Key Concepts: The fossil record shows the existence of mass extinctions: short intervals of geologic time when a sizable fraction of Earth's diversity is lost, taking millions of years to replace. Five mass extinctions stand out against the background level. These events were generated by various agents (ice ages, erosion, volcanism, asteroids), and killed by various causes (eutrophication, ocean acidification, anoxia, hypercapnia, darkness & starvation; pulses of heat; etc.). However, one basic pattern stands out: mass extinctions (and smaller crises, such as the Paleocene-Eocene Thermal Maximum) are produced when rate and intensity of environmental change exceeds Life's capacity to evolve or migrate.

Mass Extinctions
Last week we saw how the world view of Lyell (where major disruptions in the fossil and rock record must have been caused by erosion and missing time) and Cuvier (where such disruptions indicated catastrophes) were in conflict. We actually introduced you to the tools necessary to sort out these differences, but didn't highlight them. In particular, the ability to use radiometric dates eventually allowed stratigraphers to calculate the age of the units below and above the turnovers, and see how far apart in time they were. It was discovered that in at least this context, Cuvier and the catastrophists were correct: these event were rapid, on the scale of million years or less (sometimes many times less).

In the 20th Century came the recognition that these were indeed mass extinctions. Ordinary extinctions seem to happen throughout earth history: in fact, they form what is called "background extinction". But statistical analysis of the appearance and disappearance of forms through time indicated that there were a small number of events in which the loss of species was greatly magnified. Not surprisingly, these fell near or at important geological time boundaries.

In background extinctions the species dying out were often replaced by other species (sometimes close relatives, or even descendants which outcompeted them) which had the same ecological role. But mass extinctions were different: the ecological niches lost sometimes were empty for millions (or tens of millions) of years afterwards.

In order to qualify as a mass extinction, the event had to see:

Paleontologists Jack Sepkoski and Dave Raup did pioneering work in the 1970s assessing rates of extinction in the marine realm over time, and identified the "Big 5" mass extinction events (discussed below). They did recognize that there were some smaller scale disruptions in the history of life as well.

Agents & Mechanisms
Something that is important to distinguish is between the cause (or trigger) behind an extinction event, and the means by which the extinction is actually accomplished.

In particular, we should recognize:

In order to identify a reasonable causal agent for any given extinction event, we must first recognize that it would in fact be able to produce killing agents in both the terrestrial and marine realms. It also must not be overkill (after all, EVERY mass extinction event has survivors, and every organism alive today had ancestors that lived through every single mass extinction event so far!). And finally (and most important for this to be more than mere speculation), any proposed causal agent must be identifiable from the rock record independent of the extinction event itself.

The Big Five
Ordovician/Silurian Mass Extinction (443.8 Ma [millions of years ago]): The oldest well-understood mass extinction. Occurs (possibly as a two phase pulse) as a byproduct of intense glaciation and drop of habitat range. The least understood of the Big Five at present. When the occurred, there were very few land organisms.

Devonian/Carboniferous Mass Extinction (358.9 Ma): Also less well-studied than the later three. Saw a collapse of a hugely-diverse reef community (even more widespread than today's coral reefs, and at least as diverse in terms of biodiversity). Saw the loss of many invertebrate groups, and also many important fish groups. As with the O/S extinction, the D/C may have been in two pulses.

Life had only colonized land during the Devonian: the first vascular plants and the first trees date back to this Period. Indeed, it appears as this this was the significant factor in the extinction. As plants began to move away from the water's edge they formed the first deep soils. Soils store some of the carbon from the plant matter, rather than releasing it back into the atmosphere: this burial of carbon brought carbon dioxide levels lower, and thus reduced the greenhouse effect, allowing the world to cool.

However, the more important aspect was the presence of abundant new nutrients (broken down plant matter and rocks broken up by the action of plants) washing into the shallow seas. This produced eutrophication (a problem we see today, too!). This is when the nutrients from land fertilize the shallow water, leading to a bloom in phytoplankton. When the phytoplankton dies, it settles to the sea floor and decays: that process exhausts the dissolved oxygen in the water, causing "dead zones" of anoxia (non-oxygen conditions). Today we see such dead zones offshore where there is a a lot of farming or development (and thus lots of excess nutrients in the water), and today such dead zones and cloudy water are two of the main threats to coral reefs (a third being coral bleaching due to warming seas, and a fourth being ocean acidification: more about this below).

Permo-Triassic Mass Extinction (252.17 Ma): The "Mother of all Mass Extinctions". If this is a single event, between 95-96% of species in the marine realm (and 70% of terrestrial species) died out. (And consider that a species can persist with just a couple of individuals, so we are looking at the death of >99.99% of the biosphere). Produced a MAJOR overall of marine communities: the single biggest change in the biosphere. The end of the Permian brought an end to the Paleozoic Era.

The primary trigger is the Siberian Traps, a massive series of eruption of lava in Siberia. This lava field covered an area of about 5 x 106 km² and represented a volume of 3 x 106 km³: if you took this amount and spread it evenly over North America, it would cover us to a depth of 121 m (almost 400 feet)!! All this material was erupted in less than 1 Myr time (possibly much less). But the lava itself isn't the main effect: it is the greenhouse gases (carbon dioxide and methane) that was released. Estimates show 12-18,000 Gt C were emitted: in comparison, the modern atmosphere has a mere 800 Gt C and the value before the Industrial Revolution was about 600, so all the global warming from human activity so far is a mere 200 Gt C.

The net result was a catastrophic rise in atmospheric CO2, leading to extreme global warming. This warmed the ocean floor, where methane clathrates (ice with methane trapped within it) was melted, releasing the methane into the atmosphere. Methane (CH4) is an even stronger greenhouse gas than carbon dioxide, so it added to the this warming.(There is even evidence that the magma which produced the eruption ignited buried coal fields in Siberia, so some of the carbon dioxide was from coal rather than the lava!)

Additionally, the oceans had extreme ocean acidification, greatly damaging to a wide variety of sea life. The mass loss of plants on land and algae at sea resulted in a tremendous drop in atmospheric and oceanic oxygen, producing anoxic effects. (A secondary contributor to anoxia was the raised global temperature: with the world pole-to-pole hot, there was little temperature differential, which reduced wind, which reduced the oceanic circulation, allowing the seas to stagnate.) Isotopic evidence shows that the oceans of the earliest Triassic were lethally hot in the tropics (about 40°C rather than today's 30°C; warm water holds very little oxygen.)

And there is evidence of bursts of hydrogen sulfide out of the ocean: this is poisonous to land and sea life, and damages the ozone layer, allowing harmful UV radiation to stream in. The sulfur from the eruptions and the ocean produced catastrophic acid rain levels.

Triassic-Jurassic Mass Extinction (201.3 Ma): Aka "Permo-Triassic, version 2.0". The environmental changes were from a very similar cause: a huge eruption of lava called the Central Atlantic Magmatic Province (CAMP), produced when North America + Eurasia ripped away from Africa + South America, splitting the supercontinent Pangaea and opening the Central Atlantic Basin. It clobbered life on land and in the sea, and opened up the terrestrial realm for dinosaur dominance. The environmental effects of CAMP were essentially the same as for the Siberian Traps, on a (just slightly) smaller scale.

Cretaceous-Paleogene Mass Extinction (66.0 Ma): Not the largest mass extinction, but easily the most famous. In older stratigraphic nomenclature, called the "Cretaceous-Tertiary (or K/T) Extinction". Essentially all land animals >5 kg died out (including all dinosaurs other than modern-style birds); in the marine realm, major extinctions of plankton and the part of the food chain that fed directly through plankton (rather than bottom-feeders).

There is a huge volcanic event associated with the Cretaceous-Paleogene, the Deccan Traps of western India. At least 2 x 106 km³ were erupted, producing effects similar to those of the Permo-Triassic and Triassic-Jurassic. The eruption began slightly more than 250 kyr before the Cretaceous-Paleogene boundary event. There is some evidence of global warming produced by the Deccan Traps in the latest Cretaceous fossil record.

However, the main event was first identified from chemical traces at Gubbio, Italy: the impact of a ~10 km diameter asteroid in the Yucatán Peninsula of Mexico. This produced a crater with a diameter of about 180 km (long since buried, but detectable by drilling, seismic scanner, etc.) The energy released was equivalent to 1.8 x 108 Mt (megatons) of TNT: this is about 1000-10,000 times the destructive power of the entire planetary nuclear arsenal at the height of the Cold War! This blasted out vast amounts of material high into the atmosphere, some of which rained down right away, and the rest of which stayed up for months.

The infalling debris burned up in reentry, causing a thermal pulse out of the skies with heat levels 8-10 times that at noon at the equator. Every spot on the Earth would have been baked as with a heat lamp for minutes to hours. The soot that stayed in the atmosphere blocked incoming solar radiation for weeks or months, chilling the surface of planet. And when the atmosphere cleared, the elevated carbon dioxide levels from the Deccan Traps and from the vaporization of carbonate rocks at the crater produced a global warming event.

The Paleocene-Eocene Thermal Maximum (PETM)
The Paleocene-Eocene Thermal Maximum (PETM) was a very short term global warming event 56.0 Ma in which for a geologically-short period of time (<10,000 years) the level of CO2 jumped to 3-4x the previous background level. (This jump is comparable to the upper end of anthropogenic greenhouse gas models, but from a higher starting point.) PETM saw a temperature increase of +4-5K at the tropics, +6-8K at the poles, and even +4-5 K in the deep sea.

Some consequences of this event:

The likely cause of PETM was due to a massive release of methane clathrates from the sea floor. (This matches the isotopic signature of the carbon increase.) In turn, this is likely due to a sudden burst of submarine volcanism in the northeastern Atlantic (between Greenland and Europe).

Mass Extinction & Other Crises as the "Game of Thrones"
Why do we have mass extinctions at certain times in Earth History? It isn't necessarily that the environment changes, because they change all the time. The main reason is the intensity and rate of the change: Thus, the shared cause for mass extinctions and other crises is: environmental change too severe in intensity to allow adaptation.