Читать книгу Extinction: Evolution and the End of Man - Michael Boulter - Страница 11

For the dinosaurs, ammonites, and lots of other groups, life ended after an event that happened in Mexico one warm spring day. We know it was a spring day because a magnolia flower was found in the wreckage. A chunk of rock from outer space, 20km in diameter, had hit the earth off the Yucatán peninsula in south-east Mexico. If the meteorite had splashed in deep sea perhaps it would have been different, but the shallow-water impact made it one of the most severe physical crises in Earth’s history. The explosion shook the planet and the firestorm quickly spread up across most of North America. The fall-out of dust and smoke was blown east to Europe and beyond. We know that the projectile itself had come from a westerly direction because bits that broke off during the descent through the atmosphere have been found in sediments of the Pacific Ocean.

Flames and heat fanned around the planet. Rock and soil, steam and vapour, charred splinters, roast meat, were scattered into the acrid atmosphere right around the northern hemisphere and most of the south. Weather patterns changed, there was no light on the surface of the Earth. Life stopped or went into hiding; nothing thrived. The thick clouds of debris took many years to clear; the burnt vegetation on land, the acid and ash raining into the sea, halted the majority of life.

There are no graves like Pompeii’s, no swamps for easy preservation, so most of the remnants are gone and we are left to speculate. We don’t really know how long the fires lasted, their power, their full geographical extent. Like accident investigators or forensic scientists, paleontologists sift through the wreckage of 65 million years ago, looking for evidence. But in this case, we found the evidence before we understood the cause. To make matters worse there were very few rcfugia, places to hide, that survived the inferno, and of those many have broken up through the intervening 6 5 million years. Many are yet to be discovered as the fossil record unfolds.

After the explosion the most vulnerable passengers on Earth were the largest animals, the ammonites and dinosaurs, and none survived. The oceans lost much of their dissolved oxygen, so most marine species died within weeks of the impact, and became extinct. Once the black clouds and the fires had subsided new life came to the planet. Some of the carnage itself, both on the land and in the sea, remains as sediment for us to explore and thereby to understand more of what went on through those terrible times. It is at the Cretaceous-Tertiary boundary 65 million years ago, commonly known as the K-T boundary (the K coming from the German ‘Kreide’, meaning chalk), that we see remains of the catastrophe preserved in rock outcrops from many parts of the northern hemisphere, especially north of Mexico, to the east of the Rocky Mountains.

The thin band of sediments is rich in iridium, an clement rare on Earth but common in some asteroids. There are glass globules, fractions of a millimetre in diameter, the relics of molten silicates after the explosion. It also contains burnt vegetation, as well as the rare spring flower. We can even detect remnants from the lump of rock itself, several kilometres beneath the surface of the Chicxulub crater off Yucátan.

It’s only in the last few years that pretty incontrovertible evidence for all this has come together. Scientists from very different disciplines have become involved, from all over the world, bashing the story into shape. The idea of the K–T impact event began in 1977 from a coincidental conversation between two people from different disciplines. Walter was a young field geologist and he was talking about a new specimen with a famous Nobel physicist called Luis. It was strange that it should happen at all: geologists don’t usually talk to physicists. But in this case they were father and son, the Alvarezes. So the contact was accidental, not part of a planned experiment.

Walter Alvarez had collected a chunk of rock from near the mountains of Umbria, north of Rome. It had three layers: a chunk of white limestone, a thin layer of clay, and then more limestone, red this time with no fossils. The lower white layer was full of microscopic seashells well known to be Late Cretaceous in age. The red rock was like that found all over the region, known to be Early Tertiary. The clay lay between these two differently dated rocks. Did it come from the Cretaceous or the Tertiary? Were all three layers laid down in one continuous sequence? Or docs the clay layer represent some kind of break in the process, a gap?

Walter knew that his father’s laboratory was testing a new method of dating rocks that contain traces of heavy metal elements. The rock specimen is bombarded with neutrons, breaking down the metal’s atoms and making it radioactive. The level of radiation gives the rock’s age. The Alvarezes’ intuition inspired them to test the Umbria sample for metal elements. Iridium is rare on this planet, only occurring in very small quantities, so it came as a great surprise when the clay layer showed relatively massive quantities of the metal, nine parts per billion. Suddenly the dating experiment didn’t seem to matter. The important thing now was to explain the high concentration of iridium. What made that observation really exciting was the knowledge that most iridium comes from space, carried on Earth in micrometeorite showers. So this was obviously some shower. Even greater excitement lay ahead when they had the fragment dated by the new method. Luis Alvarez’s rock sample from Umbria was 65 million years old, from the K–T boundary itself.

From calculations based on the amount of iridium in the sample, the Alvarezes’ colleagues in the University of California at Berkeley proposed a theory that a 20km-diameter extraterrestrial object hit the Earth, spreading the iridium it contained in its spray. Throughout the 1980s and 1990s what we think is the same ‘iridium layer’ has turned up in North America, Essex, Denmark, Asia and elsewhere. A speculative idea came from a small amount of evidence and some highly imaginative but testable ideas. What began as a risky expression of grey fantasy by the Alvarezes has turned into a largely black or white fact well supported by other evidence from a variety of greyer disciplines.

Not least impressive is evidence that buried remains of the meteorite have been detected in Mexico. Rediscovered is a better word, for the structure was first mapped in 1962. Then, the ash and lava flows were thought to be part of a normal volcanic structure. There was no reason to get excited and the maps were filed away for normal everyday use. Then, in 1981, five years after the Alvarezes’ suggestion of an impact, two petroleum geologists found the Chicxulub crater off the Yucatán peninsula. Impact craters don’t lead to oil, so they were told to go away and get on with something else, not quite in disgrace, but not knowing how close to fame they were.

Other evidence of the impact includes glass beads that have also been found in the thin iridium layer sediments throughout the world. It’s been known for centuries that when you shoot a hot cannon ball into sand the energy melts the sand and the glass forms into small beads that splash out all around. With big chunks of rock like the Chicxulub meteorite it happens on a different scale, producing a wide variety of different minerals. Some of them form crystals less than a millimetre in diameter and are made of very particular materials such as magnetites containing nickel, as well as quartz, only found in remnants of meteoritic material. A range of clues like these can be identified very easily by looking at the shape of crystals in a scanning electron microscope. Very unusual crystals are well known in the scientific literature and are found frequently around meteorite impact craters of very different ages. They’ve also been found on the moon, where of course there’s no shortage of such craters. Other larger beads are found in the iridium layer at several localities as well as at the site of the impact, while others are pure glass.