A METEOR the size of San Francisco hurtles towards the Earth at 20 kilometres a second, smashes into the tropical lagoons of the Gulf of Mexico and gouges a hole 20 times as deep as the Grand Canyon. The mammoth crater quickly collapses and a tidal wave surges outwards. Fires sweep across North and South America and tonnes of debris are ejected into the atmosphere, blocking out the Sun and plunging the Earth into a frigid darkness that lasts for months.
This is the classic explanation for the event that wiped out the dinosaurs 65 million years ago, but is it true? Everyone agrees that the Earth suffered a large impact towards the end of the Cretaceous. Yet more than 20 years after a meteor was proposed as the cause of the Cretaceous-Tertiary (K/T) mass extinction, scientists are still arguing over what really killed the dinosaurs.
On one side are the 鈥渃atastrophists鈥�, who say the impact snuffed out the majority of life on Earth in the space of months or a few years. On the other are 鈥済radualists鈥�, who point out that the fossil record shows a steady decline in species diversity starting several hundred thousand years before the end of the Cretaceous. They don鈥檛 deny the impact happened, but they say it wasn鈥檛 what caused the extinction.
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The debate has remained polarised and acrimonious, with neither side managing to produce conclusive evidence. But it could be resolved soon, thanks to a feat of engineering that has given geologists their first good look at the impact site. By boring through 1.5 kilometres of rock, drilling contractors have pulled out a rocky core that tells the full story of the impact and its aftermath. Geologists are now queuing up to analyse the core in the hope of answering the key question about the impact: was it devastating enough to kill the dinosaurs?
Throughout the world the fossil record shows abrupt changes around 65 million years ago. The most prominent is the disappearance of the dinosaurs. But other major groups vanished too: pterosaurs, plesiosaurs, mosasaurs and ammonites, along with many families of fish, plankton and plants. Over 70 per cent of species didn鈥檛 make it. Globally, a film of debris several millimetres thick seems to have settled out at exactly the same time as the extinctions. The loss of biodiversity is known as the K/T mass extinction.
Back in 1980 Luis and Walter Alvarez, a father and son team at the University of California, Berkeley, proposed an impact as the cause. They based the idea on unusually high concentrations of the rare element iridium they had measured in K/T boundary clays near Gubbio in Italy. Iridium is scarce in the Earth鈥檚 crust-it combines readily with iron so gravitates towards the core-but is relatively abundant in meteorites and in volcanic material from deep within the Earth. Meteorites and deep lavas also contain other rare metals-platinum, ruthenium, rhodium, palladium and osmium-but in different quantities. The iridium layer at the K/T boundary had the unmistakable signature of meteorite dust. Since then high levels of meteoritic iridium have been recorded at more than a hundred K/T boundary locations worldwide.
The next big question was, where did the meteorite hit? In 1981 two geophysicists, Glen Penfield and Antonio Carmargo of the Mexican oil company Pemex, accidentally discovered a massive crater whilst searching for oil in the Gulf of Mexico. Though nearly 200 kilometres across, the crater hadn鈥檛 been spotted before because it was buried beneath 1100 metres of limestone. Penfield and Carmargo noticed that the rocks below Chicxulub, a fishing village on the north coast of Yucat谩n, had unusual magnetic and gravitational readings normally associated with impact craters. Gravity readings were low, indicating rocks of low density, perhaps like a pile of rubble. Meanwhile the magnetic readings suggested lots of iron, which is typical of impact melt rock.
The right site
It wasn鈥檛 until 1991 that geologists suggested this crater might be the sought-after K/T impact site. Alan Hildebrand and his colleagues at the University of Arizona in Tucson made a connection between the readings and some unusual 65-million-year-old rocks at the Brazos River in Texas, in which a turbulent sandstone bed appears between quiet layers of marine sediment. This had puzzled geologists for years. Some proposed it was a 鈥渢sunami bed鈥�, created from material washed along by a massive wave, and conjectured that there would be a crater nearby. Hildebrand joined the dots and suggested that a meteorite had hit Yucat谩n 65 million years ago, generating a tidal wave that swept across Central America.
Following this proposal a flurry of scientific activity confirmed that the Chicxulub crater could indeed be the site of an impact big enough to cause the K/T extinction. Seismic surveys revealed a prominent circular structure 170 kilometres across, which is now thought to be the crater鈥檚 rim. Radiometric argon-argon dating of melt rock retrieved from a borehole close to the rim put the crater at 65 million years old, give or take 100,000 years-the same age as the K/T boundary. Tsunami deposits up to 4 metres thick showed up right around the Gulf of Mexico, from Alabama to Guatemala.
Most convincing of all were rock samples from deep inside the crater itself. These came from two cores that Pemex drilled into the Chicxulub crater in the 1980s. Pemex鈥檚 primary aim was to find oil, not craters, and it only sampled small parts of the rock. But what it pulled out was enough to prove that Chicxulub was an impact crater.
Underneath the limestone layer Pemex found rocks known as breccias, made up from a jumble of broken chunks cemented together. Breccias are usually associated with impacts or eruptions: the chunks are bits that were blasted into the air. But there was something else too. 鈥淲e noticed that the breccia contained quartz crystals that are criss-crossed with tiny cracks. The only explanation is an impact,鈥� says David Kring, a geologist from the University of Arizona. Deeper down, Pemex found a glassy material called melt rock, which can only result from fusion of target rocks under the huge pressure and temperature of an impact.
And so the evidence that a large object hit the Earth 65 million years ago is unequivocal. But was the impact solely responsible for killing off the dinosaurs? For the catastrophists, there are two big problems. First, they don鈥檛 know how intense and widespread the impact鈥檚 effects were. To prove their theory, they would have to show an extreme global change that lasted for at least a year. Secondly, meteorites were not the only things stirring up trouble all those years ago. Deep beneath the Indian subcontinent the Earth was having a spot of indigestion. And this, the gradualists argue, was the real cause of the dinosaurs鈥� demise.
Around 65 million years ago an area known as the Deccan Traps in what is now western India was enduring one of the most intense periods of volcanism in Earth鈥檚 history. A 鈥渉ot spot鈥� deep in the mantle was producing plumes of superheated lava that rose to the surface and burst through the crust, inundating 2.5 million square kilometres of land. Layer after layer cooled to form a vast flood basalt which is still 2 kilometres thick. In the space of just a million years the Deccan Traps spewed out about a million cubic kilometres of lava-equivalent to the 1980 eruption of Mount St Helens 500,000 times over. Bubbling up with the lava were vast quantities of the greenhouse gases CO2 and water vapour.
The Deccan lavas were laid down over a period of around a million years straddling the K/T boundary. In 1981, Dewey McLean of the Virginia Polytechnic Institute in Blacksburg proposed that the Deccan Traps triggered intense global warming and a mass extinction.
In support of the theory, gradualists point out that this is not the only episode of supervolcanism that coincided with a mass extinction. At the Permian-Triassic boundary 250 million years ago, over 90 per cent of marine species went extinct-just as what is now Siberia was being flooded with lava.
Living fossils
Earlier this year more evidence emerged in support of a gradual extinction. A team of geologists in China discovered dinosaur eggshells in rock layers above the K/T boundary. Team leader Zu-kui Zhao of the Chinese Academy of Sciences in Beijing says this is evidence that some species of dinosaur survived for 250,000 years after the impact (Palaeogeography, Palaeoclimatology, Palaeoecology, vol 178, p 1).
For the past twenty years the debate has raged, with geologists neatly dividing into two camps. 鈥淯nfortunately politics has overwhelmed science,鈥� says McLean. However, the recent drilling programme promises to end the debate once and for all, by allowing researchers to model the exact environmental effect of the impact.
In December 2001, a team of geologists started to drill a hole deep into the Chicxulub crater, close to the southern rim (see Map). 鈥淭his location is thought to be the best place to understand both the cratering event and the consequences for the biosphere,鈥� says Philippe Claeys from the Free University in Brussels, one of the project鈥檚 science team leaders. Drilling finished in February and the contractors pulled out a continuous core, 1112 metres long and 7.6 centimetres in diameter. This tells the full story of the impact and its aftermath, starting with the rocks that the meteorite hit, through the pulverised melts and breccias and the debris of a tsunami, and ending with marine sediments that document the first life after the impact. 鈥淭he rocks are excellently preserved and certainly promise some scientific fireworks,鈥� says Jan Smit, a geologist at the Free University of Amsterdam. For the first time, scientists will be able to unravel exactly what happened to Earth on that fateful day 65 million years ago.
As New Scientist went to press, research teams were gathering in Mexico City to bid for the bits of core they want to analyse. No piece of rock will go unturned. One sought-after segment will be the melt rocks. 鈥淭hese will allow us to duplicate the dating and confirm the age of the crater,鈥� says Claeys.
The most coveted section, however, will be the pristine sedimentary rocks at the bottom of the core. These promise to answer a key question about the impact: what kind of rocks did the meteor hit? Computer models show that when a large meteorite strikes the Earth the heat and pressure cause the land and sea to vaporise, sending millions of tonnes of climate-altering gases into the atmosphere. 鈥淏ased on the size of the crater, we calculated that approximately 200,000 cubic kilometres of the Earth鈥檚 crust was instantly vaporised, melted or ejected from the crater,鈥� says Kring.
But the exact climatic effect of this debris depends on the target rock. At the end of the Cretaceous, Yucat谩n was a shallow tropical lagoon with waters just 10 to 20 metres deep. Under the clear blue waters, sedimentary limestone was being laid down while on land large deposits of anhydrite rocks-largely calcium sulphate-were forming as the Sun evaporated pools of brine.
The proportions of these two minerals in the target rocks isn鈥檛 known. But the relative quantities matter. When limestone is vaporised it releases CO2 into the atmosphere, which triggers long, slow global warming. Anhydrite, in contrast, can unleash an apocalypse. Vapourised anhydrite turns into sulphate aerosols in the atmosphere, which assist cloud formation and drastically reduce the amount of sunlight reaching the Earth (New Scientist, 2 February, p 13). Coupled with dust, large amounts of sulphate would plunge the world into intense cold and darkness for months or even years. What鈥檚 more, sulphates are eventually converted to sulphuric acid, which rains down and poisons rivers, lakes and soils.
Another key question is how energetic the impact was. By looking for evidence of the explosive force-exactly how much melt rock and breccia is left in the crater, the size of the chunks in the breccia and the stress patterns on the quartz crystals-the researchers hope to calculate how much of the debris was vaporised, and therefore the exact quantity of climate-altering gases released. The force of the impact will also tell them how high the debris was thrown into the atmosphere. This is another important piece of the puzzle: to cause a global catastrophe, the dust would have to reach at least into the upper atmosphere from where it could spread, unchecked, around the globe.
Once they know the force of the impact and the proportions of carbonate and anhydrite in the target rocks, the team will finally know how extreme the impact鈥檚 environmental and climatic effects were. And that should decide the debate once and for all. A highly energetic impact producing lots of sulphate suggests the catastrophists are right. Anything less and gradualism holds the day.
Most of the researchers working on the project are catastrophists and are confident that the results will confirm their version of events. 鈥淚 view this as the cherry on top of the cake,鈥� says Claeys. Gradualists, of course, are not so sure. 鈥淲hether the drilling will provide relevant new information on the cause of the extinctions remains to be seen,鈥� says McLean.
But already the core has thrown up a surprise. The layer of breccia is much thinner than expected-around 100 metres rather than the 200 calculated from gravity readings. So what鈥檚 going on? Jaime Urritia-Fucugauchi of the National Autonomous University of Mexico in Mexico City, one of the project leaders, says it鈥檚 possible that the breccias are less dense than usual. Or maybe the meteorite hit the Earth at an oblique angle, creating an asymmetric crater with a thicker layer of breccia on the other side. As yet no one knows which of these is true, nor what they mean for the debate. But one thing is clear: there鈥檚 still plenty to learn about the Chicxulub impact. Time to sit back and enjoy the fireworks.
