When we read archeology or paleontology news, one of the most important pieces of information is the age of the creature or object found, an estimated number whose discovery is a work totally anchored in chemistry and physics. How do scientists know the age of a fossil or ancient object, anyway? Briefly, counting radioactive atoms. But the answer can get much more complex than that.
In the most recent artifacts and creatures, within what we conventionally call human history — from the beginning of agriculture to the development of civilizations —, carbon-14 (C-14) dating is used, which works very well to know the age something from the last 55,000 years. After that, it is necessary to use radioactive isotopes, such as uranium-238, uranium-235 and thorium-232. Before we get into them, though, let’s explain how dating works.
How does radiometric dating work?
Don’t be fooled by the size of the atoms of the tiny elements that make up everything on our planet — they contain very important information about the time and events through which their nuclei and electrons have passed. This is related to radioactivity and the frequency of light absorbed by atoms throughout their existence.
Atoms of radioactive elements, such as uranium, contain a lot of energy. So much so, in fact, that their cores, or core, are literally flying everywhere, missing tiny bits all the time. This loss of matter is known as “decay”. Each of these elements decays at a steady, unchanging rate, i.e., no external aspect changes that rate. This only occurs while they are unstable, a characteristic common to radioactive elements.
As they lose particles, these elements change and end up becoming atoms of other elements, generating by-products until they finally become stable. Uranium, for example, passes through countless elements until it finally becomes lead. With a lot of observation and knowledge about chemistry, researchers are able to know the proportions of lead and uranium present in minerals, and, knowing that the more uranium decays, the more lead there will be. The element’s fixed decay rate tells us how old the mineral is.
When talking about radioactive decay, the term “half-life” is usually used. It defines how long it takes for half of the radioactive atoms in a sample to decay, a portion that gives a great idea of the age of an artifact. Elements can have half-lives that are incredibly short, like helium-7 (23 yoctoseconds, or trillions of a trillion of a second), or incredibly long, reaching an age greater than the current age of our universe, as is the case with hydrogen. atomic.
Atoms of the same element can have different shapes, called isotopes, with different masses and people. Uranium-238, for example, the most abundant and long-lived isotope of this element, has a half-life of 4.47 billion years. With it and its “brother”, uranium-235, which has a half-life of 704 million years, we were able to discover the age of ancient minerals from Earth and the Moon and of meteorite fragments that allowed us to estimate the age of our planet — 4.54 billion years, with a margin of error of about 50 million years.
How do radioactive elements mix with human remains and objects?
Maybe you’re wondering, “But aren’t radioactive elements rare? How can a dinosaur or saber-toothed tiger have the estimated age if it is not radioactive?” Well, radioactive elements are everywhere, actually, but in low enough amounts that they don’t harm us. They reach living beings and objects through food and sediments, for example.
Carbon-14 and food
Let’s take the example of carbon-14, more common in the recent past. Its half-life is about 5,730 years, decaying to nitrogen-14. It all starts, oddly enough, from space! From it, cosmic rays arrive, particles coming from the cosmos, which reach the atoms of the terrestrial atmosphere and release their neutrons, which arrive at the nitrogen-14 atoms and form the radioactive carbon-14.
Once formed, carbon-14 is combined with oxygen in the atmosphere and forms radioactive carbon dioxide, which plants absorb and use to manufacture energy so they can grow. Herbivores and omnivores eat plants, absorbing radioactive carbon while they are alive. When they die, they stop absorbing the substance, letting its accumulation inside the body begin to decay until it becomes nitrogen-14. Thus, we can know when the living being died.
Sediments and dinosaurs
For fossils older than 55,000 or 60,000 years, more than 99% of their carbon has already been lost to the environment, leaving very little for scientists to detect. How, then, are measurable elements preserved in ancient fossils?
Well, most very ancient fossils are not actually composed of the original organic material, such as bone, cartilage, skin, and soft tissue, but rather are a mineralized “mold” that, under the right conditions, has solidified into the shape of the bones. remains of the creature that, upon dying, was buried under sediment.
This mineralization around the ancient animal or plant contains its own isotopes, as well as the surrounding sediments, giving us the opportunity to at least estimate when the fossil ended up buried with the minerals. We may not be sure when this happened, but we get a pretty good approximation by figuring out when the creature lived and what other life forms it shared its environment with.
Uranium and thorium, for example, are large isotopes, present everywhere and always unstable, allowing us incredible discoveries like a 4.4 billion year old zirconium crystal in Australia.
Luminescence dating
Finally, a last method allows us to estimate not exactly the age, but rather when a mineral stopped being exposed to sunlight, being enveloped by sediments and rocks, and starting to receive radiation from these new neighbors. Some electrons end up back in the atom, but others get trapped in holes and other defects in the dense atomic cloud around them. A second exposure to heat or sunlight is needed to “kick” these electrons back to their original position — and that’s what scientists do.
When an electron returns to its old position, it emits light, or a luminescent signal, giving the method its name. The longer an artifact or fossil was buried, the more radiation it received from its surroundings. Being exposed to a lot of radiation, the fossil will end up having a lot of moving electrons, emitting a lot of light when it returns to its place.
This method is common in geoscience, useful for calculating changes in landscapes over millions of years and discovering, for example, when a glacier appeared or retreated, depositing rocks in a valley, or when a flood dumped sediments in a river basin.
Source: NIST, LiveScience
