The biosphere has used five of its lives

kw: book reviews, nonfiction, geology, geologic history, earth history, mass extinctions

Beginning Geology students have to learn a plethora of new terms, including the geologic ages: Pre-Cambrian, Cambrian, Ordovician, Silurian, Devonian, Carboniferous (in the US, divided into Mississippian and Pennsylvanian), Permian, Triassic, Jurassic, Cretaceous, Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Recent. I remember learning a letter-mnemonic for Cambrian through Recent: COSDMPP-TJC-PEOMPPR. Over time we learned that each such age has characteristics that differ from the others, including different prevalent rock types and different fossils.

Later on we learned that the significant differences from age to age are a result of great revolutions in the kinds of living things, and the transitions are marked by large increases in the rate of extinction. Further, five of these are called "the Big 5" and "the Great Extinctions". Here is a summary of these as shown in a poster prepared by Bud Charles, in use by many web sites that discuss "the Big 5"

The "Death Rate" reported in the diagram is the number of species that were driven to extinction. The percent of the biosphere that was destroyed is a much harder quantity to determine, and I have not found any published estimates. Considering the certain damage to each species that survived any of these events (or periods; some "events" took a million years or more to unfold), it is likely that the actual reduction in the "living biosphere" was 90% or greater for each of these, and may have exceeded 99% for the biggest, the end-Permian mass extinction.

The recent book The Ends of the World: Volcanic Apocalypses, Lethal Oceans, and Our Quest to Understand Earth's Past Mass Extinctions, by Peter Brannen, is a very well-written travelogue through the geologic ages, focusing in some detail on the processes of each of the Big 5. The photo plates show scenery of the existing geology resulting from them. It would have been very beneficial to have a chart such as that above, with even more detail, because more is known now (in 2017, the date of publication), compared to just a few years earlier when the poster was produced.

It would also be useful to have a visual of where these fit in time, such as this diagram from an article in the Washington Post, reproduced here at a small size as allowed by copyright law.

The section on the right labeled "Tertiary" includes all the ages from Paleocene to Recent. The dips and wiggles show how extinctions less severe than the Big 5 have played a part in the transition from each age to the next. Also, the little dip in the middle of the Carboniferous represents a significant change in plant life that accompanied a lesser extinction event, which marks the divide between the Mississippian and Pennsylvanian ages as recognized in the US.

In this diagram, the metric is "number of families", rather than number of species. Look particularly at the end-Permian, where the number of families has dropped by about half, as compared to the number of species, which, as noted above, dropped by 95% to only 5% of its former value. The taxonomic Family is two levels broader than the Species, so if even a single species in a family survived, that is a surviving family. Nonetheless, the end-Permian event is clearly the most severe.

The kinds of things that nature can throw at Earth and its biosphere are described clearly in the book. In many cases, the primary causes may differ, but the critical factor is a great change in global temperature. In the chart above, "rapid global cooling" is mentioned twice, and "rapid global warming" is mentioned twice. In the case of the end-Cretaceous event, it is likely that almost instant world-wide broiling occurred, followed by decades or centuries of "impact winter".

Three of the Big 5 were caused, at least in part, by the eruption of flood basalts, a tame term for the sudden release of 100,000-1 million (or more) cubic miles of lava. Even the end-Cretaceous event, which destroyed all the dinosaurs (except a few that became birds) plus the flying and aquatic reptiles, which "everyone knows" was caused by an asteroid crash 65 million years ago, coincided with the eruption of the Deccan Traps ("Trap" is another word for flood basalt), in India. Enough is known about the Deccan Traps to calculate that if the lava had been evenly spread over the Continental US, it would have been 600 feet deep. That makes the scale of the Yellowstone "supervolcano" a cap pistol by comparison. As big as that was, the Siberian Traps that we think were the primary cause of the end-Permian extinction, 251 million years ago, were as much as ten times larger. The third flood basalt event left remnants in New Jersey (the Palisades) and around the East Coast of the US at the end of the Triassic about 200 million years ago. It was smaller than the Deccan event, but big enough to rate as one of the Big 5.

What can cause volcanism on such a scale? Generally speaking, the splitting of a continent will fill the bill. Prior to about 1966, the continents were thought to be stable and immobile, and all kinds of wild ideas were in vogue at various times to explain the origin of mountains. Plate tectonics was "discovered" in the early 1960's, after being proposed in 1912 by Alfred Wegener, and just when I began to study geology in earnest (1970), this new paradigm changed everything. It was an exciting time to study geology. One offshoot was the study of supercontinents. They seem to form periodically.

The mid-Atlantic Ridge is pushing that ocean's coasts apart at a rate of 2.5 cm/yr., which amounts to 25 km per million years. The Atlantic Ocean's width varies from under 5,000 km to 6,000 km or so, which implies that the breakup of a supercontinent that once included Africa, Europe, and the Americas adjacent to one another along a "seam", occurred beginning about 250 million years ago, and was completed after 200 million years ago. That 250 million-years-ago figure is suspicious. Another "seam" appears to have ruptured at the same time thousands of miles away, and apparently it was the more "active". That would be the source of the Siberian Traps. The 200-million-years-ago figure, relevant to the Equatorial Atlantic, would point the finger at the end-Triassic event that covered much of the eastern US with lava. I think that is enough to make the point.

The mid-Atlantic Ridge isn't the fastest. The Pacific Plate is presently being absorbed under Asia at a rate of 8 cm/yr or 80 km/million years. Other plates are moving in other directions, with Africa as an apparent pole of stability. Other plates and their continents show evidence of passing over hot spots in the mantle, such as the one that produced the Hawaiian Islands and the Emperor seamounts, the one currently under Reunion Island, and the one that produced Yellowstone and a chain of calderas stretching at least as far as Idaho. Africa doesn't seem to have any hot spot traces, but it is apparently beginning to rift apart near its eastern margin, in the beginnings of another cycle of continental splitting. 

The crust of the Earth undergoes cycles of continental mash-up into supercontinents, followed by fragmentation. Note that we are in an era of partial fragmentation; Asia occupies about 2/3 of all the continental area of the Earth, and is likely to gather the rest of the continents into one after another 100 million years or so. The Pacific Ocean is shrinking at a rate of about 80 km/million years, so it won't last forever!

Considering this, I wondered if the breakup of earlier supercontinents might be implicated in other big extinctions. Here is a list, summarized from several articles and other documents. Note that Pangaea spans the end-Permian and end-Triassic extinctions. I find it curious that Pannotia came together just after the Deccan Traps erupted, so perhaps continental construction is as effective as continental splitting at producing biosphere-threatening volcanism.

The earlier supercontinents occurred before the Cambrian, which means before easily-found fossils. They could not have been involved in the Big 5. But there were earlier mass extinctions; they are hard to detect, because only bacteria (and archaea) lived then. I suspect a number of Precambrian great extinctions happened. Numerous Precambrian geology specialists are busily working on it.

Atmospheric chemistry is a different driver of extinction. It may be that the biggest mass extinction of all occurred when oxygen began to accumulate in the atmosphere. Photosynthesis began about 3.5 billion years ago. There are several types of photosynthesis, and at first, only one, called "C3", found today in algae and most plants, produced oxygen directly. Other types, such as that used by green sulfur bacteria and purple sulfur bacteria, have other chemical results. For a billion years, none of the oxygen made it to the atmosphere. It was all consumed oxidizing iron and its sulfides, and other reduced materials, forming the great "red beds" and other oxide ore accumulations, for example. About 2.5 billion years ago, most reduced materials had been oxidized, and oxygen began to enter the atmosphere.

We can see from this chart, found in Wikimedia Commons, that a low level of oxygen, in the 3%-5% range, prevailed for more than a billion years, and it seems life kind of stagnated once it got used to that. This era is called the "boring billion." The upward inflection shown here at about 900 million years ago probably occurred a little earlier, maybe at 1.1 billion years. The red and green lines are limits proposed by various workers; the red line is the more probable.

Animals and plants apparently evolved just about a billion years ago, with a metabolism fueled by increasingly abundant oxygen. The big uptick occurred during the Permian, when oxygen probably exceeded 30%, compared to 20% today.

The other significant gas is carbon dioxide, CO2. This is one of numerous diagrams showing the level of CO2 over the past 600 million years. In The Ends of the World it is stated that the primary mechanism for absorbing CO2 is weathering of basaltic rock. However, it is slow, taking place on a scale of 100,000 to a million years. Based on this chart, one would expect that fresh basalt was made available in the mid-Cambrian, all during the Ordovician and Silurian ages, and in the early Carboniferous. That doesn't seem so. Rather, in particular for the Carboniferous age, the huge expansion of forests that produced nearly all the coal in the world occurred. I'll leave it at that because this is a side point.

Apparently, the dip in life's diversity in the mid-Carboniferous that I mentioned earlier happened when CO2 was drawn down to modern levels (between 200-400 ppm), at which point trees, which rely on C3 photosynthesis, have a hard time growing rapidly. At the far right, the CO2 drawdown that trended during the Tertiary (Paleocene until the present) was apparently a result of new types of green photosynthesis, called CAM and C4. Grasses, the main sort of C4 organism, and their relatives are very happy with 200 ppm of CO2 or less, while trees are happiest when CO2 is in the 1,000 ppm range. Experiments with rice plants have found that rice grows best when CO2 is 2,000 ppm.

The author makes much of the gyrations of CO2 that occurred in the past. While he is right in general, he tends to be alarmist. I almost added "polemics" to my labels for this review, but I decided not to. The book may get a little polemical, but it is much more than that, and contains a great amount of interesting and useful information about the worlds that preceded us, particularly how different each one was from what we live in now. For example, we are accustomed to reefs based on hard corals that host mollusks and fishes of many kinds. Permian reef backbones were masses of brachiopods and mollusks, with corals scattered among them, hosting more ammonites than fishes. Cambrian reefs were even more weird, made of piled-up animals that looked like individual coral polyps, lots of brachiopods (they look like cockles, but with much lower metabolism), and the main swimming things were trilobites and nautiloids, which look like straightened out ammonites (both resemble squids with big shells).

I was particularly interested in the analysis of one person cited, who said we are nowhere near a sixth great extinction. He has the numbers to prove it. So it is a little early to call the past century or two the beginning of the Anthropocene age. But we would do well to be wary. The greenhouse effect is real, and it is pretty certain that we are contributing to it. Whether it will lead to catastrophe or instead ameliorate the next ice age is yet to be seen.

How much can we heat the planet if we continue to burn petroleum and coal? As a teen I reproduced the calculations of Arrhenius, the one who first publicized the CO2-induced greenhouse effect. Later, using differential-albedo modeling (such as how sunlight heats something that absorbs visible light better than infrared, or vice versa), I verified something I read: "If we push CO2 so much that we 'close' the 'window' of its absorption bands in the infrared, the maximum warming would be 4°C". Statements about 9°C and even 12°C are not realistic, nor mathematically possible. Now, four degrees is significant. Will it melt Antarctica? Probably not.

We are also not going to push CO2 into the tens of thousands of ppm range, just by burning fossil fuels. Some look at the oxygen in the atmosphere as being entirely "carbon debt". That 20% is 200,000 ppm. CO2 weighs 37.5% more than oxygen does. If we could really extract (and burn) that much carbon from oil and coal fields, the actual mass of the atmosphere would increase 7.5%, and this new, heavier atmosphere would be 25.5% CO2, or 255,000 ppm. We would all have died from anoxia long before that point. But the total extractable carbon in the crust is only a fraction of this, a few percent. Most of the carbon from decaying "stuff" was carried into ocean trenches and is deep in the mantle, perhaps making gigatons of diamonds!

I have a quibble of a different nature: on page 189 it is stated that the asteroid that did in the dinosaurs "put a hole in the ground 20 miles deep—deep enough...to puncture the earth's mantle...". This should state "puncture to the earth's mantle", which begins around 20 miles down, and extends 1,800 miles deep.

On a happier note, the author cites Mark Richards as proposing that the asteroid impact actually triggered the formation of the Deccan Traps. A more modest level of flood vulcanism had begun before the impact, but it took off right at that time. I suggested this to one of my geology professors some 40 years ago, and he was very skeptical. At that time the "impact theory" was accepted, but the location of the crater in Yucatan wasn't yet known. I suggested a possible impact antipodal to India, somewhere in the South Pacific, perhaps 1,200 miles southwest of the Galapagos Islands. Dr. Richards does say that an antipodal arrangement would yield the best "focusing" of the seismic disturbance caused by an impact, but since the asteroid's impact was like a magnitude 11 or 12 earthquake, such focusing was not needed to trigger an ongoing eruption into a mega-event, releasing its pent-up lava over a much shorter interval.

Another significant theme is that all of the Big 5 mass extinctions were multi-factor "perfect storm" sorts of things, with the end-Permian event being the "most perfect". I'll leave that for readers to discover as they enjoy this readable and informative book.