We are now back at Roedvig at the world-renowned geological locality of Stevns Klint. Here we will use the layers in the cliff as a backdrop to explain current scientific thought on the great mass extinction 65 million years ago. This was at the boundary between the Cretaceous and Tertiary periods and upwards of two-thirds of all species on Earth perished in one of the greatest mass extinctions of all time. Stevns Klint is especially relevant for explaining this mass extinction because it was an investigation of the boundary layer between the Cretaceous and the Tertiary periods here that triggered and revitalized the concept of mass extinctions. Studies here have since followed on with numerous studies all over the world, which have not only brought us understanding of the mass extinction 65 million years ago, but all the other great mass extinctions on Earth. The investigation here took place in the late 1970s and was published in a seminal paper in 1980 in the journal Science. A team of scientists led by physicist Louis Alvarez studied the boundary layer here at Stevns Klint and other boundary layers between the Cretaceous and Tertiary periods in Italy and New Zealand. In the boundary layers they found highly increased quantities of the rare element iridium. Iridium is extremely rare in the Earth's crust. However, it is very abundant in the planet's core. Iridium is also found in asteroids and other extraterrestrial objects where it is much more common. Alvarez and colleagues argued that the increased level of iridium in the boundary layers between the Cretaceous and Tertiary periods around the world was due to an extraterrestrial impact such as an asteroid hitting the Earth. The extraterrestrial impact sent dust into the air containing iridium which ended up in the boundary layers. They also suggested that the extraterrestrial impact was responsible for a great mass extinction of many organisms at the boundary between the Cretaceous and Tertiary periods, including the dinosaurs. Their study immediately sparked a flow of new scientific investigations by geologists and palaeontologists around the world, who sought to either prove or refute Alvarez and colleagues' theories. The results of these studies and many more that followed have shown conclusively that the end of the Cretaceous period was marked by a great mass extinction where in upwards of two thirds of the Earth's species disappeared in as little as a few tens of thousands of years. Today, it is generally accepted that an asteroid some 10 to 15 kilometres in diameter, with an impact speed of between 20 and 70 kilometres per second, releasing the equivalent of around 100 million megatonnes of TNT. This impact released a huge amount of dust and ashes, which spread into the atmosphere blotting out the sun's rays completely. Physical evidence for the asteroid impact site was published in 1991. Below the Yucatan Peninsula in Mexico, at a site called Chicxulub, geologists found a 170 kilometres wide, ring-shaped structure. The structure was an impact crater situated at a depth of some 15 to 20 kilometres, as evidenced by the impact breccia - smashed and churned rock below it. The size conformed to the previously predicted computations of the asteroid's size. Geological dating puts the crater right at the Cretaceous-Tertiary boundary. Further evidence supporting the impact was the discovery of shocked quartz and glass spherules in boundary layers for several 100s of kilometers around the impact site at Chicxulub on the Yucatan Peninsula. Shocked quartz grains contain sets of microscopes fractures, which can only be produced by either violent impacts or volcanic eruptions. The glass spherules are microscopic grains of molten rock deriving from the limestones that lay at the impact site in the prehistoric Caribbean. However, long before the devastating asteroid impact took place, another longer-lasting process had been stressing the Earth's ecosystems. In India a massive volcanic field called the Deccan Traps had been erupting. At the time, the Indian continent was situated below the Equator, in the middle of what is today the Indian Ocean. At that time, the continent lay directly above a mantle plume which can still be found underlying the ocean. The presence of the mantle plume below the continent led to massively increased volcanic activity. The result was the Deccan Traps; a volcanic field whose lava flows today cover most of northern India. All-in-all, a total of 3,000 square kilometers of lava were produced in repeated eruptions along with even greater volumes of noxious gases. Individual lavas flows from just one eruption of the Deccan Traps have charted with thicknesses of 150 meters and a range up to 160 kilometres. Recent studies have narrowed the greatest outpourings of the Deccan Trap eruptions to a phase of just 30,000 years immediately on the Cretaceous-Tertiary boundary. It is thought that these eruptions emitted vast amounts of carbon dioxide - both a green house gas and acidifier of marine environments, as well as sulphur dioxide, which acts as an atmospheric coolant but also results in acid rain. Today, the mass extinction event at the Cretaceous-Tertiary boundary is probably best understood as a two-part process. First, the massive eruptions of the Deccan Traps in India emitted vast amounts of gas and ashes into the atmosphere. Carbon dioxide acted as a greenhouse gas, but more importantly the elevated levels of CO2 in the atmosphere acted as an acidifier of the upper parts of the water column of the sea. Sulphuric dioxide conversely cooled the global temperatures rapidly, but also resulted in acid rain, further acidifying the already distressed water column, both at sea and in freshwater on land, as well as damaging the plant biomass on land. Furthermore, ash from the volcanic eruptions contributed to a haze partially blotting out the sun's rays and cooling the planet slightly. Then the asteroid impact happened in the Mexican Gulf. Apart from severe local destruction, the impact sent massive amounts of ash and impact debris into the atmosphere, spreading out across the Earth and completely blocking out the sunlight. The plants on land and algae in the water were already stressed by acidification and acid rain. Now their primary energy source - the sunlight was completely blocked out. They died off in large amounts and with them went the base of the food chain both on land and in the sea. The layers here at Stevns Klint tell us exactly about the sequence of events that took place at the end of the Cretaceous during the great mass extinction. I am sitting on a layer of white chalk. The chalk is made up of more than 90% of the shells of tiny marine algae called coccolithophores. They were extremely abundant in the seas in the late Cretaceous and were the primary producers and the basis of the food chain. The chalk layer here at Stevns Klint actually continues some 900 meters below me into the subsurface. These 900 meters of chalk represent something like 30 million years of continuous chalk deposits. The shells of the tiny coccolithophore algae ended up on the sea bottom on their death. However, here at the grey clay layer, the deposition of chalk ends abruptly; the grey clay layer is almost devoid of carbonate and once we find a new layer of limestone on top of the grey clay layer, it is no longer chalk - it's a completely different kind of limestone. At the grey clay layer 85% of species of coccolithophores of the calcareous algae become extinct and in fact the grey clay layer represents how the late Cretaceous sea bottom would have looked without all the algae shells. This is just clay that was washed out from the nearby land masses. The colour of the grey clay also tells us something about the environment in which it was deposited. It is dark grey and the dark colour comes from large quantities of organic material that was buried within it. Now, this is in contrast to the white layer of the white chalk below and the light layer of the limestone above. The light colour is because the organic material - the remains of dead animals, plants and other organisms that ended up at the sea bottom were eaten and burrowed through by other animals. However, when we come to the grey layer, we find a lot of organic material with dark colour. This organic material was not eaten or consumed by bottom-dwelling organisms. Instead it was just buried along with the layer and gave this dark colour. If we look closely at the grey layer we might actually find that it becomes slightly lighter towards the top of itself. This is because slowly after the extinction event, which took place at the time of the bottom of the layer, animal life returned and started burrowing into the layer. When we come to the limestone above the grey layer, animals are now burrowing through the bottom, and the organic material inside is being consumed and the layers light. But the limestone we find here - the so-called Cerithium limestone - is completely different and that is because the small algae at the bottom of the food chain are no longer the most abundant animals in the sea. Other organisms have taken their place and this carbonate, this limestone is mostly made up of shell fragments of bivalves, of brachiopods and snails and other invertebrate animals. Many species survived the great mass extinction at the end of the Cretaceous and were able to evolve into new species and new forms. Although survival during a mass extinction is generally a matter of luck and not one of special adaptations, there are some scientific studies indicating that some species were able to survive more easily at the end of the Cretaceous mass extinction than others. Those are species that were living in marine or freshwater environments and were attached to a food chain based on detritus. Detritus is the unprocessed organic matter that can be found at the sea bottom or the bottoms of streams and lakes. Interestingly, some animals - although they seem to closely resemble some of the forms that became extinct - survived. For example all the shelled nautiloids survived the mass extinction while their cousins - the ammonites - who were also shelled cephalopods all became extinct. In a similar vein, sea turtles appear to have survived the mass extinction with very few species lost, as did the then very common ocean-going crocodiles. Most studies of the great mass extinction at the end of the Cretaceous were made at marine sections, geological studies made of rocks deposited in the sea like here at Stevns Klint and most of the world. The reason that most studies have been made on this kind of rock is that they are by far the most abundant in the world. On land - in terrestrial environment - the only good studies come from the Hell Creek deposits in Montana in North America. The Hell Creek deposits tell a similar story to that of the sea deposits. Here land plants suffered a major turnover and replacement in species. Freshwater animals that we know today are susceptible or vulnerable to acidification of their environment suffer total or near-total extinction levels. Amongst mammals for example, mammal groups or mammal species, whose teeth show that they were adapted to living off plants, fresh leaves or seeds, suffered between 50 and 90% extinction levels at the species range, while mammals that were adapted to eating insects and small invertebrates seem to have suffered much less from the mass extinction. And of course the last of the dinosaurs went extinct. The plant eaters died once their food withered and wilted under the onslaught of acid rain and when the sunlight was eventually totally blotted out, and with their plant-eating prey gone, the meat-eaters soon followed suite. However, if the Cretaceous/Tertiary mass extinction had not taken place, evolution would have taken a completely different turn and the world would not have looked as it does today. Mammals would not have been able to thrive and evolve into the multitude of forms and shapes if their dinosaurian competitors had not become extinct some 65 million years ago. And that in turn would not have meant that an unlikely, unplanned, random chain of events would allow for the origin and evolution of human beings like you and me. And that is something we will return to in the last lecture of the Origins course.
