How we date dinosaur bones

Fossils are the bones of our next kids science project. We will, of course, be making bones, burying them, and digging them up in this our fun fossil activities – but what about taking it one step further? What about discovering how old our dinosaur fossils are?

In our Mummification STEM lab that looks at osmosis, we certified our mummy as real (or fake) using the method of carbon dating. We built a pretend mass spectrometer for kids to play with using the true physical concepts of a real mass spectrometer. Can we use the same method to date dinosaur bones? Not quite. Carbon 14, the isotope that is used in carbon dating has a half-life of 5,730 years. If you are wondering if this is fast or slow, or how far you can date with C14, then I have news for you. C14 is considered a fast decay, and it can only date objects that are younger than 50,000 years old. This presents a problem since dinosaurs are 65-250 million years old. That means the Carbon14 in the bones is non-existent.

How do we date dinosaur bones?

That is a great question. If we can’t use carbon 14 dating, what can we use? There are two main fossil dating methods. One method is similar to Carbon14 dating in that it looks at ratios of isotopes that decay. In this case, scientists use K-AR dating, that is, they look at the relative quantities of Potassium40 (K) and Argon40 (Ar). Potassium40 to Argon40 has a much much longer half-life than Carbon14 to Carbon12 – 125,000,000 years. If we want to use this method we will need to find out how much Potassium40 and Argon40 is in our dinosaur fossil sample. This can be tricky, and in fact, we use one technique on potassium and another technique on argon. When it comes to potassium we can burn it and look at the colors of light emitted. Every element has a very specific set of colors, and we can pull out potassiums fingerprint (and thus the quantity) from an overall data set. Argon40, on the other hand, uses the same technique we learned about in our other STEM lab for kids – mass spectrometry. And, indeed, in this lab, we will run pennies and nickels through our mass spectrometer, look at the data, and determine how old dinosaur fossils are that we dug up.

Another method of dating dinosaur bones is looking at the geologic layers dug up when finding the fossils, a method called geochronology. As time goes on, sediment accumulates over the lands. This sediment can turn into limestone, shale, and other types of rock. By looking at the layers, and knowing the age of certain types of fossils we find in those layers, we can learn the approximate age of the dinosaur fossils. You can imagine that the top layers are the youngest (as they were just laid down), and the bottom layers are the oldest. This is why we don’t expect to find dinosaur fossils poking out of the ground – they have been covered by many layers of sediments over millions of years, making them reside deep into the ground. As the method of geochronology has grown, there have been many dating methods used on the actual rock surrounding dinosaur fossils. These methods include:

• Radiometric dating – like that of K-Ar dating and C14 dating.
• Fission track dating – where scientists look at tiny tracks of neutrons running through the material.
• Cosmogenic nuclide geochronology – looking at the interaction of cosmic rays with Earths materials and the materials in question.
• Luminescence dating- looking at the light emitted from pieces of quartz, diamond, or calcite that were found near the fossil.
• Magnetostratigraphy dating – looking at the magnetic field locked into surrounding rock.
• Stratigraphic dating – looking at the layers and fossils found in those layers to gain an idea of the approximate age of the fossil.

We will only look at a few of these methods in our hands-on kids science activity that does lots of labwork and deep learning projects – perfect for homeschool science curriculum.