K-T extinctions: the end of dinosaurs?
Since the early decades of the 19th century, it had been known that different groups of organisms dominate different periods of Earth history. One of the more notable groups was the dinosaurs, and there was a steady reinforcement from palaeontological surveys of the idea that none were to be found in rocks younger than the end of the Cretaceous period (approximately 65 Ma). In fact, it came to be recognized that the very end of the Cretaceous Period, leading into the Tertiary Period (now universally referred to as the K-T boundary) marked a major time of change. Many species became extinct and were replaced in the Early Tertiary by a diversity of new forms: the K-T boundary therefore seemed to represent a major punctuation in life and consequently a mass-extinction event. The types of species that became extinct at this time included the fabled dinosaurs on land, of which there were many different varieties by Late Cretaceous times; a multiplicity of sea creatures, ranging from giant marine reptiles (mosasaurs, plesiosaurs, and ichthyosaurs), to the hugely abundant ammonites, as well as a great range of chalky planktonic organisms; while in the air the flying reptiles (pterosaurs) and enantiornithine birds disappeared forever.
Clearly it was necessary to try to understand what might have caused such a dramatic loss of life. The flip side of this general question was just as important: why did some creatures survive? After all, modern birds survived, so did mammals, and so did lizards and snakes, crocodiles and tortoises, fish and a whole host of other sea creatures. Was it just luck? Up until 1980, most of the theories that had been put forward to explain the K-T extinctions and survivals ranged from the sublime to the ridiculous.
One of the more persistent of the pre-1980 theories revolved around detailed studies of the ecological make-up of the time zones closest to the K-T boundary. The consensus suggested that there was a shift to progressively more seasonal/variable climatic conditions at the end of the Cretaceous Period. This was mirrored in the decline of those animals and plants less able to cope with more stressful climatic conditions. This was linked, rather inconclusively, to tectonic changes towards the close of the Cretaceous Period; these included marked sea-level rises and greatly increased continental provinciality. The general impression was that the world was slowly changing in character, and this eventually culminated in a dramatic faunal and floral turnover. Clearly such explanations require a longer timescale for the extinction event to take place, but the Achilles heel was that this did not adequately account for the simultaneous changes seen in marine communities. In the absence of better-quality data, arguments waxed and waned with no obvious resolution.
In 1980, this field of investigation was completely revolutionized by, of all people, an astronomer, Luis Alvarez. His son Walter, a palaeobiologist, had been studying changes in plankton diversity at the K-T boundary. It seemed logical to assume that the interval between the Late Cretaceous and Early Tertiary might simply represent a longish period of ‘missing’ time – a genuine gap in the continuity of the fossil record. To assist Walter in his studies concerning the changes in planktonic communities at this critical time in Earth history, Luis suggested that he could measure the amount of cosmic dust that was accumulating in boundary sediments in order to be able to provide an estimate of the extent of this presumed geological gap. Their results shocked the palaeontological and geological world. They found that the boundary layer, which was represented by a thin band of clay, contained enormous quantities of cosmic debris that could only be explained by the impact and subsequent vaporization of a gigantic meteorite. They calculated that this meteorite would have needed to be at least 10 kilometres in diameter. Considering the effect of the impact of such a giant meteorite, they further proposed that the huge debris cloud generated (containing water vapour and dust particles) after the impact would have shrouded the Earth completely for a significant period of time, perhaps several months or even a year or two. Shrouding the Earth in this way would have shut down photosynthesis of land plants and planktonic organisms, and triggered the simultaneous collapse of terrestrial and aquatic ecosystems. At a stroke, the Alvarezes and their colleagues seemed to have found a unifying explanation for the K-T event.
As with all good theories, the impact hypothesis generated an impressive volume of research. Throughout the 1980s, more and more teams of researchers were able to identify cosmic debris and violent impact-related signals in K-T boundary sediments from the four corners of the globe. By the late 1980s, the attention of a number of workers was drawn to the Caribbean area. Reports showed that on some of the Caribbean islands, such as Haiti, deposits of sediments at the K-T boundary not only showed the impact signal, but immediately above this an enormous thickness of breccia (broken masses of rock that had been thrown together). This, as well as the greater thicknesses of the meteorite debris layer and its chemical signature, prompted the suggestion that the meteorite had impacted somewhere in the shallow sea in this area. In 1991, the announcement was made that researchers had identified a large subterranean meteorite impact crater, which they called Chicxulub, on the Yucatan Peninsula of Mexico. The crater itself had been covered by 65 million years of sediment, and had only been visualized by studying seismic echoes of the Earth’s crust (rather like the principle of underground radar). The crater appeared to be approximately 200 kilometres across and coincided with the K-T boundary layer, so Alvarez’s theory was vindicated in a most remarkable way.
From the early 1990s onwards, study of the K-T event shifted away from the causes, which then seemed to have been established, to attempting to link the extinctions at this time to a single catastrophic event. The parallels to the nuclear winter debate are fairly clear. Advances in computer modelling, combined with knowledge of the likely chemical composition of the ‘target’ rocks (shallow sea deposits) and their behaviour under high-pressure shock, have shed light on the early phases of the impact and its environmental effects. At Yucatan, the meteorite would have impacted on a sea floor that was naturally rich in water, carbonate, and sulphate; this would have propelled as much as 200 gigatons each of sulphur dioxide and water vapour into the stratosphere. Impact models based on the geometry of the crater itself suggest that the impact was oblique and from the south-east. This trajectory would have concentrated the expelled gases towards North America. The fossil record certainly suggests that floral extinctions were particularly severe in this area, but more work elsewhere is needed before this pattern can be verified. Alvarez and others’ work on the effects of the impact suggested that dust and clouds would have plunged the world into a freezing blackout. However, computer modelling of atmospheric conditions now suggests that within a few months light levels and temperatures would have begun to rebound because of the thermal inertia of the oceans, and the steady fall-out of particulate matter from the atmosphere. Unfortunately, however, things would have become no better for some considerable time because the sulphur dioxide and water in the atmosphere would have combined to produce sulphuric acid aerosols, and these would have severely reduced the amount of sunlight reaching the Earth’s surface for between 5 and 10 years. These aerosols would have had the combined effects of cooling the Earth to near freezing and drenching the surface in acid rain.
Clearly these estimates are based only on computer models, which may be subject to error. However, even if only partly true, the general scope of the combination of environmental effects following the impact would have been genuinely devastating, and may well account for many aspects of the terrestrial and marine extinctions that mark the end of the Cretaceous Period. In a sense, the wonder is that anything survived these apocalyptic conditions at all.
Perturbations
While much of the work in recent years has focused on explaining the environmental effects of a large meteorite on global ecosystems, work is still continuing at the Chicxulub site. A major borehole has now been sunk into the crater to a depth of 1.5 kilometres in order that detailed examination of the impact zone can take place. What is beginning to emerge is slightly disturbing to the general pattern that has been explained above. One set of interpretations of the core data indicates that the impact crater may have been made as many as 300,000 years before the K-T boundary. The interval is represented by 0.5 metres of sediment. This evidence has been used to propose that the end Cretaceous event was not focused on a single large meteorite impact, but several large impacts that occurred right up to the K-T boundary – the cumulative effect of which may have caused the pattern of extinctions.
Clearly these new findings indicate that more research and more debate will undoubtedly take place in years to come. Not least among these are the data concerning massive volcanic activity that coincided with the end Cretaceous events. Parts of India known as the Deccan represent a gigantic series of flood-basalts that have been estimated as representing millions of cubic kilometres. Quite what the environmental impact of such enormous volcanic outpourings was, and whether this was in any way linked to the meteorite impact on the other side of the world, is still to be established.
Mass extinctions are fascinating punctuation marks in the history of life on Earth – nailing down exactly what caused them is, not surprisingly, very difficult.
Dinosaur research now and in the near future
It should be clear by now that a subject such as palaeobiology -certainly as it is currently being applied to fascinating creatures such as dinosaurs – has a decided unpredictability about it. Many research programmes in palaeobiology can be planned, and indeed have an intellectually satisfying structure to them, in order to explore specific issues or problems; this is normal for all the sciences. However, serendipity also plays a significant role: it can lead research in unexpected directions that could not have been anticipated at the outset. It can also be influenced strongly by spectacular new discoveries – nobody in the early 1990s would have been able to predict the amazing ‘dinobird’ finds that were made in China in 1996 and continue to the present day; technological advances in the physical and biological sciences also play an increasingly important part in research, allowing us to study fossils in ways that were, again, unimaginable just a few years ago.
To take advantage of many of these opportunities it is important to have at hand people who share a number of characteristics. Above all, they need to have an abiding interest in the history of life on Earth and naturally inquisitive temperaments. They also need some training in a surprisingly wide range of areas. While there is still an importance in the individual scientist working and thinking creatively in some degree of isolation, it is increasingly the case that multidisciplinary teams are needed to bring a wider range of skills to bear on each problem, or each new discovery, in order to tease out the information that will move the science a little further forward.
And finally …
My message is a relatively simple one. We, as a human race, could simply chose to ignore the history of life on Earth, as can be interpreted, in part at least, through the study of fossils. There are indeed many who adhere to such thoughts. Fortunately, I would say, a few of us do not. The pageant of life has been played out across the past 3,600 million years – a staggeringly long period of time. We as humans currently dominate most ecosystems, either directly or indirectly, but we have only risen to this position over the past 10,000 years of life on Earth. Before the human species, a wide range of organisms held sway. The dinosaurs were one such group and they, in a sense, acted as unwitting custodians of the Earth they inhabited. Palaeobiology allows us to trace parts of that custodianship.
The deeper question is: can we learn from past experiences and use them to help us to preserve an inhabitable Earth for other species to inherit when we are finally gone? This is an awesome responsibility given the current global threats posed by an exponential population increase, climatic change, and the threat posed by nuclear power. We are the first species ever to exist on this planet that has been able to appreciate that the Earth is not just ‘here and now’ but has a deep history. I hope sincerely that we will not also be the last. The one thing that we can be sure of, after studying the waxing and waning of species throughout the immensity of the fossil record, is that the human species will not endure for ever.
From our origins as Homo sapiens approximately 500,000 years ago, our species might last a further 1 million years, or perhaps even 5 million years if we are extraordinarily successful (or lucky), but we will eventually go the way of the dinosaurs: that much at least is written in the rocks.
