Apocalypse in Chicxulub

It seems that the dramatic and sudden extinction of the dinosaurs, which occurred 65 million years ago, was in large part the result of a meteorite or comet impact. According to a theory put forward by the geologist Walter Alvarez and others, an object 10km or so in diameter impacted the Earth at this time. If a meteorite, it would have been travelling at around 20 km a second; if a comet, perhaps as fast as 80 km per second. It is perhaps more likely that it was a meteorite, because the unusual elements found in sediments from that time - osmium and iridium - are, according to Michael Allaby and James Lovelock, less likely to be present in comets.

[The following 2 paragraphs need a bit of updating]

The impact theory is certainly an appealing one; but it still has many critics, notably those who suggest that massive volcanic outpourings in western India may account for the extinctions. There are many people who have spent far more time considering these matters than I have, so if you want to find out the truth of the matter for yourself, I would advise you to check out the two books on the subject which I review here and here, Dewey McClean's page - which takes the volcanic point of view - , and other volcanic theory literature. The fact which most impresses me is the most immediately striking one: the presence of a 65-million-year-old, 310-km-wide impact crater at the Chicxulub site on the Yucatan Peninsula - a crater which implies a pretty huge catastrophe.

This fact (and perhaps others, for which you should consult the literature) shifts the balance in favour of the impact theory; but much as I would like to be able to make a decision one way or the other, it may not be appropriate to do so. Although I like the impact theory, there may be something in the arguments that the dinosaurs were in decline anyway, and that the impact delivered the coup de grace.

Think of the difference between what happens when you place a large stone on sloshy mud and what happens when you drop that stone onto the mud from a height of, say, 50 feet. Now keep making that stone travel faster and faster, and imagine how the mud splatters. Now make the stone bigger and bigger, and the bystanders will be a tad peeved. If you continue in this vein, the bystanders will soon be incapable of annoyance, due to death. Now lash Everest and K2 together, take them outside the Earth's atmosphere, and hurl them at the mud. Your arm is so immensely strong that the 2,500-billion-ton object travels its own height in much less than a second, hurtling towards the Earth with ferocious energy. Although you may have second thoughts about what you have done, you will most likely not be as worried as you should be; as Alvarez himself notes, our experience of falling objects and impacts is in many respects not a useful guide when it comes to events of the scale in question. The KT boundary impact was a calamity beyond imagining*.

Let’s make a slow-motion observation of the few seconds surrounding the impact of 65 million years ago. The object hurtles towards Chicxulub on the Yucatan peninsula in Mexico (and bordering the Gulf of Mexico), at about 60 times the speed of sound and 20 times the speed of a high-velocity rifle bullet. It passes through most of the atmosphere in a couple of (real-time) seconds; the air between it and the Earth’s surface is squashed sideways at huge velocities, creating winds that are orders of magnitude off the Beaufort scale. This generates a sustained roar, perhaps the loudest ever to traverse the planet. Most of the air, however, does not have enough time to escape sideways. It is therefore massively compressed, squashed into an area thousandths of its original size. It becomes hugely dense and hot - four or five times hotter than the surface of the Sun. Atoms are crushed together, and their outer layers stripped off; in other words, a cloud of plasma is formed, boiling furiously as the meteorite forces it towards the Earth's surface.

The object impacts the Earth. The violence of the impact is equivalent to the detonation of 100 trillion tons of TNT, and to an average of around 10 Nagasaki bombs expoded on every square kilometer on Earth. 65 million years into the future, Reagan and Gorbachev between them would not command 1/1,000^th of the destructive power¹. If the meteorite had approached the atmosphere at a large angle, all this energy would have been spent in the atmosphere, and the effects would have been immensely more devastating; it has been suggested that the atmosphere itself would have been blown away.

¹ Allaby & Lovelock remark that to "compare such an explosion to that of the largest existing H bomb might be like comparing the eruption of Mount St. Helens with the firing of a child's cap gun"

Half a second after your trussed-together mountains hit the ground, their back end has vanished beneath the surface, and is still travelling. The Earth literally shakes - around twenty minutes later, the other side of the Earth shudders in an hour-long earthquake. Rocks in the immediate area simply boil away to vapour. Farther away, they melt. Farther away still, they fly apart. Plasma and blazing ejecta tear through the atmosphere, forming a mushroom cloud whose stem measures perhaps over 200 kilometres across. From orbit, they slowly rain over the spinning globe. The sky rages with millions of fragments vaporising on re-entry. At the impact zone, limestone is compressed so much, so quickly, that billions of chemical bonds between two of its constituents - carbon dioxide and calcium - are shattered. Even as the atmosphere charges back into the hole ripped by the ejecta, the fissure in the sky is exploded once more by an outrushing ball of carbon dioxide. As the shockwave leaves the impact zone to spend its vast energies elsewhere in the Earth’s crust, the underlying mantle rebounds, hurling up a lofty peak in the centre of the tumultuous impact crater. The peak is too high; its sides promptly collapse downwards and outwards, forming concentric ripples. Near the impact zone, entire forests are vaporised by the shock; farther away, they are merely ignited by the incandescent air, and blazed to grey powder. Oxygen is devoured by the flames traversing the planet. A colossal tsunami, perhaps 1000 metres high, roars out across the Gulf of Mexico at hundreds of kilometres an hour. Its keel gouges the sea floor, turning the wave dark and spiky with debris. When it reaches the shore, the continental margin shudders under the impact. The remnants reverberate back and forth through the Caribbean. The savage tremors create tsunamis in all the world's oceans, which devastate every exposed coast in the world. The tsunamis arriving from near the impact site are hundreds or perhaps even thousands of metres high by the time they hit continental shores - they travel at a fearsome 700 kilometres per hour.

A ruinous deluge of nitric acid issued from the sky, having similar effects to a far-reaching acid rain*. "The result may well have been desert - a virtual absence of vegetation that would persist until acid-loving species colonized it"². Photosynthesizing plants were devastated as the sun's rays were shut out for perhaps as long as three months, and as much of the remaining light was blocked by the dust which coated their leaves. Later still, the climatic see-saw swung again; as the dust gradually drifted down to earth, and the sun became able to penetrate once more, all the newly-ejected carbon dioxide and water vapour in the atmosphere trapped the sun’s heat, on a far greater scale than the greenhouse effect currently being fostered by many of our businesses. The Earth became insufferably hot; the unnatural heat did not abate for centuries.

* Acid rain is actually not associated with rain; a more accurate name would be 'acid dust'. After the K/T impact, though, the acid was carried by rain.

Unsurprisingly, much of life on Earth was unable to survive these events; about three quarters of all species were made extinct.

But you are here, reading this; many lifeforms* did survive. What was it about your distant ancestors that enabled them and others to pull off the feat of surviving the calamity?

They needed to be able to to survive the shock wave, the massive fires, the tidal waves, the acid rain, the darkness, the food shortages, and the heat. To survive the shock wave, fires, tidal waves, and (to a lesser extent) acid rain, sheer distance from the impact site would have been a major benefit. In general, it would also have been preferable to be located to the east of Chicxulub; the Earth’s spin meant that much of the ejecta landed to the west. A subterranean or (non-Caribbean!) deep-water existence would have provided some refuge from the storm of fire; but the noisome acid would most likely have permeated deep into the sea.

* I will mainly focus on animals, both because more has been written about them and because, I must admit, I generally find them more interesting (or should that be 'more accessible'? I love watching time-lapse photography of plants).

Photosynthetic organisms were the most immediately vulnerable to the darkness. In the sea, for example, phytoplankton (tiny surface-dwelling creatures) would have died with the sunlight, and with them the entire food chain which relied on them. The result was the collapse of several food chains, with dire repercussions for many of the large animal species. Many land plants survived the extinction; some species would have been able to survive the catastrophe because the germination of their seeds or sporess would not take place until the sun broke through once more. However, if most of a species’ spores had germinated at the time of the impact, or if dormant seeds could not survive nitric acid poisoning, it would have been at great risk. Many seeds would have germinated at the time of the carbon-dioxide-inspired hot spell, and subsequently been parched. Live plants would have been devastated, and few but the hardiest can have survived the fires, the lack of sun, and the prolonged heat. Among the better-equipped for this task may have been the kind of plants which actually depend on fires for their propagation; when fires kill all around them, they promptly reproduce, rapidly exploiting the the lack of competition and the abundance of organic debris.

There does not appear to be evidence that the Great Darkness was accompanied by a corresponding Great Chill; perhaps surprisingly, it appears that being 'cold-blooded' may actually have been an advantage at this time, since a cold-blooded animal only needs to eat about a 10^th as much as a warm-blooded animal of the same size. Allaby & Lovelock make the interesting point that ". . . those reptiles that were [cold-blooded] would have fared much better, and perhaps that is why all most modern reptiles are cold-blooded." They go on to suggest that because heat regulation is a major problem for any large animal, it is likely that many of the larger dinosaurs would have been warm-blooded, and by implication less capable of pulling through the famine which the impact caused. To my knowledge, though, studies of the type of internal structure associated with cold- and warm-bloodedness have, if anything, suggested that the dinosaurs studied were probably cold-blooded. Information welcome.

On land, only animals weighing less than about 25 kilograms survived. I can think of two reasons for this. First is the simple fact that small animals are far more numerous. Secondly, they don't need to eat as much. The smallest dinosaurs were about the size of a goose; especially if these were warm-blooded, their chances of surviving the famine were pretty slim. So, one can argue, they died.

DIET: Those animals which only ate live plants would (assuming that they had survived the cold until then) have died soon after the plants themselves. Creatures which were able to feed on decaying vegetable matter would also have been well-equipped to survive. In rivers, then as now, much of the food chain was based on organic detritus such as rotting leaves; although the situation was hardly ideal, this intact food source would have given river-dwellers some chance of pulling through the dark days. If they were capable of living off rotting meat, so much the better. Omnivorous animals may have been in a stronger position than either, as they would have been able to eat whatever foodstuffs were available, on an ad hoc basis. Another asset would have been the possession of large fat reserves at the time of the impact. Animals which stored food for long-term use (in the manner of squirrels) may also have been advantaged - that is, if the Great Darkness did not persist for too long.

Depending on the duration of the Darkness, animals which could hibernate may have been able to save energy for healthier times. However, they would still have needed reasonable fat reserves if they were to hibernate successfully. Nocturnal animals (such as many of that time’s mammals) would in general have fared better than diurnal creatures.

One result of the Yucatan impact was to wipe out almost all large creatures; and almost all large creatures - with exceptions like the plesiosaurs and crocodiles - had been dinosaurs. Before the disaster/blessing of the impact, mammals had had to compete with the dinosaurs. Now, for the first time, the small rodent-like mammals could venture from their burrows and trees with little risk of being devoured. There was a vast gap in the ecosystem - the 'large animal niche' was pretty much vacant (although there were still some large predators such as the ancestors of crocodiles and monitor lizards), and some of the mammals proceeded to fill it, transforming over thousands of generations from small, cuddly animals into large, marauding predators.

Cataclysmic though the K/T boundary impact of 65 million years ago was, it was tame in comparison with several earlier disasters.

With some justification, the earliest part of the Earth's history is called the 'Hadean Era'. The Hades of that time has not entirely gone away. If you microwave a cup of water to boiling point, it will cool down pretty quickly; if you boil a kettle, it will take longer. If you heat a body with a circumference of 40,000 kilometres or so to temperatures of c.5,000 degrees Celsius, it will take a very long time indeed to cool down. Not too far below our feet, molten rocks and scary temperatures are the order of the day. Once in a long while, the interior busts out in a big way.

As you may know, the crust of the Earth is made up of tectonic plates, very roughly 20 in number. Some of these plates are comparatively light, and therefore protrude from the seas, which rest on the heavier (oceanic) plates. The mechanisms of plate tectonics are rather ill-understood; as I understand it, the currents of hot rock within the Earth move these plates around at speeds of perhaps 10 centimetres a second. These currents of molten rock are thousands of kilometres thick; their pace is agonisingly slow, but they are enormously powerful. It takes a lot to halt one of the plates which ride atop them.

245 million years ago (we speak of these timespans so easily! I find it remarkable and admirable that our species has been able to measure these vast ages with such precision), it is contended, many of these plates came together to form the 'supercontinent' named Pangaea. As you might expect, the currents under the plates did not suddenly stop when the plates above them met; their vast momentum crushed the plates together. Under this unbearable strain, the plates heaved and buckled and shuddered. In places, huge mountains were formed; in many places at or near plate boundaries, the plate simply could not endure the stresses, and broke - millions of tons of boiling magma and dust shot forth, blackening the land and obscuring the sun. A great chill descended over the Earth.* 95% of all species died; 52% of all families of species were exterminated.

* This is all very biblical isn't it? Scientists such as Mike Baillie of Queen's University, Belfast maintain that some mythical events are in fact contorted memories of natural catastrophes.

Tectonic plates have been hugely important for the evolution of terrestrial intelligence. As well as opening a plethora of niches by annihilating species, their activities have continually brought together and separated vast tracts of land, thereby joining and splitting whole ecologies. They have also produced huge climatic changes; for example, the coming-together of North and South America precipitated climate changes around the world, among them the important deforestation in Africa, which impelled some of humanity's ancestors to take to the savannah.

The effect of such changes is to favour innovation; for large animals, it is quicker to learn than simply to inherit behavioural instincts. Brains which can learn need to consume more energy in order to carry out the requisite calculations; but when environmental niches change, the animal which learns is often better-equipped to seize the new opportunities, and offset its greater energy consumption. Where an environment is stable, it will still be sufficiently complex to provide room for intelligence; but when new challenges come along, the smart big animals and the quickly-reproducing microorganisms will often be best positioned to adapt before they are wiped out.