11.3: Other 'Mass' Extinctions

Peter Brannen’s excellent book The Ends of the World recaps the Big Five, but also looks in detail at numerous other mass extinction events, including some in the Precambrian and some that have yet to occur (i.e., in Earth’s future). A few notable examples outside the traditional Big Five are briefly described below:

The Great Oxidation “Event”

Mass extinctions are relatively easy to detect when the organisms dying (and replacing them) leave behind body fossils with hard parts. The picture is much more difficult to discern when all organisms were microscopic. In the early Proterozoic Eon, Earth transitioned from an oxygen-free atmosphere to one with a decent amount of free oxygen (\(\ce{O2}\)). Though this is called the Great Oxidation “Event,” [GOE] it is not a very sudden event at all. Traditionally, it is viewed as the span from 2.4 Ga to 2.0 Ga: that’s 400 million years, a very drawn-out “event.” Other names include the Oxygen Crisis, the Oxygen Catastrophe, and the Oxygen Revolution, each of which puts its own spin on interpreting the rise of this important gas.

Figure \(\PageIndex{1}\): Deformed banded iron formation, Soudan, Minnesota. Chemical sedimentary rocks like these imply an oxygen-free atmosphere. (Callan Bentley photo)

Prior to the GOE, the planet’s atmosphere lacked significant free oxygen. We know this because of unusual chemical sedimentary rocks that could only form in an ocean that is largely oxygen free (banded iron formations [BIFs]), as well as grains of minerals like pyrite and uraninite in clastic sedimentary rocks. These signatures of the absence of oxidizing conditions imply a very different early Earth.

Figure \(\PageIndex{2}\): Detrital pyrite in conglomerate of the Steyn Reef Placer, Witwatersrand Supergroup of South Africa. (Photo by Frances Deegan; reproduced with permission.)

Those anoxic conditions are preserved in Archean strata and those of the early Proterozoic. But by the middle of the Proterozoic, the BIFs and detrital pyrites peter out, and we see the first appearance of oxidized terrestrial sediments (red beds). These are river and floodplain sediments deposited in contact with copious oxygen – enough \(\ce{O2}\) to rust their iron content through and through.

Figure \(\PageIndex{3}\): Modern stromatolites, Shark Bay, Australia. (Photo by Alicejmichel via Wikimedia)

So where did all this free oxygen come from, and what was the impact on the biosphere? Oxygen is a waste product of photosynthesis, so any photosynthetic organism produces it in proportion to the amount of glucose it generates. Of particular note are cyanobacteria, which are preserved from Archean and younger times as stromatolites. Long before true plants evolved, photosynthetic cyanobacteria was busy for billions of years pumping oxygen out into the oceans (and thence by diffusion into the atmosphere).

This oxygen, being the highly reactive element it is, immediately reacted with whatever was suitable and available: carbon, for instance, making \(\ce{CO2}\). Or silicon, making \(\ce{SiO2}\). Or maybe iron dissolved in the seawater; that would oxidize and settle out as magnetite or hematite. But the rates of production matter: if oxygen was being churned out at a rate greater than the availability of these other reactive elements to buffer its rise, then it eventually began to accumulate in the atmosphere, available for reactions, but finding insufficient reactive elements to bond with. It built up and built up.

But while life was chemically fine for cyanobacteria, surrounded by their waste. Sulfur-reducing bacteria, for instance (the ones implicated above in end-Permian euxinia) cannot survive in the presence of free oxygen; it’s a deadly poison to them. They are not alone: many groups of microbes cannot tolerate free oxygen. Some persist today in isolated low-oxygen settings such as swamp muck, but many others probably went extinct as the GOE took hold. How many exactly, and what were their characteristics? We’ll never know: microbes just don’t fossilize with the level of detail we would need to answer those questions. It seems reasonable to infer it was a great many, but we must become comfortable with the uncertainty about the specifics.

The Cambrian explosion

The Cambrian Explosion is justifiably celebrated as a great blossoming of diverse animal life forms, well recorded in the geologic record by their profusion of shells and other hard parts. But it was preceded by a time when soft bodied, sessile animals were widespread across the globe. These were the Ediacaran biota, a diverse group of organisms presumed to be animals. Some resembled jellyfish, and others looked similar to modern sea pens. Some might have been the ancestors of mollusks. But the suddenness of the Cambrian Explosion implies that it was an adaptive radiation, and therefore took place in an ecological space that had recently been cleared out of its previous inhabitants. The Ediacara had no chance against their Cambrian competitors, not only because they lacked armor, but also because the Cambrian critters destroyed their ecosystem.

Figure \(\PageIndex{4}\): Artistic rendering of Ediacaran shallow seas. (Creative Commons Attribution-Share Alike 4.0 International license.)

 

The Ediacaran period was a time of microbial mat-dominated ecospace, apparently with very few organisms exploiting higher levels in the water column, nor were there any burrowing very deeply into the sedimentary substrate below. The lack of bioturbation in Ediacaran sediments suggests it was a “sealing” ecosystem, largely anchored to the sediment/water interface, and rarely deviating from it.

Figure \(\PageIndex{5}\): The fundamentally different ecosystem structure of the Ediacaran biota as compared to the Cambrian Explosion. (Watercolor painting by Richard Bligny for an article by Jean Vannier; inspired by an original in Fedonkin et al. 2007.)

Figure \(\PageIndex{6}\): Treptichnus pedum defines the Ediacaran / Cambrian transition at the GSSP at Fortune Head, Newfoundland. (C. Bentley annotation of Martin Smith photo via Wikimedia.)

Once vascular body plans and hard parts had evolved, however, the diverse animals of the Cambrian were freed from being bound to the bottom of the sea. Some swam high above, such as the fearsome predator Anomalocaris, while many others burrowed into the sediment below. The latter group includes inarticulate brachiopods and various worms. These organisms dug into the sediment, using its bulk as protections against predation. In so doing, they churned up the sediment, disturbing the continuity of the bedding. This bioturbation is a “trace fossil” signal of the Cambrian Explosion. Indeed, the very oldest beds of the Cambrian are defined by the first appearance of the trace fossil Treptichnus pedum. (These burrows are presumed to be the probing burrows of priapulid worms, which continue to make similar traces today.)

By all means, celebrate the glories of the Cambrian Explosion, but spare a thought for the poor, doomed Ediacara that came before!

Eocene-Oligocene transition

Figure \(\PageIndex{7}\): Antarctica’s circumpolar current keeps it cold. (Modified by CB from a NASA public domain image.)

Halfway through the Cenozoic Era, Earth’s climate shifted into a cooler state. The most obvious cause for this cooling was the separation of Antarctica from southern Australia and southern South America. This divergence opened up a swath of open ocean that encircled Antarctica on all sides. A new ocean circulation pattern developed, the circum-polar current, which wrapped around Antarctica like a doughnut. These endlessly orbiting waters cooled and cooled, isolating Antarctica from the rest of the world. In as little time as 100,000 years, deep sea ocean temperatures dropped globally by 4\(^{\circ}\)-5\(^{\circ}\)C. By 30 Ma, there were new glaciers growing on the southern continent.

Figure \(\PageIndex{8}\): Schematic cross-section through the Chesapeake Bay Impact Structure, Virginia, USA. (Callan Bentley cartoon (2003) after Wylie Poag original (1997).)

Another series of events that may be relevant are at least three substantial bolide impacts in the northern hemisphere: Popigai in Siberia (35.7 \(\pm\) 0.2 Ma), Toms Canyon offshore from New Jersey (~35 Ma), and Chesapeake Bay, Virginia (35.5 \(\pm\) 0.3 Ma). As mentioned in the discussion of the end-Cretaceous impact, dust and debris may be thrown up to the stratosphere by an impact. Suspended in this rain-free portion of the atmosphere, the particles can screen out a portion of incoming solar radiation, reducing global temperatures.

Figure \(\PageIndex{9}\): Mesohippus, an ancestral horse, at the time of expanding grasslands at the Eocene/Oligocene boundary. (Public domain; Art by Heinrich Harder (1858-1935), via Wikimedia.)

This shift in the climate resulting in a turnover of organisms, both in the sea and on the land. In the oceans, spiky foraminiferida were replaced by forms with less surface area. Bivalves adapted to warm conditions died out, replaced by more cold-adapted species. On land, grasslands increased, taking over land previously covered by forest. Mammals living on land had to adapt to these cooler, drier conditions – and we see a transition from browsers (that eat leaves off bushes and trees) to grazers (that “mow” the grass) at this time. Horses, rhinos, camels, and antelopes all spread at this time.

Pleistocene megafauna

Unlike the preceding examples, the most recent mass extinction skipped sea life, skipped plant life, skipped small mammals, and skipped nocturnal animals. It just focused on large mammals and birds. The dead include giant ground sloths, mammoths and mastodons, glyptodonts, huge bears, saber-toothed cats of several varieties, and (in Australia) marsupial equivalents of many of these. There were even vultures of unusual size, who presumably fed on the corpses of these great beasts. These last presumably enjoyed a great glut for a short while, and then saw their smorgasbord go bare. Other huge birds included the phorusrhacids, the elephant bird, and the moas.

Figure \(\PageIndex{10}\): This moa was one of several dozen species that inhabited New Zealand prior to being killed off by humans. (Photo mashup of public domain imagery from Wikipedia.)

Various causes have been proposed for the extinction of these “megafauna,” from a new ice age to a meteorite impact. The ice age hypothesis is particularly weak, however, since the most recent Pleistocene glacial advance was just one in a series of dozens that stretch back in time to ~2.6 Ma. Why these animals did fine for the first several dozen but succumbed only at the most recent one is not explained. Evidence for a meteorite impact has been documented in recent years, but while it corresponds in timing with the North American extinction of the megafauna, it post-dates the Australian extinction event, and pre-dates the Madagascar extinction event.

Figure \(\PageIndex{11}\): A Woolly mammoth (Mammuthus, left) and an American mastodon (Mammut, right) facing each other, showing the physical differences between the two animals. (CC-BY; Image by Dantheman9758, via Wikimedia.)

Figure \(\PageIndex{12}\): Timing of Pleistocene megafauna extinctions relative to humanity’s arrival, organized by landmass. (Public domain; Diagram by ElinWhitneySmith (2006), after Martin (1989), via Wikimedia.)

Key to understanding the Pleistocene megafauna extinctions is the fact that the timing for the extinctions varied from continent to continent. Further, it seems to be strongly tied to the arrival of the first humans to arrive there. It happened earliest in Australia (~45,000 years ago), and most recently in the Americas (14,000 to 13,000 years ago), and more recently still in isolated islands, such as Madagascar or New Zealand. The Pleistocene “overkill” hypothesis (first articulated by Paul Martin) links the predatory habits of our own species with the loss of these giant beasts. This explanation is controversial, as some people see it as unfairly maligning indigenous people around the world.

The one place where the Pleistocene megafauna extinctions happened least was the one place where the megafauna had hundreds of thousands of years to get used to Homo sapiens: Africa. Africa is famous today for its surviving megafauna: elephants, hippos, rhinos, lions, and giraffes are the sizes and sorts of animals that used to be found on the other continents, too. This is a classic example of the adage “the exception that proves the rule” — where “the rule” is that when humans arrived in a place that was home to animals with no natural fear of humans, the humans rapidly killed the animals. Africa is “the exception,” as this is humanity’s birthplace, and the animals there have evolved in tandem with humans, developing a healthy aversion to them.

The modern biodiversity crisis

Right now, you and I may be living through another mass extinction, one brought on by humans’ severe impact on the natural world. Dubbed “the Sixth Extinction” to denote its severity as approaching the Big Five, the modern biodiversity crisis is harder to get a handle on, since we are in the middle of it. The tools we use to assess mass extinctions recorded in the deep time record of rocks don’t work for the present. So this is a trickier comparison to make. But there is a strong case that humanity’s influence has been both geological in scale and negative in effect. Some geoscientists have teamed up with environmental activists to suggest that we should consider the current moment as a new, distinct geologic epoch, the Anthropocene.

Elizabeth Kolbert’s Pulitzer-prize-winning book The Sixth Extinction is a well-written account of the various relevant factors. Uniquely among books about the current environmental crisis, it is exceptionally well grounded in the geological context. She notes numerous factors that contribute to the modern crisis:

• Habitat destruction and fragmentation
• Invasive species
• Disease
• Pollution
• Climate change

Rates of extinction documented in the modern day (using extant species and a historical timeframe) match or even exceed rates calculated from fossil turnover during the Big Five mass extinctions. However, these accelerated rates have only been operating for a geologically brief amount of time (a few centuries). The numbers are alarming, but it’s not an ideal scientific contrast, since all the confounding variables haven’t been controlled for. It’s comparing “apples to oranges.” That said, what other choice do we have? Humans cannot help the fact that they live in the present.

Figure \(\PageIndex{13}\): A comparison by Barnosky, et al. (2011) shows rates of species loss in modern groups, as compared to the “Big Five” in the fossil record. Numbers next to each icon indicate % of species. White icons indicate species ‘extinct’ + ‘extinct in the wild’ over the past 500 years. Black icons add currently ‘threatened’ species to those in white; the amphibian percentage may be as high as 43%.

In an attempt to assess the actual levels of extinction that have so far occurred and can reasonably be “blamed” on humanity, Barnosky et al. (2011) don’t actually find very many. They examined species documented by the main international body the focuses on conservation biology, the IUCN. (This is the group that maintains the “red list” of endangered species.) How many of those species have gone extinct in the past 500 years? How does that compare with extinction rates determined from fossils in stratified rocks at each of the “Big Five?” Thankfully, they show that there is a pretty big discrepancy between modern numbers and ancient numbers. But the question of time hangs over this comparison – 500 years is almost nothing in the geological sense of time. It is hard to compare a modern rate of 1 species per 500 years with an ancient rate of 75% of all species per 100,000 years. The timescales are too radically discrepant.

If the modern rates play out for another 500 years (or 1000), will they cross the threshold of being a mass extinction? If the answer is yes, and we wait until then to decide it’s actually a big problem, then it would be too late. Asking the question now allows us to take action to prevent a sixth mass extinction from becoming a reality.

The last life on Earth

Far in the future, perhaps 2.5 billion years from now (maybe sooner), life on Earth will cease. This will be the final mass extinction our planet is capable of pulling off. Two sets of processes need to be considered, those of Earth’s interior and those of our star, the Sun. Plate tectonics is responsible for continuously revitalizing Earth’s surface, but it is driven by heat from the planet’s interior. Heat production is maintained through time by radioactive decay of unstable parent isotopes, such as \(\ce{^{40}K}\), \(\ce{^{232}Th}\), \(\ce{^{87}Rb}\), and \(\ce{^{238}U}\). But there are fewer and fewer of these isotopes every day. Our planet is cooling off, and eventually it may get too cool to maintain plate motions. Estimates of this sad date put it about 1.45 billion years in our future. A cooler core means less liquid outer core through time, as the solid inner core grows larger. Eventually, when the core becomes completely solid, the magnetic geodynamo of the outer core will cease, and our planet will lose its atmosphere-protecting magnetic field. This exposes the atmosphere (and the surface hydrosphere) to erosion by the solar wind. Life won’t last much longer in those conditions – Earth would increasingly come to resemble Mars.

Figure \(\PageIndex{14}\): The Sun is forecast to grow in volume in the far distant future. This does not bode well for life on Earth. (C. Bentley modification of an original by Oona Räisänen, via Wikimedia.)

Regardless of these “local” details, the fate of our planet ultimately hinges on the future the Sun. The Sun is a G-type star, and those grow in volume through time as they burn through their hydrogen fuel. Ironically, as they lose mass, they swell up to a larger size. Less mass means less gravity holding them in a tight, dense ball; older stars have relaxed and stretched out into space.

In our solar system, that will mean that Mercury and Venus are likely fated to be consumed by the Sun as it expands outward. Earth’s location is likely about where the surface of the Sun will reach in ~5 billion years when it becomes a elderly red giant, but a reduction in the Sun’s mass also means a reduction in the Sun’s gravitational hold on Earth, and so the planet will likely shift into a more distal orbit, further out from where we currently orbit. Nonetheless, the larger, closer Sun will fry the Earth, evaporating away its oceans and sterilizing its surface.

Franck, et al. (2006) made estimates of how Earth’s temperature will respond to this swollen star, and compared these estimates of its future temperature to the tolerances seen in modern examples of various domains and phyla. Multicellular life is estimated to have 800 million years left, according to that study, or possibly up to 1.2 billion years, while single-celled eukaryotes will last longer, perhaps 1.3 to 1.5 billion more years. Prokaryotes are hardiest of all at high temperatures; it is thought they will persist until about 1.6 billion years from now.

And that will just about do it for life on Earth.