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RTD info logoMagazine on European Research Special Issue - April 2005   
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SEDIMENTOLOGY
Title  The disaster detective

How far back does memory reach? Not the memory of the original Big Bang that created the universe but, more prosaically, that of a planet called Earth and the life that appeared on it. Its history is that of a long evolution, punctuated by repeated and sometimes violent biological and climatic upheavals. The most recent – a particularly strange and spectacular one – occurred 65 million years ago with the mass extinction of 90% of the species, large and small, that inhabited the planet during the age of the dinosaurs. It is a very specialised family of geologists – the sedimentologists – who hold many of the keys to research into this unique biotic disaster. RTD info spoke with one of them, Jan Smit of the Vrije Universiteit – Amsterdam (NL), who has gathered evidence of a giant meteorite crashing into the planet in the Mexican Yucatan, with lethal consequences on a global scale.

How does one become an investigator into the disappearance of the dinosaurs? 

A geologist specialising in sedimentology, and especially the stratigraphy of major events, Jan Smit teaches at the Faculty of Earth Sciences at the Vrije Universiteit Amsterdam (NL). He was co-leader of the Chicxulub Scientific Drilling Project under the International Continental Scientific Drilling Program (ICDP). He has received a number of awards for his work, including, in 1999, the Mary Clark Thompson Medal of the National Academy of Sciences, Washington DC (USA).
A geologist specialising in sedimentology, and especially the stratigraphy of major events, Jan Smit teaches at the Faculty of Earth Sciences at the Vrije Universiteit Amsterdam (NL). He was co-leader of the Chicxulub Scientific Drilling Project under the International Continental Scientific Drilling Program (ICDP). He has received a number of awards for his work, including, in 1999, the Mary Clark Thompson Medal of the National Academy of Sciences, Washington DC (USA).
In the 1970s, I was a young researcher working mainly in Spain, studying the sediments of the Maastrichtian phase (between 74 and 65 million years ago). This sediment is the last layer to be deposited before the end of the Cretaceous geological period and the beginning of what is known as the Tertiary period. This boundary between these two ages of the Earth – which scientists refer to as the K/T Boundary – is a very unique period in our planet’s history. It marks the last and one of the most brutal mass extinctions since life appeared, hundreds of millions of years previously. This not only wiped out the extraordinary variety of giant dinosaurs but also all the micro-species, such as the phytoplankton and foraminifers, with which the oceans were teeming. At the 25 or more Spanish sites we were studying at the time, the very clear trace of this K/T Boundary was present in the form of an immediately identifiable quasi-horizontal line above which all the fossil remnants of these creatures, whether massive or microscopic, disappeared completely. 

From that time – and in this I was far from alone – I became increasingly interested in the significance of this divide. At the time the jury was still out on the causes of this mass extinction. We lacked proof. There was – and still is – one school of thought that supported a chain of events triggered by renewed volcanic activity, the traces of which are particularly apparent on the present Indian sub-continent, but not associated with a simultaneous climate change. Some suggest that a phenomenon interfering with the biomineralisation of calcium may have contributed to an accelerated degeneration of the huge skeletons of these giant species. But these causes, even if they really did play a part – and I repeat we have no proof of this – do not explain the exceptional brutality of this extinction.

Which made it possible to postulate a cause linked to a specific event rather than a form of geological evolution?

Indeed, the other school of thought favoured a disaster that struck from the exterior in the form of a giant meteorite that crashed into the Earth. During its very long history of over 4 billion years, other celestial bodies have struck our planet on a number of occasions. Some were of considerable size and caused very violent shocks. Evidence of this is found in traces of very old craters, such as the Sudbury crater in Canada, which is 250km in diameter, or the Vredefort crater, in South Africa, which is 300km across. It was not until 1978 that this previously speculative idea began to take shape, in particular thanks to the research carried out by Luis Alvarez (winner of the Nobel Prize for physics in 1968) and his son Walter in Berkeley. It is they who discovered a significant and repeated concentration of iridium at a number of sites associated with the K/T Boundary that were being studied all over the world, as well as corroborating evidence in the form of components of micro tektites(1), quartz, etc. The simultaneous presence of these elements amounts to a genuine ‘signature’ of an extra-terrestrial cause, that is, a violent collision between the Earth and a celestial body.

But it was still a number of years before the existence of such a collision could really be linked to the extinction dated to the K/T Boundary and the discovery – at the time of prospecting for oil on the Yucatan Peninsula and in the Gulf of Mexico – of the huge and very famous Chicxulub crater. Measuring almost 185km across, it soon became clear that this circular imprint, half of it lying beneath the peninsular sediments and the other half beneath the waters of the Gulf, was created by a meteor crashing into the Earth’s crust. Today, the vast majority in the scientific world regard this as fact.

So this was in a way the ‘missing link’ in unravelling the mystery of this mass extinction?

That is what had to be demonstrated, by carrying out painstaking research throughout the past decade into the traces that this terrible collision must have left in rock formations and sediments, not only at the point of impact but also throughout the surrounding region. The research area in which I participated as a European researcher along with many other sedimentologists – in particular Americans, as they are very familiar with the geology of this part of the world – extended from Belize, in the south of the Yucatan Peninsula, to the heart of the United States, including underwater drilling in the Caribbean under the auspices of the US IODP drilling programme.

Did this produce a scenario of what happened on the fatal day the meteorite struck?

Fossilised dinosaur footprints. © MNHN
Fossilised dinosaur footprints.
© MNHN
The study of what we call the ejecta – that is all the solid or molten matter that came from the shattered meteorite, as well as the Earth’s crust, and that was strewn over an area of several hundred kilometres around the crater – shows that the collision triggered an unprecedented shockwave, accompanied no doubt by violent earthquakes and intense heat. If the size of the Hiroshima explosion is put at 4 million tons of TNT, the energy released by this event must have been about 100 000 million tons of TNT. A second key point is that the collision did not take place on land but beneath the relatively shallow sea that at the time still separated the two blocs of the north and south US sub-continents, Central America having not yet been formed. This is the worst possible disaster scenario, as we have identified that the impact produced a giant tsunami compared with which what happened at the end of 2004 in the Indian Ocean is no more than a ‘ripple’. The waters submerged all the present great plains of the United States as far north as the Rocky Mountains and the Appalachians. A wave, no doubt smaller in scale, also certainly crossed all the oceans of the world.

But how did this essentially local disaster subsequently and so rapidly bring about the global extinction of the dinosaurs and most other living species?

We are now entering the realms of conjecture rather than fact. What seems certain is that the heat generated produced two phenomena. First, the propagation of huge fires that consumed vegetal matter and covered an area that we do not know. But, above all, a huge release of dust and smoke composed of sulphurated and carbonised particles, saturated with steam, that radically changed the atmosphere of the entire planet for a sufficiently long period. What effects did this have on the amount of light that penetrated to the Earth’s surface and, thus, on the life of the micro-organisms in the sea? There is some evidence that points to a cooling effect produced by this dust and sulphur smoke, lasting several months or several years. But was there then a greenhouse effect, and thus a climate warming that proved lethal to all the ecosystems? Or were there effects from the destruction of the ozone layer that caused extinction through sterilisation? These are all questions to which we still do not have the answers.

But is this dramatic scenario not currently being disputed by some scientists?

Indeed, the most recent opposition – it is called the Chicxulub controversy – came from a group of geologists who do not deny the evidence of the impact but believe that it did not occur at the K/T Boundary of extinction, but about 300 000 years previously. But we are working on the same sedimentological data as collected over the past decade. And I cannot find proof of this difference in time anywhere.

(1) The tektites originate in pieces of the Earth’s crust that, due to the impact of a meteorite, are vaporised and projected over distances of thousands of kilometres. In the upper atmosphere, these vapours condense and can form micro tektites that remained in the stratosphere for very long periods. 

    
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