What really happened to the Dinosaurs?
There’s something that immediately captures the imagination when one thinks of giant reptiles roaming the earth upon which we now live. They may not get quite as much attention as adorable videos of kittens or puppies on YouTube, but the success of movies such as Jurassic Park clearly show that dinosaurs have a way of engaging with society and causing us to wonder what life would have been like 65 Million years ago (Ma). But how much do we actually know about the demise of the dinosaurs? What brought about their extinction, and how sure are we really?
The second largest Mass extinction ever
Go back just 30 or 40 years and the debate about what killed off the dinosaurs was still raging. After thriving for over 135 million years, suddenly the dinosaurs, along with almost 80% of species on the earth, became extinct. Occurring 65 Ma, this event defined the end of the Cretaceous Era and beginning of the Tertiary Era, and is known as the K-T boundary. Scientists have long argued about the cause(s) of this mass extinction, with the two favoured theories being massive volcanism or an extra-terrestrial impact. Other factors such as climate change and ocean anoxia have also been proposed, although the general consensus is that these may have been only contributing factors, rather than the primary cause of the extinction.
Evidence for a giant asteroid
Whilst it is known that there was massive volcanism occurring 65 Ma in India, it was the giant impact theory which became the favoured theory in 1980. This followed detailed studies of a thin, clay-like layer of sediments found globally at precisely the K-T boundary in the geological record. This layer was found to contain an extremely high concentration of the element Iridium, an element which is extremely rare on earth, but has been found in high concentrations in asteroids. The scientists inferred that a worldwide Iridium layer at precisely the K-T boundary could have only been produced by the vaporisation of an asteroid upon impact with the earth. A few years after this initial discovery, another group of scientists found evidence of both shocked quartz and microtektites contained within the very same layer. These are both well reported phenomenon in geology which occur with large impacts.
Where did the asteroid hit?
The only question remaining was where this impact occurred. Whilst you might think a crater from such a significant event would be easy to find, 65 million years of sediments have been building up on the surface of the earth since impact, and no such crater had ever been found. At least, that was the case until 1991 when a team of scientists doing geophysical surveys in the Gulf of Mexico identified a gravity anomaly matching the expected dimensions for a crater produced by the K-T boundary impact. Named the Chicxulub crater, this feature was dated using boreholes to drill through the overlying sediments. This confirmed the age of the crater to be 65 Million years old, placing it right on the K-T boundary, and providing what was thought to be the final piece of evidence confirming the giant impact theory.
End of the story?
However, a paper published in 2004 found that based on their dating methods, the Chicxulub impact actually predates the K–T boundary by about 300 000 years. All of a sudden, questions were again being asked about where the asteroid hit, and the cause of the K-T mass extinction was again being debated. Many people still believe that the Chicxulub crater is indeed the impact site, whilst an alternative crater site in India has been discovered and proposed as a possible location. Some scientists are now even questioning the giant impact theory altogether, proposing that high Iridium concentrations can be sourced from the deep earth, and carried to the surface in the lava extruded from volcanoes. Intriguingly, the largest ever mass extinction, the Permian-Triassic mass extinction, is thought to be a result of massive volcanism in Siberia, and links between mass extinctions and volcanism are relatively frequent throughout the geological record.
There’s always more to learn
Science is an ever changing field, in which today’s theories can soon become tomorrow’s history lesson. New results and discoveries cause old theories to be re-examined, modified and sometimes thrown out all together. So what exactly killed off the dinosaurs? Unfortunately we still can’t be sure.