"Nature was quick to pass the sponge of her deluges over these awkward sketches [i.e., dinosaurs and other Mesozoic megafauna], these first nightmares of Life. And yet, what curious prints might have been made of all these creatures! Alas, the vision is lost forever." -- Auguste comte Villiers de L'Isle-Adam, L'Ève future (1886)
"Night comes to the Cretaceous." -- J.L. Powell, 1998
BIG QUESTION: What caused the Cretaceous-Paleogene extinction and the end of the Age of Dinosaurs?
(By the way, this event is by no means the largest of mass extinctions: we'll see that one next lecture! It was also not the final extinction of the dinosaurs, for as we'll see later in this course Dinosauria is still alive and flying!)
This event goes by many names:
Some of the major victims and survivors of the event:
In the marine realm, among the main victims were:
In contrast, there does not seem to be too much in terms of extinction among smaller bottom-dwelling organisms.
In the terrestrial/continental realm, major victims include:
In the terrestrial realm there is a transition from a gymnosperm-dominated flora to an angiosperm (flowering plant)-dominated one. The toothless crown-group birds survived, as did the living groups of amphibians, turtles, lepidosaurs, and crocodylians. The long-snouted champsosaurs (distant kin to the archosaurs) survived and thrived in the early Cenozoic, but have subsequently died out.
Some Older Ideas
Many hypotheses proposed for the K/Pg Extinction. In evaluating the hypotheses, must consider:
Here are but some older proposed causes for the K/Pg event:
Modern Approaches to the Cretaceous-Paleocene Extinction
The global nature of the K/Pg extinction would seem to favor some causal agent which could affect the whole planet. Cosmic (extraterrestrial) phenomena might be a good possibility.
1971: Suggestion by Dale Russell (dinosaur paleontologist) and Wallace Tucker (astrophysicist): a supernova killed the dinosaurs.
Supernovae are exploding stars: put out TREMENDOUS amount of energy. If a star in a nearby solar system exploded, it would bombard surface of planet with radiation, bringing radiation sickness, cancer, etc.
Modern analogue: during 1950s through 1970s, greatest fear about nuclear war was radioactive fallout.
Fits prediction. However, problem because it is an untestable (and thus non-falsifiable) hypothesis:
So, remains as a potential but no reason should be supported. Was the leading candidate during the 1970s.
Confirmed Potential K/Pg Causal Agents
While the above phenomena are largely insufficient to explain the event, there are three confirmed large-scale environmental changes during the end of the Maastrichtian that potentially are involved with the extinction. In order of appearance (and of increasing severity) they are:
The Maastrichtian is last Age of Late Cretaceous Epoch; regression refers in this context to any period of sea-level drop. In this particular case, it was triggered by reduction in mid-ocean ridge activity. As the mid ocean ridges shrank, the water that they had displaced onto the continents throughout the Late Cretaceous drained away.
For the marine realm, this caused a very large drop in the area for the shallow marine communities. Additionally, it removed a major source for marine productivity and modified global circulation patterns. For the terrestrial realm it made climates more continental (hotter summers, colder winters) because now not every place was essentially close to the sea shore. This would also mean that habitats would shift.
The Maastrichtian regression unquestionably occurred (although as with all such changes, there were local and global smaller scale sea level rises mixed in with the pattern of the drop.) One would predict that the environmental effect of the Maastrichtian regression would play out over the period of a few million years.
Deccan Traps Volcanism:
The Maastrichtian has long been known as a period of intense volcanism in parts of the world (see "Global Diastrophism" above). For instance, in North America, associated with change in mountain building in Rockies (the beginnings of the Laramide Orogeny). But the biggest aspect of this volcanism is the Deccan Traps.
The Deccan Traps were a GIGANTIC series of lava flows in western India. They were the most major flood basalt event since the Siberian Traps at the Permo-Triassic boundary and the Central Atlantic Magmatic Province volcanism that formed by the break up of Pangaea at the Triassic-Jurassic boundary. In some places the Deccan Traps are 2.4 km (1.44 MILES) thick. An area of 2 million km3 was covered. As with all flood basalts, it was not a single continuous event; instead, there would be eruptions; periods of cooling from sulfates; global warming from excess greenhouse gases; stabilization of the environment after the eruption had ended; then a new eruption.
The Deccan Traps began around half a million years before the K/Pg boundary. The lowermost beds erupted during a global magnetic normal time (subchron C30n), while the impact of the asteroid and the boundary itself would be later in a magnetic reversed interval (subchron C29r). Since the switch from C30n to C29r happened 350 kyr prior to the boundary, the asteroid impact could NOT have been the cause of the Deccan Traps. That said, there is some evidence that a few thousand years after the impact the intensity of the eruptions increased: this timing would be consistent with magma chambers being disrupted by impact-generated mega-earthquakes, and then percolating slowly up to the surface.
As with the Siberian Traps and CAMP, the Deccan Traps would provide both a cooling and a warming component. There is a 5-6°C increase in annual temperature in western North America during the pre-impact eruptive phase, that might reflect the volcanic greenhouse contributions. Furthermore, we would expect ocean acidification as a side effect.
So, Deccan Traps themselves were a MAJOR event, and might have contributed to the extinction event. Had just this eruption plus the Maastrichtian regression occurred, there probably would have been a mass extinction. But perhaps it would have been much less severe, and not an "era-ending" one. But Nature had one more, far more spectacular event to unleash.
The Chicxulub Impact:
The discovery of an asteroid impact at the end of the Cretaceous began in the latest 1970s and was published in 1980. Geologist Walter Alvarez was investigating a layer of clay in Gubbio, Italy at the K/Pg boundary. He wanted to determine length of time represented by the clay layer. He knew that the limestone below was latest Maatrichtian and those above were earliest Danian (early Paleogene). But how much time was there for the clay layer? A few years? Decades? Millennia? More? (The biostratigraphy above and below showed it couldn't be more than a million years.) Being clay he couldn't radiometrically date it, and there is no magnetic flip-flop right near the boundary. So he consulted his dad (Nobel winning physicist Luis Alvarez) for possible solution. After teaming up with a few chemists, the decided to look for meteoritic material as a possible clock. This was based on the following observations:
The element used: iridium (a platinum-like metal, common in metallic asteroids but very rare in Earth's crust).
When examined Gubbio clay, found a huge increase in iridium (the iridium spike) at base of clay. Using their method it would mean that there was a few million years missing, which was clearly not the case. So which assumption was not correct? They eventually realized that this wasn't a case of an average rate of infall. Instead, they hypothesized that an asteroid impacted Earth at the K/Pg boundary. Luis Alvarez had been part of the US nuclear weapons program, and later had worked with NASA on calculating the energy of impacts from asteroids and comets. Both of these sets of skills proved useful in determining the effect of the end-Cretaceous impactor
Although there had been a very good chance that the impact had hit oceanic crust that had since been subducted back into the mantle, the crater was actually recognized in 1990s. It is in the Yucatán Peninsula of Mexico. It is not visible from the surface except by radar scanning, because it is covered by 300-1000 m of Cenozoic sediments. But cores drilled during petroleum exploration had found a disrupted and melted layer right around the K/Pg boundary; with the publishing of Alvarez paper and subsequent search for evidence to test their hypothesis, these cores were re-examined and re-interpreted as being from a crater. Seismic and gravity scanning revealed the presence of a 180-km diameter crater, exactly the right size for hypothesized impactor. There are even sinkholes (cenotes) in the rock above that trace out the disrupted layer, because groundwater in the limestone preferentially drains down into the crustal fractures. The crater is named Chicxulub after a town near the first core where the disruption was recognized.
Between the initial Alvarez paper and the discovery of the crater, several proxies of the impact beyond the iridium spike were recognized. These include:
The map of the K/Pg sites and the thickness of the disrupted layers form a bullseye around Chicxulub, showing that this rather than some other spot was the source of the blast.
Effects of Chicxulub
An asteroid impact is a very different kind of causal agent, and its effects are more instantaneous than the other types of mass extinction causes we've explored. (Some of the effects, however, are extended over long periods.) Here is what we have reconstructed so far.
Phase I (the same day): Shockwave and Tsunami: A magnitude 10.1 earthquake would be expected from this blast. In the modern world this would likely topple buildings all over the planet; in a Cretaceous world it might be very disruptive to forests and cause landslides (and make dinosaurs fall over...), but wouldn't be a primary killing agent. A shockwave of hurricane-force winds would spread over southern North America and northern South America would also be locally dangerous, and the energy flash from the impact would incinerate everything in line-of-sight, but again these are not mass extinction-causers. A tsunami of 100-250 m would surge out, but again while regionally disruptive would not bring an era to an end.
Phase II (later the same day): "Easy Bake Oven" and the Canopy Collapse: However, other events of that first day would be globally catastrophic. Infalling material would mostly burn up in the atmosphere. Your average meteor doesn't put out much heat, but so much infalling material generates substantial infrared radiation. This heat raises air temperature by only about 10C° (18F°), but would be fully absorbed by rock, leaf, flesh, and any other opaque material. It is predicted that the increase in infrared radiation would be 8-10x that of high noon at the hottest spot of the Earth, and persist for many minutes to hours. Living tissue would bake, unless underground 10 or more cm (heat wouldn't have time to make it that deeper) or underwater (upper few microns of water might boil off, but that would be it). I have nicknamed this the "Easy Bake Oven" effect, and may be the reason that no land animal larger than 5 kg seems to have survived.
Related to this, some of the material WOULD make it down to surface, and seems to have sparked off global forest fires. This is called the canopy collapse, and is shown by an increase of soot and charcoal and by an increase of fern spores (the fern spike in sediments post-impact. (Ferns are excellent at recovering from periods of forest fires.)
Phase III (the first decade or so): Impact Winter: Material vaporized by the impact is kicked up into the stratosphere, blocking insolation. This was the primary killing agent suggested by the Alvarez team. Probes on Mars show big temperature drops when fine particles are spread to the high atmosphere. And in human history, the eruption of Tambora in Indonesia in 1815 produced chilling effects worldwide for more than a year later; later eruptions, such as the 1991 eruption of Mount Pinatubo, while not as severe, were better studied and analyzed.
Estimates of duration of the Impact Winter have varied from a year or so to a few months to just a few weeks, but a model published in January 2017 puts a duration for a 26°C (46.8°F) temperature drop in global surface temperature for 3-16 years and a greater than 30 year duration until recovery! Recovery on land goes much more quickly (basically once the skies are clear), but some deep-water sites show the cooling required 10 kyr to recover.
Phase IV (100 kyr or so): Greenhouse Warming: The impact site was covered by carbonate rocks; when you oxidize carbonate rocks, you release CO2 (this is the same as when limestone is converted into cement). The Deccan Traps already released enough CO2 to raise the atmosphere from ~500 to 1400 ppm; the new addition brought it up to 2300 ppm or so. Once the dust was cleared the full effect of the greenhouse gases could be in play. Recent studies show a warming of 5C° in the shallow ocean for 100 kyr; air temperature warming might be between 4.3 and 13.5C°C (average of 7.5°C). See the discussions of the P/Tr and the PETM a few lectures ago about the effects of this on the living world.
General Patterns of the K/Pg Impact
In the marine realm, plankton (including the larval ammonoids) and nekton suffer worse than benthos. Photosynthesizers, and the creatures that feed on them directly (and those that fed on THEM directly) suffer worse than bottom feeders (which eat food "stored" in sediment). Groups with symbiotic algae also suffer strongly. Among marine vertebrates, larger ones in the open seas suffer worse than those on the coast, and those with higher metabolic rates worse than those with lower ones.
(Note: the basic pattern is the exact opposite of the P/Tr, where the benthos got clobbered relative to nektonic and planktonic forms.)
In the terrestrial realm, freshwater animals and those that feed on the water ecosystem tended to do better than those which fed on land. Larger animals and medium-sized animals with high metabolic rates suffer worse than small animals and medium-sized animals with low metabolic rates.
For the terrestrial realm, it seems that "Easy Bake Oven" is the primary selective filter, with the Impact Winter taking out some survivors and the Greenhouse Summer even more. For the marine realm the "Easy Bake Oven" is likely not a factor, but the Impact Winter is the dominant selective force.
The pattern of extinction looks like it was global and essentially instantaneous: hours to days to months to a few years. This suggests that the Chicxulub impact is by far the major causal agent. But all three events (Chicxulub impact, Deccan Traps volcanism, Maastrichtian Regression) are known to occur, so the earlier scenarios may have destabilized the ecosystems to some degree.
Lessons Learned from the K/Pg Mass Extinction
Throughout the course we've asked (and will continue to ask) the question "What Good is the Fossil Record?" We previously saw that the fossil record gives us insight as to how climates form, and how the rapid introduction of greenhouse gases can disrupt global ecosystems (as in the PETM). We've seen that the fossil record is our only evidence of mass extinctions: the reality that when environments change too quickly and intensely that global ecosystems can collapse, taking millions of years to recover.
But the K/Pg mass extinction taught scientists two major lessons, often learned and studied by the same teams who initially examined the K/Pg event itself:
So understanding the world-ending catastrophes of the ancient world can help us plan to prevent them in the future.