Radioactive rocks and the dinosaur detectives
Science feature by Professor Glenn Patrick
A RECENT newspaper headline caught my attention where it was reported that the world’s first complete dinosaur skeleton found on the Jurassic Coast had finally been studied in detail and a place found for it in the family tree of dinosaurs.
The article described the well-known local story of how fossilised bones had been collected beneath Black Ven by James Harrison – an amateur collector who lived in Charmouth, and now buried in Monkton Wyld.
In 1858, Harrison sent the bones to Sir Richard Owen – an expert at the British Museum who even coined the word “dinosaur”. Although Sir Richard published two quick papers on the find and even gave the specimen the name of Scelidosaurus harrisonii (‘Harrison’s limbed reptile’), he did not investigate it in detail and the bones were just stored in the Natural History Museum.
Luckily, Dr David Norman from the University of Cambridge came along and spent the last three years analysing the find and published four papers in 2020 on different parts of the skeleton. These not only reconstructed what the real-life Scelidosaurus looked like 193 million years ago, but also revealed it to be an early ancestor of ankylosaurs – the armour-plated dinosaurs of the late Cretaceous Period.
It has taken 162 years to complete this work and, even then, our understanding of Scelidosaurus is sketchy. For example, it remains a mystery why Scelidosaurus fossils are only found at Charmouth.
It struck me how much detective work is involved in palaeontology. It is a bit like particle physics where you never directly see the object that you are studying – just the evidence and traces of its existence, which you use to work backwards to reconstruct reality.
Strangely enough, nuclear and particle physics do feature in our understanding of dinosaurs. If you wander around Lyme Regis looking at fossils in the shops and museums, the tags will proudly proclaim their age. These typically announce that they are 190 million years old – give or take a few million years. But how do we know this?
Radioactivity was discovered by Henri Becquerel in 1896, but it was Ernest Rutherford who in 1904 realised that this newly found phenomenon could be used to find the exact age of a rock… and even potentially the Earth itself!
Rutherford went on to discover the atomic nucleus in Manchester, which we now know contains different arrangements of positively charged protons and neutral neutrons inside it.
Over 3,000 species of nucleus – or nuclide – have been discovered with different numbers of protons and neutrons inside them. Only 300 of them are stable – the rest are radioactive to some extent!
These unstable nuclei basically have the wrong proportion of protons and neutrons giving them too much energy. They correct this by decaying to another nucleus with a more stable arrangement.
Isotopes are nuclides with identical numbers of protons, but with different numbers of neutrons. Many everyday elements have radioactive isotopes – for example most of the carbon in the natural world is carbon-12 (6 protons, 6 neutrons) or carbon-13 (6 protons, 7 neutrons), but there are also minute amounts of radioactive carbon-14 (6 protons, 8 neutrons).
Each radioactive isotope decays exponentially at a characteristic rate that depends on its half-life. By measuring the relative amounts of different isotopes in an object you can work backwards and calculate its age. This is exploited in carbon dating where the activity from small amounts of carbon-14 is measured.
The Turin Shroud in 1988 was dated by this method and it was shown to be only 689 years old, and therefore not linked with the crucifixion of Christ. Similarly, in 1991 a body was found sticking out of a glacier in the Tyrolean Alps. Using carbon dating, ‘Otzi the Iceman’ – as he was dubbed – was estimated to have died between 3350 and 3100 BC.
The only problem with carbon-14 is that it has a half-life of ‘only’ 5,730 years. This is fine for relatively young objects up to 50,000 years old, but not much use for fossils, which can be millions or even billions of years old. Isotopes, with a much longer half-life, are needed such as uranium-238, which decays to lead-206 with a half-life of 4.5 billion years.
To make matters worse, these radioactive elements do not normally exist in the sedimentary rock where dinosaur fossils are found. Nearby layers which include igneous rock, must be used instead to indirectly date the fossil. Nonetheless, the oldest piece of rock discovered on Earth is a zircon crystal from western Australia – estimated to be 4.4 billion years old!
As well as their age, one thing we would all like to know is why are there no dinosaurs roaming Lyme Regis today – what caused their mass extinction?
It was the Nobel Prize-winning particle physicist Luis Alvarez and his son Walter – a geologist – who came up with an answer. In 1980, they suggested that a huge asteroid – the size of San Francisco – smashed into the Earth 66 million years ago filling the atmosphere around the globe with dust and debris. The loss of sunlight stopped photosynthesis with the result that food chains collapsed, and extinction soon followed.
Alvarez based his theory on samples of limestone rock collected by Walter in a small Italian village. He asked two nuclear chemists to determine the concentrations of elements in the rock and was surprised to discover a clay layer 600 times richer in iridium than the surrounding rock.
Iridium – a metal like platinum – is one of the rarest elements in the Earth’s crust, so how did it get there?
Spikes of iridium were soon found in clay layers from other places around the world alongside a layer of soot, pointing the finger to an extra-terrestrial source – like asteroids, which are known to contain high levels of iridium!
When the huge Chicxulub crater – 120 miles wide and 12 miles deep – was found in 1991 off the coast of Mexico, the likely site for the asteroid impact was located. Not everyone accepted the theory, but in 2010 an international panel ruled that the evidence favoured an asteroid sparking the extinction. As the estimated dates for the impact and the extinction agree, it remains the most widely supported explanation.
Other catastrophic events triggered by the impact – like volcanic eruptions and releases of toxins – are likely to have also played a part in ending the rule of the dinosaurs in Lyme Regis… and elsewhere.
Glenn Patrick is a particle physicist and science communicator who explores the quantum world of sub-atomic particles (including at the Large Hadron Collider) and now lives in Lyme Regis.
EDITOR’S NOTE: When fossil hunting, stay away from cliffs, never hammer the cliffs, look out for mud flows and check tide times. Make sure you are properly equipped and, if inexperienced, seek advice from Lyme Regis Museum or Charmouth Heritage Coast Centre.