Wenlock epochs, was pretty intense at some locations – more than half the trilobite species disappeared at a location in Sweden, and worldwide, 80% of conodont species died off. Conodonts are generally deeper-water animals, as are graptolites, which also suffered a lot in that 200,000-year event. Shallow water life, like corals, had little impact. Why is that? I don’t know. 

The Mulde Event, about 427 million years ago, between the Wenlock and Ludlow Epochs, coincided with a global drop in sea level. What caused the sea-level drop is not known. The last extinction event also did a number on conodonts but had little effect on graptolites. Go figure. But the graptolites were decimated shortly after the sea-level change that marks this extinction, at a time when sea-water isotopes changed significantly. I hope it is evident that these things are complicated – most extinctions are way more complex, and less well understood, than the end of the dinosaurs.

One interesting aspect of these little extinctions is that we actually see an increase in stromatolites during the extinction periods. You remember stromatolites – buildups of calcareous material by algal mats. The theory seems to be that when animals that ate algae were decimated, it allowed for an expansion of the algal mats. But when life rebounded after the extinctions, the stromatolites declined.

—Richard I. Gibson

March 31. End of the Ordovician mass extinction

It’s the end of March, and the end of the Ordovician Period. If you’ve been following along, you know that this time, 443 million years ago, is marked by a huge mass extinction – by some estimates, second only to the Great Dying at the end of the Permian in terms of its decimation of life on earth.

More than 100 families of marine animals went extinct. Families are groups of genera which are groups of species – so this was a LOT of diverse fauna. More than half the species of bryozoans, which had just appeared early in the Ordovician, were destroyed. Trilobites, brachiopods, and nautiloids were severely impacted. Something like 60% of all marine species died.

Ordovician life

As near as we can tell, it seems that the extinction event was worse among animals that lived in shallow tropical seas, which supports the idea that the glaciation going on at the same time was an important factor. Cooler conditions, cooler water, fewer tropical animals. Also, when water is tied up in ice on land, sea levels drop, so the shallow seas that predominated over much of North America retreated. There were many fewer niches for life to occupy. And in fact there are two phases to the extinction, which seem to be related to these two different but related causes.

We talked about some of the factors that may have led to the glacial period. The position of the supercontinent of Gondwana over the south pole, the possibility that life on land was removing CO2 from the atmosphere, reducing greenhouse conditions, and the cooling effect of some of the greatest volcanic episodes known in earth history.

There isn’t much doubt that whatever caused the glaciation, the consequences for life were pretty dire, and generally, it’s the cooler conditions and lower sea level that probably did the killing of marine life. All of the phyla of life survived and came back after the glacial period, in the Silurian that starts tomorrow.

I think it’s important to note that this is the only glacial period that is closely connected with a mass extinction, unless the modern glacial period has been a factor in the fairly recent destruction of a great many species. The jury is still out on whether or not it was climate change or human activity that eliminated things like mastodons, giant sloths, and saber-toothed tigers.

The end Ordovician glacial epoch was relatively short-lived, spanning just a few million years, and the extinction event at 443.4 million years ago is very much a sharp spike in terms of species loss. I think it’s fairly well accepted that cooling and loss of shallow-water niches were the ultimate primary causes of the extinction, and those things were caused by the glaciation. There are lots of potential factors that might have caused the glaciation, as we discussed March 28.

A recent look at gamma-ray bursts in the Milky Way Galaxy suggests the possibility that such an event might have been a factor in at least one extinction in the past 500 or so million years, so it is a possible cause for the Ordovician extinction. 

Tomorrow, the Silurian begins.

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March 31, 1850, is the birthday of Charles Doolittle Walcott, in New York Mills, New York. Walcott was the geologist who discovered and described the Burgess Shale in British Columbia back in 1909. You can hear more about Walcott in the podcast for February 12.

—Richard I. Gibson

Photo by Ryan Somma under Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0)  

February 28. Cambrian-Ordovician extinction

The beginnings and endings of the subdivisions of geologic time are usually well recorded in the rocks. Many of them are major changes in the life of the time, as indicated by fossils. And many of those changes are mass extinction events.

Extinctions occur probably almost continuously, but there’s clear evidence in the fossil record for relatively short time spans when the rate of extinctions ramped up dramatically, killing many more species than usual. These mass extinctions punctuate the geologic record. You’re probably familiar with the mass extinction at the end of the Cretaceous period, 65 million years ago, when most of the dinosaurs died, and the even greater event at the end of the Permian period when about 96% of all marine species vanished.

But there were two or maybe four mass extinctions during the Cambrian period that were probably worse than any later events except the Permian one. The last of these events was about 488 million years ago, and is taken to mark the end of the Cambrian.

Many of the marine animals that we described in the Cambrian explosion of life died. Brachiopods and trilobites especially saw a serious reduction in the number of species, and this is clearly recorded in the fossil record.

What caused it? Until quite recently we haven’t been able to point to smoking guns, explicit causes for mass extinctions. You’re undoubtedly familiar with the idea of an asteroid impact causing the end Cretaceous extinction. Other extinctions are not so clear cut.

There is evidence for increased glaciation at about the start of Ordovician time, and that’s been cited as a possible cause or factor in the mass extinction. Besides colder temperatures, glaciation lowers sea level by locking water up in ice, so there would have been fewer of the popular shallow water niches for trilobites and such to live in. Cooler water is also less able to hold oxygen, so oxygen depletion is also cited as a possible factor in the end-Cambrian extinction.

Bottom line: we have some reasonable well thought-out ideas for causes of the Cambrian mass extinctions. But we really don’t know. UPDATE: New dating evidence ties one of the middle to late Cambrian mass extinctions, the one at 510-511 million years ago, to volcanic eruptions in Australia. Here's the link.

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Today, February 28, 1743, is the birth date of René-Just Haüy, at St-Just in Picardy, France. Haüy was a mineralogist, often called the Father of Crystallography. He studied the regular way minerals break apart, a property called the mineral’s cleavage, and applied mathematical approaches to crystal forms, anticipating the much later understanding of molecular crystal structure. He was imprisoned during the French Revolution, but survived, and under Napoleon became a professor of mineralogy. He died in 1822. 

—Richard I. Gibson

Image from Wikipedia under GNU free documentation license