People have IDs and social networks remind us of our birthdays every year. The trees have their famous rings on the trunk. And we are used to being told what species it is, what it was like, and how many years have passed since it lived when a new fossil is discovered. But, as you can imagine, it is not an easy task to date organic remains from millions of years ago that have been petrified for posterity. How do we know the age of a fossil? There are several methods and we depend on some factors to be able to get closer or closer to a correct chronology. What you can be clear about is that, in the absence of fossils that come with DNI, their dating is only approximate , never exact and, the older the fossil, the more difficult it is to determine its age.
From the outset, we can distinguish between two types of dating and the methods for their study: indirect methods , which offer us a relative dating ; and direct methods , which give us an absolute dating. For now there is no perfect method, therefore, paleontologists are usually not satisfied with a single result, but use several techniques in order to contrast and gain the greatest possible precision.
A cake are surprises
With relative dating we can identify to which moment in Earth’s history the fossil belongs. We would therefore obtain a segment of millions of years in which the fossilized organism would have lived. Stratigraphy will be familiar to you: it is the branch of geology that studies the different levels of rocks ( strata ) that have been superimposed over the millennia. The theory is simple: if you cut the earth like a cake with different layers of flavors, the layer below is older than the ones that overlap. In practice, things can get a bit complicated, since nature does not usually build flat layers to make life easier for us, but we can imagine that someone crushes the cake on both sides at the same time, undulating, mixing and making it more difficult to differentiate the different layers of flavors. Luckily we have the geologists.
We have some help in this chaotic puzzle: the guide fossils . They are the remains of species or even objects that we have already clearly identified in time. For a fossil to serve as a guide, it must be abundant enough on Earth to be able to compare it with the rest of the finds related to it and thus be able to assign the fossil to a short period of time (“short” in geological time are million years, of course). For example, we have the dinosaurs , which we know only lived in the Mesozoic, if we find remains of any of them, we already know that everything we find in the same stratigraphic level is between 252 and 66 million years old for sure. Then the dating can be narrowed further with guide fossils of specific species that we have better located chronologically. This is one of the reasons why it is vital to study fossils in situ, in the context in which they are found, since everything around them provides clues and information for research.
Fine tuning with physics and chemistry
Science advances by the day and, of course, we have more precise techniques that manage to date fossils in a window of hundreds or tens of thousands of years. But, as we have said, they cannot always be used, but rather depend on some factors.
Radiometric dating stands out. It is a technique by which the percentages of two elements that are part of the fossil or the rocks around the fossil are compared . When a rock cools during its formation and, in the case of the fossil, when it is petrifying, minerals are formed. Minerals are made up of different elements, some of these elements are transformed into others and this transformation occurs over a period of time that we know. Therefore, by comparing the elements that remain intact, with those that are transformed, we can estimate a chronology for fossils and rocks.
You will quickly understand this mess with an example. Carbon-14 dating will ring a bell. Well, organisms have Carbon-12 and Carbon-14 while we are alive. When dying, as C14 is more unstable, it gradually disappears. We know that in 5730 years half of the C14 that the organism contains will have been transformed into nitrogen. After another 5,730 years, half of the Carbon-14 remaining in the organism will have disappeared, that is, it would have a quarter of all the Carbon-14 it had when it died. Since the amount of Carbon-12 remains the same as when it died, by comparing the amounts of both elements we can date the fossil fairly precisely. Problem: We can only date fossils up to 60,000 years old with this technique . For older fossils, other elements that have longer periods of decomposition are used, using in the same way the comparison that we have given as an example. Thus, the percentages between Beryllium-10 and Aluminum-26 are studied for fossils up to 15 million years old, Potassium and Argon, or Calcium-41.
more weird names
After this pleasant memory of when you studied (or it will be your turn, don’t worry) the periodic table, we can end by mentioning other dating techniques to finish blowing your mind. Paleomagnetism dating is based on the changes that the Earth and its magnetic pole have undergone. Thermoluminescence dating and simulated optical luminescence detect when minerals were last exposed to the sun or the heat of a fire. And to finish with another technique with a bombastic name, electron paramagnetic resonance exposes the fossil to radiation to measure the energy absorbed throughout its history.
What did you think? 3.5 billion years of life were not going to be dated with birth certificates.
Abril, J. M. et al. 2012. Radiometric dating of recent sediments: beyond the boundary conditions. Journal of Paleolimnology 48, 449-460. DOI: 10.1007/s10933-012-9622-5.Marsicano, C. A. et al. 2015. The precise temporal calibration of dinosaur origins. PNAS 113 (3), 509-513. DOI: 10.1073/pnas.1512541112.Sues, H. 2016. Dating the origin of dinosaurs. PNAS 113 (3), 480-481. DOI: 10.1073/pnas.1523058113.